Alloha ,

I just freshly rebuilt a GTR engine from 71 for a friend of mine, I should add that I'm more used to rebuild kitted vespa engines...

Almost everything have been changed, rings included, but when I start the engine, I am surprised how low is the compression...

There seems not to have any air, oil or mixture leaks...

Engine is working ok, but there is very little power at low RPM, I guess it is related to the compression as I have also double checked the ignition timing (22°). Besides that, points, condenser and ) HT coil are new. Carb (SI20/17) has been cleaned, but same disapointing results were found with a SI20/20...

Mid and High RPM seem to be ok, cruising at 85-90km/h top speed... Exhaust is from PX, but it is not new.

Any hints why the engine would be soooooooo lazy at low-RPM ? The deck clearance is more than 1mm in the cylinder head, could it be interesting to reduce the overall squish to something like 1.5-2mm by turning the cylinder head ?

Thanks !

KTy

Maybe your crankshaft-housing is damaged...

Did you hone the cylinder before installing new piston rings? What´s about piston to cylinder clearance? Excessive end gap of the piston rings possible?

Maybe your crankshaft-housing is damaged...

Could be... But the engine was in very good shape before we changed the bearings, very little play and very little ware... Besides that, it idles fine, doesn't smoke too much...

Did you hone the cylinder before installing new piston rings? What´s about piston to cylinder clearance? Excessive end gap of the piston rings possible?

Nop, again, cylinder/piston were in very good condition, we just changed the rings. Of course it would have been better, at least, to hone the cylinder, I just gave quick turn of sand paper...

Is the PX exhaust a good choice for the GTR ?

Is the PX exhaust a good choice for the GTR ?

Normally it should be a good one. Maybe something is wrong with your idle mixture or ignition ist not working properly at low rpm (poor magnets, incorrect coil to magnet position at pointers opening).