Hi there Moped Army,
I might get my hands on a 65cc engine. It has not run in years, but before I go take it home, I was wondering what the correct compression should be? Any ideas on how to figure that out? If it helps at all, its not for a moped, but close enough to one, it’s a 75’ Benelli Motorcycle. If anyone would know, I know it’s the MopedArmy forums.
Thanks,
Johnny
Like almost any other small motor , it'll need about 100 pounds to run . Any above that is a plus .
alotta car compression testers don't really work on tiny engines, use a little dirtbike one. kick it over, see what it read. you can turn it over with a drill too. 100's a good number to shoot for. some less is OK and could be recovered with a bore or hone, much less is probably broken or gouged things.
without getting technical you can also feel for it. if you stick your finger over the plug hole and turn it over, if it's got decent compression you'll feel it puff/speak your finger off the hole, but if it's f'd there will be little or no pressure behind it
alotta car compression testers don't really work on tiny engines, use a little dirtbike one. kick it over, see what it read. you can turn it over with a drill too. 100's a good number to shoot for. some less is OK and could be recovered with a bore or hone, much less is probably broken or gouged things.
without getting technical you can also feel for it. if you stick your finger over the plug hole and turn it over, if it's got decent compression you'll feel it puff/squeak your finger off the hole, but if it's f'd there will be little or no pressure behind it
150 is ideal, most will put that out if the engine is pretty tight. 100 will run ok but be a bitch to start in colder weather. Low compression on an old barn find bike could mean that the rings are stuck in the grooves. Best to clean it up first before starting so the cylinder doesnt get scored. If the top end isn't broken in yet, could be below 90 until it's run a few heat cycles.
If you have time for a compression test then you would have time for a leakdown. A leakdown for vacuum will tell you a lot more than a compression test. If you are using a vacuum source stronger than lung power (vacuum syringe) you can suck the piston up with the vacuum syringe if the wall is wetted and the rings are above the roof of the exhause port. If the rings leak you will not be capable to draw the piston up.
You can also check for the carter gaskets and seals condition with a test at BDC with intake and exhause blocked off.
Otherwise the thumb on the sparkplug hole test will suffice for the top end compression check. Even rotating over the engine by hand you would feel resistance of compression if the rings were good.
Old engine that has been sitting usually needs gasket-seals-rings within a year of purchase, 40+ year sold is 40+ years old, just look at how cracked and dead half a century old tires look - the seals are likely going to be in the same dried and cracked condition.
Want to post in this forum? We'd love to have you join the discussion, but first: