Jonsered 670 issue

alabamatriker

alabamatriker

ArboristSite Operative
Joined
Jun 18, 2015
Messages
122
Location
sw alabama
Have a 670 that will start and run for about 20 -30 seconds then die out. I don't know the history of the saw as it was purchased as salvage. Have cleaned and then rebuilt the carb. Runs the same after cleaning and rebuilding. Starts normaly, choke and throttle lock, 2-3 pulls it will pop. then un choke and it will crank and run. Good throttle response as soon as it starts and then seems to get weaker and then will quit. Will pop but not start back up. Will repeat next day. I had an atv once did that and it was low compression. Start cold then die and not start back when warm. Checked compression after it quit and had 120 psi. Have not checked it cold. What should compression be on this saw?
 

TLG

Komatsu Iron Life
Joined
Jan 26, 2010
Messages
943
Location
Western Pa.
That's kind of a loaded question. Depending on who has been in there before you and what they might have done. I've found they are not a real high compression in stock form. Normally 155-165psi in stock form. It will not run at 120psi and probably won't start either. Have you inspected cylinder//piston?? I would suggest looking for a sticking ring especially if compression is higher cold and it starts and runs for a bit. This is only a starting point but a good baseline to start. Lots of other input will soon follow. @Cantdog is a good resource of information.
 
Cantdog
Joined
Feb 17, 2009
Messages
15,861
Location
Beautiful Rockbound Coast of Maine
Is your 670 an early one with the impulse running through the carb boot or the later type with an external impulse line? 120 is certainly low on comp. If it's the external type be certain the hose has no leaks and fits tightly on both ends. I would pull the muffler and look the piston over to see if you have any scoring, excessive carbon build up or as said, perhaps a partially stuck ring. Make certain your exhaust screen is clean and not plugged with carbon. Another thing to check is the tank vent. Also a bad coil can act this way too.....Or a bad spark plug......a lot of things can cause this issue and it will be a process of elimination to find the cure. I'd start with the easiest and least expensive and work from there.
 
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