1987 Yamaha 70 Long Saga of Repairs

Repair or modification of Boston Whaler boats, their engines, trailers, and gear
Davo785
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1987 Yamaha 70 Long Saga of Repairs

Postby Davo785 » Tue Aug 11, 2020 12:10 pm

[Moderator's note: all posts by this author have been extensively edited and re-written in order to present the information in a more concise and readable manner, to remove repeated use of pronouns without any clear antecedent nouns, to follow the normal rules for punctuation and capitalization, and in general to remove extraneous information whose inclusion had no possibility to exert an influence on the engine behavior that is being discussed--jimh]

[A 1987 Yamaha 70-HP ETLH outboard engine will not accelerate to more than] 1,500-RPM at maximum throttle setting. I have had this outboard for over 15 years. The Yamaha 70-HP is on my 1988 SUPER SPORT 15. I know this Yamaha 70-HP engine and I maintain like crazy.

The initial run of this engine in 2020 was across lake. The engine could only accelerate to 3,000-RPM at full throttle.

Later [in that same time frame] the maximum engine speed decreased to only 1,500-RPM. Three days later, the engine could accelerate to 4,000-RPM. After that engine speed was reached, I stopped the engine. Then when I tried full-throttle again the engine would only reach 1,500-RPM

I don't believe the Yamaha 70-HP engine is overheating because the problem [occurs] before the engine temperature rises.

On August 7, 2020 I bypassed the [the temperature sensor on the power head].

In 2018 I installed a new throttle cable.

In 2015 I installed a new 703 control box.

In 2019 I replaced the gear case with a rebuilt gear case.

I rebuilt the water pump [at some earlier time].

[At some earlier time] I have replaced the thermostat, the fuel pump, cleaned the fuel filter, and I inspected the fuel lines.

Three different fuel tanks have been used while running the engine.

I [mix] STA-BIL 360 Marine [additive] in the fuel at all times.

I have [inspected] every [electrical] ground and [electrical] bullet connector. I eliminated any corrosion found.

I have tested spark. The spark seems good but that's only while dry cranking.

I have checked the throttle cable and all motion of linkage including magneto advance while running.

I do not have a test tank.

I wonder about the [capacitor discharge ignition] unit.

I wonder about possibility that the [capacitor discharge ignition] suddenly lost dynamic timing.

I wonder about any part within the magneto system that would allow the Yamaha 70-HP enginet to start, idle, but not go past 1500-RPM.

I am not ruling out carburetors.

All seems normal except the 1500-RPM [engine top speed].

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Phil T
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Re: 1987 Yamaha 70 ETLH Won't accelerate

Postby Phil T » Tue Aug 11, 2020 12:26 pm

I think it may be only running on 1 or 2 cylinders.

Take the boat out and shoot a video of the engine idling, planing off and runnning wide open. Post a link or email to me and I will post it for you.
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1992 Outrage 17, 1992 Evinrude 115

jimh
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Re: 1987 Yamaha 70 ETLH Won't accelerate

Postby jimh » Tue Aug 11, 2020 12:47 pm

I don't see any mention of any work done on the carburetors. The carburetors deliver the fuel to the combustion chamber. Without adequate fuel the engine speed could be limited.

Regarding ignition spark timing, you will need a timing light to check that. Two-stroke-power-cycle engine speed is very much influenced by proper spark timing advance for higher engine speeds. Typically on older engines the spark timing advance might have been implemented mechanically by rotation of the stator plate via a throttle linkage. I used to own a pair of 1987 Yamaha 70-HP engines, and they were very nice runners. They loved to run around 4,100-RPM, where the engine note had a nice resonance or ring. My only electrical problems with the engine were with the battery charging rectifier-regulator assembly.

dtmackey
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Re: 1987 Yamaha 70 ETLH Won't accelerate

Postby dtmackey » Tue Aug 11, 2020 10:40 pm

Diagnosis by internet is amusing. Being there to hear, see and feel makes a big difference in narrowing it down quickly.

