Checking compression on a non running 750

Thank you all again for your valuable insights. I have since realized my error in attempting a compression test to a non-running motor. Piranha I totally agree w/ your advice. A friend will stop by to do a leak down test that should indicate the motor's state. I have the Green Book and will check the cam timing if I feel I can do it. So far, the valve clearances are in spec which is reassuring. I just spoke to the mechanic who did the engine rebuild and he's assured me that it was a thorough job and said that the previous owners didn't ride it much afterwards- hence the 15K miles. I'm enjoying the learning process so far- Laverdas are beautiful machines!
 
We should add polls to these threads: I’m going with stuck rings given that it apparently actually ran after the rebuild which argues against incorrect cam timing. Time to break out the Marvel Mystery Oil!
 
Good news that you are able to contact the mechanic in person. Valve clearances will be spot on even if cam timing is out. Go with the compression test and taker the next step then. An SF will run no problem (for years) with the cam timing a tooth out, probably depending on whether it's advanced or retarded. We're all keen to se what the issue is!
 
piranha-bro2 said:
WARNING SIGNS RIGHT HERE!! Understanding that you bought this bike as is and have no prior confirmation of exactly what was done, i would be removing the motor, pulling the top cover and checking that the cam timing is correct - piss easy job that could save you major time and grief. The number of bikes with 'won't idle' issues that have been caused by one tooth out on the cam timing outnumber the ones that have had a simple idle mixture circuit problem. Far more logical than BOTH carbs having exactly the same idle mixture circuit issues.

While the top cover is off, I'd pull the head and barrels and give the whole thing a decent going over. Don't trust the 'was recently rebuilt' line unless you personally know the mechanic who did it. All it will cost you is a few gaskets.
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You've mentioned this before with my SF2 non-idling problem but wow, is it completely necessary to remove the engine in order to check the cam timing? That is a big job!
 
Motors I have found with a tooth out it will not create the "pressure then vacuum" as the piston passes TDC compression. With the spark plugs out, cover the spark plug hole completely with a finger. When you turn the engine forward from before TDC it should create notable pressure against your finger, and after it passes TDC it should create a sort of vacuum - enough that if you whip your finger off it sort of pops. A motor with no compression due to other causes will also lack this, but either way it is a sign that you need to attend to things inside the motor.
 
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