1986 70 hp yamaha 2 stroke 3 cylinder

[Zmoore88]

I have a 1986 yamaha 2 stroke 3 cylinder that I recently brought back to life. I replace everything from water pump down. Had the carburetors rebuilt by a marine mechanic, replaced all spark plugs and fuel lines. The boat runs great idiling, and will run up to about 1/3 of full throttle. Then it seems to have no power, will not plane the boat out. It acts like a weedeater does when its cold, you give it full throttle and it kind of roars, you can back off the throttle and it runs great. Doesn't miss a lick, the plugs are not oil fowled. Only messes up when you try to give it the throttle, the motor doesn't die, but it seems like it has no power. Motor has less than 18 hours on it. Help Please.

 

[99yam40]

Re: 1986 70 hp yamaha 2 stroke 3 cylinder

What is the compression?

test for spark on all plugs with an inductive timing light to see if it is dropping spark to any cylinders while problem is occurring, and while you are at it make sure the timing is advancing like it is suppose to.

Spraying a fuel mix into the different carbs while problem is occurring will tell you if one or more cylinders are running too lean

 

[Zmoore88]

Re: 1986 70 hp yamaha 2 stroke 3 cylinder

Why would the timing or advance be off? The motor looks brand new underneath the cowling, it belong to my grandfater. Is there anything i can try without having to take it to a mechanice? I do not feel comfortable doing that sort of stuff. The boat just runs too good for it to be a major problem. It ran for over 3 hours yesterday and didn't miss a lick. Is there anything i can do to point me in thne direction of fuel or electrical?

What is the compression?

test for spark on all plugs with an inductive timing light to see if it is dropping spark to any cylinders while problem is occurring, and while you are at it make sure the timing is advancing like it is suppose to.

Spraying a fuel mix into the different carbs while problem is occurring will tell you if one or more cylinders are running too lean

 

[99yam40]

Re: 1986 70 hp yamaha 2 stroke 3 cylinder

If it is running lean fuel/air mix on any cylinder it will pop a piston and if timing is off it can do the same thing.
Running a motor 3 hours that will not function properly is a bad idea.
you may have already hurt the motor.

I would take it to someone that can check it out if you are not able to do it yourself