compression tester comparison

[GardenKit]

Who needs someone to tell you what compression a saw should make or a manual: -

 

200+ tuned

170+ very good

150+ normal well run in

130 pretty shagged

100 buggered

75 well buggered

 

One of the first checks to do on any machine in for service IMO!

In general, yes Steve, I agree. But not all engine run at the same pressures. I have seen engines run perfectly well as low as 110psi and this could be where they should be, but without the correct figures the tests are just a guide.

 

For instance, according to these figures copied from Eddy's PDF we would be writing the K1260 off straight away.

 

Compression pressure

 

Average value for new engine: K650, 700 = ca 150 psi, K950 = app. 130 psi,

K1250 = app. 110 psi.

[spudulike]

In general, yes Steve, I agree. But not all engine run at the same pressures. I have seen engines run perfectly well as low as 110psi and this could be where they should be, but without the correct figures the tests are just a guide.

 

For instance, according to these figures copied from Eddy's PDF we would be writing the K1260 off straight away.

 

Compression pressure

 

Average value for new engine: K650, 700 = ca 150 psi, K950 = app. 130 psi,

K1250 = app. 110 psi.

 

Yes but these big engines can be pretty hard to pull over so is this figure with using the decomp valve in or out. I know the MS660 for instance often runs in good condition around 145psi, the MS460 can have around 170psi in well used condition - 110psi is pretty low in my books unless a decomp is used or it has an auto decomp valve. I have seen tht the bigger machines will run ok with much lower compression than much smaller engines, then there is heat - it knocks of around 20 - 30psi hence starting a hot machine with low compression is usually a pig.........anyway, you know all this:sneaky2:

 

I have seen an old Poulan having 130psi as normal and ran fine also but my general rule of thumb is the above and generally stands good.

 

On bigger machines, a simple drop test on the recoil can tell you a fair bit on the state of the cylinder and piston!

[GardenKit]

OK, so I started doing what Eddy does best, and trawled through PDF's looking for compression figures.

Not so easy to find for some machines, but found the Tanaka ones. I have serviced and repaired 1000's of Tanaka's for 25 years and never before have I found the figures!

I have always gone by 'general rule' of 150psi being good, and indeed some do pull that, but many have run perfectly as low as 110psi. Using 'general rule' I would have condemned these machine to serious repair cost.

 

But in fact, the accepted figures on the brushcutters are actually 105 to 140psi.

 

So the point is, just be a little aware of not relying on compression figures unless you know what they should be.

 

Found Echo saws at between 135 and 145 on a PDF.

 

Still found none for Stihl, would be interested to know if anyone has any.

[BILLSMOWERS]

i always remove the exhaust even if good compression as i was shown by my ex boss a saw that had 150 psi compression but a badly scored piston the only way it was running was the rings had not stuck yet but it ran like a pig when hot

i try to always pull the exhaust in front of the customer if i can to show them the condition it come in like

[GardenKit]

Me too Bill, same with dirty fuel. Otherwise they don't always believe you when you phone them up with the bad news.

[spudulike]

I think we all know what we do and all three of us are similar in our methods, one neat little trick is to use one of those flexible single LED inspection torches and shine it down the plug hole - it is a very quick and easy way of telling if a saw is good or bad.

 

I have had saws up to 180psi still run like a bag of XXXX, there is the seal from the ring and a secondary seal from the piston body and skirt. On this 180psi saw, the skirt was extremely worn but the ring was fine. It had lots of compression, would start but no torque whatever - just goes to show what a worn piston will do.

 

Yes, you can get good compression figures on worn/seized pistons but it is the inspections that we all do and our experience that separates us from the much less experienced. If you have measured 100+ machines, you kind of build up an expectation of what you should get before even putting in the gauge and pulling the engine over!

 

The PDF in question does mention warming up the machine before measuring. I tend to always measure stone cold and only measure hot if the machine is a bitch when warm.

 

Probably no one method is right and it is the fixed and reliable machine at the end of a repair that is the proof of a job well done:thumbup: