84 CR500 Engine Probs

Joined
Sep 2, 2001
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#1
I have an 84CR500 that has been nothing but a big pain in the butt. I got it knowing it needed the engine to be rebuilt (low compression) so I did that. I found in the rebuild that the head was leaking so I had it milled just enough to smooth it out and had the cylinder bored. I also replaced the piston and rings naturally. Once I got the thing together the compression felt good, but it ran horribly. I tinkered with the main jet and the timing (which I shouldn't have touched) and got it running better but it still ran like crap. It would either bog on the low end or high end. Well, after a few hours of riding it (basically all just test rides after the changes) the engine lost a ton of compression and would barely run. I checked it and there is only 41 lbs of pressure (how much should it have anyway). I took it apart and the rings are flat on one side and the plug is in bad condition (I thought the piston had melted due to the plug condition but it didn't). What went wrong? Now I need to do this all again and I want to make sure this time it works!!!
 
Joined
Jan 25, 2001
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#2
Ignore the compression meter it will only tell you when something catastrophic has happened. Like it did.

Don’t quite understand the description of the damage but guessing detonation.

Oh dear where to start?

The problem could have been that the compression was too high after machining for the gas you were running in + it raises the MSV which you want to keep low on an engine of this sort & in combination with timing that may have been too far advanced & maybe too far down on the main jet.

PS the main alters only ~full throttle fueling. Ignore revs, the pilot jet can be way too rich & changing the main too lean won’t affect low throttle position jetting.

Put the timing & jetting back to std for a while & you may have to machine the head squish area to lower the compression & the MSV. Did you measure squish clearance after putting it back together?

Then work on getting low throttle position right & working up to main.

Hey just thinking re reading, -it didn’t just seize the rings on the exhaust port bridge & smear them into the lands?
 
Joined
Sep 2, 2001
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#3
1) I did not measure the squish after it was machined, how do I go about doing this?
2) The engine did not seize on me. THe rings were flattened on the side opposite the line up pins.
3) it seems that there is too big a ring gap (about 1/8 inch) when I put them back in the cylinder without the piston in.

Thanks for the help