Pioneer P21 starts and runs but after a few minutes and several cuts it won't stay on

[RRRocketMan]

Interesting new problem for me since in winter I had no issues with this saw. Now in the summer I fire it up, tune it a bit, cut some wood and then it dies. I try to get it going again only to find it just sputters but won't stay on. After relieving a **** load of gas pressure and playing with the choke I manage to find the sweet spot for it to idle but as soon as I give it any throttle at all, it dies right away. I've become accustomed to detecting the lag between throttle apply and revving up and I can tell you the saw won't respond to throttle in this condition. It was nice and zippy when I first turned it on but after some use, it's like there's no gas at the high end.

Just as footnote to the above, is there any way to tune the high side on these carbs? No screw for it. But that aside, I'd love to know if this is a common issue because there are hints that this happens to other people from a quick google search.

 

[Old2stroke]

Interesting new problem for me since in winter I had no issues with this saw. Now in the summer I fire it up, tune it a bit, cut some wood and then it dies. I try to get it going again only to find it just sputters but won't stay on. After relieving a **** load of gas pressure and playing with the choke I manage to find the sweet spot for it to idle but as soon as I give it any throttle at all, it dies right away. I've become accustomed to detecting the lag between throttle apply and revving up and I can tell you the saw won't respond to throttle in this condition. It was nice and zippy when I first turned it on but after some use, it's like there's no gas at the high end.

Just as footnote to the above, is there any way to tune the high side on these carbs? No screw for it. But that aside, I'd love to know if this is a common issue because there are hints that this happens to other people from a quick google search.

That's a problem with the carb on these saws, fixed high speed jet with no adjustment. Runs fairly clean on 16:1 mix but is always running too rich on modern 50:1 ratios. These saws normally start easily and run reliably with the low speed mixture screw set as lean as possible without getting a lot of hesitation when snapping the throttle open. Sounds like you have a fuel starvation issue when it gets hot, might be time to rebuild the carb but if the saw runs for awhile and then starts to run out of fuel it can be a faulty tank vent that's not letting air into the tank. When you "relieve" tank pressure, are you sure it's air coming out of the tank and not air rushing in? Tank vents are only supposed to let air in. If the vent is an issue, just try cracking open the cap on the tank when the saw acts up.

 

[RRRocketMan]

yeah the pressure is definitely coming out and in fact it looks like the fuel's boiling as there are a lot of bubbles coming from somewhere. I would rebuild the carb except I just rebuilt it this winter. Everything is brand new there. Running 32 to 1. If throttles causes it to die doesn't that mean lean? Too much air from opening the plate? Because the throttle is a leader and the carb response lags, which is why the low screw has to be a bit rich so the saw doesn't "whaaaaa" and die right? Mine doesn't make that sound but it just dies with throttle... and again this is only when it's done a few minutes of work already. Full tank.

 

[camel2019]

Crank seals? I’d definitely find a different carb these are the same saw all the way up to p28 if memory serves me correct and the 1073 and 1074 my 1074 has a fully adjustable carb.

 

[Old2stroke]

The fuel in those saws tends to get very hot but usually only after extended hard cutting at full power. A fuel tank that gets as hot as you describe might mean the saw is running too hot, probably because it's running lean. Could be an air leak (not too likely since it idles well) or fuel starvation. Time to pressure test the crankcase and check into the carb and fuel delivery. The saw sounds like it is having vapor lock due to fuel that is way too hot.

 

[RRRocketMan]

After doing my own looking around, I thing vapour lock sounds like the best diagnosis too. The only thing that confuses me is that with so much pressure in the tank, you'd think gas would be gushing into the carb readily... unless it's vaporizing on its way to the venturi and never makes it as a liquid. I might be able to idle it barely because I'm only getting fumes, which might just be enough.

 

[Old2stroke]

After doing my own looking around, I thing vapour lock sounds like the best diagnosis too. The only thing that confuses me is that with so much pressure in the tank, you'd think gas would be gushing into the carb readily... unless it's vaporizing on its way to the venturi and never makes it as a liquid. I might be able to idle it barely because I'm only getting fumes, which might just be enough.

Pressure building up in the tank can't force fuel into the carb until it exceeds the pop-off pressure of the inlet valve. When pressure testing the fuel inlet to the carb, it should hold a minimum of 10psi and that should be high enough to prevent fuel being forced into the carb, however, if you have boiling gas in the tank, the pressure could be higher than that.

 

[pioneerguy600]

Fuel boils very easily in these small all metal Pioneers, especially bad if one tries running gas bought in the winter, best to never run winter bought gasoline in the warmer months as it will boil easily when the temps rise.

 

[cscltd]

Dirt build up under starter and around flywheel and cylinder also caused these to boil the fuel.
The cylinders on this model was famous for having Crome peel causing poor running when saw got hot