Thread: Low compression

Originally Posted by catpower

Hi everyone I have an old stihl ms 660 that won't start did a compression test just on 100psi is this to low to start I'm thinking worn rings.
Any help would be appreciated.
Thanks Tim

It depends.
It should still start on 100 psi but it may not run too well.
If the crank case pressure is shot it may not start.
Does it show any signs of firing, e.g. cough air splutter?

Seems to be dead as a dodo will strip it down and have a good look at it

If you've no spark, I wouldn't strip it down just yet, pull the flywheel off and check that inside is clean and everything OK, have had it where a wire has come adrift and earthed out, causing no spark.
Kryn

Check spark, check fuel. If you have spark get some Start Ya Bastard spray or similar and give it a go, that stuff will kick start an Elephant. If it shows sign of starting look at the fuel system. I would have thought 100 PSI was a good number for a chainsaw. Another thing, make sure that the carburettor is tight on the barrel, if not air leaks will stop it starting.

I have never tried a comp test on a 2 stroke, but I think 100PSI would be fairly good. Remember that the piston is not compressing anything in the cylinder until it has closed the ports in the cylinder wall (exhaust and transfer ports from crankcase), about halfway through the stroke.

Also, what type of compression tester were you using for the measurement, something with a hose between the plug adaptor and gauge, or a direct mounted one. The hose style ones are more convenient for hand cranked motors, but the air volume of the hose can approach the air volume of the piston/cylinder head combination at Top Dead Centre, throwing out the indicated reading substantially. Hardly an issue with large displacement car engines etc, but potentially quite an issue with sub 100cc engines.

The issue with 2 strokes is that the fuel/air mixture is drawn from the carb into the crankcase as the piston rises to TDC, then compressed and blown into the cylinder via the transfer ports as the piston descends. If the crankshaft seals or crankcase seals are at all iffy, you have less vacuum to draw a charge into the crankcase, and then less pressure to push it into the cylinder, so charging the cylinder becomes unreliable and the engine either won't run, or runs poorly.

I think however that your issue would be either fuel system or spark, as I doubt you would get 100PSI on a comp tester if the cylinder was not charging well. Maybe install a clean dry plug, and pull the motor over a few times with the choke set, then pull the plug and inspect it. A dry plug suggests a fuel/carby issue, a damp plug suggests that there is plenty of fuel flowing through the carb, leaving a spark issue as the likely problem.

Try hard to get it going before your strip it down. Fuel, spark, compression and timing all needed but compression seldom is suddenly lost so other factors need close checking to help with diagnosis. Plus check that hornets haven't built a nest in the exhaust.

Originally Posted by malb

I have never tried a comp test on a 2 stroke, but I think 100PSI would be fairly good. Remember that the piston is not compressing anything in the cylinder until it has closed the ports in the cylinder wall (exhaust and transfer ports from crankcase), about halfway through the stroke.

Also, what type of compression tester were you using for the measurement, something with a hose between the plug adaptor and gauge, or a direct mounted one. The hose style ones are more convenient for hand cranked motors, but the air volume of the hose can approach the air volume of the piston/cylinder head combination at Top Dead Centre, throwing out the indicated reading substantially. Hardly an issue with large displacement car engines etc, but potentially quite an issue with sub 100cc engines. .

The 066 has a compression ratio of 10:1 so it has a theoretical compression of 145 PSI but that is a good point about if the compression test has been done with or without a connecting hose.
I have a hoseless tester and I get about 135 PSI on my 066.

The smaller hose connect type test units have a small hose volume (i.e. 5cc) so this reduces the theoretical max compression on an 066 to around 137 PSI.
The bigger units with the longer hoses have volumes of as much as 50cc so that would more than halve the theoretical pressure on a 90cc saw.