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72 Commando Fastback 750 Compression readings

Submitted by ian_hind on Sat, 30/06/2018
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Commando

Hi all, my Dads 72' Commando 750 Fastback started running rough a couple of days ago, after intital tests of plugs, ignition etc.. which all appeared OK we decided to check cylinder compression. The left hand cylinder was at 80 PSI and the right one at 130 PSI, oh dear it seems we might have a valve or piston related issue.

Does anyone know what the actual compression readings should be ?

Kind regards,

Ian

Submitted by john_holmes on Sat, 30/06/2018

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When you have more than 10% difference between cylinders you have a problem in the low reading cylinder, you now need to do a leakdown test to find where the compression is leaking. As it's relatively sudden my guess would be a loose piston circlip has worn a groove in the bore but it's a guess.

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Submitted by andrew_heathwood on Sun, 01/07/2018

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Ian

I agree with what John says - further test is also known as a leakdown test.

For info readings on my '72 Roadster were 120psi on both cylinders with a cold engine.

Andy

Submitted by ian_hind on Sun, 01/07/2018

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Thanks for the advice guys, stripped the head and barrel, the bores and pistons are good. After filling the combustion chambers with petrol it seems that the valves are passing. Going to check all the valves and replace and reseat where necessary . Cheers Ian

 


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