1986 Johnson 90 Makes Knocking Noise

Repair or modification of Boston Whaler boats, their engines, trailers, and gear
Kend
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1986 Johnson 90 Makes Knocking Noise

Postby Kend » Mon Aug 01, 2022 10:02 am

On July 31, 2022 I ran my Montauk 17 with a 1986 Johnson 90 HP VRO at full throttle with the engine trimmed up to see what boat speed could be obtained: the boat accelerated to over 50-MPH.

Then after a few minutes I could hear a knocking in the engine, I slowed down and within five seconds the noise went away.

After the knocking stopped, I increased engine speed to 3/4-FULL throttle for ten minutes with the engine [trimmed down]. The engine seemed fine .

Q1: what caused the noise?

Q2: why did the noise occur?

Q3: is the appearance of the noise related to the engine having a variable rate oiling system (VRO) and a fault in the oiling that caused the engine to not have sufficient oil at high engine speed?

Q4: if the VRO is the cause, is there a remedy?

Note: previously I have operated the engine at full throttle but not with the engine trimmed up.

jimh
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Re: 1986 Johnson 90 Makes Knocking Noise

Postby jimh » Mon Aug 01, 2022 12:00 pm

Before I can speculate about the cause of the knocking noise from the 36-year-old Johnson 90-HP engine, I have four questions, which are all related and dependent on the answer to the first of my questions:

Q5: does the engine have a tachometer gauge connected to it?

Q6: if there is a tachometer gauge, were you observing the tachometer gauge as you operated the engine at full throttle and trimmed up the engine?

Q7: if there is a tachometer gauge and you did observe the reading when you operated the engine at full throttle and trimmed up the engine, what was the indicated engine speed in RPM?

Q8: if there is a tachometer, and you did observe the reading when you operated the engine at full throttle and trimmed up the engine, and you know the engine speed in RPM, and assuming you know the manufacturer's recommended maximum engine speed in RPM, how does the observed engine speed in RPM compare with the manufacturer's suggested maximum engine speed in RPM?

Kend
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Re: 1986 Johnson 90 Makes Knocking Noise

Postby Kend » Mon Aug 01, 2022 1:42 pm

jimh wrote: Q5: does the engine have a tachometer gauge connected to it?
There is no [tachometer].

I assume the engine [speed] was in excess of 7,000-RPM due to the engine being trimmed up.

To get the speed to 50-MPH took a while.

This engine was rebuilt in [2007]. Since 2007 I estimate the engine has about 20 to 25-hours of run time.

I own three boats. This one does not get much use.

dtmackey
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Re: 1986 Johnson 90 Makes Knocking Noise

Postby dtmackey » Mon Aug 01, 2022 3:55 pm

"Knocking" is a very generic term that could me different things to different people.

To me a knock is a pre-ignition condition, but to others "knocking" could be a mechanical noise of a bad rod bearing or a dislodged piston ring alignment pin that's backed out and made its way between the piston and the head and is being mashed everytime the piston makes top dead center. That noise stops when the piston is expelled through the exhaust ports.

I've also heard people explain "knocking" for the click that the VRO system can sometimes make.

Until you provide a better description of the "knocking" it might be best to have the rebuilder take a listen/ and look at the motor since they were the last people to touch it.

D-

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Re: 1986 Johnson 90 Makes Knocking Noise

Postby jimh » Mon Aug 01, 2022 4:21 pm

I doubt that any rebuilder of an outboard engine offers a warranty that exceeds one years, and a warranty for 15-years after a rebuild would be incongruous. The manufacturer of the engine when new only warrantied it for a year or two. No rebuilder would warranty an engine for over 15-years.

For an engine to only run for 20-hour in a 15-year period is not particularly good for the engine. Engines like to be started, get up to temperature, and run for a while. Running for 90-minutes once a year generally is not an operating regime that leads to prolonged engine life. Engines that are operated for extended amounts of time on a regular basis are the ones that exhibit long-life.

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Re: 1986 Johnson 90 Makes Knocking Noise

Postby jimh » Mon Aug 01, 2022 4:24 pm

Kend wrote:I assume the engine [speed] was in excess of 7,000-RPM...
On that basis I assume the engine speed exceeded the manufacturer's recommended maximum engine speed.

