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Minimum chainsaw compression for starting and running.

Okie

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What would be the approximate minimum compression for a chainsaw to start and run?
I have a itty bitty Homelite super 2 that has about 80-90 lbs compression after about 6 pulls of the rope, spark seems ok and it won't even make a promise pop when cranking by adding mixed gas in the carb throat. I've rebuilt the carb.
It briefly started and ran at wide open throttle once for little while before I carefully installed a carb kit, thinking the carb kit would get it going at low and high speed, but now won't even make a promise pop????
Acts like it got mad at me and sulled up!!!!!!!!!
I changed spark plugs and checked ignition spark several times.

Next question: If a saw requires high compression for starting why does a Homelite EZ with the manual compression release start and run easily at reduced cranking compression???
 
ChoppyChoppy

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I had an ms360 with about 125psi compression. Ran fine, but if the decomp got pushed it wouldn't start. Also was fussy if I left it outside when it was cold.

I have seen saws with only 90psi run, but most wont.
 

AVB

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Okie, I see out on other forums. LOL. Not using that crane to lift that saw are you?

Usually for it takes around 100 psi for most 2 cycle to start. Pull the muffler and take a look see at the piston and cylinder for damage or just a stuck ring. You be surprise how many stuck rings I free over the years on trimmers and chainsaws.

As I said on the other forum my Little Red has 110 psi on only a couple pulls.

Those decomp valves are metered ports that allow compression to bleed off so it makes the engine easier for the operator to get the engine up to starting speed. Think of it like this try blowing a small tube. Blowing thru it slowly is easy but try blowing thru it fast and it a harder as produces back pressure. I had a Husky that I rebuilt that had over 100 psi with the decompression valve completely missing. It started and ran but ran a lot better once I install the decomp valve.
 

Okie

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This question is pretty model specific. I'm thinking 90 psi is pretty low on just about any saw. Have you checked other chainsaws to see how your compression check compares on those? (gauges can have a lot of variance)

Yes about the gauge variance readings. Automotive compression gauges would only go to about 35lbs in 6 pulls with a dry cylinder and to 125-140 with oil in the cylinder of the super 2. I finally bought a Actron cp7828 compression gauge kit after I was given a hint that most automotive gauges will read very low. the shrader valves in the cp7828 are different (weaker springs on the valves I suspect) than standard automotive compression gauges. I pulled the muffler and looked at the piston and cylinder and they appear ok and the piston only has one ring which is not stuck and what little I could see looked nice and shiny with no scratches or gouges.. I'll try adding spoonful of oil to thru the spark plug hole and see how much the comp increases on the new Actron gauge kit. I suspect it will probably increase to the 140 area indicating a worn out saw. I did not try this again because I thought 80-90 should at least produce a pop?
The saw was gave to me because it would not start.

I could not check the good running Homelite EZ compression using the manual release vs no release because I could not get the long screw in spark plug adapters into the space with the handle on the saw was why I asked about such.
I'll retest with little 10W oil in the cylinder to see how much the compression increases so as to get an idea of cylinder ring wear. Just tugging on the starter rope of the super 2 seems to indicate weak compression as compared to others saws that don't have use a compression release.
 
Rx7man

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I have an old homelite XL12, it feels like it's got no compression (I didn't bother checking it), but it starts up with about 3 pulls every time... doesn't make much power in the cut though... it's funny how some saws with low compression seem to do surprisingly well, while others just quit if they have less than 140psi
 
pioneerguy600

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I have an old homelite XL12, it feels like it's got no compression (I didn't bother checking it), but it starts up with about 3 pulls every time... doesn't make much power in the cut though... it's funny how some saws with low compression seem to do surprisingly well, while others just quit if they have less than 140psi


I have seen that more on the older all magnesium chainsaws, for the most part they had longer stroke and heavier flywheels plus they had points and condenser+ coil ignition systems that made a fatter blue spark when pulled over. The newer saws with electronic ign modules seem to produce a weaker spark when just pulling them over with the recoil.
 
dsell

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Probably flooded. They won't pop when they're drowning. Perhaps the inlet needle isn't closing. Also, possible to pull bar oil up in the crankcase, depends on the design. The super 2 will run on 90 psi but it won't have the power it should. That's assuming it's from wear. This is from my experience of the 10 XL's I've owned. If it's scored bad, you'll probably get mixed compression results and it won't run. I've seen very misleading compression results on very badly scored saws. It should be around 120 to 130. I might have a new cylinder and crankcase for $30, but no piston.
 
Rx7man

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I have seen that more on the older all magnesium chainsaws, for the most part they had longer stroke and heavier flywheels plus they had points and condenser+ coil ignition systems that made a fatter blue spark when pulled over. The newer saws with electronic ign modules seem to produce a weaker spark when just pulling them over with the recoil.
Our old Husky 65 that was bought the week I was born had 0.080" ring gap on a single ring and it still ran, but was hard to start and didn't have much power... I never took a compression reading but I'd guess around 100-110... I'm surprised it ran at all, or long enough to wear the ring that much... besides the ring was so thin around the exhaust port it was very close to snapping and hanging up, and a little scoring. That P&C became the donors for my 'wild' husky 65, since they weren't perfect I wasn't too concerned about going a little too far with the porting. 90* exhaust, 85* intake, 3/4" carb, .014" squish and 140 PSI now, it runs pretty good, though I think my squish is a little tight... I do hear the piston touch a little bit once in a while I think... I'll take it apart at some point and see if that's what I'm hearing, or perhaps detonation? Either way I'm not terribly concerned.
 

AVB

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I have an old homelite XL12, it feels like it's got no compression (I didn't bother checking it), but it starts up with about 3 pulls every time... doesn't make much power in the cut though... it's funny how some saws with low compression seem to do surprisingly well, while others just quit if they have less than 140psi
Come to think of it I had a Poulan 3716 here a few weeks ago that would not start and it had 110 psi but when I pull the muffler I found why; totally trashed. The only I was getting such a reading was the excess fuel in the cylinder chamber.
 
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