1990 Yamaha 250 VX 76. Power drops off.

[Mokingbird 421]

Engine starts and idles fine. When advancing throttle engine accelerated then power drops off quickly. If throttle is not retarded then engine might surge up and down, Engine might vibrate excessively. The oil system is removed and gas is mixed. All oil lines are capped both oil tank plugs wires jumped. Spark plugs have been replaced and wires checked. 3 mechanical fuel pumps are brand new. TPS sensor was replaced. New fuel bulb. Totally at wits end trying to figure out if it might be the electric fuel pump or a stator problem.

 

[GasCan II]

Engine starts and idles fine. When advancing throttle engine accelerated then power drops off quickly. If throttle is not retarded then engine might surge up and down, Engine might vibrate excessively. The oil system is removed and gas is mixed. All oil lines are capped both oil tank plugs wires jumped. Spark plugs have been replaced and wires checked. 3 mechanical fuel pumps are brand new. TPS sensor was replaced. New fuel bulb. Totally at wits end trying to figure out if it might be the electric fuel pump or a stator problem.

 

[GasCan II]

Had the same problem,did everything we could think of,and it turned out to be the stator.

 

[99yam40]

Had the same problem,did everything we could think of,and it turned out to be the stator.

care to explain what part of the stator was the problem?
charge coils or maybe pulser coils?

 

[GasCan II]

care to explain what part of the stator was the problem?
charge coils or maybe pulser coils?

Changed only stator

 

[99yam40]

Changed only stator

well what all does this stator do
lighting coil furnishes voltage/current for battery
charge coil furnishes voltage to CDI
Pulser coils tell CDI when to fire spark plugs

Edit: apparently all are set in one piece on this old motor

 

[Mokingbird 421]

Well I changed all the electrical components stator, computer, and pulsar coil. Still have the same problem. I cranked the motor and warmed it up in driveway, checked compression afterwards and all 6 are in the 115 psi range. I am totally lost here. I even connected a portable tank to isolate the main tank and no change. She will not power up, it will surge back and forth if remote is left there , get engine vibration and have to retard back. I am at wits end with this problem. Every fuel component replaced and all electrical except thermosensor.

 

[Faztbullet]

Is it a rapid surge like hitting a rev limiter or slow surge like runs out of fuel then it returns? Also is this problem on the water or hose?

 

[99yam40]

a complete model # would help find what motor you have .
I do not see a 1990 VX250 listed
the 1990 250STXD I see has carbs,
Fuel pressure would be a good thing to check if you think the carbs are running out of fuel.

 

[Mokingbird 421]

1990 250ETXD

 

[Mokingbird 421]

This is on the lake. Will not accelerate with remote forward. Every fuel pump component on motor replaced and carburetors rebuilt. All 6 of them.

 

[Mokingbird 421]

Is it a rapid surge like hitting a rev limiter or slow surge like runs out of fuel then it returns? Also is this problem on the water or hose?

On the lake.

 

[Mokingbird 421]

On the lake.

 

[99yam40]

This is on the lake. Will not accelerate with remote forward. Every fuel pump component on motor replaced and carburetors rebuilt. All 6 of them.

but have you monitored the fuel pressure while this is happening at the fittings to the carbs?

sounds to me like there is not enough fuel in the carb bowls so it cannot spool up or the main jets are plugging up

 

[Mokingbird 421]

but have you monitored the fuel pressure while this is happening at the fittings to the carbs?

sounds to me like there is not enough fuel in the carb bowls so it cannot spool up or the main jets are plugging up

I have never checked the fuel pressure while driving the boat . All the carburetors were cleaned and completely rebuilt. The fuel pressure regulator was replaced also. I have changed all the fuel filters, all the fuel hoses, all the fuel pumps one electrical and 3 mechanical. I am totally at bay. I read something about the thermosensor but not sure if this could be culprit.

 

[99yam40]

testing things is better than throwing parts at a problem to see what the problem is.
if pressure does not stay in spec, you have something to chase.
apparently all of the money you have sent replacing parts was in vain from what you have told us

 

[Faztbullet]

Does it have a power reduction feature if it gets hot? Could be sensor tell it to shut down...

 

[Mokingbird 421]

testing things is better than throwing parts at a problem to see what the problem is.
if pressure does not stay in spec, you have something to chase.
apparently all of the money you have sent replacing parts was in vain from what you have told us

I actually took the boat to a so called mechanic. He rebuilt the carbs and charged me $850. I took the boat back to the lake and I was in shock to still have the same problem. I contacted him and he said he would need to check the electrical. I just decided I would replace the parts myself since everything was changed in fuel system and supposedly the fuel pressures checked by the mechanic. So this is where I am . Still with same problem.

 

[Mokingbird 421]

I actually took the boat to a so called mechanic. He rebuilt the carbs and charged me $850. I took the boat back to the lake and I was in shock to still have the same problem. I contacted him and he said he would need to check the electrical. I just decided I would replace the parts myself since everything was changed in fuel system and supposedly the fuel pressures checked by the mechanic. So this is where I am . Still with same problem.