Chainsaw Repair

How To Basics - Carb Fixes + Mods - IPL and Service Manuals => How To Basics and Fixes => Topic started by: pigpen60 on October 06, 2014, 12:29:10 pm

Title: min. compression?
Post by: pigpen60 on October 06, 2014, 12:29:10 pm
whats the min compression for a saw to run/ run right?
Title: Re: min. compression?
Post by: mdavlee . on October 06, 2014, 01:01:15 pm
At 100 it may run. 125 or so seems to be better.
Title: Re: min. compression?
Post by: Cut4fun . on October 06, 2014, 01:39:29 pm
I have had worn rings that showed 120psi pulled over easy but ran pretty good.  Below that I dont know. Never had one run below 120 that I can remember that would start and run.
Title: Re: min. compression?
Post by: pigpen60 on October 06, 2014, 04:25:23 pm
I thought I had seen it posted somewhere. I need to do a comp. test on my mak 6421. just doesn't have the ass I think it should. least ways its a good place to start.
Title: Re: min. compression?
Post by: farmboy on October 06, 2014, 10:21:53 pm
Somewhere close to 100# is lowest. I've had em start and run @ 95# today won't start tomorrow.  @ 115# or so I pull the cyl.
Shep
Title: Re: min. compression?
Post by: Cut4fun . on October 07, 2014, 06:44:33 am
LOL had to add this to my above post (  that would start and run ). Have had many under 120 LOL.
Title: Re: min. compression?
Post by: KilliansRedLeo on October 08, 2014, 01:37:54 pm
If it runs at 100, it is only a matter of a short time before it goes below 100 so I tear them apart right then. IMHO Most will run at 100 (how well is another question), @120 they will run pretty good, @ 140-150 they will run well.
Title: Re: min. compression?
Post by: pigpen60 on October 08, 2014, 04:17:35 pm
checked my mak 6421 and it had 180#. Ive had a couple of leaf blowers that would start to and not tomorrow. should have kept them to test on. but no not me I tossed them across the yard and set em on fire! before I had patience and wanted to understand the little beasties.
Title: Re: min. compression?
Post by: Al Smith on October 08, 2014, 08:34:55 pm
Blowers are a horse of a different color just like weed wackers .They sit more than they get ran unless you own a lawn care business .Most times the carbs get funky from inactivity and old stale gas .Very seldom with either one do they get worn out .
Title: Re: min. compression?
Post by: pigpen60 on October 09, 2014, 07:29:05 am
I recently was given a weedeater brand blower used by a local business every week to clear the p-lot. it was wore out, it met its demise like I previously mentioned. when it ran it beat the rake, when it didn't it got beat!
Title: Re: min. compression?
Post by: 3000 FPS on October 09, 2014, 09:41:42 pm
I have found at 120 psi they start and run ok.
At 115 psi they start getting alittle hard to start.
At 110 psi down to 100 psi they become difficult to start or it may not start at all.
Below 100 psi down to 90 psi you might get it to start on priming it thru the carb and it will not run right and will probably die.
Below 90 psi I doubt that it will start at all. 

At work all I do is work on 2 cycle engines and if I find one that is hard to start off the carburetor and has below 120 psi I let the customer know that there is a major problem and I do not proceed any further.
Title: Re: min. compression?
Post by: 3000 FPS on October 09, 2014, 09:45:45 pm
Blowers are a horse of a different color just like weed wackers .They sit more than they get ran unless you own a lawn care business .Most times the carbs get funky from inactivity and old stale gas .Very seldom with either one do they get worn out .

I find a lot of stuck rings on weed wackers and blowers.
Title: Re: min. compression?
Post by: man of stihl on March 07, 2015, 06:14:18 am
I was working on a guys 272 Husqvarna. He tells me the coil is bad because when he runs it for a bit it just dies. So I listen to him and swap the coil, fire it back up and it runs but wants to stall. Its hard to re start. Difficult to keep running. I'm adjusting the carb and its way off where stock settings should be to keep it running.
So I decide to comp test it and the tester reads 100psi. I pulled the muffler and we have scoring on the piston.
New piston and ring installed, clean up cylinder, carb back to 1 full turn out on the H and L. Now she purrs like a kitten.
Lessons learned. Don't listen to the customer ;) and low compression saws are hard to tune and when they get warm, they drop more compression and go below the required comp to run.
Title: Re: min. compression?
Post by: 660magnum on March 07, 2015, 06:24:17 am
+1
Title: Re: min. compression?
Post by: 67L36Driver on March 07, 2015, 11:29:03 am
The really, really old Devils will start and run fine at 70-90 psi.  But, that is smack in the 'normal' range for 1958.

I truly believe they were designed to run on Coleman fuel in a pinch.
Title: Re: min. compression?
Post by: 3000 FPS on March 07, 2015, 11:29:21 am
Yep you never know.