stock 49cc 2 stroke compression

[colemanbales]

Hey guys, i did a compression check on my engine today just because, and was a little concerned when i found that the engine was putting out 80psi. Now this sounded ok to me at first, but after seeing a video on youtube saying that these engines come stock with around 100, to 120 psi, i thought it would be good to get some feedback from you all. Now as far as the engine goes, the bottom end (including piston and piston rings) is brand new (only 1 tank of break in ratio fuel through it so far), but the head and jug (cooling fins, idk what their called really), are from my previous engine that i accidentally ruined the bottom end of. the only modification aside from that is i sanded the head with 400 grit sandpaper to make a higher compression head (i did this before even putting the engine together). does this seem right to you? will the compression go up once the break in is complete? thanks

 

[Theon]

Can you do a squish test?
tell us which crank your running,
Straight fire head?
did you give the barrel a light hone? was it glazed?
Compression may go up a little with breakin, but I'd be looking for more than 80 Psi on a rebuilt motor, check your squish!, if you replaced the crank? is it the same stroke?

 

[mrfubs]

Compression will go down slightly once broken in, ido not recommemd deglazing these cylinders sense they are only flash chromed and are very very thin. If it runs fine then dont worry, also you could have a in accurate guage.

 

[colemanbales]

Well it's a grubee skyhawk slant head 49cc engine if that helps. Is it ok that I put together a brand new piston and piston rings with a used head and piston jug?

 

[Theon]

What condition was the bore in?
slant head should be giving more compression than 80.
Gauge could be wrong.

 

[SmokerX]

used head and piston jug?

Sounds like the bore of the jug is possible worn causing "blow by" on the piston rings.

Spray carb cleaner or starter fluid around the intake manifold to jug connection. if it idles up here, tighten those bolts cuz there's an air leak.

pull the head and watch the piston cycle up and down. See where the top of the piston stops rising and starts pulling back down. Use the spot on the bore wall as an indicator line. The wall should visually be the same thickness above and below this line. If it slopes in below, its worn. Move the piston head to the bottom of its stroke. Are there grooves in the bore wall, cuz there shouldn't be.

Take the jug off and examine the piston below the rings. If there's exhaust deposit here it's blow-by.

If it's blow by you need to replace the jug/bore the engine bigger and match with appropriate ring size or stick a feeler gauge between the ring and bore wall. What ever the fattest gauge you can fit in the gap, divide by 2 and find compression rings that much bigger outer diameter. Example: 40mm OD ring+1mm feeler gauge= 40.5mm OD rings. Lube the bore wall when you reassemble.

 

[crassius]

did you remember to hold the throttle open when testing?

does it run well?

 

[2door]

I never recommend mixing new pistons/rings with a used cylinder. Once the cylinder is broken in with a set of rings then installing it on an engine with new rings it's almost certainly not going to seal as well as with the original rings. They wear in together. I'm thinking you're seeing some compression loss due to the mis-match of those components.

My recomendation is to not worry about it if the engine is running good. Unless you're looking for all-out top performance it should give you miles of use as is.

Also I agree with checking the accuracy of the gauge you're using. Is it a known accurate gauge?

Tom