Topic: compression test question

[fergie]

I recently picked up a 350f.I checked the compression on each cylinder and they all read just below 125 psi. however it took me about 20 or so kicks (on each cylinder)to get it up there.I cant check it with the starter turning the engine over because it does not work.My question is this: is my 125psi reading acurate or am I just falsly getting my hopes up.thanks, Fergie.

[Bob Wessner]

Will it start with the kick starter? If so, you should run and warm the engine first.

[Einyodeler]

It also depends on the type of compression gauge tool that you use.I bought a gauge that had a hose about 18" long between the gauge and nipple and got very low readings,then tried a friends` tester that had the gauge attached to the nipple without the long hose and lo and behold my readings jumped by 40 to 50 lbs.The closer the gauge to the nipple the more accurate the reading. 

[Dennis]

Fergie,
Did you have the throttle wide open when you checked?

Yodeler.
Sort of, it depends on the gauge and where they put a "check valve" in the assembly.
There should be a valve right at the threaded fitting. If it is at the gauge with a long hose what you noticed will occur.

[fergie]

my gauge has the 12" hose .fergie

[Dennis]

That only matters if the check valve is up by the gauge. Can you see a valve (like you would see for filling a tire) at the fitting where you screw it into the spark plug opening. If you answer yes, then the long hose has minimal effect.
Someone posted a link to a long dissertation (including references to gas laws and other stuff that I vaguely remember from freshman physics) on this subject on the old site but I have no idea how to find it.

[fergie]

there are two release valves one at the fitting and one at the gauge.fergie.

[pmpski_1]

So what about the reading if you kick it 20 times? If the engine doesn't run and you're trying to see if you have bad compression, should you keep kicking it until it gets to the point where it doesn't go any higher? Or should you just kick it a couple of times?

Thanks,
Brian

[heffay]

Fergie~
i've always been told (by my instructors and by my compression test)  your highest reading is your best... if you consistently get a high reading (no matter whether it takes 20 kickes each time) or not!  you should trust your equipment... YOU HAVE TO GET A CONTROL!!!!!... meaning, get your best test... and work from there. 

[heffay]

p.s.  dont give a crap about the long extensions etc.... if you have quality tools you will get the same readings.

[x2qwk4ux]

I agree with the warmup procedure...then once it's warm, remove all plugs before doing the test so the engine spins more freely.  A quick way to see if your rings are shot, is add a teaspoon of motor oil into the cylinder with a turkey-baster & repeat the compression test.  If it goes up a LOT, you might want to consider re-ringing & honing.  125 sounds like it's in the mid-to-low range of acceptable to me...and if it don't smoke, just RIDE it.  I'm thinking you should would have 140-175 if everything was new.  Might also want to try adjusting those valves before the test.

Josh   

[fergie]

Thanks for the advice guys.Fergie