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drop in pressure after rebuild but cyl not scored??

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1989 Cr250r - rebuilt last winter with a knock-off piston as this is just a casual bike. It had blown up due to waterpump failure. Replaced everything, got a new cylinder that still had hone marks off ebay. It went through its break in fine, with 5 heat cycles. Tons of power. Took it on a 15kms run down the gravel road - great. Rode it to my buddy's place 5kms away and as I was getting there power faded fast, soo I was in 5th gear gunning it and going a jogging pace. when it shut off it would not fire again. Pulled the plug and found flakes of metal. since I melted the last piston and had gobs of metal on plug I assume I had wrecked the piston and possibly the cylinder. pressure tested and it was at 65. assumed it was lunched and parked it.

That was 9 months ago. I finally got around to taking off the head (I said this was a casual bike) and the cylinder walls look good, no scoring. Put the head back on and pressure tested it. 145. hhmmmm.

Without taking the cylinder off to look at the piston and rings can anyone think what might be happening?? The air screw had been stripped and so was running lean I know but too lean?? This was a bike that ran good two summers ago and I have not changed setting besides the air screw (I had srcewed it all the way in, they tried to back it out the 1 1/2 turns but it jammed and snapped off the slot.

So what I am asking is if I could have started to fry the piston - could I NOT have scored the cylinder?? could I somehow be getting 145 psi with a partially fried piston??

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hey chopper,

not sure what you mean by stuck the rings? I am assuming that I have a ring problem but I assumed that problem rings may in turn mean scored cylinder. The bike DID run great for 20 minutes or more. crisp and tons of guts after the heat cycles. Then I tried to mess with the set screw to stop the spooge and then shortly after the compression died.

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that one of the rings somehow got seized into the ring groove. this can happen due to a lean condition/lean oil condition.

at times all itll take to get the ring to pop back is to take the piston out and itll pop out by its self. and itll seem fine even thow it is hopped. and in same cases wont hurt anything in the motor such as the cyl. wall.

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Just thinking out loud here. What happened when it was blown up? What parts exactly were replaced? I'm wondering if the lack of proper cooling (due to waterpump failure) caused the head to be warped a little.

Or simply the head nuts loosened up after the heat cycles/riding it the last time. When you rebuilt with new cylinder and piston/rings all seemed well. But then when it got some more heat in (and after the heat cycles) the head loosened up a bit and you lost compression, thus the power drop. When you pulled off the head and put it back on it was now sealing well again and the compression jumped to 145.

Does it run now and you're trying to figure out what happened or does it not run at all anymore? Also, the potential lean condition could've caused it to run hot resulting in the head (or base?) gasket to lose its seal.

After re-reading your 1st post, I forgot you said you found metal...oops! Do the piston/rings look damaged in any way?

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if you tourqu the head right it should never loosen off to the point were it looes compration.

if that happend it would be noticible because coolent would be leaking from the head.

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if you tourqu the head right it should never loosen off to the point were it looes compration.

if that happend it would be noticible because coolent would be leaking from the head.

What if it's warped instead?

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coolent would more then likly be ither leaking into the cyl. or out ontot he cylinder in both cases.

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true...

Sounds like maybe those metal pieces were actually from the rings sticking like mentioned above? Seems possible assuming no coolant loss.

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i bet its peaces of the big end bearig in the crank that ****ed off then got sucked up.

I was also thinking that, but would that affect compression that much if there's no visible damage to the cylinder/piston? I suppose if the bearing was bad enough it wouldn't allow the piston to travel enough and the compression would be low, but it's just weird that the compression went back up to 145 once he re-installed the head. I'd like to see some pictures of the parts...

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It sounds like I shoudl go ahead and pull the cylinder.

Details - The engine (a 88) had the mag waterpump and so the original cylinder was scored when the piston shattered and went down into the bottom end. I replaced the cylinder, water pump cover, head, clutch, from a '91. All went on great despite some people saying a 91 is not compatible. Trust me it is.

I did forget to fit the rings properly, meaning press them in the cylinder and measure the gap - stupid me but I did not do this. I thought afterwards that maybe these swelled and tore into the cylinder but of course this did not happen.

Also When I took the head off today the nuts were friggin tight and there was NO leakage of coolant. I did NOT test comp first - the 65 was back in June when the problem first happened. for all I know the cold tightened it up enough and pulling the head off and putting it back on did nothing.

I am busy tomorrow but Sunday I will try to get around to pulling off the head and checking the piston. I think it is a namura or a pro-x. It was only 65 big ones off ebay. The metals flakes were just that, fine flakes on the plug. When I pulled the head today I could not find any. everything looked very clean and solid.

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hey chopper,

not sure what you mean by stuck the rings? I am assuming that I have a ring problem but I assumed that problem rings may in turn mean scored cylinder. The bike DID run great for 20 minutes or more. crisp and tons of guts after the heat cycles. Then I tried to mess with the set screw to stop the spooge and then shortly after the compression died.

what set screw were ya playing with?

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well if they are really fine shavings then chances are its off iether the piston/rings or cylinder

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what set screw were ya playing with?

aka air screw - on the micro fiche they call it the set screw (I just ordered a new one so I thought I would use correct terminology...)

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oh ok just wondering if you could have leaned it out too much?

That was my original thought last June - that I had messed with the mixture and ran it too lean and cooked the rings/piston. I have been too busy cutting wood to pull the head off and it is freezing out there now - will do it tomorrow when it warms up.

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