'99 380 cuts out at high RPM after new top end

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#1
Ok, I'm looking for some brainstorming ideas. I have a '99 380 which I've had since it was new. This winter I decided to replace the piston and rings. It still ran really well, but after two years I figured I'd pull it apart and see what things looked like. Things looked pretty good: one ring end gap was 0.013", the other was 0.019". (Spec is 0.012 to 0.015".) I replaced the piston and rings with Wiseco.

Ever since replacing the piston and rings, it has run poorly at high rpm and doesn't rev out as far as it used to. Can anybody shed some light on this? Hear are my thoughts:

Jetting: I don't thing it's related to jetting. I keep very accurate records, and I'm running jetting that I've used before at this altitude and temperature with good results. Just in case, I've played with it a little bit, with no improvement.

Reeds: It ran great before the top end, now it doesn't rev out. The reeds are still stock, and perhaps are wearing out a little, but that would be a gradual decline in performance, not an overnight change. I don't think it's the reeds.

Power valve: I have removed the cover to make sure the power valve is actuating properly, which it is. I guess it's possible that I put it together one tooth off or something, but I thought I was careful about that.

Piston: I noticed that the Wiseco piston's skirt was shaped quite differently from the stocker. Could this affect port timing or something and cause it to cut out at high RPM?

X & Z: I *think* I got these exactly as they were before. I'm sure I could be off by a few thousandths, however.

It's fully broken in, and compression is good. (190 psi at 4500 ft altitude.)

I've replaced the plug with a new one just in case the problem was related to the plug starting to foul or improper plug gap. No improvement.

One last detail: I'm doing an oil experiment. I noted the wear and deposits after two years using Golden Spectro and MC-1. After doing the top end, I switched to a really cheap oil and I'm monitoring compression to see if the top end wears faster with the cheap oil. Hmmn, I can't see how that should affect high RPM performance, but I though I'd mention it in case.

Ok, any comments you might have are welcome.

Burt
 

CRGuy

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#3
Hmm that's a mystery. Pull your head off once more. Check to make sure you didn't put the power valve one tooth off. This is what It sound's like. Move it one tooth up and fire it up. If it has original power then there you go but if it's worse then go down two teeth.
Good luck,
CRG
 

marcusgunby

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#4
I would go striaght to the reeds as they are a known weak point on the 99-they were very thin and chip at the edges-replace them with 2000 reeds or get a V force for better reliablity and more performace with smoother power.I advised someone recently on a 99 125 who had identical problems as you and his reeds were shot.
 
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#6
Burt - on my 98 380 EXC the power valve adjustment setting, which I use and like is only about one-half a turn from the point where the power valve will not open when the bike is reved out. Is it possible that you may have slightly tightened the power valve spring and eliminated it from opening?

Good luck and I hope it is something simple like this!!
 
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#7
Good thought, but I removed the left-side power valve cover and watched it actuate properly when I revved it up. It opens all the way, and the cutting out occurs after it is fully open. Thanks for the input, though.

Burt
 
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#8
If new gas and oil does not straighten out your bike upper revving problem you may want to borrow someone's black box (ignition) to see if yours is starting to fail.

I had a YZ 250 and the first symptom of the black box's failure was lack of high rpm followed 2-3 rides later with starting to foul plugs at lower rpms. Replaced the box and ran the bike for two more years without trouble. Just a thought!!
 
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#9
Thanks, I'll keep that in mind, too. And I did foul a plug the other day for the first time on this bike. 'Course, that could be fuel/oil related, too. Thanks!
Burt
 

CRGuy

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#10
I doubt the fuel/oil would foul a plug just this time. It would have fouled plug's before this problem. Could it be that you need to drill hole's in the piston. Does wiesco drill them?? there are supposed to be holes drilled or you have to drill them for the port bridge or something like that. I think that might be it.
CRG
 
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#11
That's just it, though. I switched oil at the same time I did the top end, so it could have been starting to foul ever since the top end.

I think the hole you are referring to (added forthe '99 models) is the hole in the cylinder to ease starting. I can't think of any other holes or differences between the stock and the Wiseco, anyway.

I'm going Wednesday, so I'll get some fresh gas and see what happens.
 

CRGuy

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#12
You have to drill some hole when you do a topend on a CR125 I am not sure if the KTM's have the need for the hole but I gave it a shot.
CRG
 
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#13
Piston Skirt Differences

You first mentioned the piston skirt was much different. This would have an affect on your port timing. Are you sure you got the right piston? Is there more than one for the 380?

I also had a power valve installed incorrectly on a CR250. It ran fine down low but did not rev at all. It reminded me of an old 70's DT250 or something like that. It sputtered terrible trying to rev it.
 
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#14
Say, a couple of years ago I rebuilt my 250sx top end (96) and while I was at it I took the power valve assembly apart to clean it and replace the O rings. I got the same results as you with plenty of bottom end but nothing on top. It turns out that the power valve spring was backwards. With the end cap off and the bike running it appeared to be working, but it wasn't working right. I took my cylinder back off and took it to the dealer who showed me the way it went. Instant top end power just like b-4! Hope this helps.
 
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#15
Thanks for the tips. I called Wiseco and there is only one piston available. According to Wiseco, the skirt differences shouldn't be causing the problems I'm seeing. (If nothing else works, I'll put the stock piston back in. If it cures the problem, Wiseco's name will be mud...)

Mikromo, I'm not sure I understand what you meant by saying the power valve spring was backwards. I did take the whole power valve assembly apart and replace the O-rings, so I could have messed something up. The only spring that I remember is on the right side, a torsion spring that wants to keep the valves in the closed or low-rpm position. The centrifugal actuator oversomes this spring to open the valves as the rpm increases. Is that the spring you're talking about? With the linkage disconnected, I can actuate the valves and the spring seems to be doing what it's supposed to. I took the pipe off and looked in the exhaust ports with a flashlight, and everything seems fine. Maybe the '99 design is different from the '96?

Burt