'91, 40 HP, 2-Stroke, Missing at High RPM

[Houseboy22]

Hello. I've got a 1991, 40PLRP, 2-stroke, 3-cylinder Yamaha outboard pushing a 20-foot Sweetwater pontoon. When I first fire it up, warm it up, and then open the throttle up all the way, it runs fine, hitting on all 3 for the first 30-seconds, and then it stutters and loses power. It almost feels asthough it is going into limp mode because of over heating, but I've never had an alarm that would indicate that. I've checked the fuel pump and I've got good fuel flow and preasure. Clean fuel filter basket, clean fresh fuel. New plugs as well. Water pump is new and I've got a good strong witness stream. When the engine started misfiring I pulled the plugs to look for clues. The top plug was wet and the bottom 2 were dry, I switched number 2 plug with number 1, fired it up and ran wide open firing on all three for the usual 30-seconds. When it faltered again I pulled the plugs and again the top plug was wet. I swapped the top coil with a used working one and had the same result. Compression is around 125 psi/cylinder. My question is, is this the sign of a bad CDI unit? And if it is, is there a good way to bench test it? I would hate to drop that kind of money and have it not be the problem. Thanks for any help you can give.

Dave

[99yam40]

You need to test inputs and out puts of the CDI while it is still on motor.

You will need to pick up a service manual and a peak reading voltmeter made for testing these ignition systems, or a DVA adapter and regular meter for testing properly.

A inductive timing light will show you if it is retarding the timing like when over temp. or low oil. It will also show you if it is dropping spark if you move it to the different plug wires.

Have you tested your alarm system lately to make sure it is functioning properly?

If you have good compression, good spark at all times, and the timing is doing the proper thing, then you can look at the fuel problem

[Houseboy22]

Thanks for the information. I'll have to look for a meter. Ah... this wonderful hole in the water in which we pour money.

[boatmangc]

Just to be sure, I would also test the horn to be sure it works!
If not maybe you are in PPM reduction,have you put your hand on top of the head right when/after it occurs?

[Houseboy22]

Thanks, boatmangc. I never bothered to touch the head, because I had a strong witness stream. It never occured to me that it might be overheating. Can you please tell me what exactly is going on mechanically/electronically when the engine goes into PPM reduction (or limp mode?). Could it be that I have a faulty temp sensor?

Thanks again in advance.