Carb help

[Leerod83]

Does anyone know the pop off pressure for a walbro(I believe) carb on a ms310 It's a knock off carb and seems to pop off around 25psi and falls to about 22psi. It will do this repeatedly. Thanks

 

[HarleyT]

Does the saw run?

 

[HarleyT]

Don't waste much time on the "popoff" thingy....

 

[HarleyT]

It is a distraction....

 

[Leerod83]

It does but tuning is inconsistent and it starts acting up after it gets hot. Checked tank vent, and pressure/vac tested to 10 psi both ways through the impulse. It all checks out. Only thing I haven't done is swap coils or checked compression(the comp. tester I have is one you hole in the plug hole). It's a new build, no scoring or anything.

 

[HarleyT]

So you did a leakdown test of the crankcase? Can you describe that? Just asking.
Replace the fuel line? Can you put up some pics and details?

Popoff pressure is not an issue. {will probably start a fight}.

Did you put in just a new piston?

What caused the failure in the first place?

 

[Leerod83]

The saw had an issue when I got it. Did a complete engine swap with a hutzl 390. The previous owner had done the same but only the top end. I assumed(yeah yeah I know what that means) it needed seals or wasn't sealed up well. The impulse is new the fuel line checked ok on pressure. On the pressure/vac test I hooked up my tester to the impulse ,blocked the intake and exhaust with scrap inner tube and put 9-10 psi on the whole engine for 10 minutes and then the same 9-10psi vacuum for 5 then rotated the motor over. It held.

 

[Leerod83]

Seems like it's losing compression when it warms up is my logic but it's new stuff. I know not oem but I figure it would have enough to run. And both engines have the same symptoms to the t. I believe the other is nwp maybe....... it has no markings. Neither is scored whatsoever. I think it's cursed.

 

[Thommo]

If it holds 22Psi it should be good. I have run carby's that only hold 10Psi and run good. Some Chinese carby's that i have tested held 40Psi and i have found that by clipping a coil off the spring to drop the blow off pressure to around 20Psi will give the saw more torque. I would try a new genuine impulse hose because i have been caught out with clone saws that do exactly what you are describing , and the solution was to replace the impulse hose with a better quality hose. The chinese impulse hoses are too soft and get softer as the saw warms up, so they suck in or kink over when the saw warms up so your fuel pump does not work properly. Dont use Tygon hose for the impulse because it will melt.