When to rebuild top end?

Fultoncm · Oct 17, 2005

I'm thinking it's time for a top-end rebuild. I bought the bike used and have had it out about 10 times. Who knows when it was rebuilt last, but my performance is getting worse with every ride. I am loosing bottom end power and some top end power. It's becoming more difficult to start also. It's definitely not the carb or air/fuel delivery, because I've gone over this thoroughly with the needle, clip, main jet, and slow jet...it's all set properly.

I can't seem to find anything that says...This is when you rebuild the top-end. So from my symptoms, do I need to rebuild?

nickyd · Oct 17, 2005

its hard to say if you don't know when it was last done - the easiest thing it to order a gasket kit and tear into it - if it all measures oK, then just clean it and replace the gaskets - worst case you get a clean p-valve. best case you fix a problem before it arises and then you have a fixed starting point to say that you have XXX hours on the top end and the compression was XXX when I started and now its YYY.

I once did this with a bike I had - it ran great, no issues and on a whim I decided to pull the top end - good thing I did, the skirt of the piston had a hairline crack about 1/2 the lenght of the piston - had that cracked, my routine top end would have become a split the cases ordeal.

Fultoncm · Oct 17, 2005

Thanks Nickyd. I'm going to order the entire top end kit to have a starting point like you said. When I check the compression, I hold the throttle open and crank it a few times right?

Also, what gauges will I need for this job? I've looked at several instructionals and those guys have every gauge under the sun, but I just want to get the necessities.

Thanks.

Spine_Thrust · Oct 17, 2005

You need to hold the kill switch as well when testing compression.

Fultoncm · Oct 17, 2005

Really? How would it start without a plug?

Spine_Thrust · Oct 17, 2005

You don't start the engine when compression testing...

Fultoncm · Oct 18, 2005

I understand that. I just never could figure out why to hold the kill switch, there's a compression tester in the plug hole, what does holding the kill switch do when the plug isn't in place to fire over the engine?

nickyd · Oct 18, 2005

protects the ignition.

Fultoncm · Oct 18, 2005

Didn't think of that. I'll definitely do that from here on out.

I pulled the top end apart yesterday and it all looked good, but the piston did look pretty burnt on the top. I ended up just ordering all the OEM replacements.

Somethign strange on my exhaust valves though. After getting them all cleaned up it did feel as though there were very, very slight indents on the side in different places. Almost as if from wear. Do you think they need replaced? I'd like to keep them since they're $75 each.

83MX80 · Oct 18, 2005

my KDX is a biatch to start at the moment. it has to be pulled to get it to fire up. and my dad and i played with some stuff on it, and nothing. and today after i dropped my mom off at work i went over to a local dealer and they couldnt help me.but said do a compression test on it. and then i went over to the yamaha shop and they said top end was fockked. now i told my dad that i thought it was the top end. but he said no it wasnt. but it is the TE. but i think im going to just pull start it for now. and wait until my dad's shop is done and in the winter im going to tear it apart and fix a few things.

nickyd · Oct 18, 2005

the p-valves do sometimes have some "indents" and shapes to them - put them in the cylinder and see how they fit in the exhaust port - they should mirror its shape. manually turn the mechanism to see how they retract - are they hanging up on anything, etc.

When to rebuild top end?

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