That the engine was allowed to accelerate to an engine speed that exceeded the maximum recommended engine speed specified by the manufacturer is very likely related to the onset of the engine making an unusual noise while operating at that excessive speed.

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Re: 1986 Johnson 90 Makes Knocking Noise

Postby jimh » Mon Aug 01, 2022 4:31 pm

Regarding the engine having variable rate oiling (VRO): the VRO pump that mixes gasoline and oil was designed before gasoline fuel was begun to be diluted with alcohol in the form of corn-derived ethanol. The components of the VRO pump include rubber and other materials that can be harmed by ethanol-gasoline blended fuels.

Q9: was the engine being run for the last 15-years on ethanol-gasoline blended fuel?

Q10: was the VRO pump in the engine the original pump from 1986?

dtmackey
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Re: 1986 Johnson 90 Makes Knocking Noise

Postby dtmackey » Mon Aug 01, 2022 9:04 pm

Kend wrote:I assume the engine [speed] was in excess of 7,000-RPM...
I must have missed the running speed in excess of 7000-RPM. How did this occur, wrong prop selection? Prop out of the water? Revving in neutral? I have concerns at an enginer RPM over 7000RPMs.

Jim mentioned the low hours, but if the motor was rebuilt properly, this is not a problem at all.

VRO systems were also not the best and I'm sure those hoses and carb components are not holding up well with E-10 ethanol fuels.

D-

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Re: 1986 Johnson 90 Makes Knocking Noise

Postby Vance's Revenge » Mon Aug 01, 2022 10:17 pm

dtmackey wrote:VRO systems were also not the best...
Absolutely agree about VRO Systems not being the best.

Some say VRO's got a bad rap and if maintained correctly they were good. I say there is no reason to take a chance on ruining your outboard engine.

I bought my 1992 Johnson GT 200 new and disconnected the VRO immediately after the warranty period and began premixing during fuel fill up. This motor has been ran hard (most of the time with a jet pump) ever since and never had a problem. A lot of other friends that depended on the VRO did not do well and ended up with powerhead damage. Most of those motors went to the scrap yard years ago.

If there is no damage to your powerhead, I highly recommend disconnecting your VRO and premix your fuel. Premixing is easy at 50 to 1 ratio and powerhead damage is incredibly expensive and not worth the worry.

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Re: 1986 Johnson 90 Makes Knocking Noise

Postby jimh » Tue Aug 02, 2022 6:04 am

dtmackey wrote:I must have missed the running speed in excess of 7000-RPM. How did this occur, wrong prop selection?
The thread initiator stated he thought the engine over-revved. Without a tachometer, no one knows what speed the engine reached.

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Re: 1986 Johnson 90 Makes Knocking Noise

Postby jimh » Tue Aug 02, 2022 6:09 am

No determination has been made for any of these questions:

What was the engine speed? --It's not known.
What is the source of the "knocking"? --It's not known
What is the remedy for the "knocking"? --It's not known
Is the VRO to blame?--It's not known.

The only things known:
The engine was run at full throttle and trimmed up to produce more boat speed.
The engine made a knocking noise.

That's all the facts.

Before jumping on the VRO-is-no-good bandwagon, perhaps some actual investigation into the present condition of the engine would be appropriate. The engine's present condition cannot be deduced by readers of this thread. We can only speculate.

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Re: 1986 Johnson 90 Makes Knocking Noise

Postby jimh » Tue Aug 02, 2022 7:10 am

Another anecdotal data point about VRO engines: I bought a 1992 Evinrude 225-HP engine with VRO. The VRO worked perfectly. In 2009 I sold the engine. The 17-year-old engine was working fine, but I strongly recommended to the buyer that as a precaution the VRO pump be replaced with a new pump due to concern about using gasoline diluted with ethanol possibly causing deterioration in the original pump.

More about VRO pumps in

The VRO Story
The Myth of the Mixer

https://continuouswave.com/whaler/reference/VRO.html

dtmackey
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Re: 1986 Johnson 90 Makes Knocking Noise

Postby dtmackey » Tue Aug 02, 2022 8:32 am

The original VRO was a unreliable system that had many failures giving VRO a bad name. VRO2 was quickly launched with improvements that resolved the failure concern, but like a Ficht, the market already soured on VRO.

