Motorcycle Repair: kick start, air fuel mixture, crankcase breather

Question
I just restored (cleaned carb,new tires,coated gas tank)my 1965 Sears imported 106 Gilera from Italy.

It was riding good.
Now When I kick start it does not have compression. It builds up but after 1 or two kicks if it does not start I lose the compression I can start it by popping the clutch going down the driveway.
What is causing this?
The Gilera is 4 cycle.
Thank You for any help
Jack

Answer
Jack, I actually do know of the bike, as a friend in high school had one way back when....

Compression is a function of:
Valves sealing in the cylinder head
Piston rings sealing in the cylinder bore
Cam/Valve timing

The other factors would be that the engine is able to inhale air/fuel mixture (plugged air filter would restrict air flow) and if the exhaust system was plugged up with carbon or the muffler collapsed inside, blocking the exhaust gas outflow.

The first thing to do is to adjust the valves to whatever the correct tolerances are....

If the valves had little or no clearance, then that could bring your compression back up.

If the valves have correct clearance, you could have a burned valve.

Your valves could be okay and the piston rings could be worn. You would see a lot of blue smoke from the exhaust and the breather would have a lot of fumes coming out.

A lot of Italian bikes had little or no air filters and that cause premature wear on the intake valve guides, valve faces/seats and piston rings and cylinder bore.

If everything is to specifications externally, get a compression check done to see what the numbers look like.
They pretty much won't run under 100 psi. If someone has a compression gauge adapter, you can remove the check valve and use it with low pressure air to pressurize the cylinder when the valves are closed at TDC. Listen for where the air is escaping...
From the carb, intake valve leaking...
From the muffler... exhaust valve leaking...
From the crankcase breather... piston/rings worn

These are pretty easy engines to disassemble the top ends due to the pushrod design. Just lineup some gaskets for the top end and then take it apart to see what is worn...

Bill Silver