Super Jet im running low number compression some one help

[fast superjet chase]

Im running a 760cc motor in my superjet just rebuilt it and put 33cc domes in my ada head im only getting 150psi out of it any one have any clues or direct me in the right way. Thanks

 

[swapmeet]

Headgasket or O-rings? Is it a used head?

 

[Waternut]

Did you rebuild the engine with domed or flat top pistons?

 

[Matt_E]

What's your piston to walk clearance?

 

[fast superjet chase]

my piston clearance is .070, i think they are dome pistons they are just wsm .75mm over bore. and it is new domes for my ada head, it is o ring.

 

[Matt_E]

That's pretty big clearance for cast pistons. Probably not big enough to cause that low compression, though.
Is the starter cranking the motor over nice & fast when you do the compression check?
Is the cylinder ported?

 

[Freakrider]

what kind of tester and what's the altitude there in CO...

 

[Matt_E]

Hah, good eye. I bet that's the "problem".

 

[fast superjet chase]

ya cranks over fast i have 35cc domes in my other ski and it is at 180psi, one of my friends ran these domes in his ski and his was at 195psi, we are almost at 5000 feet here, the cylinders dont have a port job. i pulled my head and the pistons dont come flush to the top. is there some thing else that i need to do when building a frankenstin motor 62t bottom end and 64x top end. do i need to deck the top of the cylinder .020.

 

[Matt_E]

Considering your elevation, the compression sounds right.

 

[fast superjet chase]

ya but my other ski with bigger domes has more compression with 35cc and this one has 33cc domes, plus my friend was running 195psi with these domes.

 

[Matt_E]

What cylinder does the other ski have?
What about your buddy?

 

[fast superjet chase]

all running the same cylinder.

 

[fast superjet chase]

so im going to deck the cylinders .020 and that should fix the problem.

 

[Freakrider]

if you deck it like that you could run into squish probs and need special cut domes forever...you need to find out why 33 cc domes on a 760 is so low on compression...something is not right ! anybody else have opinions on this ?

 

[fast superjet chase]

well with the squish being at .070. and i talked with ada they said it should be at .050 i think decking it .020 will solve every thing my other ski i checked and it was at .040 i dont like running them that tight so .050 will be perfect i think.

 

[Matt_E]

Are you running a really thick base gasket our maybe two standard ones?

 

[fast superjet chase]

no im just running one standard base gasket. i built tons of motors and they all came out perfect thats why im asking if any one has had this problem before. or has run into this before. its a weird one i know.

 

[Matt_E]

Only thing I can think of is excess piston to wall clearance or bent rods.