A VRO2 pump will make a tapping or clicking or knocking sound (call it what you want) which can be unsettling and does not sound like a noise it should be making. I've worked and rebuilt enough of these motors over the years to have seen many VRO anomalies, but ran [the VRO system] on my boats when I owned Johnson and Evinrude engines.

D-

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Re: 1986 Johnson 90 Makes Knocking Noise

Postby jimh » Tue Aug 02, 2022 8:48 am

If a two-stroke-power-cycle OMC engine like a 1986 Johnson exhibits an anomaly when running at full throttle with the engine accelerating to unusually high engine speeds, one possible cause of a problem is a restriction in the fuel supply line. The fuel demand for an engine at its full throttle setting will be much greater than at lower throttle settings, even throttle settings that can maintain the boat on plane. The full-throttle fuel rate might be double the normal rate. If there is any restriction in the fuel supply, the lack of sufficient fuel will show up at full-throttle.

If a fuel-air mixture is too lean, that is, there is not enough fuel, and the cylinder temperature is already hot, there is a strong tendency for pre-ignition or engine "knock."

Since no one but the OP had heard the noise, no one knows if the noise was engine pre-ignition or "knock" caused by the fuel-air mixture being too lean at full throttle and caused by a restriction in the fuel supply system.

A good place to look for a restriction in the fuel supply system is at any fuel filters in the supply hoses, and at the supply hoses themselves.

Another possible cause of reduced fuel delivery is ingress of air into the fuel system. The fuel pump suction pressure (negative pressure or vacuum) will be highest at highest engine speeds. This greater vacuum could lead to air being sucked into the fuel system and producing foaming in the fuel.

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Re: 1986 Johnson 90 Makes Knocking Noise

Postby jimh » Tue Aug 02, 2022 8:55 am

If this engine we are discussing were my engine, here is what I would do:

--since I had the engine rebuilt 15-years ago by a dealer, and

--since the engine had been running fine for 15-years, and

--if that dealer is still in business, then

--I would contact that dealer and make an appointment to have the engine checked by the dealer, and

--I would explain to the dealer the exact circumstances that produced the onset of the "knocking", and

--I would let the dealer use his experience and wisdom to assess the engine with regard to the present condition of the engine, and

--make an assessment in regard to what might have caused the "knocking" and

--make an assessment if there is any damage present from the "knocking" and

--make a recommendation on how to proceed.

A dealer with enough skill to successfully rebuilt the engine and have it run for 15-years afterward, should be in a good position to make all of the above determinations and give a recommendation. Of course, some readers might be, too, but readers cannot examine the engine in person; readers can only speculate.

Kend
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Re: 1986 Johnson 90 Makes Knocking Noise

Postby Kend » Tue Aug 02, 2022 10:23 am

All the replies were helpful.

To answer a few questions:

--the VRO was rebuilt at the time the engine was rebuil

--the noise or knocking sounded like a Diesel engine starting but stopped as soon as the engine came back to a lower speed

--I ran the engine again for about five minutes at a high speed but not all out and it seemed to okay.

Q11: could the problem be the VRO system was not giving the motor enough oil?

Q12: would it make sense to add a small amount of oil to the fuel tanks that would compensate for the VRO not adding enough oil?

The fuel tanks are each 12-gallon capacity.

The gasoline is premium grade gas station fuel.

Vance's Revenge
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Re: 1986 Johnson 90 Makes Knocking Noise

Postby Vance's Revenge » Tue Aug 02, 2022 11:26 am

Kend wrote:All the replies were helpful, to answer a few questions, the vro was rebuilt at the time the engine was rebuilt, the noise or knocking sounded like a Diesel engine starting but stopped as soon as the engine came back to a lower rpm , I ran it agin for about 5 minutes at a high speed but not all out and it seemed to ok, Question,, could the problem be theVRO system was not giving the motor enough oil and would it make since to add a small amount of oil to the gas tanks that would compensate for the VRO not adding enough oil, I’m running with 12 gal tempo tanks using premium gas station fuel .


