CR125 Leakdown and Compression

Ray Lovestead · Posted: Sat Mar 09, 2013 8:49 am Post subject: CR125 Leakdown and Compression

I picked up a compression gauge and a leakdown tester recently. They already did their jobs and told me that I do indeed have worn rings (I can see a particular worn spot).

But here's a question: How good is good enough? Can anyone give me some numbers on their compression level (psi) and leakdown (%) on a Honda CR125?

I know that 0% is impossible. What is possible? And what is good enough?

thanks!

Ray
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"Karting Expert Since 2014"

Randy Mckee · Posted: Sat Mar 09, 2013 12:55 pm Post subject:

Is this a stock motor? It might make a diff in the compression verses a mod motor.

Also what level will you be racing? If club level, it's not that important to get those last little bits of power out of your motor. Driver and setup will always be more important at that level. If racing on the national tour (and you're competitive at that level), then you'll want the motor to be at it's best.

I don't have compression #'s for a stock honda. (sorry). I do use a leak down to test for air leaks after a rebuild. Usually pump it up to around 6 or 7 psi and it should hold for several minutes with little or no drop in pressure. Typical leaks are around the exhaust manifold or reed cage. Spray simple green or soapy water to find the leaks (while it's pressurized).

Hope that helps.
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Randy
#123 TonyKart - S4 Stock Honda

Ray Lovestead · Posted: Sat Mar 09, 2013 1:22 pm Post subject:

Randy

This is a mod moto. I'm not running the compression terribly high (maybe 10:1). But it's a pretty worn out jug. I've stuck a few pistons over the past year.

I'm only doing the leak down test at TDC to check the leakage past the ring.

I'm getting 45% leak down. I replaced the ring and now I'm getting 40%. Whoopee. Seems to me that we won't get great results considering the size of the gap in the ring. But I don't know what a new piston, new ring and new cylinder would get so I can decide when it is time to replace more parts.

I'm not concerned about leakage past the reeds (they are new), but don't you get lots of leakage past your pipe viton rings (I know I do!). Also, how do you plug up the pipe's exhaust end?

Ray
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"Karting Expert Since 2014"

Randy Mckee · Posted: Sat Mar 09, 2013 1:54 pm Post subject:

My bad. I read that as testing for air leaks in the motor. You're doing a cylinder leak down test (duh -- exactly what you said)

I was referring to testing the motor for air leaks. You plug the intake and exhaust and then pressurize the cases via a pump hooked up to the pulse fitting. Different test.
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Randy
#123 TonyKart - S4 Stock Honda