KDXRIDER.NET

So here's my bike:
86 KDX200
Replated cylinder (original bore) w/ wiseco top end
DG Pipe and Silencer
VForce 3 Reeds
New Carb (still 34mm VM Mikuni, just wanted an air screw)

Here's my problem/question:
So yesterday we were all riding, and at the time I was not riding the bike but I asked my buddy plenty of questions. The bike was running beautifully all day. Needed slight carb fine tuning adjustments (slight lean bog in pilot circuit, but the main was screaming perfection), and it was running better than it has in a while...until the afternoon. I pulled up and my buddy was kicking the bike. He said he slowed down to turn the corner he pulled the clutch and the engine just slowed until it shut off. When I pulled up he was kicking it and you could hear it sputtering wanting to start but not quite there. Then when he kicked it made this really loud pop/backfire out of the exhaust, which I'd never heard before. I said I don't know keep trying it. He did and it wasn't sputtering or throaty or anything anymore. It sounded wrong. I grabbed the bike tried to kick it and NO compression at all. I reached down and could push it down with my hand. Checked the plug, it looked fine, caramel brown but dry, and no leaks anywhere. The carb boot on the engine side is a little dry rotted but last time I checked it it was smooth and no cracks on the inside. If anybody has a reasonable solution to repair the boot I'd appreciate that too. But the bike was done for the day.

I'm thinking the pop either blew back threw my reeds and messed them up? Or blew the piston rings? I don't know. When I did the top end I didn't touch the bottom end, so I believe its the original 30 yr old bottom end, as it still had the original piston in it when I got it (having said that the bike wasn't ridden too much, sitting up most of the time of it's life, so I wasn't too concerned with it at the time). But could this also be indicative of crank seals or anything there?

Before I pull the cylinder off and check the rings, has anybody heard of this or care to give me guidance to a starting point?
Thanks.

The correct starting point is as you mention, tear it down. If you've lost compression, something let go.

Got to go with Julien here. Just pull the head and I'm guessing you'll see the problem. Or if you have access to a bore scope you can just look in the spark plug hole.
Good luck.

DO NOT, I repeat, DO NOT tear it down just yet. Do a pressure test and a vacuum test on it before disassembly.

Good luck on whatever you do, and let us know how it turned out.

What were your carb specs, in detail?

Thank you.

I was considering a pressure/vacuum test. So I will definitely do it first. That should tell me if my crank seals are good too right? If I did blow a crank seal could that be the loss in compression? I'd sure love to not have to pull the cylinder, but I will do what is necessary.

Carb Specs:
Mikuni VM34 off of Bike Bandit.
I've probably got a combined total of 20 hours in getting the jetting right. That's been a pain but I just could not stand the fact that there was no air screw on the original. I can't exactly tell you what the jetting is, my notebook is at the house. But it was very close to running perfect.

This rebuild was a huge learning process for me. I've worked plenty on bigger engines, like my car and truck. But this was my first dirt bike and first time working on and rebuilding a small engine and a two stroke. I love two strokes now btw they are simply amazing.
But when I was assembling it I put the ring in the cylinder, used the piston to push it squarely in the bore and took feeler gauges to measure the ring end gap. I didn't file any off of it or do anything else to it as it was within the specs in my service manual. Is this common? Or should there have been some adjustment? I'm second guessing my engine assembly skills now. Haha but I guess that's how you learn..

A very similer thing just happend to a freind of mine and it turns out the people that rebuilt it put the piston in backwards, the arrow goes towards the exhaust. the cylinder and power valve were destroyed. I know it sounds stupid and it is, funny huh.

Hm. Yea that'd suck because power valve assemblies for this bike are a pain in the butt to find. But I remember noticing the arrow when I was putting it together. I don't exactly remember doing it but I'm confident in myself that I would've remembered that. But I'll definitely check it! I'd already done the break in procedure and ridden it a few times before it did this though. Nothing blew any no super bad engine noise. Just a loud pop from the exhaust and lost compression.


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those pops happen when there is a leak at the head/cylinder junction.

When you did the top end did you use a copper head gasket or the fiber style? The fiber ones can blow out and lose compression. It will also make the pop you described as it blows out.

Hmm I used a fiber gasket I'm pretty sure. Whatever comes with the wiseco top end kit. That'd be phenomenal if it was just a gasket! I'll check it when I get the chance. I'll try to find a copper head gasket somewhere. Parts for this old thing are slim. I think the cylinder base gasket is fiber too. Could that have blown out or is it just the fiber head gaskets that do it? Also if I had to could I spray a fiber gasket with the copper sealant spray?


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Paper base gaskets are good. The proper copper head gasket is easy to get still. Base gasket blowout wont cause low compression on its own.

Thanks! I found some copper gaskets on eBay. I'm gonna get a couple for future use as well. I sure hope that's all it is. Will a blown head gasket cause that much loss in compression? Because it was quite easy to turn over by hand.


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Update:
Well with my over excitement and my amateur experience level I'm a little embarrassed to admit that I found the problem..
As bone7615 said above, I installed my piston backwards..
Luckily however, my cylinder is still flawless and my power valves are still actuating perfectly. The piston is ruined but I will happily get a new one knowing my freshly replated cylinder is ok. I'm desperately hoping my rings didn't break and fall into the bottom end but I will have to wait and see when I pull off the cylinder. I'm just happy I found the problem, even though I had to learn the hard way. Thanks bone7615 and everyone here for their thoughts!