D-

Davo785
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Re: 1987 Yamaha 70 ETLH Won't accelerate

Postby Davo785 » Wed Aug 12, 2020 10:25 am

Compression is okay. Spark is okay and tested while idling. Carburetors have not been serviced for some time, but I do my best to keep them clean. Gear case was rebuilt. To get older smaller gear cases rebuilt is hard now. I put on [the rebuilt gear case] in Fall 2019. The engine ran well. I drained gear case oil later and found nothing wrong in the oil.

The 1500 RPM failure started in June 2020.

[After June 2020] the gear case oil was again drained and found clean as can be

I hesitate to over-rev [without a load on the engine], but on August 11, 2020 I tried. The engine quickly accelerated to 3000-RPM. I [reduced] throttle fast for fear of damage. {The limitation to 1,500-RPM in engine speed] seems to only occur under a load.

jimh
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Re: 1987 Yamaha 70 ETLH Won't accelerate

Postby jimh » Wed Aug 12, 2020 11:14 am

As PHIL has already suggested, the engine's inability to accelerate under load is probably related to loss of power in one or two cylinders. If the cylinders have good spark, then the most reasonable inference for the cylinders not firing is lack of fuel.

Davo785
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Re: 1987 Yamaha 70 ETLH Won't accelerate

Postby Davo785 » Wed Aug 12, 2020 4:05 pm

This 1987 Yamaha outboard engine has a speed control. The speed control has a signal that will provide a malfunction indication in the event the speed control functions when not intended. If [the speed control] receives a signal from the engine temperature sensor or overheat sensor low-oil sensor, the engine speed is automatically reduced.

I cannot find where [some component in the speed control that was only described as "it"] is [i.e, exists] when activated to test it [unclear, perhaps is trying to say whether or not the speed control is being activated].

I eliminated heat sensor with test; the oil warning light is green.

Phil T wrote:Take the boat out and shoot a video of the engine idling, planing off and running wide open.


I am going to take out the boat for a run with a mechanic on August 14, 2020. If the mechanic cannot help me figure out the cause then I wi'll try to [make a motion picture and audio recording] .

jimh
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Re: 1987 Yamaha 70 ETLH Won't accelerate

Postby jimh » Wed Aug 12, 2020 5:55 pm

I believe your intent is to describe an alarm or alert system that also implements an engine speed limitation when certain alarm conditions exist. My advice is to obtain an electrical schematic diagram of the engine. Note that there is an electrical schematic diagram of the engine in the owner's manual or operating manual. Inspect the engine electrical wiring. Look for an alarm module. Look for a circuit from the alarm module that carries a signal to the ignition module.

biggiefl
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Re: 1987 Yamaha 70 ETLH Won't accelerate

Postby biggiefl » Thu Aug 13, 2020 9:55 am

I have a better idea. Disconnect the wiring harness and pull start it. Anything electrical will be eliminated and you will have to choke it to stop it or reconnect the harness.
On my 24th Whaler. Currently in the stable: 86 18' Outrage, 81 13' Sport(original owner), 87 11' Sport, 69 Squall(for sale cheap).

Davo785
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Re: 1987 Yamaha 70 ETLH Won't accelerate

Postby Davo785 » Fri Sep 11, 2020 1:41 pm

I replaced the gear case last fall--which is the last time it ran great--with a remanufactured [gear case] that now has around 50-hours use. Have drained [the lubricant from the gear case] twice. The lubricant looks great.

The acceleration problem persists under load.

I have replaced all fuel, fuel tank fittings, new OEM fuel pump.

I have personally gone through every [electrical] bullet connector and fixed any corrosion.

I had my mechanic re-check the cylinder pressure; 100, 105, 100-PSI. Those figures are not [as high as would be obtained in a] new [engine,] but [they are] not terrible.

He also checked spark timing and advance. He cannot find any electrical defects

Got out my wallet again and had the mechanic rebuild all three carburetors. Then I went for a lake test, The now empty empty boat with just myself still cannot accelerate onto plane.

