84 KDX 200 Questions - Piston Rattle? Replate or Rod?

[Chuckrfish]

Hello all, long time lurker first time poster.

I've been reviving an 84 basket case since last fall and I'm about 80% done. When I got it, it somehow ran, but had bad main bearings and crank seals, the cylinder had rough brown spots and had sizable chips out of the top and bottom of the exhaust port where plating wore away.

I disassembled the motor and replaced crank seals and main bearings, kicker mechanism, the rusty crank with a good looking used crank from a spare bottom end, and replaced the original "A" stamped cylinder with a good looking used 85 "B" cylinder. The cylinder has uniform plating and seemingly no flaws but presumably it's still the stock plating.

Anyway. Bike starts, runs and rips pretty well as far as I can tell. When I did a compression test with the Wossner piston after break-in, I got 160-165 PSI with the engine cold.

The only problem I'm chasing down is a rattle or chatter that I hear when letting off the throttle while I'm in the powerband. Everything sounds fine when the engine is under load, but most times when I go from loaded to unloaded at a mid to high RPM situation, I hear a rattle that sounds like it could be the piston slapping. Does not matter how warm the motor is or how long I ride it. If I take the head off, put the piston at TDC and push on the intake or exhaust side, I can wobble it back and forth slightly to hear a similar tapping sound (I do not know if there should be any forward / backward piston rock at TDC or not).

I started the rebuild with a brand new Wossner 8093DA piston at the suggestion of the KDXGeoff post on this board and did 3 heat cycles and some test rides around my field before riding it for about 5 hours at a local enduro event.

After hearing the rattle all day at the enduro ride, I found a seller on Ebay selling cast stock reproduction pistons for the 83-35 KDX's and decided to try that to see if that made any difference, and it really didn't. It may be quieter overall than the forged Wossner, but I can still get the short chatter when I go from heavily loaded to unloaded quickly, like snapping the throttle shut. If I go up the revs in neutral until I hit mid to high RPMs and hold it, I can reproduce the chattery sound as well. It does not chatter all the time that I can tell.

Since I didn't replace the rod when I had it all apart the first time (rookie mistake, absolutely kicking myself now) could a sound like that possibly be big end / small end play, out of spec?

Is it possible for a stock 40 year old bore to appear visually fine but be worn enough for excessive piston to cylinder clearance without being worn through the plating?

Last bit of information I just remembered. The copper head gasket that came in my Vesrah kit does not align perfectly with the cylinder hole, part of it overhangs the intake side by a mm or 2. I didn't think it would be an issue since the piston doesn't seem to reach the gasket but hey, it's another variable in my experiment. Could the piston actually be hitting that gasket under no load, high rev situations?

When I did a compression test with the Wossner piston before switching to the stock reproduction piston, I got 160-165 PSI with the engine cold.

After months of exhausting internet research for non-existent 83-85 parts, I now have a Wossner conrod kit and recently eBay'ed a NOS 83-85 "B" piston and rings ready to go in but wanted to run it by the experts before I drop $400+ on a Millennium re-plating. I also understand if these are things only a shop could tell me by measuring my specific parts and clearances.

Link to video - Idling after rebuild

Hope that ramble all makes sense. I've been working on this thing by myself for quite a while and I'm finally breaking down to ask for advice!

Thanks

[Kipsy]

Sorry I can't answer your entire situation as I don't all the causes of . But in answer to your question:

"Is it possible for a stock 40 year old bore to appear visually fine but be worn enough for excessive piston to cylinder clearance without being worn through the plating?"

Yes, it is. When I bought a 96 KDX200 it was in pretty good shape. It had been stored inside all its life and looked well-used but better than your average 90's KDX. I opened it up for a top-end and decided to send it to Millennium just because I wanted to keep this bike for many years and figured it should have a fresh cylinder. I was surprised to find that I needed to pay extra for them to rebuild the cylinder it was so worn. The bore had worn oval. The cylinder looked really good when I tore it down - no deep scratches or dark patches. The bike ran really well. Yet that cylinder was really worn out. I didn't check compression before rebuild. With the wear on the top-end, I ended up just rebuilding the bottom-end, too. New rod, and main bearings. One main bearing was spinning roughly so I'm glad I did.

