670 champ running fine then starving

[jacrawley]

My 670 champ has a newly rebuilt carb. It ran great through a tank of fuel. On the next use it would drop out mid cut and start to race at idle and bog on acceleration. Then it would recover and make it about halfway through a cut then repeat. I tried venting the tank cap to make sure it wasn't pulling a vacuum but no effect. I dumped the fuel and there was a few tablespoons of water at the bottom. Swapped that out and no change. Could I have jacked up the metering valve setting? It probably wouldn't hurt to swap the fuel filter. I haven't seen this kinda variable behavior before.

 

[cbfarmall]

Check the fuel line, impulse lines and manifold for air leaks. And it never hurts to double-check prior work.

 

[Woodtroll]

Did you drain the fuel tank when you rebuilt the carb? The water in the fuel is problematic, especially "a few tablespoons" worth. Whether it was in the tank already, or came out of the fuel can that you refueled with after using the first tankful, your fuel is now highly suspect and may have had other trash in it. If you're lucky you may be able to start over with dumping the fuel from the saw and your can and using a fresh mix from a clean source. If you're unlucky you may have to go through the carb again and clean it out.

As cbfarmall noted, it may not be fuel related at all, but with water in the fuel that's what I'd start with.

 

[huskihl]

Maybe the fuel filter is filling up with water and not letting enough gas through. Or possibly the screen in the carburetor is getting plugged up

 

[RedneckChainsawRepair]

Like others said and check that impulse line close. I had this happen on mine.

 

[HiOctane]

Check intake rubber is not torn as well pulse line.

 

[jacrawley]

Did you drain the fuel tank when you rebuilt the carb? The water in the fuel is problematic, especially "a few tablespoons" worth. Whether it was in the tank already, or came out of the fuel can that you refueled with after using the first tankful, your fuel is now highly suspect and may have had other trash in it. If you're lucky you may be able to start over with dumping the fuel from the saw and your can and using a fresh mix from a clean source. If you're unlucky you may have to go through the carb again and clean it out.

As cbfarmall noted, it may not be fuel related at all, but with water in the fuel that's what I'd start with.

Yeah it was a shocking amount of water. I dumped it and checked my fuel can by pouring into a glass jar first. It looked good. I think it was a **** load of condensation from fuel stored for too long in a can with a slight leak.

 

[jacrawley]

I did a pressure test on the fuel line and found no leaks. So I pulled the carb apart and cleaned it out with carb cleaner. It looked a little shabby inside but not too bad. The diaphragm on the metering side has a nub but it isn't the style that locks into the metering lever. Is that odd?

After this I reset the h/l screws and did a retune. It was very similar. It would get a nice idle and then start to starve and surge like a vacuum leak but then it would calm down and idle well again. On acceleration it would hit about 9500 and after a few seconds lean out and run up to 12500. It kept this behavior up through a few cuts. I've never seen this type of oscillation.

I'll check the impulse line. I have a fuel lineand filter on its way. I also have a spare carb I could try but I'm sure it needs a rebuild kit.

I did a leak down test when I rebuilt the carb and swapped the piston and it passed but that was also cold. I will check that again as a leak could have developed sense then...