compression test help!!!

[guen84]

I recently installed a 50mm bbk, just got the motor back together today. so i did a compression test. i had the head torqued down to 9ft/lbs, i used a cordless drill to crank the motor. my reading was 122psi, so i wanted to just snug the head bolts (for good luck) to atleast 10ft/lbs. so i did, re-ran compression test, it was 80psi. So i crank head bolts to 12ft/lbs, i still hit 80psi on test. The cordless drill battery seemed like it was dying so i got out a corded drill, compression hit 180psi. So in doubt of all readings i hooked starter up and got 122psi. The manual says to crank the starter 3x's for compression test so thats what i did and got 122psi, so im in range of manual, but why did i get the 180psi reading??? does the rpm's make a difference in compression? theres no way that drill ever hit 8000 rpms, which is what this bbk is capable of hitting... so can someone please explain to me whats going on!!!! what compression should i be looking for with the 50mm bbk..

[Bear45-70]

How do you crank a bolt or nut to any number PSI??? Maybe foot/pounds or Newton Meters or Kilograms/Meters But never PSI. PSI is what compression is read in. I would suspect that head and or the jug are not flat.

[guen84]

How do you crank a bolt or nut to any number PSI??? Maybe foot/pounds or Newton Meters or Kilograms/Meters But never PSI. PSI is what compression is read in. I would suspect that head and or the jug are not flat.

there you go bear all fixed up for ya, i was typing that in a hurry with a screaming 6 month old in one hand and trying to type with the other... Now when you say they may not be flat, is that the installers fault? or manufacturers? How can i check if its flat or should i just try to remove it all and reinstall??

[bakaracer]

Did you put some 2 stroke oil on the piston,rings,and cylinder wall before you installed it?

[guen84]

Did you put some 2 stroke oil on the piston,rings,and cylinder wall before you installed it?

yes i just dipped my finger in the bottle then smeared it all around.. What will that matter for compression?

[bakaracer]

For one metal scraping on metal,not good. 2 the oil helps make that extra seal and lube the piston,rings to wall contact .do you have the manifold and reed cage bolted up?

[guen84]

i know bout metal on metal just didnt know oil would help it seal. yes i do have them bolted on now, but i need a new manifold gasket. will this make a difference? isnt the compression just between the piston, rings, head, and head gasket not anything below

[bakaracer]

You can loose some of that compression if not all through the reeds.as the piston goes up,the pressure shuts the reed close.if its open you can loose that air pressure.that's why the reeds are there for.if its in there and the leak is not that bad I suspect the head gasket went bad or the head is warped or your seal onyour tester is bad.

[guen84]

You can loose some of that compression if not all through the reeds.as the piston goes up,the pressure shuts the reed close.if its open you can loose that air pressure.that's why the reeds are there for.if its in there and the leak is not that bad I suspect the head gasket went bad or the head is warped or your seal onyour tester is bad.

Awesome thanks for the help.. the head gasket was bent a little when i got it, after i squashed it it looked like it flattened out. maybe be not though.. is there any way to see if the head is warped? also i removed the head to see what was going on and noticed the piston does not clear any of the ports. to clear the ports at bdc i would have to raise the jug 2mm then that would leave me like 3mm from the squash band at tdc. as it sits right now piston is 1 mm from cylinder head at tdc Raising the jug 2mm would make the piston only clear the intake ports, not the exhaust or boost port. What do you think about this??? i was reading a thread earlier about this situation, and the way he took care of the problem was with a mini stoker crank and thats definitely not in the budget.. THANKS!!

[bc117]

can you take a pic thats alittle hard to believe, you might be mistaken by what your looking at id say just get it together and see if it runs or an easy way to tell if the head is warped/deformed is to put some fine wet sand paper on a glass table, mirror, or other large perfectly flat surface and gently rub your head on the paper and see if it is evenly sanding the entire head, if it sanded it evenly then try the top of the cylinder, also did you actually squish the head gasket that could be a problem but your probly fine if you just bent it back into place by hand, lastly take your base gasket off it and see if it is clearing the ports it could be too thick try a thinner one if you can find one, heck maybe just put some liquid gasket on the case and run with out a base gasket thats what i did on my cr80 dirtbike it gave it more power by raising the compression

[Bear45-70]

You can loose some of that compression if not all through the reeds.as the piston goes up,the pressure shuts the reed close.if its open you can loose that air pressure.that's why the reeds are there for.if its in there and the leak is not that bad I suspect the head gasket went bad or the head is warped or your seal onyour tester is bad.

Awesome thanks for the help.. the head gasket was bent a little when i got it, after i squashed it it looked like it flattened out. maybe be not though.. is there any way to see if the head is warped? also i removed the head to see what was going on and noticed the piston does not clear any of the ports. to clear the ports at bdc i would have to raise the jug 2mm then that would leave me like 3mm from the squash band at tdc. as it sits right now piston is 1 mm from cylinder head at tdc Raising the jug 2mm would make the piston only clear the intake ports, not the exhaust or boost port. What do you think about this??? i was reading a thread earlier about this situation, and the way he took care of the problem was with a mini stoker crank and thats definitely not in the budget.. THANKS!!

Depending on the port timing baka is telling the truth. 2 strokes are supercharged by design. The fuel/air mixture is first compressed in the crankcase as the piston drops in the bore and this is why the reeds must be installed to do a compression test. When the intake ports open the fuel/air mixture is under pressure as it enters the cylinder. So the compression numbers will be higher than the bore, stroke and combustion chamber volume will indicate.

[guen84]

can you take a pic thats alittle hard to believe, you might be mistaken by what your looking at id say just get it together and see if it runs or an easy way to tell if the head is warped/deformed is to put some fine wet sand paper on a glass table, mirror, or other large perfectly flat surface and gently rub your head on the paper and see if it is evenly sanding the entire head, if it sanded it evenly then try the top of the cylinder, also did you actually squish the head gasket that could be a problem but your probly fine if you just bent it back into place by hand, lastly take your base gasket off it and see if it is clearing the ports it could be too thick try a thinner one if you can find one, heck maybe just put some liquid gasket on the case and run with out a base gasket thats what i did on my cr80 dirtbike it gave it more power by raising the compression

no time to take pictures this morning, maybe this afternoon. if i take the jug any lower to the block, the ports would open even less.

[bakaracer]

What else parts are you running with that 50mm kit?

[guen84]

What else parts are you running with that 50mm kit?

28mm v8 pipe, oko vari, 8.4-1 gears, no restriction air filter, 1.5k center spring, 1k clutch springs, and hopefully a 28mm oko carb intake combo from ebay

[bakaracer]

Which v8 pipe? Can style or chamber?