Yamaha VX76 225HP 1994 Problems-Looking for suggestions

[Viking53]

Yamaha VX76 225HP 1994

Engine starts fine when cold. Choke system appears to work perfectly. After 10 minutes of use (engine warm) the engine idle drops to 400 RPM and eventually turns off. Once symptom appears it continues until engine is stone cold. Restarting the engine yields 400rpm for 15-20 seconds before engine turns off.

1. Replaced low pressure fuel pumps (past due anyway)
2. Replaced fuel check valve
3. Yamaha Blinky harness does not display any error codes
4. Swapped CDI unit from Starboard to Port ? Same
5. Swapped Rectifier Regulator from Starboard to Post ?Same
6. Tested Crank Position Sensor ? within specs
7. Tested Throttle Position Sensor-within specs
8. New Spark plugs

When the emergency(red) CDI bypass connector is disconnected motor appears to run fine. This would lead one to believe that the problem is a sensor feeding back information to the CDI. Any suggestions would be appreciated.

 

[Caseb]

Re: Yamaha VX76 225HP 1994 Problems-Looking for suggestions

You said you replaced the check valve. Which one?

 

[Viking53]

Re: Yamaha VX76 225HP 1994 Problems-Looking for suggestions

You said you replaced the check valve. Which one?

Fuel check valve before the low pressure fuel pumps. Also replaced the priming bulb.

 

[multimech]

Re: Yamaha VX76 225HP 1994 Problems-Looking for suggestions

Look down the throats of the carbs and see if any are leaking. (Boat been sitting awhile?)

Enrichment valves working as designed?

Check your lift pumps for leakage (Rich mixture into cylinders)

First and foremost what is the compression?

Lastly, I had the same engine (75 degree if I am not wrong) that had the same symptoms which was recently rebuilt. All the pistons were in the wrong orientation. Took a lot to find that one. Compression fine.

 

[Viking53]

Re: Yamaha VX76 225HP 1994 Problems-Looking for suggestions

Look down the throats of the carbs and see if any are leaking. (Boat been sitting awhile?)

Enrichment valves working as designed?

Check your lift pumps for leakage (Rich mixture into cylinders)

First and foremost what is the compression?

Lastly, I had the same engine (75 degree if I am not wrong) that had the same symptoms which was recently rebuilt. All the pistons were in the wrong orientation. Took a lot to find that one. Compression fine.

Thank you for the suggestion but the problem appears to be sensor/electrically related. I discovered that when the engine gets warm the 3 coils on the starboard bank of cylinders do not fire. Even though it was highly unlikely that 3 coils would go bad on the same side of the engine I decided to replace them out of desperation. Unfortunately that did not solve the problem either. The ECU was swapped, fuel pumps replaced, rectifier swapped and still not luck.

 

[multimech]

Re: Yamaha VX76 225HP 1994 Problems-Looking for suggestions

A synch and link is very important. Make sure your TPS (throttle position sensor) has about .5 volts at idle. Make sure all throttle plates are closed when in idle position.

I am not sure about this particular model, but Yams use knocking down cylinders at neutral and idle position to conserve fuel. Be sure yours is not one of them or you are chasing ghosts.

 

[99yam40]

Re: Yamaha VX76 225HP 1994 Problems-Looking for suggestions

Do you have the Yamaha service manual for your motor?

It should have the steps needed for testing the inputs and outputs
and tell you the test equipment your will need to do the testing

Sounds like something under flywheel maybe is heating up and going out of spec

 

[mygrady]

Re: Yamaha VX76 225HP 1994 Problems-Looking for suggestions

Correct me if I'm wrong but I thought that TSP was supposed to be set with throttle linkage t fully closed or idle screw fully backed out

 

[Viking53]

Re: Yamaha VX76 225HP 1994 Problems-Looking for suggestions

Thank you for all the input. Based on the ignition logic it would appear that the stator coil fails when the temperature reaches a certain point. A new stator is on the way. Many of the test tools are no longer availabe. Will report back when the stator is replaced.

 

[99yam40]

Re: Yamaha VX76 225HP 1994 Problems-Looking for suggestions

I believe you need a peak reading meter or a DVA adapter with regular meter to take voltage readings coming out of charge, pulser, and lighting coils coming out from under flywheel. Those are all still available.
Ohm meter to take ohm reading are simple to find also

 

[multimech]

Re: Yamaha VX76 225HP 1994 Problems-Looking for suggestions

I hope the stator fixes it, but I have not really had many stators that changed their electrical properties once they got heated up. They are simply a coil of wire and their cores. Not to say it couldn't happen, it just seems to be a very expensive way of solving the problem if it is not correct. No one will take electrical parts back once they are even opened. Usually electronic components are effected by heat rather than simple wired wrapped cores.

I agree with 99yam40, the pulsers and stator are simple to test electrical as they are coils of wire. Ohms checks should be first and usually show a problem before the running voltage tests.

 

[Crab boat]

Did you find a fix same problem.
Thx

 

[jbcurt00]

Crab, start a topic about your motors problems. That way you'll get advice specific to your motor.

Try to avoid posting into inactive topics. Its been nearly 3yrs since Viking posted this and the last time they logged onto iboats was 30 days later. I doubt hevll get or reply to your question. Thats the reason we discourage posting into inactive topics.

Thanks. Welcome to iboats, good luck w getting your motor running great.