96 XP cutting in and out at 3/4 - full throttle

[tingraham]

Hello all,
I am trying to diagnose a long running issue on this dang XP that I have. It has been fighting me for probably 2 years and I am super close to throwing in the towel on it. The issue has slightly transformed which can be heard by listening to the first video and then listening to the second. It cuts in and out like it is just dropping spark. It used to almost be like a rev limiter that was at like 5200 RPM and when you would hit a wave, it would let it rev out until you bring the throttle back down and try it again and it would do that same once again. Now it is just stuck at maybe 5,000 rpm and if you hit a wave it might let it spike for 1 second but comes back down to being stuck at low rpm. It goes about 40MPH when it is doing this and will not go any faster.

I have replaced the following with no success:
entire E-box
Stator/mag cover/trigger
mag cup
cdi
swapped another set of carbs on it and it didnt change the issue
voltage regulator
Probably other stuff I'm forgetting

Please help!

 

[jhjesse]

When was the last time you cleaned the rave valves?

 

[tingraham]

When was the last time you cleaned the rave valves?

That's another thing I forgot to add to the list of parts. I put different Raves on it with zero change. They were cleaned before install.

Also, compression is at the proper 150 on each cylinder.

 

[Jeffspx]

Also have you done anything else with the fuel system? You may have tried different carbs, but if the fuel system isnt flowing properly, different carbs arent going to matter.

 

[tingraham]

Also have you done anything else with the fuel system? You may have tried different carbs, but if the fuel system isnt flowing properly, different carbs arent going to matter.

I've went through it a bit across the couple years but can't remember specifically what I have done at this point. I know I have checked the bowl filter thing and cleaned that. I have had the sending unit out of the tank to check that over. I've also had the entire tank out of it to make sure there weren't any like pin holes or something. It doesn't have any grey gas lines left except for on the vents.

I was steered away from the fuel system when it would rev out fine after hitting a wave or whatever but now that it doesn't really do that I'm not sure.

 

[Jeffspx]

Im not the most familiar with later x4s and 787 motors. Ive always wanted one, but never wanted to spend the money for one as around they always want top buck or found one for a good enough deal for my cheapness, plus I've always liked the simplicity of my sp/spi with single carb and no rave valves, so hopefully some one more experienced can chime in.
It does sound like its missing up top, But my thoughts, seeing as you replaced most electrical items would be:
Try bypassing the fuel valve, confirm that fuel pickups are clear, if you believe the fuel lines are good maybe leave them, and rebuild your carbs or try the known good carbs like you have before.
Also, im going to assume you've tried a new set of plugs?
Have you tried removing the spark plug boots and trimming the wire back just a little and reinstalling the plug boots?

 

[Sportster-2001-951C-Stock]

Sure sounds like an ignition miss to me, as if CDI is rev limiting. I'd also mist the iggy coils and plug wires with spray water while running, especially if it's breaking down more as things heat up over a period of time.

Also don't forget, bad voltage regulators are known to cause similar symptoms but this old ski only uses a rectifier, if I recall. Along those lines, a bad battery (generally one that's incapable of starting the engine) or harness connection can also cause CDI to become unstable.

Also, the lanyard switch might be flaky?

 

[Sportster-2001-951C-Stock]

Also, im going to assume you've tried a new set of plugs?
Have you tried removing the spark plug boots and trimming the wire back just a little and reinstalling the plug boots?

Plug replacement is always top on the list and an important place to begin, great comment. I will say, in my experience high speed missing usually isn't plugs but never say never.

 

[Matt Braley]

If you have unscrewed your spark plug boots and clipped back to good wire, then screwed them back in tight then look at where the wires go into the coil. I hate messing with that end because the plastic retainer clip always breaks but you can clip back that side too and then put a zip tie around the whole coil and the wires to keep them in place since they just push in on that side. Also look for loose or rusty grounds on the base of the coil. Make sure the white signal wire on the + side of the coil is OK.

 

[Sportster-2001-951C-Stock]

Oh wow, I glossed over your parts replaced list. There must be a lose connection somewhere... Okay so how about "replacing" the wiring harness? Okay well don't just stand there, get busy!

 

[Chester]

Check for plugged exhaust. Remove the waterbox and shake it to see if any baffles broke loose. Check the exhaust hose to see if the inner wall has collapsed.

Chester

 

[tingraham]

Wow, thanks for all the replies guys. As for trimming the wires, I have before on both ends but I have also replaced the entire e-box including the wires, wire ends, coil with literally zero change in behavior. I went through my grounds last night with an ohm meter and they all showed good from what I can tell. The white wire for coil signal seems fully intact.

I’m interested in the idea of checking the exhaust. Might try that. I have noticed the hose that goes from the water box to the exhaust outlet is ever so slightly kinked. I can’t imagine enough to make any different but I don’t fricken know at this point.

I’m also wondering if it’s something like internal in the engine. Like could water be slipping through one of the gaskets and cause this?

 

[Matt Braley]

So the E-box you replaced was the rear/black one? That leaves the front grey box a suspect.

 

[tingraham]

So the E-box you replaced was the rear/black one? That leaves the front grey box a suspect.

Correct, the rear black box I replaced. As well as the cdi abs regulator in the grey box. Connections look good. Can mpem cause any issues with cutting out or not really?

 

[Matt Braley]

Yes the spark does run through the MPEM. I've lost spark do to a faulty MPEM before so it's possible yours is messing with you.

Only one regulator/rectifier swap? It seems like the ignition miss and symptoms that the rectifier causes. You didn't replace it with a used one did you?

 

[tingraham]

Yes the spark does run through the MPEM. I've lost spark do to a faulty MPEM before so it's possible yours is messing with you.

Only one regulator/rectifier swap? It seems like the ignition miss and symptoms that the rectifier causes. You didn't replace it with a used one did you?

Not a used one but it is one of the probably cheap ones off ebay. I still does the same with the red wire unhooked. I also checked for a/c voltage at the battery and there is none.

I decided to rule out the motor so I pulled her and tore it down this morning. Looks pretty good inside, seems to be all stock and no excessive wearing. Going to replace all gaskets and put her back together, see if anything changes (probably wont). But at least I know now that the motor looks good inside and if I decided to sell it I can at least say its been torn down and inspected.

All that being said, anyone have an mpem with lanyard for sale?

 

[tingraham]

We have success!!!!! It always drives me nuts when people don’t close out a post with the solution so I wanted to be sure I did.

I pulled the motor and tore the top end down to check it out for the heck of it. In doing so I went ahead and deglazed/honed the cylinders as the piston and cylinder walls still looked pretty decent still. I got a top end gasket kit and replaced all the gaskets in reassembly. It appeared that the motor had never been apart before, at least the top end. After assembling it and reinstalling in the ski, the problem is completely gone!

My theory is that either the cylinder base gasket or one of the o-rings at the top of the cylinder was allowing water to slip by at a certain water pressure/rpm and into the combustion chamber thus making it run like trash at a particular rpm.

Now, if you are reading this in the future and your ski is having a similar issue, this is how I should have tested it- put the ski in the water and confirm it is still having the issue. Clamp off the water supply line to the engine and run one quick pull with it to see if it goes away or at least changes behavior. That would have immediately pointed me in the direction of ripping the engine apart sooner.

Thanks for all the tips guys!

 

[Sportster-2001-951C-Stock]

Good one! Yep, no arguing about it an inward water leak will certainly cause missing! BTW, thx for posting your conclusion.

 

[James96xp]

Thanks for not skipping out on the real jewel of the thread, the solution. Glad it's running like it should finally.