97 GTX Surging / Low RPM

[hioctane]

* I posted this in the premium section but didnt receive any responses so this is a repost *

About a month ago I posted this: http://www.seadooforum.com/showthread.php?t=16405 regarding an issue we're having with our 97 GTX.

I completed the following list noted in the older post:

1 - check RAVE valves (a very slight bit of carbon... cleaned)
2 - check pulse line (hose was a bit loose)
3 - check rectifier (voltage was fine & no AC present)
4 - *** test ride after this step went fine... it ran great ***
5 - clean fuel selector (no noticeable gunk... cleaned)
6 - clean filters inside carburetors (maybe 40% clogged.... cleaned)
7 - Replace all gray fuel lines
8 - *** test ride after this step went fine... it ran great ***

We've been out on the lake 3 or 4 times (not including the test rides) since all this was completed. The last time was about a week ago. It has run perfectly each time until today.

The surging/low RPM problem is back in force. The really odd thing is it goes from running bad to running fine back to running bad all in the 20 minutes of riding this afternoon. It's acting exactly like it did before with a little more surging... oh and it did run fine at WOT for 5 minutes or so in the middle of running bad.

I'm at a loss as to why it's run fine for the last month and now the problem is back and I'm wondering what else I need to check or do to fix it?

I checked the compression and both look good - MAG: 147psi / PTO: 145psi.

Attached is a pic of the plugs. Plug reading isnt a skill I have but they dont look right to me. These plugs have been in since the begining of the season... ~5 hours on them.

Thanks!

-Scott

 

[shakedown]

I had about the same problem, Disconnect the wire between the mpem and coil leaving the coil connected to the stator and see what happens. where the wire plugs into the coil, the same wire has a junction connector in it that will allow you to unplug the mpem, but still keep the engine plugged into the coil.
If that clears it up you have a mpem problem(rev limiter bad). Also remember if you do that you have to choke it to kill the engine since the kill circuit will be bypassed.

 

[lilngineer]

I would put money on it that it is the MPEM. I have a 96 challenger 787 doing the exact same thing and spent probably about 100+ hours diagnosing only to find out it is the stupid MPEM.

I hope I am wrong for your wallet's sake.

 

[lilngineer]

When you get the symptom, pull each of the plug wires on at a time and see if it is running on one cylinder. That's what the MPEM did to mine, it would miss the MAG compression stroke every 360 degrees below 3500 RPMs.

 

[hioctane]

Solution...

After a complete carb cleaning & rebuild she's shooting out of the water like a rocket. Success!

I also went ahead and replaced the manifold gasket and the gasket between the manifold and tuned pipe. I knew the gasket at the tuned pipe was leaking and I could see water marks around the other after the carbs were off so I replaced both. Not a hint of water leaking after the test ride.

One note related to the carb rebuild for others... do not assume the pop-off will be the same if you use the same spring with a new needle valve. The MAG carb was 35 pre-rebuild and it dropped to 30 with the same spring and a new needle valve. To verify I put the old valve back in and retested and sure enough it was back up to 35. On a related note... I think it was worth the cost of building the homemade pop-off tester which came in at around 25 bucks... and 30 plus minutes wondering around Lowes trying to piece it together

-Scott