Jonsered - revs very high at start but runs good at WOT

[Deanpeddle]

Hi folks, I have a 5 year old Jonsered CS2240 S (Same as husky 435 I’m told). Saw was running perfectly until about one week ago, when it started to race with no throttle applied. I was able to adjust the idle screw to almost get rid of the turning Chain but not quite. Ive also set the high and low screws to factory settings (manual says 2 out for H, 2.5 for L). Now the saw starts up and runs at about 5500 rpm for a few seconds but If I don’t touch the throttle it will shut off. If I hold the throttle wide open it will run up to about 11500-12000rpm and stay there. I tried several slight adjustments to the high and low but it’s not improving the condition. (However I’m not very experienced with this). Couple other notes: (1) to eliminate the Carb as a problem I installed a new one ... but nothing changed (so put old one back on) (2) compression is 120psi.

could this be the result of an air leak? My research indicates that adjusting H/L with an air leak is just masking the problem. What do you think?

 

[merc_man]

Yes. Sounds like an air leak. Time to pressure/ vac test it.

Sent from my SM-G950W using Tapatalk

 

[Thumper88]

Air leak somewhere. Get it vac checked and you’ll find it

 

[Robin Wood]

This symptom is usually leak before the carb, when you hit the throttle it sucks the boot or whatever in there hard and it seals the leak. When idling the suction is weak and exposes the leak

Compression is definitely low
Yes tuning masks minor air leak, but its common and is fine as long as leak not too severe

 

[Piotr Pakula]

A bit of air leakage plus adjusting the carburettor is a free saw tuning ;-). This is how I used my first chainsaw in 10 years . The disadvantage is that then the saw itself works about 5 seconds at idle speed and goes out

 

[Deanpeddle]

So I was getting ready to do a pressure test when I found the following ... 2 of 4 screws (clutch side) holding crankcase (?) in place are missing. see attached pics. is this the cause (or contributor) to air leak problem?

 

[TBS]

So I was getting ready to do a pressure test when I found the following ... 2 of 4 screws (clutch side) holding crankcase (?) in place are missing. see attached pics. is this the cause (or contributor) to air leak problem?

Could definitely be the cause because that's a good spot for an air leak.

 

[Deanpeddle]

Could definitely be the cause because that's a good spot for an air leak.

thanks for replying. Sure Hope that's it!

 

[J D]

Those bolts hold the cylinder to the crankcase so they're pretty crucial. I would replace them & pressure test it (check the bolts on the other side too as they will have been given a hard time). You will probably still have a leak around the bearing as the sealant will have been split, in which case the cylinder will need to come off & be cleaned up and re-fitted with new sealant. Getting them to seal on used bearings can be a PITA & you may be better off putting a new set in if you have it apart. I'd also be checking the cylinder & piston for damage, 120psi isn't great & it might be time for a new piston or at least a new ring & clean up the cylinder while you're in there. Good luck, let us know how you get on

 

[Deanpeddle]

Those bolts hold the cylinder to the crankcase so they're pretty crucial. I would replace them & pressure test it (check the bolts on the other side too as they will have been given a hard time). You will probably still have a leak around the bearing as the sealant will have been split, in which case the cylinder will need to come off & be cleaned up and re-fitted with new sealant. Getting them to seal on used bearings can be a PITA & you may be better off putting a new set in if you have it apart. I'd also be checking the cylinder & piston for damage, 120psi isn't great & it might be time for a new piston or at least a new ring & clean up the cylinder while you're in there. Good luck, let us know how you get on

Thanks JD appreciate the great advice.
so... I replaced the 2 missing screws and put blue locktite on all 4. I don’t have a mityvac (yet) to do a pressure/vacuum test, so I decided to put the saw back together (had the time anyway), and try her out. I figured, as you did, that the damage would have been already done. But, surprisingly, the saw started up and is running great. I’m going to do a bit more testing tmrw but I’m hoping I got very very lucky! If I don’t post tomorrow it’s because I’m back in the woods!

 

[Deanpeddle]

By the way JD, if the saw ISNT working tomorrow,I’ll do everything you suggested in your post...

in fact, when the mityvac gets here maybe I’ll do the breakdown anyway. She’s due some TLC

 

[J D]

I would at very least re-tune it. If it does have a bit of a leak you'll end up tuning it a bit fat to mask it but that will help minimize any further damage. I'd also pull the muffler & have a look for any piston scoring &/or do a compression test if you can. Unless you'll be pulling it down again in short order put some of your blue loctite on the muffler bolts too