Checking compression

[NCPT]

Does it matter if the control switch is on or off when checking compression? Seems like I have read to make sure it is in the off position for some reason. What about holding the throttle open vs leaving it closed? Thanks

 

[Clemsonfor]

Switch is irrelevant. Some say to hold wide open as you do it. I don't think it matters as much but it makes sure it can suck all the air it can without a restriction.

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[deye223]

If you want a true reading do it when the engine is hot not stone cold

 

[redline4]

Also, do you guys give a certain number of pulls, or pull until the guage reads the highest?
On cars, 5 or 6 compression strokes is general rule of thumb.

 

[Clemsonfor]

Just keep going till it's the highest. But for me that's always a few pulls at most.

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[redline4]

Just keep going till it's the highest. But for me that's always a few pulls at most.

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Thats what I have been doing, just wasnt sure if it was "right".

 

[deye223]

And I know this is a stupid thing to say but don't push the decomp in ......... I seen a bloke do exactly that it was just out of habit haha

 

[Clemsonfor]

And I know this is a stupid thing to say but don't push the decomp in ......... I seen a bloke do exactly that it was just out of habit haha

Haha!! "I don't know why this saw has such bad compression...it runs perfect"

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[huskihl]

Throttle position doesn't matter. It'll suck it through the exhaust if it can't get it through the intake

 

[Al Smith]

You all have to remember what's been talked about is static compression reading ,dynamic or under running conditions could be higher .For several reasons .First two cycles use trapped compression because the piston is closing an open port .Using the port time area open variables it would make sense that at running speed would be of shorter duration .So what might appear to be a low reading might not be entirely accurate as far as running under power conditions .It could run fine even showing low on static .Usually though if the static is real low like 80 psi it indicates something is wrong . 120 from a normal 140 probably not much if any .

 

[Keith Littlepage]

I'm working on a Poulan Wildthing 2375 chainsaw and the compression reading I'm getting is 70. Would you say it's for sure time to do a top end job? Thanks if someone replies

 

[Chainsaw Jim]

If the switch controls the choke then yes it does matter. Choking it while doing a comp reading will flood the hell out of it and throw the reading off. It can also fill your compression tester up with mix.

 

[jk14]

This may not necessarily pertain to the compression test itself, but I've heard it's not good for the coil to be triggered without a path for the energy to go to ground. Either through through the plug, the kill wire or grounding the plug wire. But I'm no expert so....

 

[066 redeye]

I always ground the plug wire when doing tests.
To prevent damage to coil electronics.

 

[Larry B]

I always either ground the plug wire or put lever on stop. The old LawnBoy coils were bad for crapping out if the plug wire was not connected. Energy wants to go somewhere.

 

[Al Smith]

Briggs and Stratton opposed twins had a tendency to burn up the advance winding running an open wire ,like to test the spark .Damned things would fail on full advance and smoke the electric starters . I think they called them a "magnatron".Self advance but need to be cranked over at least 600 RPM before they fired and fired both plugs at the same time like a Harley .

 

[Duane(Pa)]

Good thread!

 

[Stump Shot]

Turning it off is sufficient for to test compression for the coils sake. I also like to put a zip tie on the throttle trigger if for no other reason to keep it from pulling fuel. I like to see a good bump the first pull before seeing where it quits getting higher at. I've also an adapter for small plugs with a valve built in so as to not get a false low reading. Normally checking to see if there is enough compression to run as a diagnostic, if it has about 130 pounds or so it still has some service life left, if down to 100 pounds it will most likely run, but it's on its last leg, if under 100 it's most likely not enough to run or run well enough to get some work out of, time to repair.