no top end...

[arjay]

Hi there - I'm fettling my aircooled 87 KDX/ 86 KX hybrid a bit more and was after some advice.

The bike makes ok bottom end (a bit weak) then pulls hard in the midrange, then shuts off early without any over rev. It kind of dies out at around 7000 rpm and slows down a bit. Top end doesn't sound great anyway - fluffy, hollow, sort of 4 stroking when you push it hard.

So I'm trying to work out where the problem might lie - which is hard on a bike with so many cross matched parts.

Carb - have an aftermarket 35mm Air Striker but with KDX jetting - all seems ok. Jetted on rich side for WOF running.

Pipe - KDX FMF Gold series with 1.5 cm cut off stinger and has an FMF short KDX silencer - would shortening stinger take away top end?

Ignition - the spark is weak - barely shows up in daylight. On a new plug it starts and runs fine. Hard to start bike once it has been run. Could poor quality spark affect power at top end?

Thanks for any thoughts...

 

[julien_d]

You could lose some power somewhere with the shortened silencer. I doubt it's enough to cause your current problem.

Jetted rich could lose some top end. If it's all blubbery up top, could be too rich.

weak ignition could cause loss of top end. Check the condition of the magnets on your stator.

Check those KIPS valves!!!

 

[dirt bike dave]

Check those KIPS valves!!!

x2. You should have a cover on the right side of the kips valve. Pull the cover off and rev the motor. You should be able to see the kips move at high rpm.

IMO, it's that or jetted way too rich on the main jet.

What main is in there now? I'd be tempted to drop it two sizes and take a test run to see if it is step in the right direction.

 

[sr5bidder]

Check those KIPS valves!!!

x3

also research what size carb came on the 87 and the jetting it used

 

[EYY]

Check those KIPS valves!!!

x4!

 

[Dirtdame]

86 and 87 model came with a 34 mm Mikuni. The 88 changed the porting a bit and went with a 35 Keihin. Don't know what jets an 87 came with, but the 86 had a 320 main jet, so it would be hard to cross refer that to Keihin jetting sizes, as they are numbered much differently. You might want to check what the jetting was on an 88 model instead, since that was a Keihin, and work from there.

 

[arjay]

Thanks for the posts Kips looked like it was working. The rod pulled put at high rpm but not before high mid range. Main jet is 160. The top end isn't too blubbery, just doesn't rev out crisply, like a top end flat spot. Maybe it's the way the 87 kdx was. I had an 88 back in the day and I remember some kind of top end power. I'll try lengthening the silencer and getting a healthier spark.

 

[wake_rider]

It could definitely be spark. Also, this sounds ridiculous, but I was having a similar problem and found out that a chunk of muffler packing had made its way down and mostly plugged the end of my spark arrestor. My bike ran great other than 3/4 to 1/1 rpm range, then it just sputtered and bogged out. It could be as simple as something plugging the exhaust a bit...

 

[sr5bidder]

86 and 87 model came with a 34 mm Mikuni. The 88 changed the porting a bit and went with a 35 Keihin. Don't know what jets an 87 came with, but the 86 had a 320 main jet, so it would be hard to cross refer that to Keihin jetting sizes, as they are numbered much differently. You might want to check what the jetting was on an 88 model instead, since that was a Keihin, and work from there.

true that :cool:

 

[julien_d]

Agreed on the jetting, that's sound advice.

The kips rod moving does not give any indication that the valves aren't sheared off though. You need to pull the pipe off, move the rod by hand, and look through the exhaust to see if the valves are actually moving as they should.

 

[whenfoxforks-ruled]

It does sound like an issue with the exhaust valves. But, a shorter stinger does mean bye bye to the top end. The silencer would just be a jetting issue. Double check your grounds on the electrics? Vintage Bob