1000 rpm power loss. 1987 yamaha JD50ESH

[Bigfun]

I have a 1987 yamaha model JD50ESH.
For the last 2 months I have been trying to fix a loss of power problem.(800rpm). Here is what I have done/tried.

confirmed fuel supply, including pumping the primer bulb while running at speed, and squirting fuel directly into carb while running. No change while pumping. Bogged down when fuel was squirted into each carb.

I replaced the head gasket. I then took it out for a trial run and thought I had fixed the problem. The motor ran great thru out all ranges untill the motor reached max. temp. or about 10 minutes.

I have checked the plugs many times and they appear equally fine.

appears to have good water flow for cooling.

Although the power loss is the same while idling and full power, during full power runs sometimes (usually while running with little added weight in the boat) there is a split second gain of power every few seconds.

I was going to re-build the carbs but after replacing the head gasket and then having it run fine for the first time in months tells me that the carbs may not be the problem.

Whould a warped head cause the symptoms described? I checked for water intrusion after replacing head gasket and found no sign of water.

cylinder compression while cold is 120-115.

I bought the motor new and have not changed any original settings.

Any suggestions will be apreciated, thanks

 

[Ray Neudecker]

Re: 1000 rpm power loss. 1987 yamaha JD50ESH

Those models have had CDI issues where the fire drops out intermittently. Check the fire with a timing light when the motor is acting up.

 

[Bigfun]

Re: 1000 rpm power loss. 1987 yamaha JD50ESH

Those models have had CDI issues where the fire drops out intermittently. Check the fire with a timing light when the motor is acting up.

Thanks for reply Ray Neudecker,

The fire does not drop out. The engine while under load sounds fine except for the loss of rpms (lugging) and the ocasional intermittent gain of power. While idling the engine sounds fine but their is a loss of rpms.

Whould the timing light determine cdi problem while idling? Or am I going to have to have my wife drive the boat while I time the engine? Just priced a cdi,$ 270.00 ouch.

I keep thinking that it's something to do with the replacing of the head gasket. The engine ran great after replacing the gasket untill it got hot.

 

[Ray Neudecker]

Re: 1000 rpm power loss. 1987 yamaha JD50ESH

The timing light will work when the problem is there. Often the timing light test will only work under a load. The darn CDI problems on those engines were intermittent and were hard to catch because of that. It may come down to having the wife drive the boat as bad as that sounds. Borrowing a test wheel and doing it on the trailer is an option.