CR500R loss Compression

Nope, but you do. That hole got in there from either detonation or an air leak. Even slight mechanical up keep could have prevented this. Been running regular gas? Check the plug, listen to the motor. The old 500 is about indestructible, can last seasons without top ends. BUT,,, it could have been worse. Vintage Bob

tstapp said:

Can anyone give me some advise. I had my 1984 CR500R opened up on a stretch the other day and it lost all compression. Does the air cooled 500 have a problem burning a hole thru the piston? :think:

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If you verify that there is a hole in the piston ......

Any 26 year old bike could burn a hole through the piston if it happens to be running lean for some reason (leaking ignition side crank seal, air boot leak, leaking base gasket, etc.). Some would say to check the jetting, but there's no point in jetting around a problem like an air leak.

I have a '79 husky that runs a bit lean. I rode it for for a 25 mile loop and noticed pinging, so I did not ride too aggressively. I need to fix any air leaks before I start messing with jetting or ride it again.

I've assembled the parts for a leakdown test kit, but I've been too lazy to put it together and test. I tested the air boot and base gasket for leaks by spraying them with starting fluid while the engine was running. If there were leaks at those spots, the starting fliud would have changed the idle speed significantly. There was no change in the idle speed, so I'm guessing that it's the ignition side crank seal that's leaking. The next step is to do a leakdown test. All parts for the tester can be assembled from items bought from the local Home Depot for about 20 bucks.

If I get around to assembling put the leakdown tester, I'll post pics of my version. Meanwhile, you can search various dirt bike sites for "leakdown tester".

The black rubber clean out fittings, that have the hard plastic tapered insert, the more you turn it in, the harder it compresses. Schrader valves thread in sweet. They come in a lot of popular sizes. Vintage Bob

tstapp said:

FYI... The 1984 Cr500R did have a problem with overheating, that is why after 1984 all Cr500's are liquid cooled. A small amount of research on your part would have revealed this. I do agree with you on one point, the bike is almost indestructible since I have been riding it for the past 15 years balls to the wall without any problems other than a few broken bones. Thank you for the advise.

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FYI, most of this type of information comes from guy talk and magazines like mx action and such. Waste of time, completely. Less you feel up for a laugh? Some may be true, I would bet that a modification to the cylinder/piston would resolve this also. But there is always a first? (30 seconds later) Not this time either, sorry. Its hard to get out of that habit, but it can be done. It worked for me! In Eric's book about MX and Offroad Performance Handbook, clearly states the issue and solution on page 177. Not 1 word about chronic overheating by the way. You made me get the book! Vintage Bob

Theyn did have an air scoop that may have helped. had a couple of these bikes only time one locked up on me was full on 80 mph fireroads. make sure your bike had the correct float valve (size) and level inthe carb Some kiehins float valves were pressed in and not removable but if someone did change it to smaller size you could have a problem filling the float bowl . its probably dried up crank seals or gasket or something but id check the carb out and clean the screen in the fuel tap. Might keep you from buying piston #2 i bet theres alot of old desert riders that have fixes for this bike I really liked mine.