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Discussion Starter · #1 ·
Hi everyone I'm new here I was recently given a 1976 honda cb500t and have been working on it so it will be my first bike when I turn 16 in a month. So I pulled the motor out rebuilt it using newer used pistons with 10,000 miles since my bike has 28,00 and the motor was seized when I got it. I honed the cylinders and put standard size rings on the newer pistons. Put the motor back together with a new cam chain and didn't really do anything to the head except for clean it up and throw it on. I then painted the motor gloss black and put it in the frame and did a compression test and got basically nothing out of both cylinders. I then adjusted the valves correctly and got 130 pounds out of the left cylinder and 30 out of the right which is not right at all. I expect one of the valves not seating right? What should I look for if I have to take the head off again? I'm not too familiar with the dohc torsion bar type engine so don't really know what to look for. I expect the compression to go up in the left cylinder since there isn't any oil in the crankcase but only oil to lubricate the cylinders and the rings still need to seat. I would really apreciate the help and sorry for the long post!
 

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Not knowing the original running condition of the engine since it was stuck, after you honed the cylinders did you have them checked for size?

When you got the rings did you put them in the bores and check the end gap?

If the gap is to large or the cylinder bore is to far out of shape that can cause leakage which can cause low compression.

Not checking the valve seats on a stuck motor is another no no. There's reason they stick and if it was due to water that will also cause the valves/seats to corrode and not seal correctly.

I know that's some things you may not want to hear but they are fixable. Might be that you still have the valves too tight, there is a thread active right now that talks about setting valves for a 450, process is the same for yours.

Welcome to the forums btw, feel free to post up in the introduction section with plenty of pictures. :D
 

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Ok first drop some oil into the badly performing cylinder and see if that makes any difference to 30psi. If so then you have a real problem with rings or bore. If nothing changes then it's likely to be valves. Now drop some oil into the better cylinder and see if this improves at all and if so to what. 150psi is the lowest you should really have most here get 160-180ish
So if it turns out to be valve related then you have to consider why. Did you use .05mm / 2 thou or the manual spec of .03 or 1 thou. That is too tight.
It could be a badly seating valve. It could be a bent valve. It does seem likely that you will have to have the head off again IMHO but lets try and isolate the fault first.
 

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Discussion Starter · #4 ·
Yes I did measure the gap for the rings in the cylinder and it was mint. So I'm thinking it's valve related. I adjusted the valves correctly to .005 in. I think it could be I valve not seating correctly or possibly a bent valve. I am almost positive the compression will go up when I put oil in the cylinders and the rings also still need to seat. Thank you for the replies I hope we can get this figured out
 

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.005" is NOT the correct valve clearance adjustment......0.05mm (.002") is....
 

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Sensei
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.005mm is STILL wrong.... It's 0.05mm...... .005mm would likely not allow the valves to fully close once warmed up....
 

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Sensei
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So true.....But, back to the problem at hand......You COULD add 160 PSI to the cylinders with air through the spark-plug hole and check the intake and exhaust ports for escaping pressure with the valves APPARENTLY closed (TDC on compression stroke for that cylinder).....A bent or non-seating valve will be obvious......If most of the pressure comes out the breather, it's the rings...I'd take out the dipstick so as NOT to overpressurise the cases as well and it would leak out there too.....
 

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It seems to me that you might as well pull the head and see what's really wrong. The top end has to come apart anyway to fix the issue whether it is rings or valves.

I'm betting on valve problems. Easy to check with some acetone after you get the head off.
 

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Discussion Starter · #11 ·
Thanks for the replies guys. I pulled the head and lapped all the valves and all looked mint except when I got to the left intake valve thing just didn't look right. There was a gap between the seat and valve. Did a few tests and the valve is abviously bent.
 
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