Could the [capacitor discharge ignition] just not be responding correctly?

Can the remanufactured gear case be the cause?

[Can the hull having] bottom paint be the cause? The bottom paint needs to be repainted but bottom paint has never slowed down the boat before

I can't give up at this point.

biggiefl
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Re: 1987 Yamaha 70 ETLH Won't accelerate

Postby biggiefl » Fri Sep 11, 2020 1:49 pm

Lower unit could have a frozen bearing on the drive shaft but if that is the case it would usually stall going into gear like you hit bottom. Pull the plugs, put it in gear and turn the prop by hand or foot. If it will barely turn, you found your problem.
On my 24th Whaler. Currently in the stable: 86 18' Outrage, 81 13' Sport(original owner), 87 11' Sport, 69 Squall(for sale cheap).

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Phil T
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Re: 1987 Yamaha 70 ETLH Won't accelerate

Postby Phil T » Fri Sep 11, 2020 2:07 pm

A month ago, in the first reply I suggested taking a video (use your phone) of the engine idling, accelerating and post.

Why not do that instead of throwing parts at it?

I have helped 5 owners this summer via videochat in their boat underway or at the dock solve a problem.
Member since 2003
1992 Outrage 17, 1992 Evinrude 115

Davo785
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Re: 1987 Yamaha 70 ETLH Won't accelerate

Postby Davo785 » Fri Sep 11, 2020 2:30 pm

I have done that prop test; it seems OK.

What about the [inference that the cause of the engine being unable to accelerator is from the] stator?

Davo785
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Re: 1987 Yamaha 70 ETLH Won't accelerate

Postby Davo785 » Fri Sep 11, 2020 4:04 pm

My mechanic and many others said [the problem was] the fuel pump or the carburetors.

And as for planing out, the engine is running very smoothly, doesn't ever just die, and when I use my weight at the bow to get it going, the engine accelerates slowly.

Now the mechanic thinks the high end of stator may have gone bad.

My mechanic is not great with they way Yamaha wires things.

very motivated to figure this out and don't think this problem is unsolvable.

Tthe only thing that didn't need to be done so far was the fuel pump. That is only a $40 part and the easiest thing so far.

The work has been my own time, going through wires, various testing I could think of, and figuring out the wiring on a new RPM gauge.

I'm an old guy that plans on this continuing being my fishing boat.

Davo785
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1987 Yamaha 70-HP

Postby Davo785 » Tue Oct 13, 2020 4:06 pm

I called an outboard engine repair company that specializes in older outboard engines for advice on the repair of my 1987 Yamaha 70-HP engine. They suspect the cause of the problem is borderline compression. During a half-hour conversation the person at the outboard repair company commented that if the engine still will not accelerate under load after all the prior repairs done, and with cylinder pressures of 100, 105, and 100-PSI, then the engine compression is borderline, and he speculated the Boston Whaler boat is too heavy for the engine. The fellow thought the Yamaha 70-HP engine would probably would work well on a lighter boat.

At first I thought that was crazy, but the more I think about how the boats acts as I throttle up depending on the weight in the boat, this might just be true.

jimh
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Re: 1987 Yamaha 70 Long Saga of Repairs

Postby jimh » Wed Oct 14, 2020 8:54 am

The reason you have to move weight forward to get the boat to plane is the engine cannot produce enough power to accelerate up the bow wave with the boat in a bow-high trim. Reducing the bow-high trim by moving weight forward reduces the uphill nature of passage of the boat over its own bow wave, and permits the boat to reach hydroplane with less power output from the engine.

Any engine, and particularly a two-stroke-power-cycle engine, will exhibit the ability to accelerate to extremely high engine speed when the load on the engine is reduced to a low level. The ability of the engine to accelerate to its rated engine speed under load is the best indicator that the engine is able to produce its rated power output. Clearly the problem with the engine under discussion is that it is unable to produce its rated power output of 70-HP (which actually might be more like 65-HP if one allows for the variations in power rating allowed under the ICOMIA standard generally used to rate engine power) under the normal load. Here I am assuming that at some time in the past history of this boat and engine combination, the engine was able to accelerate the boat unto plane without difficulty and in a reasonable time frame.

