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'85 RM 125 compression

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I'm looking at an '85 rm 125 for sale locally that looks good on the outside and seems to ride good. But I've been burned before on old bikes that looked good!

I want to test the compression on the bike before buying it. A bike mechanic that seems to know told me it should be ok if I can get a reading over 100 psi cold on the test. I'm not expecting a 26 year old bike to have the compression of a new one, but do you think 100 psi or above would be a good number to look for to hopefully avoid problems caused by low compression on this bike?

I found a spec sheet that says this model should have 8.4:1 compression. So new should have been something on the order of 7.4 x 16 = 120 psi.

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It does not matter how old the bike is, cylinders can be replated to better than factory spec and pistons can be replaced. Any year or model can be freshen up to factory new condition. That bike should make 140-170 psi. What you don't know is if anyone has shaved the cylinder or head to increase or decrease compression. Even tho its an 85 model your looking at or if it is a 2011, its all in how much love that bike has had from the owner. Good luck.

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Thanks Padgett!

I brought the bike home. I'll take time and sort it out over the next month or two. At least she runs for now. And runs pretty good for a 125 I guess. I'm used to a Husky wr250 and it seems you have to rev the 125 to about 5 million rpm to get to the real power.

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84-85 rm125's were all top end power motors, they can be ported to run strong @ lower rpm's, but then they eat rings at an alarming rate!

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I just purchased my 1st dirt bike last Saturday, 1987 RM 125.

I know just enough to do simple wrenching and I always ask advise from various forums when I don't know what to do.

With that said, I tested my '87 cold and got a 155-160 reading. I rode the bike for the 1st time yesterday @ Milestone MX park in Riverside, CA. (my 1st time riding) and bike ran great.

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No update yet. I took the Husky out to ride with my son last Thursday and it ran great as usual. What's even better is that my son's kdx 200 had no issues. He had just bought the bike the previous weekend and it was his first time out with it.

On the RM, I've got a few things to do to it before it goes out. New chain, maybe a bigger rear sprocket, get a missing muffler spring, and adjust the suspension. Then I'll take it out and see how it does.

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