Low compression problem

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Crofty1984
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Low compression problem

Post by Crofty1984 » Sun Mar 10, 2019 10:14 pm

What could cause low (but not no) compression? The Starfire had been a bit reluctant to start then just died after about 5 minutes of riding. I've got fuel getting there, sparks, I've checked the EI hasn't slipped on the tapered rotor, so spark timing is OK. I'm showing about 75psi.
I checked the pushrods are free to spin as per Rupert ratio so I don't think the valve clearances have closed up. Put a bit of oil down the bore and that's impeded the compression based on kickstart feel, I'd put the test away by then.
And then how do I fix it? Thanks in advance all!

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Re: Low compression problem

Post by skippy » Sun Mar 10, 2019 10:45 pm

Crofty1984 wrote:
Sun Mar 10, 2019 10:14 pm
Put a bit of oil down the bore and that's impeded the compression based on kickstart feel, I'd put the test away by then.
I take it that impeded is improved, if this is so your rings are worn. I had this problem with mine, it had been started and run without the bore being checked and the rust slowly ground ground thing away.
Doug
Should never have sold them old motorbikes

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Re: Low compression problem

Post by mlb50 » Wed Mar 13, 2019 6:47 am

Knackered piston rings take a while to wear down and start to smoke badly.
Sudden loss of compression can be due to a chip off a valve face or eroded seat. Doesn't take much to bleed off PSI. Pull the head and have a good look, best also to remove the valves and either replace or recut.
B50 @ 10:1 - what's a leg between friends?

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kommando
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Re: Low compression problem

Post by kommando » Wed Mar 13, 2019 8:12 am

Even a small loose piece of carbon stick to a valve will do it, once it comes off normal service will be resumed. Had it on a C15 that stopped on way to work, 50 easy kicks later it restarted.

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