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Author Topic:   2000 Mercury 90-HP FOURSTROKE Specifications, Problems, and Solutions
pcrussell50 posted 02-24-2011 04:16 PM ET (US)   Profile for pcrussell50   Send Email to pcrussell50  
I have [a 2000 Mercury 90-HP FOURSTROKE] on my new Alert. I have the expensive, factory service and repair manual, but I do NOT have the operator and owner's manual. So I have a couple of basic operator questions:

What is the redline RPM?

The previous owner said that with this motor, you don't choke, even though it's a carburetor four-cycle engine, and there's no fast idle and warmup lever, like in carburetor two-cycle engines. He says you simply turn the key and it enriches itself, starts and warms itself up, just like a fuel injected motor. Indeed, it did that just fine, when he demoed it to me, pre-sale, and when I started it in my driveway when I got it home last week. How does this system work without a computer and EFI?

I understand about "making oil". How do I know if I've run this motor long and hard enough to heat the oil enough, to burn off precipitated water? On my race car, I have an oil temperature gauge.


number9 posted 02-24-2011 07:48 PM ET (US)     Profile for number9  Send Email to number9     

You can download the OM from the Mercury EU site.


jimh posted 02-24-2011 10:41 PM ET (US)     Profile for jimh  Send Email to jimh     
We do not have an organized collection of historical data about outboard motors. I have an old Mercury catalogue dated model year 2000 that lists a 90-HP FOURSTROKE as having an maximum engine speed range at full throttle as 5,000 to 6,000-RPM.

Your seller probably demonstrated the engine starting after it was warm. Most all engines start easily when warm.

You need to get the owner's manual to learn the starting procedure. Just about every engine has its own unique starting procedure. When you follow the procedure that is recommended, the engine will usually start better than if you just make up your own starting procedure.

The old carburetor systems often have enrichment circuits with pumps that will try to help with starting and running smoothness. Since you have the repair manual, the whole fuel system should be carefully described in the manual. You will know more about it than anyone.

You probably want to monitor engine temperature to see if the engine cooling system is working properly. On outboard motors there is really an infinite supply of cooling water, and the cooling system uses regulation of cooling water flow to keep the engine operating temperature in a warm range when running at moderate or low throttle. Typically the engine cooling system will try to keep the engine around 140-degrees-F at slow speed operation. When the throttle is advanced, the cooling system tends to pour on the cold water, and this will drive engine temperature lower as engine speed increases.

In Mercury engines there are usually two regulating valves in the cooling system. One valve operates on temperature. This is called the thermostat. It opens and closes based on temperature. A second valve or valves operates on pressure. When engine speed increases the pressure of the cooling water system increases. These pressure valves respond to the increased pressure so as to increase cooling. They are often called poppet valves because of their general shape and design. They open or close in response to water pressure in the cooling system.

If the thermostat malfunctions it often leads to the engine running too cold at low speed. If the poppet valves malfunction it often leads to the engine running too hot at high speed.

If you monitor engine temperature you will see it go up and down with engine speed. The most important situation for preventing oil dilution or making oil is to have proper operating temperature at low speeds. The engine should be around 140-degrees when running at low speeds.

jimh posted 02-24-2011 10:47 PM ET (US)     Profile for jimh  Send Email to jimh     
It is not hard for a motor to have an idle speed control circuit based on engine temperature which does not use a choke in the carburetor. Typically there is a temperature sensitive switch which connects to the ignition control module. Engine speed at idle often is sensitive to spark timing. You can create a fast idle speed by changing spark timing. When the engine warms up, the temperature sensitive switch closes. This signals the spark controller that it is time to change the spark timing at idle speed back to normal, which slows the idle speed.
jimh posted 02-24-2011 10:48 PM ET (US)     Profile for jimh  Send Email to jimh     
Since you have the service manual, you should be able to find a description of how the fast idle speed circuit operates.
95Outrage17 posted 02-25-2011 12:46 AM ET (US)     Profile for 95Outrage17  Send Email to 95Outrage17     
Hi Peter,

The "redline" for your motor is 6000 RPM. My 2004 version reaches this RPM trimmed out at full throttle.

The previous owner of your motor was essentially correct about starting it. If it's in good tune it will start simply by turning the key, cold or hot. When the motor has been sitting unused for a while it may idle a little cranky at first, but will smooth out after a few minutes. Mine rarely stalls, but can sometimes take a few turns of the key to start in very cold weather. Not as smooth as EFI, but nothing like an old carbed 2-stroke with manually operated choke when cold. I've never read enough about the system to understand exactly how it works, but there is a computer at the front of the powerhead behind the intake plenum. The only problem I've experienced with the system involved fouled carbs which caused idling problems. There are some tricks to improve this situation before tearing all the carbs apart. Of course it's easiest to take measures to keep the carbs clean in the first place though. I use stabilizer in the fuel if the boat is going to sit unused for more then a few weeks.

