Thread: Compression check Maybe ut oh??
09-06-2008, 12:38 PM #1
Compression check Maybe ut oh??
I just did comp. ck. on my 94 750 slt Came up 80 lbs on each cylinder. Is this low?? Seemed like it based on what I've been reading around here. Thanks folks. You all are a big help.
09-06-2008, 12:43 PM #2
80 is low but they are all with-in 10% so that is good. Try a different gauge some gauges read lower than others.
09-06-2008, 12:46 PM #3
- Join Date
- Jul 2007
- near Toronto, Canada
You sure your gauge is accurate?
You tested with a strong battery, and held the throttle wide open while cranking until the reading stabilized?
Good compression numbers, using a known accurate gauge and strong battery, would be 120PSI or higher, and should be consistent across all cylinders, within 5-10 percent.
09-06-2008, 12:47 PM #4
Brand new gauge. Carbs are off. Putting back on tomorrow.
09-06-2008, 12:48 PM #5
all three read btwn 75-80 lbs. Maybe gauge is funky.
09-06-2008, 02:04 PM #6
- Join Date
- Sep 2006
Does your guage screw into the head the same distance as the spark plugs?? ( some guages have short reach threads on them)
If not then thats why your readings are low !!!!!!
09-06-2008, 03:26 PM #7
My experience has been that motors become difficult to start under 100 psi compression, and won't run below 85-90 without help from starting fluid/ether.
Is your ski running? If so, you've probably got a bad gauge or a leak on the hose somewhere.
Missing or cracked O ring on the end that screws into the head? Loose hose where it threads into the gauge?
EDIT: Oh, wait a minute, carbs are off? Are reeds off too? Unsure if that would cause low compression, but I sure think so!
BTW, I have tested compression with hot vs cold motor, open vs closed throttle, etc. I never got more than a few pounds difference by doing any of those... But I never tried it without reeds!
Last edited by delagem; 09-06-2008 at 03:32 PM. Reason: Had another brainstorm. Brilliant!
09-06-2008, 03:48 PM #8
09-06-2008, 04:08 PM #9
While I was building mine I tested w/o reeds installed & found 150 psi on all 3. Measured the same when the reeds, intake and carbs were installed.
09-06-2008, 11:59 PM #10
- Join Date
- Oct 2006
- Cleveland OH
True reading should be taken on a warm engine. Cold engine will yield lower compression due to every man's enemy..... shrinkage!!!!!
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