VBB Won't Start. 50psi Compression Test.

[tranquilitybase]

Hi,

I'm working on a '64 VBB 150 Vespa Sportique which has had an engine rebuild despite its condition being very good. The speedo shows about 5k miles and I believe this to be genuine judging by the lack of signs of wear in the power unit etc.


It refuses to show any signs of starting/firing.


I have replaced the piston/rings and had the cylinder honed by an engineering company. I was careful to put the (deflector) piston in the right way round.



New points/condenser were fitted and I triple checked the timing when I reset it.



There's a good spark at the plug but a compression test shows only 50psi (about 4 bar).


I've tried squirting "Easystart" directly into the carb to eliminate fuel starvation.


It seems strange to me for it not to have a head gasket. Could this be the cause of the low compression? I have read that the mating surfaces can be ground in, like valves on a car.



Any advice will be gratefully received.

[scooterg]

Sounds like it
Not as if the gaskets expensive
If it don’t work it’s not cost a fortune
And you can tick another idea off the list

[alan34]

Hello my 2008 Px 125 doesn't have a head gasket when I put my cylinder head back on I put some locktight oil and petrol proof sealer on not to much just a light smear then torqued it down and left it to go off.if u think u have a bad head I would try a second hand one. Not sure what the compresion should be if not in mannual try vespa club for advice.let us know how u r getting on keep trying Alan.

[scooterg]

Thought a gasket first
Easier than getting a head
I’d start easy cheap, then ?

[tranquilitybase]

Many thanks guys.

Your ideas are useful. I'll give them a try.....

[pxmatt]

Maybe try spraying brake cleaner between the head and cylinder whilst doing the compression test and see if there are any bubbles or hissing? I suggest brake cleaner instead of soapy water in case there is a leak you wont be squiting water into the bore. Brake clear evaporates/combustable. This is how I found a leak on my PX