jonsereds 621 has too much compression

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hungrybare

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Hey guys, I need a little advice from the experts. one of my buddies has a Jonsereds 621 which pulls hard, ya know like it wants rip your arm out of the socket. Take the spark plug out and it turns nice and easy and really smooth. Not getting too much gas in the carb/ muffler not plugged so I checked the compression and it is 210. What woud you suggest to put in the cylinder to disolve any carbon that might be on top of the piston? Or any suggestions would be greatly appreciated. When it starts it runs fantastic. Sorry, could a moderator please move this to the chainsaw forum?
 
I've a Dolmar 6100 (worked hard, minimal chamber deposits) which sends my compression gauges' needles to 215 psi, so your indication of 210 psi would seem to be workable, and likely would also lead to serious power output. I find that to start that Dolly first thing of a winter morn, I have to put it on the ground and step inside the rear handle. Once it pops, it gets lots easier.
I'd suggest checking that the starter mechanism is in good shape and dosed with some dry-lube.
By far the easiest way to clean chamber deposits: run it properly hot & hard for a few tankfuls. Then don't baby it. Chainsaw engines thrive on what some folks would perceive as abuse. Piston crown should have thin dry dark-beige coating. Nothing chunky.
 
Not familiar with the size of the model, but 210 psi is just fine for a saw in the 60cc range.

Just start it like you mean it. If you are afraid and go slow, it can fire and kick back on ya.

You can also try giving it a tug or two with the plug in but the switch in the off position. If there is a difference, you're getting firing kickback.
 
drill a couple holes in the top of piston!! that will fix it right up maybe then your limp arm buddy could start it hahahahhaha
 
If it's all crusted up with carbon, search & read posts about "Stihl EDTA". Kind of a lazy man's method, but reportedly it works.

I've never used it, because I always pull the cylinders to clean them out.

If it's not, run it & be happy with the high compression! :)
 
That is pretty normal compression for a healthy 621....I have two that are slightly higher than that.
I don't think your saw has any carbon issues.....the 621 uses a high dome, two ring piston that is why the compression is like this. They also have a rather large diameter pulley to give more mechanical advantage.......is the pulley full of rope or does the saw have a short pull cord? The only other thing that can make a saw seem hard to pull over is if you are trying to start it without the bar on.
 
If you are not already doing this, slowly pull it over until it hits compression then return the recoil and pull like you mean it. I had run small engines for over 40 years before I learned this technique right here on AS. It works for every engine I have 2 or 4 stroke. A real arm and hand saver.

Ron
 
Thanx for the replies everybody, I think I will tell him to try it with the kill switch off, to eliminate any ignition problems. Then try starting like rwoods suggested. Piston up on compression stroke, then pull like a beast. If that doesn't work I'll drill a couple holes in the piston like Mohick suggested. 2 half inch holes should be good huh? LOL !!!
 
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