In my experience, these motors don't "make" a lot of oil unless there's a mechanical problem. Mine had a leaking fuel pump diaphragm and another Yamaha version (2001 F100) had a corrosion problem between the cylinder block and head.
Mine has a temperature gauge which has an aftermarket sender on the cylinder head. The motor seems to typically warm to operating temp after about 5-10 min at around 3000 RPM and reaches the highest temps when run at full throttle and fully trimmed for a couple minutes. During a typical cruise I usually try to reach full throttle at some point for a short period, but not always. I usually run between 3000 and 4000 RPM. I have not had a significant problem with the engine "making" oil since the fuel pump was replaced a couple years ago. It's hard to estimate without an hour meter, but I wouldn't be surprised if my 2004 has 700-1000 hrs on it.

As a side note; I imagine you are aware that this motor's powerhead is basically a carbureted version of a Yamaha F75-F100? I work on and maintain several of them, both Mercury and Yamaha versions. The powerhead and related components are identical other then the Mercury's block being painted black instead of gray/blue.

I could never figure out why it seems the Mercury versions tend to have more carb problems then the Yamaha versions until someone brought up a valid observation recently; The Mercury cowlings are black. Imagine how hot it gets under that cowl on a warm sunny day. Yamaha cowls are gray/blue and likely don't get nearly as hot. Less heat means less chance of the fuel in the carbs evaporating. Seemed like a valid explanation to me...

- Chris

pcrussell50 posted 02-25-2011 03:59 PM ET (US)     Profile for pcrussell50  Send Email to pcrussell50     
Chris, thanks for the very informative answer. My motor has an hour meter that says it has 811 hours on it or so. Brian was a pretty hardcore user. I'll be lucky to do 50 hours a year. Probably more like 25.


jimh posted 02-26-2011 10:08 AM ET (US)     Profile for jimh  Send Email to jimh     
Chris has made a very cogent observation. Based on anecdotal reports presented here, we have seen many more complaints about the Mercury FOURSTOKE 90-HP than we have about the practically identical Yamaha F90. Chris's suggestion that the basis for this difference may be due to the color of the engine cowling is quite interesting. I tend to agree with the notion that the temperature of the engine when sitting in the sun will be greater on an engine with a black cowling. This may produce a temperature rise that could lead to more fuel evaporation.

Fuel evaporation is likely related to onset of problems with the carburetors. When fuel evaporates from the carburetor, it may leave behind some residue which can create problems for future fuel flow. Apparently once these residues dry, they become less soluble in gasoline than you might expect, and tend to restrict fuel low.

THE GAM is really not the place for lengthy discussion about engine running problems. If the topic of problems with carburetors on 90-HP Mercury FOURSTROKE engines is to be pursued further, please move the discussion to the REPAIRS/MODS forum. Thanks.

FISHNFF posted 02-26-2011 10:27 PM ET (US)     Profile for FISHNFF  Send Email to FISHNFF     
Just a quick note.

I always stored the motor with a Tough Duck Sunbrella cover.



ADKWhaler posted 02-27-2011 10:08 AM ET (US)     Profile for ADKWhaler    
I have the 75 HP version of your motor. Make sure you use an appropriately sized prop.


bloller posted 02-27-2011 10:24 AM ET (US)     Profile for bloller  Send Email to bloller     
My 2004 60HP Mercury Fourstroke EFI seemed to make oil during the first 100hrs or so of operation. I figure this was due to the fact that the motor was just breaking in or that I had propped my boat to reach minimum suggested rpm at full throttle

Since I have installed a prop that allows motor to reach max rpm at full throttle, I have not noticed the oil level rise on the dipstick.

jimh posted 02-27-2011 10:52 AM ET (US)     Profile for jimh  Send Email to jimh     
[Moved to REPAIRS/MODS.]
jimh posted 02-27-2011 10:54 AM ET (US)     Profile for jimh  Send Email to jimh     
Peter--Since you have the OEM service manual, perhaps you could describe in detail for readers the unusual warm-up circuit that is employed in the Mercury 90-HP FOURSTROKE. As I recall, the engine--which is actually a Yamaha engine--employs a somewhat novel approach to timing the engine warm-up based on melting of wax in some sort of control valve.
95Outrage17 posted 02-27-2011 10:31 PM ET (US)     Profile for 95Outrage17  Send Email to 95Outrage17     

You're more then welcome. I thought my perspective as an owner and someone who maintains several of these motors would be helpful. I'm getting to know them pretty well. Last winter I replaced an oil pump on an 1999 F80, which required removing the powerhead. It took some time, but was fairly straight forward. Apparently some of the pre-2002 motors had problems with oil pump seals as this one did. The replacement was an updated design. Anyway, that was a good learning experience.
Good luck with your new-to-you Mercury, I bet it will work well for you with proper maintenance. You'll probably find that service manual handy too. I know a few of the pages in mine are a bit dirty, haha.

