compression testing 100/110 psi minarelli engine e By masterburt (Read 3,070 times)

[masterburt]

Once again i'm relying on you lot to help me out,Recently brought a scooter that needed alittle tlc,decided to rebuild the top end and install a 70cc naraku cylinder,piston,cylinder head.ect, i took of my exhaust and someone nicked it so thats on my agenda to buy another one,tried to start it and when it does kick it its sounds jumpy then it's a sudden stop can't rev as i have no control over it,desided to do a compression test and my reading is 100/110 psi max with throttle open, cannot find the correct compression reading online anywhere so i don't know wheather this is correct or not, the engine has 0 miles on it.. i have taken the the cylinder kit off and looked for damage anywhere the only thing  ive seen is a a lttle score mark on the bore does't feel pitted of gruved just a mark,, do you have to compression test with the exhaust on ? any help please

[Bashan]

Regardless of engine specific, that compression is too low. I say 120 is the absolute bottom and I like to see 150+. I'm not sure why your reading is so low but I think until you get that off of the table you won't be able to assign blame to your running problem. Rich

[jct842]

You don't suppose the 70cc kit is the problem do you? If the dimensions are not right as far as piston, cyl height, head it may be that kit on your particular crankcase is not going to work. Unless the rings are not sealing that is all I can see it could be.

Tonight I went to start my 3 or 4 year old weed wacker. It ran good last year and had good compression. Tonight it has almost no compression. I assumed the rings were stuck. I sprayed carb cleaner in the spark plug hole and let it sit. Later I put a squirt of 30 weight oil in on top of the piston and let that sit. When I went to pull it over there was so much compression I could hardly get it to turn over and in 3 pulls it was running. Smoking like hell but running good. I warmed it up and burned off all the smoking oil and shut it down. Pulled it over and it started right up with good compression. Your kit is new but it still might not hurt to put a little 30 weight in on top of the piston and see if the compression changes for the good. It would be good to tip the engine up so the piston top is level at least for a min so the oil can get all around it. Out side of stuck/leaking rings there is not much else that would cause low compression except if the kit was the wrong one.

[Bashan]

John makes a good point about the kit. In answer to the question I missed, the compression test is pre exhaust, having that on or off wouldn't change things.

[masterburt]

Right Guys have a few decent photo's for you to look over see what you think,1# at top dead center is this correct where the flywheel should be just off the raised piece on the flywheel. 2# cylinder wall notice the score type mark on the cylinder wall,feels flush is this right in somewhat as the engine has 0 miles on it,3# the arrow on the piston does that point down towards exhaust or upwards toward's carb 4# on the piston them little nipple looking things is that where the piston rings seat at both ends ect, sorry about the biography just want second opinions,ps dont worry about the 3 bolts that are missing replacing them as we speak

[masterburt]

[masterburt]

[masterburt]

[masterburt]