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I had to rebuild my carburetor last spring and ever since then, the motor seems to be running rich. When we are idling for a period of time, I smell fuel….and not just the normal exhaust smell, this is raw fuel. I’ve taken the cover off while the motor is running while in the water and do not see any fuel spillage on or around the carbs. I went to the local Yamaha dealer last week and their lead mechanic said there wasn’t anyway to adjust the fuel/air mixture for my motor and that it is factory set. I walked away scratching my head as this information from him just didn’t seem right. There is one distinct screw for each carb that looks like a fuel adjustment screw…as they all have springs on them. When I rebuilt the carbs, I was carful to count the rotations of each screw when I removed them for cleaning. I also made sure that I put each screw in with the same number of rotations as when I took them out. The plugs look pretty good though they are black.

Am I able to adjust the fuel air mixture for my motor? If so, how do I know I have the three carbs set up correctly? Do I need vacuum hoses etc?

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Does anyone know about a Yamaha 40HP carb?

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Cliff,
Do you have a shop manual for the engine?
Mike

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hi, i had the same motor. you are right, the screw with the spring on each carb is to adjust the fuel/air mixture.

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Cliff , I would try a different shop. I always use a guy that I have found that can do repairs on about anything. Unless your motor is still under warranty I would take it else where. New Yamaha's can not be adjusted because of emission laws. However if you found some tech that was over forty it we be worth your time. BG

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I think you're right, I will take it to another shop. The guy I talked to has been with this shop for 30 years and is a grey hair....so it struck me funny that he told me what he did tell me. I guess business is good for this guy.

I have a maintenance manual that I bought from Ebay a while back but it is for a 4 stroke motor of the same vintage, I have a two stroke.

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Cliff, Next time that happens and you smell fuel, take a look in the water around the engine and see if that's where it's coming from. (sometimes you can see the "sheen" on the top of the water).

Also, how do your plugs look, any excess carbon?

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Dave. I do see a "sheen" of fuel behind the motor. I figured that was from it being a two stroke. I didn't see any fuel leaking down the side. I should probably replace all my fuell lines anyway, they seem to be stiff.

I'll take a look at the plugs tonight. The last time I looked at them they were black, but no excessive carbon.

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I ran the engine today and then removed the spark plugs to take a look at them. Each plug looked normal to me. Each plug was lightly blackened with an oily residue on it which for a 2 stroke, I felt was normal.

While the engine was running, I noticed the water that was coming out of the was had some oily or fuel type sheen to it while the water flowed down my driveway. I also figured this was normal as it's an old 2 stroke. Maybe there's too much fuel and oil in the exhaust. I think I'll take it to another shop as the Yamaha dealer in Plano TX keeps blowing me off when I go in there to see if they will service my engine.

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How's it run otherwise? Notice any changes recently? Any chance you can do a compression test to check the integrity of the rings, etc?

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Dave,

Well I feel like the motor should have more power to it than it does. When I was running the motor yesterday while on the hose, I increased the throttle a bit from idle. The motor seemed to be skipping and not running smoothly. After I shut it down is when I checked the plugs. Do you think I may need to completly rebuild the carbs?

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Well if you can run a compression test, it will check the integrity of the rings and make sure the extra fuel isn't due to blow by. Also, you may want to take a look at the choke circuit. I've seen the solenoids malfunction and let extra fuel in that way.

When you say completely rebuild the carbs, what was the extent of your rebuild last spring?

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Thanks Dave.

I had the carburator cover off yesterday while running the motor. I did use the choke switch to start it and the three flapper valves worked fine and did not close while the engine was running.

When I rebuilt the carbs last spring, I tore it down, cleaned all the parts with carb cleaner and then re-assembled. I did not replace any gaskets, springs or O-rings at all.

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If you cleaned out all the passages, you're probably OK there. I'd focus more on the float setting and needle valves. It's a good idea to always swap out the valves as they have rubber tips that can get grooved and worn, and check the float setting while you're there. If either is an issue, the float assy won't be able to stop the fuel flow into the float bowl once it's reached its level, and could be the source of too much fuel. While it's all apart, new gaskets are a good idea too, eliminates that variable....

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Dave.

Well I think I solved my issue. I leaned out all three carbs by 1/2 turn on the mixture screws. I no longer smell fuel when at idle and also my fuel efficiency seems to be much better than before. Thanks a million for your help ONCE AGAIN.

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Cliff,
Keep an eye on your plugs. Black is too rich(like they were before). You should get sort of a golden or honey color. If they are too light, like almost a white burn, then you are too lean and that's not good. Check them after running at a good speed.
Mike