96 XP intermittent spark

[esahlin]

Heres a tough one that I have spent alot of time on and cant figure out.

I have a 96 XP where I just rebuilt the engine over the winter and just now am trying to get it on the water.

First start up after rebuild engine install it started easily and ran fine in the garage.

Sometimes it will start and when it does it runs but you can tell it is misfiring then it will eventually die.
Sometimes it will just crank with no spark to the plugs. Sometimes after a crank but no spark episode, I try it and it will start right up but missfire and eventually die. Its even backfired a couple of times.

I have a good running 96 GTX 787 next to the 96 XP on the trailer so I have swapped in good parts to try with the 96 XP.

Here is what I have tried:
Tried 6 different spark plugs
2 different coils same result one was brand new
Spark plug wires cut back and new ends put on.
Tried a different CDI box from the good running GTX (CDI box from XP works in GTX) same result.
Tried the good running GTX MPEM same result.
Battery is newer and has 11.5 volts during cranking.
White spark wire from grey box to ignition coil is brand new.
Ground wire from rear black solenoid box is brand new
Removed the red rectifier wire same result.
Checked the Dess post and the start button with the OHM meter and both are within ohm specs.

Removed the front cover to inspect the stator and the mag pickup. Didnt seem to be any metal particle interferrence. Both the stator and the mag pickup are brand new aftermarket and were installed when the engine was rebuilt. Ohmed out the mag pickup and all the yellow wires from the stator etc and all are within ohm spec of the manual.

Put the front cover back on and still intermittent spark problems.
I'm not convinced that the new mag pickup in the front cover is functioning properly (eventhough it meets ohm specs) so I tap into the yellow with white stripe wire coming from the mag pickup and goes to the CDI box. I tap in after all the connectors and just before the CDI box where I hook up my volt meter.

Volt meter set to volts AC: I get 1.5 volts AC on the yellow/white stripe wire to the CDI box when the engine cranks. Ive gotten readings of constant 4.8-5.0 volts when it ran but engine ran with misses. Now Ive read that I should see 0.3 to 0.5 volts AC from this wire when it is cranking. Is this higher than normal AC volts causing havoc for the CDI?

So I went over to my good running GTX and tested the yellow/white wire from the mag pickup going into the CDI on this good running ski. On this GTX engine I got 0.8 volts AC when cranking. And 3.5 volts AC while the GTX is running smoothly.

Does anyone have experience with the AC output voltage of the mag pickup and what it should be and can about a 1 volt higher than normal difference cause the CDI and spark delivery to go bezerk?

 

[esahlin]

Well, swapped the front cover including the mag pickup and stator from the good running GTX into the XP and didnt help. First try there was no spark to the plugs. Tested the yellow/white stripe wire from mag pickup to the CDI and got 0.8 volts AC when cranking right what it was when this cover was on the GTX.

 

[esahlin]

Well....

Ding Ding Ding!

Guess what it was.........

DESS post.....eventhough it ohms out to manual spec it was the root of the pain. As a last attempt I swapped the good running GTX Dess post to the XP and the XP started right up and ran decent. Only started it twice but its the most success Ive had. So I think its it. More run testing tomorrow to confirm.

 

[esahlin]

Well it wasnt the DESS post. I swapped the ignition coil from the running GTX to the XP and now I am able to get it to start everytime. and havent had any moments where ther is not spark ata all. But it will idle when started at about 2000 rpm then slowly climb in rpms without me touching the throttle. It runs with a slight miss.

If I let it idle it will idle but will keep creeping up in rpms without me touching the throttle.......it will creep to 4500+ where I have to pull the laynard to stop it. Checked to make sure throttles arent stuck and their not.

Tried the other MPEM and same thing.

It seems like its a spark advance thing.....but thats MPEM controlled.

When I rebuilt the engine, I installed a new WSM crank , and a different used PTO flywheel. I lined up the mag cup with the flywheel appropriately I believe.

I havent checked the timing cause its not really adjustable.

So not sure what to attempt next?

 

[GGuillot]

Throttle creep going untouched with the throttle plates closed or not stuck sounds like a vacuum leak somewhere. Just like in a car an occasional slight miss is sometimes a small vacuum leak.

If timing checks out like you believe, I'd look for a possible vacuum leak...it's where I'd look next, you've tried everything else under the sun except that..

Just for process of elimination and double checking your work,, I'd pull the RV cover first to recheck the timing with the degree wheel... But wouldn't think the runaway throttling is timing related...that's more of a vacuum issue.

 

[GGuillot]

BTW - I've had (actually still have) an 80's turbo car that would act nutty like you describe, miss at times, high idle, stall, backfire, hard to restart. It was usually a vacuum line blown loose somewhere from the high boosting.

 

[esahlin]

Yeah- I read some more info and posts and the runaway seems to be a lean condition or a vacuum leak as you mentioned. Looks like I will swap carbs here next and take a look at the RV timing as I do still have the degree wheel that I used when building this thing.