And that was a 96 (5 years ago). I have an 87 and 88 also I plan to rebuild and just expect to send them away. If my 96 was that bad, I would guess just about any mid-80's bike would be even worse.

I just listened to your video. I don't think it sounds bad. I think that sound is how all my air-cooled IT's and KDX's sound. Even the ones with fresh bores and pistons. Maybe the experts hear something I don't. Air-cooled are a lot noisier than liquid-cooled. I think a rod is more of a metallic knock.

There is also "4-stroking" which my 88 had when I last ran it years ago. It sounded like someone banging a hammer on the exhaust pipe when I let off the throttle after running in the powerband for at least a few seconds. It only happened when the engine was good and warm - 20 minutes or so of riding. But that banging was irregular, almost like a backfire, and very loud, like startlingly loud.

[Kipsy]

I would guess that healthy rings can conform to an oval-shaped cylinder, keeping compression acceptable, but the piston will be able to rock.

[Chuckrfish]

Kipsy,

Thank you so much for taking the time to read and reply. I really appreciate it. That makes me feel a lot less crazy knowing you also had similar experience. Also explains why my pistons rock in both cylinders, they’re BOTH worn out.

When I first got this 84, it did run, but not well. I did a leak down and found the right side crank seal leaking and the crank had a drastic amount of vertical play. The bearings were absolutely destroyed. But it still ran! So I can totally see how a cylinder could become ovaled over the years by unsuspecting owners. The amount of smoke was absurd though with all that gear oil being burnt, which is what led me down this total rebuild path.

I will bite the bullet and get a re-plate. Thanks again for the detailed input.

After that I’m on the hunt for a new carb. It came with an Amazon PWK 35 clone on it. Brand new Mikuni VM32s are half the price of a new PWK but I believe the PWK is more modern and easier to work on.

[Kipsy]

You're welcome, I hope I steered you to the right solution. I think you're making the right decision. My attitude toward a 40 year old engine in a bike that I love is - if I'm going to work on it, just open up the wallet and do it right. A bike that lasted this long will last the rest of my life if I just rebuild it properly.

As for the carb, a PWK 35 came on the 88's and I've read they are a better carb, as compared to the Mikuni's that were on the 87's and earlier. I can't notice much difference between my 87 and 88 but neither one is running in top form and both have tired engines. I don't know about how good the clone's are. I guess it might be a crap shoot - if it's a near-perfect copy - great! If not.....

[KDXGarage]

Be mindful of intake and airbox boot dimensions, as well as length of carburetor. It might be a little better carb, but it might not be worth the hassle. Some guy on eBay makes reproduction boots for intake and airbox for some KDX's and other 80's models. Hot foot moto or something like that.

[Chuckrfish]

Thanks for the advice. It’s a pretty confusing game for the carb. The way I understand it there were 2 different Mikuni models offered between the 83-85 and the 86-87ish? One circular and one oval bore? Then they went to a 36mm PWK that is too big for my airbox boot on all of the later models I think.

I have a VM 36 that I got with the bike that fits the intake perfectly but is too big for the airbox boot so it’s a no-go.

I have a hot foot airbox boot and looks like the PO put a hot foot intake manifold on it too.

My intake manifold is 43mm and my airbox boot is 58mm, it fits the modern PWK Airstryker 33/35 dimensions correctly. They’re out of stock at JD Jetting and Sudco just closed their business so that’s a bummer.

https://jdjetting.com/product/jdkh35-ke ... w-jet-kit/



The VM32 I’m looking at on Niche Cycle is about as close to original as I think I can get brand new. Maybe you’re right, best to stick close to stock.

http://www.nichecycle.com/ncs/mikuni/ca ... -left.html


On another note, I found a local power sports machine shop who said that if I ran a new piston in a 40 year old bore, I’m I’m almost certainly hearing piston slap. As the original piston would have worn with the bore, a brand new one would not match with the decades of wear and be noisy even if the rings take up the slack. They’re going to see which of my cylinders is best and send to a partner shop for a replate.

[KDXGarage]

The 1988+ KDX's had PWK35, not 36. The 36 body is bigger, as best I remember.

I'd pull the studs and send the OEM piston along with the cylinder to Millenium or PowerSeal.

I hate to hear Sudco is gone!!