If your VRO malfunctioned at high RPM and caused powerhead damage to the point of you hearing a knock the chances of the motor not having serious permanent damage are slim to none. Rarely will a piston/rod knock go away so what you heard is odd and hopefully not serious.

If you suspect your VRO of not functioning properly I highly recommend that you do not run that motor until you get the VRO checked, changed, rebuilt or disconnected utilizing pre-mix fuel.

If you need to run the motor you can pre-mix the fuel along with the VRO but it will be running real rich similar to original break in situations on new or rebuilt motors. Premixing 12 gallon tanks is incredibly easy at 2.6 ounces per gallon. In my head rather than worrying about 2.6 oz per gallon; I revert back to the days of my youth when Johnson/Evinrude sold 16 ounce cans of oil to be mixed with their 6 gallon tanks and just fill in 6 or 3 gallons intervals. Everyone carries a cell phone with a calculator in it now so odd gas fill ratio's are easy to compute.

Another direct benefit (other than risking incredibly expensive powerhead damage) to deleting the VRO is there is no longer the need for the oil tank which takes space in your boat.

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Re: 1986 Johnson 90 Makes Knocking Noise

Postby jimh » Tue Aug 02, 2022 1:39 pm

Kend wrote:To answer a few questions...the VRO was rebuilt at the time the engine was rebuilt
If you had the rebuild done 15-years ago that would be 2007. In 2007 the VRO pump would have been the NEW MODEL--not the OLD MODEL that is the subject of so much dock talk. That c.2007 VRO pump should be tolerant of ethanol, but 15-years is a long time to bet the farm on the VRO pump. As I said, when I sold my 17-year-old engine with original 1992 VRO pump, I told the seller that the next item I was planning on servicing with a replacement part was the VRO pump, and I recommend he also plan on that. Also, much of the fuel that ran through that engine was pure gasoline NO-Ethanol fuel obtained at marina fuel docks selling REC 90 fuel.

Kend wrote:Q11: could the problem be the VRO system was not giving the motor enough oil?
That COULD be the problem or it COULD NOT be the problem. There are many things that COULD be the problem.

Kend wrote:Q12: would it make sense to add a small amount of oil to the fuel tanks that would compensate for the VRO not adding enough oil?
That COULD help, and it certainly WON'T hurt. Any time you have a suspicion that there is a lack of oil, you can supplement with pre-mixing oil. But the BEST way to determine if the VRO is working properly is to monitor the volume of oil consumed compared to the volume of fuel consumed. The ratio should be about 1:49 (oil:gasoline). Some outboard engine owners are still fanatical about adding oil to the gasoline, and recommend this be done even with modern direct-injection engines like the Evinrude E-TEC which does not mix any oil in the gasoline.

Kend wrote:The gasoline is premium grade gas station fuel.
That really does not answer the question I asked at Q:10. I asked if the fuel being used has been diluted with alcohol or ethanol. The octane rating of the fuel is not important. What matters is the ethanol content of the fuel. The ethanol in the fuel is what produces harmful effects on rubber components. If you are in Michigan, it is just about IMPOSSIBLE to buy pure gasoline fuel at a highway retail fuel vendor pump. There are just a very few highway fuel stations that sell pure-gasoline fuel, and they are typically located close to boat launch ramps where a very high volume of boaters pass by. When I have the boat on the trailer and am towing it away from home for boating, I usually plan my trip so I can stop at one of the highway pure-gasoline fuel stations on the way and take advantage of buying the REC90 gasoline from them, usually at a lower price than at the marina fuel dock. But other than those two or three stations, you won't find REC90 being sold on the highway at your average "gas station." You have to generally hunt for a fuel source like that.

If you lived in a better run state--a state not sucked into the corn lobby vortex of getting ethanol in every drop of gasoline--you might find pure gasoline at the pump at a highway gas station. You could do that in Wisconsin the last three times I was in that state. No problem finding pure gasoline there. But you cannot do it easily--if at all--in Michigan. The corn lobbyist took over here under the Jennifer Granholm administration.

