checking compression

[albawil]

Is it best to check with motor running or by just turning it over with starter & how much difference will there be between the two methods
Thanks

 

[Stephen Brussell]

You certainly don't check compression with the engine running. I pull all the plugs and crank the engine about 5 compression strokes on the gauge. Do that for all cylinders and record the readings. Then do it again after a few squirts of oil in the cylinders and record those readings. The wet and dry readings will give you an indication of the condition of the engine.

 

[CoopertownBob]

(quoted from post at 21:19:59 01/17/12) Is it best to check with motor running or by just turning it over with starter & how much difference will there be between the two methods
Thanks

I have access to the equipment to do it both ways, and I usually get about 10% higher readings with the engine running, but I prefer to do it with the engine off and cranking the engine over. The important thing is to be consistent from cylinder to cylinder no matter how it is done. It does no good to bump the engine for three revs on number one if you them turn it over ten times on number two because the readings are not comparable. That being said, what I usually do with a hand held is turn the engine until the reading stops climbing and levels off.

 

[Bern]

Back in the day, Ford recommended a running compression check, but later backed away from that, I suspect because someone got hurt doing it.

The latest recommendation that I saw was 6-8 puffs. The important thing is that you're consistent, and that the readings are all similar.

 

[albawil]

Thanks very much for the replys. I have a 800 that is running rough & I have switched carb, coil, points & cond. off good running tractor & it makes no differance Thanks

 

[DollarBill]

I have switched carb, coil, points & cond. off good running tractor & it makes no differance Thanks

Would you like some help in isolating the cause for rough run issues?

 

[souNdguy]

ditto what db SAID.

GIVE US SOME IDEAS OF WHAT IT DOES OR DOES NOT DO.

AHRD START?

MISSING?

STUMMBLES WHEN TAKING THROTTLE?

POPS THRU EXHAUST LIKE POPCORN?

sorry for caps.. hard to use this lil mobile device.. to type

soundguy

 

[daman]

(quoted from post at 02:19:59 01/18/12) Is it best to check with motor running or by just turning it over with starter & how much difference will there be between the two methods
Thanks

Usually a comp test is done just with cranking speed,also it's done at operating temp.

 

[KV Bruce]

After the engine is brought to operating temperature, without haste, remove all spark plugs. Crank engine with a strong battery. Throttle should be wide open and each cylinder should experience the same amount of cranking time. As others have expressed, the results you are looking for should be close to one and another. If you have a low result, compaired to the others, inject oil into cylinder and check again. If results improve, rings may be your culprit, if no improvement, valves should be suspected. I like to remove the positive lead to the coil to avoid excitement.