gp1200 low compression

[kman]

Rebuilt the motor in my sons 1998 gp1200 over the winter,SBT crank and gasket set,prox pistons,riva performance head ,new reed valves.We have run it about 5 hours .checked compression only 118psi on all cylinders.I was expecting about 135 psi with the riva head.compression was checked with snap on guage ,engine warm throttle wide open,strong battery.I have pulled cylinder off and rechecked piston clearance,Within spec.Every thing looks fine.Any ideas .Do you know what the piston installed hight should be?The only mod to this motor is the riva head,and the chambers in that head are obviously smaller than stock.And yes the snap on guage is right on double checked it against a Matco.

[Mpeavler1]

Kman,


You always check compression on a ICE COLD engine!

Mike

[Technical Support]

Our kit gaskets are standard size, as our rods are. There shouldn't be any variation there. Even if you decked the cylinders, the compression would have gone up, shouldn't be an issue there.

This is either going ot be a guage problem, a head problem, piston to cylinder clearance and/or ring-gap problem.

You didn't port the exhaust ports any did you?

What exact clearance and ring gap are you setup to? What cc are the heads? Did you verify the gauge on another motor?

[ May 06, 2004, 07:33 AM: Message edited by: Technical Support ]

[1fastski.com]

forget about the numbers for a min and tell how the ski runs :) is it fast out of the hole ? good topend ? or is it boggy ? compression numbers do not mean a lot as long as your even , ive built 951s with 125psi that fly with great responce . also your PSI should come up a little after the rings seat .

[anapg77]

Exactly!!!
I wouldn'y worry about the compression a whole lot, especially if the cyls. have been ported. (As long as it is running good)

If you think something is amiss buy, rent or borrow a leak down tester. Do a serch on the web for the usage.

[torquenut]

Dude on the triple good compresion will be in the 120. you are well withing range. If it was a twin than you should be concern but on the triple you seem fine.

[Technical Support]

He says he has a high compression head on it, which should give around 145.

[torquenut]

Man sorry, thought it was a standard OEM motor, it was early in the AM when I posted this reply need sleep! [img]graemlins/buck.gif[/img]

[kman]

Thanks guys ..every thing checks out ,im going to put it back togeather and run it some more,Im thinking maybe the rings just havent seated.Ring gap in spec,piston clearance in spec,Ill let you know in a couple of weeks what the compression came up too,ihope.

[1fastski.com]

iam running a high compression head on my 98xpl 951 and my psi is 150 { stock } it all depends how the motor is setup . PS i still outrun the SC :)