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Re: 1986 Johnson 90 Makes Knocking Noise

Postby Kend » Tue Aug 02, 2022 4:29 pm

I looked at my receipt from the rebuilder: the engine was rebuilt in 2009. The VRO was not changed, but a new filter and alarm system was installed.

I did notice was the primer bulb is solid and will not squeeze.

Q13: is [the reason the primer bulb will not squeeze] because it is fully primed?

Q14: or, does the primer bulb need replacement?

I tested the Johnson 90-HP engine again. I added added 5-ounces of oil to 5-gallons of gasoline left in the tank. Then I ran the engine at high speed. [The addition of the oil to the gasoline] seemed to solve the knocking.

I will fill the VRO tank to the top. Then I will run the engine on the other fuel tank that does not have added oil in the gasoline, and see what happens.

Based on the results of that test I may get an idea of what needs to be fixed.

Again thanks for all the input.

I would take [the engine] to a local shop, but [when I inquire with the local shop they tell me] "maybe you can bring it in sometime in mid- September."

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Re: 1986 Johnson 90 Makes Knocking Noise

Postby Vance's Revenge » Tue Aug 02, 2022 6:34 pm

Kend wrote:Again thanks for all the input...
Kend--I want to make sure that you realize how serious this is. If that VRO isn't oiling correctly when you switch to raw gas you can be ruining that motor for good. There is no second chance on damage due to lack of lubrication.

And, 5-ounces to 5-gallons is not near enough oil if the VRO quits mixing.

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Re: 1986 Johnson 90 Makes Knocking Noise

Postby jimh » Wed Aug 03, 2022 8:40 am

Vance's Revenge wrote:Another direct benefit...to deleting the VRO is there is no longer the need for the oil tank which takes space in your boat.
Another direct NON-benefit of deleting the VRO is the tedium of measuring and adding oil from the bottles you must carry aboard all the time to the gasoline and doing it in the correct proportions and not forgetting to do it. The VRO does all that for you, and the VRO also gives an alarm if there is a problem with the mixing or if the oil reservoir level is getting low.

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Re: 1986 Johnson 90 Makes Knocking Noise

Postby jimh » Wed Aug 03, 2022 8:54 am

Kend wrote:I looked at my receipt from the rebuilder: the engine was rebuilt in 2009. The VRO was not changed...
This is a big change in the story line. The engine was rebuilt 13-years ago. So it has been running for 13-years without a problem. And it is running on the original VRO pump from 1986. That VRO pump is now 36-years-old. On that basis, you should consider replacement of the VRO pump with a new OEM VRO pump.

But before you do that, you need to establish that no significant damage has been done to the engine. No one has any exact knowledge of what was causing the "knocking" noise. On the other hand, the knocking noise has not recurred, and that may be a good sign.

If there were very severe mechanical damage done to the engine while the engine was running at a very high speed, then that damage would likely still be present and would still be making some audible evidence of the damage when the engine was run later. That the knocking noise as not reappeared may be a good sign. At the least, you should remove the engine cowling and make a very careful visual inspection of the appearance of the engine, looking for any sign of damage or leaking of fluids. Another relatively simple inspection can be made by removing the spark plug from a cylinder and with a strong source of illumination, looking into the cylinder. You can also rotate the flywheel by hand to bring the piston to the top of the cylinder and examine it for damage.

A new VRO pump will be expensive, so before investing in a new pump, you should be certain the engine does not have any mechanical damage to the cylinders or the pistons, or the connecting rods or the crankshaft. Of course, to examine all those components is going to require an expensive diagnostic cost at a dealer. Or, you may decide to remove the VRO pump and revert to the pre-mixing. If you take that approach you will need to get advice on a method to remove the VRO properly and to prevent it from creating false signals or from harming the oil distribution system. Start a new discussion on that if you decide to forgo replacement of the VRO in revert to pre-mixing.

You are now at this point: you own a 36-year-old outboard engine. It made an unusual noise. The cause of the noise has not been determined. The noise has gone away.

The best course of action for you now is have the engine looked at by an excellent two-stroke-power-cycle engine mechanic familiar with OMC engines before running it any more.

