Frustrated- Jonsered 670

[Chris_In_VT]

Working on a 670 champ, 1997 model.
Things I have done- rebuilt the carb (tillotson HS recirculating) new fuel line and filter, new base gasket.
Have not done impulse line (it seemed ok) or the fuel return line (shouldn't matter right?)
Compression is a hair under 150lbs.

Symptoms-
Difficulty getting the idle properly set. Adjust the HS screw till the chain stops spinning and it does after I hit the throttle. Adjust the L jet so it doesn't die/bog and the chain won't stop spinning.
After a brief WOT run to try and adjust the H jet it acts like it's starving for fuel and dies.
Starts reliably.
I already tried switching the return and main fuel line, thinking I mixed them up. Nope, it was obvious when I did that.

Now I consider myself a somewhat competent saw mechanic. Not an expert by any means, but I've rebuilt a carb or two, saved a few cylinders, and split a few cases.

Maybe its something simple i'm overlooking, but anyways, I'm getting frustrated with the saw and need to step back. Back to the basics. Ask the masses at AS.
Some fresh ideas and opinions are welcome.

Tank vent?
Clutch springs?
Spark breaking down?
Back into the carb, something not right?

 

[Chris_In_VT]

It doesn't SEEM like an air leak to me but maybe?
I don't have the tools to pressure/vac test.

 

[Chris_In_VT]

Also working on a second 670 champ, not as nice condition wise as the problem child. Gave that one the same treatment, carb kit, fuel line, filter. It started, idled, and revved up fine. Go figure.
Well now I'll just swap components till I find the issue. Start with the coil, carb second.

 

[bikemike]

Start out with tuning the high jet so saw runs good load or no load then turn idle screw up without chain brake on. Then then work the low speed jet till it runs close to consistent and then lower you idle speed screw back. If your still having problems then it might be a fuel stop needle not seating and loading up. Or you may still have something still stuck in the carb that didnt get blown out. If it still dont work then its seals or gasket

 

[Chris_In_VT]

It just will not tune right. The other one tuned fine. Ruled out coil and tank vent (not as likely but easy to test)
I'm tearing back into the carb now.

I had a carb on a stihl 028 once that acted similar. No matter what I did it wouldn't run right. I ended up junking that carb!

 

[weimedog]

Based on what I have seen from you and the symptoms, my guess is carb body.

BUT having said that, I had similar conditions on a 266 that drove me nuts. Acted like fuel so I kept chasing that. Ended up being the primary (it had two) ignition coil.

Why not just swap the carb with the one that runs well? That will tell you pretty quickly.
And if it was the tank vent, it would run if you opened the fuel tank cap... (and not with too much fuel in there) Also failed tank vents takes a minute or more before the "vacuum" begins to impede the carbs ability to draw fuel.

If it was the pulse line.. it too might start and very quickly run out of fuel. Something to check...but once the fuel was out of the carb that saw isn't going to run.

A very bad case seal might react that way, a can of brake clean can quickly help u figure that one out. The one on the clutch side can be wiped when you install that oil pump. Some thing else to check AFTER everything else fails.

A vacuum test can find a lot of that stuff.

 

[bikemike]

I like the carb swap. Was gunna mention it but didnt know if that would be too much work With 2 saws sittin half apart

 

[bikemike]

Bad gas can cuase oxidation in the carb and once it starts to rot many things in that carb could be affected and ots usually the areas the matter most needle seats ports and around the plugs

 

[Chris_In_VT]

I haven't swapped carbs YET.

These 670's are new enough so they have the single coil. I swapped those, no change. Also ruled tank vent out. I changed the impulse line just because, but no change.
The bench does get quite cluttered if you have two of the same saw torn down swapping parts around

Right now the (assumed) bad carb is apart again.
Gaskets were in the proper order. It's soaking in carb cleaner.
I managed to find leftovers of a whole carb overhaul kit from when I just did gaskets on another saw with a tilly HS. I just got the gasket kit this time. So I'm gonna change the metering needle and lever, and the welch plug. See where that gets me.

Thanks for the help guys. Carb cleaner, gas fumes and a overly warm shop added to a frustrating problem can get to you!

 

[Chris_In_VT]

Went through that carb with a fine tooth comb. It was dirty underneath the welsh plugs. Got that out of there. Then I proceeded to f**k up the new plugs
Not my day I guess.
Carb swap tomorrow so I can have the nicer 670 in saleable condition, then I'll get new plugs and see if that carb will work on the other 670. Or get a new carb. Or sell it as is. I don't know. Time for a beer

 

[gulity1]

Tank Vent-- I had a 288 that would run for about 2 mins then start acting like it was lean or straving for fuel I figured it out after openening tank to check fuel level and to make sur efilter was at the bottom several times and every time same thing would run about 2 mins then start to act up and it would never hold a tune correctly seems like I was always chasing it. food for thought? Open the fuel cap, fill it half full and try to run it, does it run better same or worse?

How is the carb pulsed? could you have a week pulse ?

 

[David Young]

Hey just bought a misty vac and come out your way every weekend.
David

 

[bikemike]

Maybe the fuel pump pressure is getting low voltage. Change it to a higher amp alternator and maybe a k&n cold air intake With a new turbo valve spring rejection carburetor control power chip. Should run good as used. Maybe

 

[bikemike]

Oh and some new crome valve covers adds a couple hp

 

[Duane(Pa)]

I wish I could help. You got me unstuck a time or two. Two things: you said "step back" right off the bat, that is a good plan. Some days you just swim upstream and the current wins. Another, there are plenty of deadbeat carbs out there in the OPE realm. I learned that from a Stihl tech, that is no parts swapper (he fixes things). Some just get fubared inside and need replaced. Good Luck! DW

 

[Chris_In_VT]

Maybe the fuel pump pressure is getting low voltage. Change it to a higher amp alternator and maybe a k&n cold air intake With a new turbo valve spring rejection carburetor control power chip. Should run good as used. Maybe

Older jonsered, non turbo. I foresee complications going from NA to forced induction.

 

[Chris_In_VT]

I wish I could help. You got me unstuck a time or two. Two things: you said "step back" right off the bat, that is a good plan. Some days you just swim upstream and the current wins. Another, there are plenty of deadbeat carbs out there in the OPE realm. I learned that from a Stihl tech, that is no parts swapper (he fixes things). Some just get fubared inside and need replaced. Good Luck! DW

Yes, that might be a better course of action than getting new welch plugs for a carb that's still not known good. Tilly HS carbs have got to be fairly common. Anybody have one? HS250a recirculating but I think a non recirculating will work.

 

[Chris_In_VT]

Hey just bought a misty vac and come out your way every weekend.
David

This is why this website is great. I appreciate the offer. Another member who lives a few towns over from me also offered the usage of his pressure testing equipment.

 

[bikemike]

Older jonsered, non turbo. I foresee complications going from NA to forced induction.

Well if you put a flow master 5.0 exhaust on it and a msd ingition and a 2 speed powerglide with a 2500 stall converter and a bigger hood scoop it might help the carb issue. Dont mind me yet i haven't had my coffee yet

 

[David Young]

Chris I'd like to catch up with you and test run the 385 and 395 some time.