- Chris

95Outrage17 posted 02-27-2011 10:46 PM ET (US)     Profile for 95Outrage17  Send Email to 95Outrage17     

When I was presented with the suggestion that the black Mercury cowls were contributing to the carb problem in these motors, my eyes opened wide. It makes sense to me and certainly explains why the Mercury versions tend to have more carb problems then the Yamaha versions.
I've been keeping a Pacific blue Sunbrella cowl cover on my motor when the boat is moored. I did this to protect the cowl decals (they're pealing anyway), but I'll continue to do so now to help keep the cowl cool!

- Chris

pcrussell50 posted 03-06-2011 09:57 PM ET (US)     Profile for pcrussell50  Send Email to pcrussell50     
Original poster here...

So far, this motor has not failed to start easily and purr like a kitten in the three weeks that I've owned her. Not -that- remarkable, except that in the year before I bought her off of fishnff/Brian, I think she did a LOT of sitting, since he got an Outrage, and already had a Grady White. Maybe mine is one of the fabled ones people talk about that don't have problems? Fingers crossed.

Brian said he uses Quickleen in every tank and adds StaBil if he thinks the boat will sit.


wezie posted 03-07-2011 08:51 AM ET (US)     Profile for wezie  Send Email to wezie     
Mercury uses a different carb. than Yamaha; Japanese, but different from the same year Yamaha engine.

The black paint may have an effect.
All I know about that is that "It just ain't cool".

95Outrage17 posted 03-07-2011 10:06 AM ET (US)     Profile for 95Outrage17  Send Email to 95Outrage17     

How do you know that the Yamaha and Mercury version of this motor have different carbs? I have never noticed a difference between the carbs on a Yamaha version and a Mercury version. The diagrams online at show no difference between the Yamaha and Merc versions that I can see. Why would Mercury use a different carb if everything else on the powerhead is the same? Take a look at these pictures I took last year and let me know what you think: 1999%20F80%20and%202004%20Merc%2090/ Not sure what makes the difference between say a 1999 F80 and F100 though... Larger main jet in the carb?
As I said, the only difference I've noted on the powerhead is the paint color. Even the wiring connectors are identical. When I order a replacement voltage regulator for a Mercury version I buy it from Yamaha because it's half the price for the identical (updated) part.

- Chris

wezie posted 03-07-2011 09:33 PM ET (US)     Profile for wezie  Send Email to wezie     
Mine is one brand and the Yamaha manual shows another.
wezie posted 03-08-2011 08:12 AM ET (US)     Profile for wezie  Send Email to wezie     
Thanks for the pictures, and sharing your experiences.
One thing I did discover in the shop manual, are the 4 fuel filters/screens in the carb. fuel lines just before they enter the carbs.
Of course, they are impossible to get to without removing the whole manifold.
I "suspect" they are a potential problem. particularly since the adjustments for the carbs are so different, just to get the engine to idle.

Seriously, the carbs on the 2005 90 are not the brand Yamaha says they use. I will look this up again. Maybe they just flip back and forth with suppliers. My first cynical thought was some cheap Chinese brand, but they are Japanese.
Thanks for the parts price tip.
If I get brave enough, I may try the manifold, to get to the filters.
Garbage Gas!

Good Luck!

95Outrage17 posted 03-08-2011 09:15 AM ET (US)     Profile for 95Outrage17  Send Email to 95Outrage17     
I didn't realize there were little filters at the carb fuel inlets. Guess I never looked that closely. I always understood the low speed jets to be the location of the carb problem in these motors. They're so small they clog easily. This is also a problem on other 4-stroke carbed outboards like the pre-2007 Honda 90. As for the manifold; I assume you mean the plastic intake plenum? If so, it's not a big deal to remove if I remember correctly. I wouldn't be afraid if you have the mechanical ability (I assume you do considering you own a service manual). I don't think it's that complicated to remove. Neither is the lower motor cowling if need be. I had both of the lower cowlings removed on my 2004 last month. I was having a look at the lower engine mounting bracket while servicing. My motor is a well looked after salt water motor and that bracket is starting to show significant rust/corrosion. Mercury seems to use cheap metal for pieces like that. Same story on many others I've seen, but I have not had to replace one yet. I think most of the motors I deal with are too new. I was looking at it and my service manual trying to figure out how I'm going to replace it at some point without taking the whole motor apart. I think I have a plan.
Well, I'll take your word for it that your 2005 doesn't have Keihin carbs. Maybe because it's a 2005 and the Yamaha versions switched to EFI that year if I'm not mistaken. The Merc versions were EFI in 2006 before their 90-hp 4-stroke changed to the Mercury "Verdito" in 2007.

- Chris

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