What causes low compression

[gustafsonerik]

87 force 85hp i check my comp last month and it was 120 on all 3 cylinders now i checked it and its barely 80 all on three..i was having some starting problems but i can't imagine it running that lean?? my question is what should i look for seals? gaskets? reed valves?? also could i just get away with changing the piston rings??? thanks Erik

 

[Haywood123]

Re: What causes low compression

bad rings, cracked head, blown head gasket, burned out valves, scored cylinders.... Anything that will allow bypass will lower compression, however if all 3 are dropping at the same time, would tend to lean towards head gasket.

 

[Barnacle_Bill]

Re: What causes low compression

Recheck with a different compression gauge.

 

[Bigprairie1]

Re: What causes low compression

Recheck with a different compression gauge.

Full agreement with this. If you are using a thread-in gauge and the seal is not seating/sealing properly you will be getting a lower reading across the board. It is always a good idea to 'get a second opinion' by using another gauge...perhaps a push-in/pressure seal type.
Borrow a gauge from a friend (or rent one) and ideally have them help you do another compression test.
BP

 

[JB]

Re: What causes low compression

Don't panic. Very subtle differences in engine state (cold/warm/plugs in or out/different guages/ change in technique/ state of charge in your battery) can make as much as 50psi difference in psi reading on a healthy engine. The important thing is how they compare to each other and yours compare favorably.

I doubt that there is anything seriously wrong. Warm the engine up, pull all plugs and take new readings with a fully charged battery and a new, screw-in guage for at least 5 revolutions.

Let us know what you find.

 

[bman1bpm]

Re: What causes low compression

if all 3 are within 5-10% of each other usually you're fine, if they all dropped from 120-80 at the same time my guess would be that it was warm the first time you tested, now its cold, that would cause a drop.

 

[Frank Acampora]

Re: What causes low compression

It could also be sticking rings. After you have eliminated the probability of other problems, try the Sea foam treatment.