Measurement of cylinder pressure can vary depending on several factors, including the gauge being used to measure the pressure and the presence of any lubricants in the cylinder that might enhance the sealing of the piston rings to the cylinder walls. The usual metric for assessment of engine cylinder pressure is the amount of variation among all cylinders. In your report of cylinder pressure of 100, 105, and 100-PSI, there is only 5-PSI variation, which is a very small variation. This suggests that there is no one cylinder that has lost compression. The notion that ALL three cylinders have lost compression in a very uniform and almost perfect match seems unreasonable, although you mention that you have used this engine for 15-years, which does lend some possibility of a gradual and uniform loss in compression.

I recommend you carefully make a recollection of exactly when the engine began to exhibit problems in getting the boat onto plane. If you can determine a particular date or epoch when the behavior of the engine changed to being unable to accelerate under load from previously having been able to accelerate under load, then give thought to what might have changed or occurred at that particular date or epoch. From that information you might be able to make a better inference about the cause of the problem you now have.

jimh
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Re: 1987 Yamaha 70 Long Saga of Repairs

Postby jimh » Wed Oct 14, 2020 9:03 am

Regarding the stator as being the cause of the problem and also the notion introduced by your mechanic that there is some sort of duality in the stator in which there is a "high" or "high end" stator:

Having owned two 1987 Yamaha 70-HP engines myself, and having had to replace both their stators, I do not have any recollection that there were different windings on the stator assembly or circuits being supplied current by the stator that were employed at different engine speed ranges. This notion seems to have been introduced by a mechanic. Perhaps the mechanic was thinking of certain older Mercury outboard engines which employed dual stators and dual circuits for spark ignition voltage generation that operated at different engine speed ranges.

The stator coil assembly does contain the coil winding that will generate the primary voltage for the spark coils. Any defect in the stator which might cause the coil winding for the spark coil primary to generate a reduced voltage or current output could lead to loss of spark. Replacement of the stator will require removal of the flywheel. If the flywheel has been in place since 1987, or 33-years, removal of the flywheel may require use of special tools such as an impact driver and a special wrench to hold the flywheel in place while the flywheel retaining nut is being removed. The cost of a new stator assembly is probably over $100, but in consideration of all the other components that have been already replaced, replacement of the stator might be worth the investment. My general opinion is the stator assembly is not particularly robust on the 1987 Yamaha 70-HP engine, and my need to replace both stators is the basis for that assessment. The only other components of the two 1987 Yamaha 70-HP engines I owned that needed replacement were the spark plugs, the water pump impellers, and on one engine the propeller shaft outer seal. Otherwise the engines were quite reliable and did not need repairs, only routine service.


Loss of spark at the engine under load can be tested for and observed if a spark-gap test tool is used. I suggest you insert a spark gap test tool in the spark plug wiring and observe the engine spark under load when the engine is unable to accelerate.

I also recommend you check the spark plug spark gap distances. It is a well known behavior of engines to suffer problems in acceleration under load when the spark gap distance on their spark plugs is too great. I do not recall from your lengthy narrative of repairs a mention that you have checked the spark plug gap distance or replaced the spark plugs with new plugs (of the correct type) with the proper gap.

Regarding testing of spark: the best method to test spark is to insert a SPARK GAP TESTER in the spark plug wire to the spark plug. The air gap in the spark gap tester should be easy for a proper spark voltage to jump across. A typical spark gap tester is made by Lisle and sold in automotive parts stores. The cost is modest, about $10.

Image
20610 LISLE In-line Ignition Spark Tester

Anecdotal Comment: the only time in my history of operation of the 1987 Yamaha 70-HP engines that exhibited a problem accelerating the boat onto place occurred with there was a loss of spark on one cylinder. This problem was not evident in any other situation of running the engine. It only manifested when trying to accelerate the boat onto plane.