SOS. Apex is idling down to 500-800 RPM and shutting off

[henseljg2]

Ok, so my head gasket went on my apex last year. So I pulled the motor, and had the head gasket replaced by my a professional mechanic. I got the motor back in and the sled back together. Started right up, idled normally for about 10 seconds then idled like crap, then shut off. Then get it running, you have to give it just a little bit of throttle. When its running, you can goose it and motor responds fine until you let off the throttle, then it idles down to 500-800 RPM's and then it shits. Here is a list of what I have done.

1. Swapped the stator assembly with a spare - I have seen the pickups go bad and weak spark causes this. New stator assembly pickup has 530 Ohms which is in spec.
2. Cleaned the throttle bodies.
3. Adjusted the idle air control valve and it does nothing.
4. Swapped throttle bodies with a spare.
5. Swapped out ECU with by buddies.
6. Ran it leaner, then richer, and it just runs worse.
7. Changed out the gas with fresh gas.
8. Went into diagnostic mode and verify the Throttle Position Sensor is 16% off and 96% wide open.
9. Plugs were new last season and I have verified they are not fouled.
10. I verifed the check valve on the tank is not stuck.
11. Swapped out the injectors and boots with the throttle body but kept the same idle air control valve.
12. I verifed there was no obstructions in the idle air hoses.
13. My battery is brand new and fully charged.


I have no error codes, fuel pressure seems fine, coolant is flowing through the idle air control thermostat. I am at a loss. Unless someone has any other ideas, the plan is to replace the wiring harness, replace the idle air control valve, then maybe set it on fire if that doesn't work. Please someone provide some suggestions. The wiring harness isn't the end of the world to replace just annoying as hell. Maybe I have a short somewhere is my guess from taking it apart last year.

 

[VMX]

Check cam timing mine went out 1 tooth and it acted like that

 

[hanifnoor]

So I pulled the motor, and had the head gasket replaced by my a professional mechanic. I got the motor back in and the sled back together. Started right up, idled normally for about 10 seconds then idled like crap, then shut off. Then get it running, you have to give it just a little bit of throttle.

 

[grizztracks]

I've done many cam chain tensioners on the 3 cylinder engines and I know from experience that after the engine is reassembled if there is any slack in the chain it will jump on the intake cam gear. When this happens the engine will run fine at high RPM's but will not idle and typically starts hard. Unless your mechanic has experience with the Yamaha engines he probably did not take the necessary precautionary steps to prevent the timing chain from jumping. If you understand the dual overhead cam timing and the fact that the gears and chain are light weight you'd realize the issues in properly timing these engines. Professional mechanic or not, Yamaha Technicians have screwed this up. Pull the stator cover plugs and remove the valve cover to recheck timing marks. I believe you'll find it jumped at least one tooth.

Remember to NEVER ROTATE A 4-STROKE ENGINE IN THE REVERSE DIRECTION. The reverse rotation can cause slack in the chain and allow it to jump time.