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Motorcycle Repair/low compression

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Question
i have a  1965 cb 77 305 Honda cycle
I am having trouble with low compression

I have....installed new cylinder..rebuilt the head...new rings..leak down test and found that 100% leakage.. check the piston..no holes or warping,,,compression is 20 left and 120 right

could you suggest a fix

Answer
engine cam timing
engine cam timing  
Debbie, check your valve clearances, first.  .004" cold on the intake/exhaust set on the T mark when each cylinder is on compression stroke.. both valves closed.. not on overlap when both valves are slightly open.

A leakdown test would show you where the compression loss is... air escaping from carburetors/air filters is intake valve leak. Air sounds in the mufflers would be exhaust valves leaking.

Excess air coming from breather tube indicates ring sealing issues... broken perhaps on assembly.

If cam timing was not set accurately, then you could have bent valves against the pistons when you turned the engine over.

Cam chain should be installed when the right piston is at T mark and the punch marks on the camsprocket flats and point cam are straight up, if you envision a straight line from the crankshaft to the center of the camshaft sprocket. The flat surface on the camsprocket needs to be parallel with the cylinder head top surface.

Normal compression readings on a rebuilt engine should read out about 160-175 psi.

You can back off the valve adjusters, roll the camshaft lobes until the valves are closed on the combustion chambers and then spray carb/brake cleaner down the ports, watching for leaks at valve seats in the combustion chambers.

Bill Silver  

Motorcycle Repair

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Bill Silver

Expertise

Need help with vintage Hondas from the 1960s? I am an expert with 250-305cc bikes in particular and most all of the other pre-91 models, in general. I do NOT claim to have a great deal of experience on Gold Wings, Cruisers, ATC/ATVs and dirt bikes.

Experience

I have owned/ridden/maintained Honda motorcycles for 35 years. I have written five books on Honda repairs and collecting. I was a service manager for two Honda shops back in the 1980s.

Organizations
VJMC (Vintage Japanese Motorcycle Club) of North America

Publications
VJMC newsletter, as editor for two years and as contributing editor currently.

Education/Credentials
3 years auto shop in high school, teacher's aide in Automotive Technology in Jr. College, Diesel mechanic course in college, self-taught mechanic and automotive writer.

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