The next best course of action--if you want to keep running the engine until a mechanic looks at it--is to revert to pre-mixing oil into the gasoline at a ratio of 1:49 until you find out what caused the engine to make the knocking noise.

Kend wrote:Q13: is [the reason the primer bulb will not squeeze] because it is fully primed?

Q14: or, does the primer bulb need replacement?


You do NOT make clear WHICH primer bulb you are talking about. There are two primer bulbs. There is a primer bulb in the fuel line. There is a primer bulb in the oil line.

Q15: which primer bulb are you talking about?

Also, repeating my earlier question which is not answered:

Q16: was the gasoline you have been using in this engine for the past three decade gasoline that was diluted with ethanol?

Q17: or, was the gasoline you have been using in this engine for past three decade pure gasoline without any ethanol?

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Re: 1986 Johnson 90 Makes Knocking Noise

Postby jimh » Wed Aug 03, 2022 9:10 am

Another new question:

Q18: what is the pitch of the propeller on the Johnson 90-HP engine?

I would very much like you to answer this question.

If the propeller pitch were known, then from the report of a boat speed of 50-MPH, the engine speed could be deduced. The engine gear ratio will also be needed.

Q19: what is the gear ratio of the 1986 Johnson 90-HP engine?

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Re: 1986 Johnson 90 Makes Knocking Noise

Postby dtmackey » Wed Aug 03, 2022 4:16 pm

jimh wrote:Q19: what is the gear ratio of the 1986 Johnson 90-HP engine?


Q19 Answer - 1986 Johnson brochure shows 13:26 (.5) as the gear ratio for the 90 and 110 models.

D-

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Re: 1986 Johnson 90 Makes Knocking Noise

Postby jimh » Wed Aug 03, 2022 5:24 pm

DT--thanks for the suggestion the gear ratio is 2:1 on the engine under discussion. The propeller calculator uses the reciprocal of 0.5 so 2.00 is the proper data to enter.

If we assume the propeller pitch was 19, and we assume the propeller SLIP was about 10, then we can deduce the engine speed at a boat speed of 50-MPH from that data using the PROPELLER CALCULATOR and entering these values:

RATIO = 2.00
PITCH =19
SLIP = 10
MPH = 50

and asking for the engine speed in RPM. The calculated engine speed is then 6,175-RPM.

The OP cited the speed as being "over 50-MPH" so we can also calculate on a boat speed of 55-MPH. That boat speed produces a calculated engine speed of 6,795-RPM.

We could also modify the SLIP value, setting it lower to indicate the propeller was working more effectively. Repeating the above with SLIP = 5 the 50-MPH boat speed suggests the engine was running 5,850-RPM and the 55-MPH boat speed suggests the engine was running at 6,435-RPM.

All of those engine speeds are probably close to the limit for a 36-year-old old two-stroke-power-cycle engine to be running for extended times like over five minutes.

As a comparison, my Evinrude E-TEC engine which was first run in 2009 or 13-years ago has a cumulative operating time at 5800 to 6000 RPM of 156-seconds, and a cumulative operating time at 6000 to 6200 RPM of 28-seconds. Those faster engine speeds were all put on during brief propeller testing with lower pitch propellers, and the engine was never run for extended periods of time at a speed above 5800-RPM. which is the upper bound of the manufacturer's recommended full throttle engine speed operating range. Those speed are for a V6 engine.

If the 90-HP is a V4 design, I would expect the recommended full throttle engine speed may be lower; a V4 does not run quite as smoothly as a V6 and the allowed engine speed may be lower than 5800-RPM.

Again, this is all based on a GUESS that the propeller pitch was 19-pitch. We will have to wait for KEND to answer my earlier question, Q18, asking for the propeller pitch. I am guessing 19-pitch because that would be a common pitch for a 90-HP on a light boat like a MONTAUK.

Again, this is a speculation because we don't really know what the boat speed was. The boat speed might also be just a guess. So now we have another question for the OP:

Q20: how was the boat speed measured in the performance test cited in the initial post as being "over 50-MPH."

