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E: Engine Correct Compression? Rapide


New Website User
VOC Member
I have a 1948 Vincent Rapide.

Just checked compression:

Front was 90 PSI
Rear was 110 PSI

What should they read?

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Well Known and Active Website User
VOC Member
Cold would explain the generally low figures.
20% variation between cylinders is getting up there as half this value is considered acceptable.
Suggest you run the engine if you can to a warm temperature and try again to reduce piston to bore clearance.
Also suggest once this is done you squirt some oil down the plug holes and take another set of readings. If it increases then you've got leakage past the rings. If no change then valves leaking.
I would expect to see at least 120 psi normally in a soft road engine when measured hot.
Good luck!


New Website User
VOC Member
pushrod: Kicked over (no electric starter on this bike)

Thanks for input, Passenger.

Warmed bike up:
Front 117
Rear 125

Warmed up then oil squirted into plug:
Front 140
Rear 140
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Well Known and Active Website User
VOC Member
Try a leakdown test. You might find the engine is in good shape.
Compression tests are a bit iffy with electric start and even moreso with a kickstart.
A leakdown test will also tell you where the problem is, if there is one.
Leakdown testers are quite inexpensive.
I had an issue with low compression on a Rapide. The leakdown tester agreed with what I was noticing on kickover. The leakdown test pointed to the valves as the problem, one cylinder much worse than the other.
On inspection it was the valves that were leaking.
There were tiny bits of carbon baked onto the valve seats.
This occurred when running with worn out carbs.
A bit of valve lapping with a cordless drill fixed it and brought compression leakdown back in the green range ( excellent)

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