1989 40hp 3 cyl Yamaha idle issues...

Have you investigated the fuel pump isue mentioned earlier. If the fuel pump gets its pulse from 3, and I think it does, a pin hole in the pump diaphram can cause this problem. The leak may be small enough to have little effect at higher RPM.
There are hoses on the crankcase cover to control the pooling of raw fuel at low RPMs. be sure you confirm this system if working properly.
This type of ignition system incorporates a start up timing advance for easy starting. When the engine is started the timing is advanced for a few seconds and the RPM is raised. When the timing retards, you can hear a drop in the RPM. I'm wondering if this is the point when the engine stalls. If so, I look closer at the linkage setup or CDI box. When the engine stalls, can you instanly restart it and have it idle fine for another 15 mins. (resetting the starting timing advance)?
I remember the Pro 50s with this ignition, you had to wait for the timing to retard before shifting to avoid lower unit damage.

Oh Yah, I forgot tomention. have you checked the thermostate? A defective thermostate that will not allow the engine to reach opperating temperature could also be your problem.

[TTandMee]

Need help with my motor too

I am having similar problems with my Yamaha 40 hp 2 stroke. I got it to idle and messed with the carbs a bit. I think I made it worse. I got the motor to go to a medium speed, but it dies as soon as I open it up for top speed. What is most likely wrong? I messed with the needle valves--at least I think I they are. They are located at the top left of each carb. I used to remember a general rule of thumb with cars you close the valve and then open it up one and a quarter turns. Well as you can imagine, that can't be right on those carbs. What should I do? thanks in advance for your assistance.

[brunscopter1]

If you suspect a leak in the crankcase you may want to use propane to find the leak, I have used this method for many years looking for induction leaks on auto, marine and aircraft.

I use a small bottle with a valve and a vacuum hose and sweep all suspected areas,when you find it the cylinder will start firing---good luck

[casey1240]

I've seen where a mixture of spark plug brands can do strange things. Are yours all the same. Might want to buy new just to eliminate that possibility. Also I would check the intake manifolds for excessive carbon to make sure they are getting air into the engine on the up stroke. If these are good, I would go back to the carbs. Just some thoughts

[DougBradlow]

Crank shaft seals

I'm betting on that. We worked on it for a few more days troubleshooting the timing and coils and everything, and after we sprayed some CDC down on the bottom of the powerhead where it goes into the drive unit, the motor dies down which means there is a leak down there. We checked the compression again and it is spot-on, but the bottom cylinder just doesn't seem to be firing at all, even though it has fuel and spark. It must be sucking in air from a leaky lower crack seal and killing that cylinder. Time to pull the powerhead...

On a side note, how hard is it to replace those seals? Do you have to pull the crankshaft, or is it just on the bottom of the powerhead where you can get to it pretty easily after pulling it? Thanks for the heads up!

Also, on a side note, what does that plastic black box do that is attached to the timing/throttle linkage? It says Mitsubishi Electric on it; does this have something to do with timing/ignition?

When replacing crank shaft seals examine the crank shaft were the seals ride look for any grooves in the crank shaft.I have done this before and found grooves in the crank shaft on omc outboards.If you find any grooves were the seals ride,new seals will not fix the problem due to clearance.If you find grooves in the crank shaft you will have to replace the crank shaft and the seals.

[normana01]

i am try to check witch is the correct way to install the bottom crackshaft seal and the 2 small drive shaft seals yamaha 3cyl 1989 blue model .
when the aluimium hosing is removed and you are looking at it should the top of the seal be facing up or down as i have recently worked on a 50 cyl and that one seals face up yet mine they all seem to face down ie flat side facing down which makes more sense . ????