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Re: 1986 Johnson 90 Makes Knocking Noise

Postby jimh » Wed Aug 03, 2022 5:31 pm

dtmackey wrote:A VRO2 pump will make a tapping or clicking or knocking sound (call it what you want) which can be unsettling and does not sound like a noise it should be making.
I meant to comment on this earlier. I do not recall while operating my VRO engine that I ever heard the VRO making a noise, as the general engine noise was the dominant noise emitting from the engine--and what a nice noise that was, 225-HP roaring along at over 5,500-RPM. But I respect DT's experience with OMC engines, so if he has heard a tapping or clicking noise originating from a VRO pump, then I believe him.

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Re: 1986 Johnson 90 Makes Knocking Noise

Postby Vance's Revenge » Wed Aug 03, 2022 7:31 pm

I never hit a rev limiter or even know what the limit is set on a 90hp OMC motor from those years. Is it possible he hit the rev limit and the shutdown caused his knocking sound?

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Re: 1986 Johnson 90 Makes Knocking Noise

Postby jimh » Wed Aug 03, 2022 8:08 pm

VANCE—I think you have the answer. Your remarks are very likely to be exactly what occurred. The engine protected itself. A beautiful mind! A beautiful answer!

The sound of a rev-limiter is sometimes described as a “barking” sound because the engine suddenly cuts out, the engine speed drops below the rev-limit threshold, the engine resumes running and accelerates above the rev-limit threshold, and it cuts out again. The result is like a dog barking: arf, ARF, AAAARF, silence, arf, ARF, AAAARF….repeat.

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Re: 1986 Johnson 90 Makes Knocking Noise

Postby dtmackey » Wed Aug 03, 2022 10:21 pm

Vance's Revenge wrote:I never hit a rev limiter or even know what the limit is set on a 90hp OMC motor from those years. Is it possible he hit the rev limit and the shutdown caused his knocking sound?


1986 Johnson 90s did not have a rev limiter, so there's no way he could hit it. Factory recommended operating RPM range was 4500-5500RPM.

D-

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Re: 1986 Johnson 90 Makes Knocking Noise

Postby dtmackey » Wed Aug 03, 2022 10:30 pm

jimh wrote:VANCE—I think you have the answer. Your remarks are very likely to be exactly what occurred. The engine protected itself. A beautiful mind! A beautiful answer!

The sound of a rev-limiter is sometimes described as a “barking” sound because the engine suddenly cuts out, the engine speed drops below the rev-limit threshold, the engine resumes running and accelerates above the rev-limit threshold, and it cuts out again. The result is like a dog barking: arf, ARF, AAAARF, silence, arf, ARF, AAAARF….repeat.


Depends on the type of rev limiter used, here's a couple different ways rev limiters work and they vary by motor and model:

1. When the motor hits a predetermined RPM, spark it cut until the RPMs fall to a predetermined RPM and then spark is turned back on. This sounds like what Jim has described.
2. When the motor hits a certain RPM, the EMM, ECU or brain pulls spark from some of the plugs to limit the RPMs.
3. When the motor hits a certain RPM the spark advance is retarted (as in reduced) and the motor doesn't want to accelerate as much to a higher RPM
4. Safety - where the EMM, ECU or brain limits the RPM to a much lower RPMs due to a safety built into the motor for a variety of differnt reasons that vary motor to motor and manufacturer - Low oil level, high temp, etc.....

There are more ways to limited RPM, but these are the basic ones, but since the 1986 Johnson 90 has NO rev limiter, that does not apply in this case.

D-

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Re: 1986 Johnson 90 Makes Knocking Noise

Postby jimh » Thu Aug 04, 2022 11:23 am

dtmackey wrote:...since the 1986 Johnson 90 has NO rev limiter...
Very good to know. Darn that REV-Limiter concept was a really good explanation.

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Re: 1986 Johnson 90 Makes Knocking Noise

Postby Kend » Thu Aug 04, 2022 5:56 pm

To answer a few questions:

I believe the prop is 13x15, but will I check the pitch when I can get in the water to look.

The knocking noise was similar to a diesel engine when it first starts up. The sound was not a swishing sound.

I believe the VRO is working.

I believe when I trimmed the engine up and ran the engine at the highest speed possible the VRO didn’t put enough oil.

