Commando 750 compression readings

Over a hundred and within 10 psi of each other is a rough guide aswhat'sacceptable , the higher overlap of the 2S cam reduces the real compression at test speeds more than the higher compression ratio increases it.

Another test is the 'leak down test'. Differs to a compression test as the engine is static with piston at TDC on compression stroke. Cylinder is pressurised withair, you then listen for hissingat carb inlets, end of exhaust pipes and crankcase breather indicating less than satisfactory sealing at inlet or exhaustvalves and piston rings. A leakdown test kit comes with a gauge giving you leakage as a percentage (of what I'm not sure) but is a good way of telling you what the problem is before stripping the engine. Youdo need a compressor though to carry out the test although I have made a leak down test tool out of a spark plug and tyre valve.

Forgot to mention in addition to the above diagnostics, on a multi cylinder machine removalof the spark plug from the adjacent cylinder and evidence of hissingwould indicate internal leakage across the head gasket.

Hi Andy,

120 & 130psi are acceptable readings & would pretty much rule out valve problems as a cause of your red hot exhaust, if a valve was leaking badly enough to cause that, I would expect very little pressure on the offending cylinder, & I'm guessing that's what you're still trying to diagnose ? Which cylinder was the lower figure ? you've put them both down as right hand, & what ignition system do you have, isit still on two sets of points or has it been converted to electronic ? If on points, you can have wildly varying timing from one cylinder to the other, so worth looking at that.If you are still on points, I'd recomendfitting one of the readily availableelectronicconversions, that will eliminate unequal timing&If you're not worried about originality, a single carb conversion is a good way to go, much less hassle & better economy too & again, eliminates the possibility of them getting out of balance. That said, it should still be possible to get it running perfectly well in stock trim if all is set up correctly, it just wont stay that way for long

HTH, Regards, Tim