55psi with new piston???

[bjorn773]

I just installed a new piston into my 350 Husky. Before assembling it completely, I put the compression gauge in and pulled her over. I'm only registering 55-60 psi on the gauge. The comp release seems to be in the out position. I don't have a carb or exhaust on it, will that make any difference?

 

[PB]

What condition was the cylinder in before the new piston?

 

[bjorn773]

Cylinder had quite a bit of aluminum transfer. I cleaned it out with sodium bisulfite, which seemed to eat the aluminum. Then I honed it, quite a bit actually, perhaps too much? There still appears to be a good coating on the cylinder walls, NiCr I assume. I even added a little tranny fluid to wet test, no change.

 

[blsnelling]

Did you pull the rope repeatedly until it wouldn't go any higher?

 

[bjorn773]

Yes. Should I start looking for a cylinder?

 

[blsnelling]

Yes. Should I start looking for a cylinder?

Schrader valve at the tip of the hose where it screws into the jug? Guage reads properly on other saws? If so, then something's definately amiss with that saw.

 

[husky362]

it should build some compression without carb or muffler, ring broken, jug not torqued ,gasket not sealing or misplaced, could be gasket material in decomp valve from cleaning,,, where do you hear air leaking

 

[DougMN]

Is your compresson tester bad. I have had two go bad on me.

 

[PB]

Is your gauge any good? Can you measure a known, good running saw to compare? Did you put gaskets on?

 

[striperswaper]

you said you install a new piston, did you forget the rings?
seriously, what was the end gap of the rings?

 

[bjorn773]

I'm an auto tech by trade, my compression gauge has the schrader vavle at the end of about 18" of hose. I don't hear anything hissing, I can remove the decomp vavle and check for carbon or debris. I torqued the cylinder to 100 in pounds, didn't have a spec. They're pretty small screws, didn't want to strip anything. I used the old base gasket with very thin coat of exhaust sealer on both sides.

 

[blsnelling]

It wouldn't matter if your base gasket were leaking, as far as compression goes. I just can't imagine it only making that little compression. Many saws with major scoring and stuck rings will make that much.

 

[PB]

Yes. Should I start looking for a cylinder?

Yes.

 

[blsnelling]

Are you sure you got the right size piston? There should only be about .003-.005" clearance.

 

[bjorn773]

I don't have another saw here at work to compare. I've never checked compression on a saw with this gauge, has always worked well on cars. I have a fluke digital pressure transducer, but it plugs into the same comp. tester hose. It probably wouldn't tell me much.

 

[PB]

I don't have another saw here at work to compare. I've never checked compression on a saw with this gauge, has always worked well on cars. I have a fluke digital pressure transducer, but it plugs into the same comp. tester hose. It probably wouldn't tell me much.

Do the dummy test. Hold the pull rope and allow the saw to drop. See how low it gets and how fast. If it drops to the end, you have very little compression. If it drops one or two revolutions, it will most likely run.

Put it back together and fuel it up. If the gauge says 55 but the saw runs like a champ, who cares what the reader says.

 

[blsnelling]

Do the dummy test. Hold the pull rope and allow the saw to drop. See how low it gets and how fast. If it drops to the end, you have very little compression. If it drops one or two revolutions, it will most likely run.

Put it back together and fuel it up. If the gauge says 55 but the saw runs like a champ, who cares what the reader says.

If that's the case, then the guage would be wrong. A saw won't run with 55 PSI compression.

 

[bjorn773]

I pulled it back down. The ring gap is about .018 and the piston looks to be the right size. I can't get a .015 feeler between the piston and cylinder wall. Granted, this is at the base of the cylinder. The ring is in good shape, not cracked. I haven't put a bore gauge in to check if the top of the cylinder is enlarged do to the previous piston damage.

 

[blsnelling]

.018 is more than I like to see, but defintely not enough to cause that poor of compression. Forget the bore guage. Just push the ring on down into the upper part of the jug above the exhaust port and see what the gap is. I like to use the piston to push the ring in to keep it square to the bore. Take care not to snag it on a port.

 

[bjorn773]

I'm starting to doubt my gauge. I have a press in style at home. My work gauge is old and has a soft hose. It has always worked on cars, but you're displacing a lot more air in a car cylinder.