As already mentioned, I added some oil to the [gasoline fuel] tank to see if it would help. I ran the engine again at full RPM. The engine ran without [making the knocking noise]. I also added a quart of oil to the VRO tank which was at Half-full before I did the repeat test at full RPM.

Before I take the engine in for service, I plan to "weed out" all the possible causes of the knocking noise. My method for the next test [the third test] will be

  • I will fill the VRO tank to the top.
  • I will [switch the fuel supply to the other gasoline tank, the one that does not have any added oil].
  • Then I will see what happens.

When I complete [the third test] I will post the results.

I’m in Michigan and the weather has been stormy. [To get the third test done] make take several more days.

Thank you for all the input.

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Re: 1986 Johnson 90 Makes Knocking Noise

Postby jimh » Fri Aug 05, 2022 8:17 am

Kend wrote:...I believe the prop is 13x15
In order for an engine with a gear ratio of 2.00 to turn a 15-pitch propeller at a speed sufficient to propel the boat at "over 50-MPH", the engine speed would need to be in the range of 7,800- to 8,600-RPM. That seems like a preposterous engine speed.

You also omitted an answer to my earlier question.

Q20: how was the boat speed measured?

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Re: 1986 Johnson 90 Makes Knocking Noise

Postby Kend » Mon Aug 08, 2022 10:52 pm

To measure boat speed I used a smartphone with a GPS speed app.

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Re: 1986 Johnson 90 Makes Knocking Noise

Postby jimh » Tue Aug 09, 2022 8:16 am

Kend wrote:To measure boat speed I used a smartphone with a GPS speed app.
That should be accurate to about plus or minus 2-MPH. On that basis your report that the boat was able to accelerate to a boat speed of "over 50-MPH" is likely to be accurate.

If you achieved that with a propeller pitch of 15-inches, you probably set the world record for highest crankcase speed with a 36-year-old Johnson 90-HP two-stroke-power-cycle engine while doing that. I would not attempt that feat too many times.

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Re: 1986 Johnson 90 Makes Knocking Noise

Postby Kend » Sun Aug 21, 2022 11:01 am

I changed the spark plugs. I changed the the gear case lubricant. I added oil to the the VRO tank to FULL; the tank was at 1/2-FULL.

Then I got the boat underway and ran the engine at a high speed for 20-minutes. The engine trim was set down. The engine ran without making the knocking sound heard before.

I am not a mechanic. Common sense suggests [the cause of the knocking noise heard once before was because] the engine speed was greater than the recommended engine speed.

I am now concerned because the [primer bulb for the ] VRO [oil supply line to the engine] is hard [and it] will not squeeze further.

[Moderator's note; the next sentence was marked with a question mark but not written as a question. This is ambiguous and the sentence could be read two ways. Both possibilities are shown below.]

As a statement: the engine is getting oil.
As a question: is the engine getting oil?

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Re: 1986 Johnson 90 Makes Knocking Noise

Postby jimh » Sun Aug 21, 2022 12:06 pm

The in-line primer bulb on the oil supply line from the oil reservoir to the engine will generally become very hard when the line is primed. As I recall, on the oil line from the oil reservoir tank on my Evinrude engine the primer bulb has been very firm for above five years, and no further priming has been necessary during that five year period, nor would the bulb yield to compression. Many gallons of oil have been consumed in those five years, so I am certain the oil system is working properly, and the situation with the primer bulb remaining very firm is completely normal.

On that basis, if you have a concern about the VRO supplying oil to the engine because the primer bulb is very firm, I don't think there is a problem. But, as always, the VRO warning system would alert you if the VRO pump was not operating properly. You can also visually confirm that the oil level in the oil tank goes down as the engine runs.

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Re: 1986 Johnson 90 Makes Knocking Noise

Postby jimh » Sun Aug 21, 2022 12:16 pm

Have you any more information about the propeller and its pitch?

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Re: 1986 Johnson 90 Makes Knocking Noise

Postby Kend » Sun Aug 21, 2022 2:03 pm

Jim, and everyone else that replied thanks for all the info, I will get the prop dia and pitch next time I get in the water, the boat stays in the hoist so the only way to see the prop to get in the water.