Compression test

S

Snowcon1

Location
Lake Orion Michigan
So I have been using my friends and other people’s compression testers to test compression on various jet skis over the years but I always end up getting wrong or messed up compression numbers so can one of you guys please explain to me how to do a compression test I now own my own and I would like to do it correctly for once
 
iQRTurbo

iQRTurbo

Main thing is be consistent in what you do. Obviously every engine and every gauge is different. I test all my engines the same way regardless of dirt, snow or water and record all my engines numbers to know if something is going away on me. Spark plugs removed from cylinder head but plugged back into ignition wires and grounded on head. Cold engine for consistency, typically numbers drop as engine temps go up and can give varying readings. Crank, kick or rope it over until number stops climbing and you got what you see.
Fresh rebuilds that have not been run always show high right off the bat as assembly oil seals rings like crazy. Typically after it has been run numbers will drop but come back up as rings seat.
 
Harbor

Harbor

Location
Stockton
I'll add, use the correct length threaded hose or adapter on your hose. Long reach for most newer aftermarket heads, short for a stock head. It's roughly a 7 psi loss with the short reach hose/adapter in a long reach head. The length of threads on the hose/adapter should match the depth of threads in the head.
 
iQRTurbo

iQRTurbo

bird said:
No difference on my 50cc moped, kick start.
No difference on my SXR either.
Click to expand...

Apparently this WOT thing needs some clarification for the guys who feel the need to post just for the sake of arguing.
Yes, you will eventually arrive at the same max compression number with open or closed throttle plates. But on an a real high performance race or freestyle engine like the stuff I work on with 250+ psi I prefer to open the plates to arrive at the max number in as few kicks or recoil pulls as possible because it’s not fun crushing the instep of my foot on 15 kicks when it can be done in 7 or 8...
I’m sure you probably wouldn’t notice the difference in a scooter engine with a piston the size of a thimble on an engine with a compression ratio designed to basically run on junk fuel while commuting the public around China and India though...
 
bird

bird

walking on water
Site Supporter
Location
La Crosse, WI on the BIG River
iQRTurbo said:
Apparently this WOT thing needs some clarification for the guys who feel the need to post just for the sake of arguing.
Yes, you will eventually arrive at the same max compression number with open or closed throttle plates. But on an a real high performance race or freestyle engine like the stuff I work on with 250+ psi I prefer to open the plates to arrive at the max number in as few kicks or recoil pulls as possible because it’s not fun crushing the instep of my foot on 15 kicks when it can be done in 7 or 8...
I’m sure you probably wouldn’t notice the difference in a scooter engine with a piston the size of a thimble on an engine with a compression ratio designed to basically run on junk fuel while commuting the public around China and India though...
Click to expand...
Wasn't trying to argue, just stating my findings. I still hold it open, sometimes I just forget to do it...
 
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