Walbro WT Carb Stops Pumping Fuel Intermittently?

[SteveSr]

Hello,

I have a Walbro WT-875 on a small 25cc blower that is driving me nuts! The carb has been rebuilt with an OEM kit. Fuel line has been replaced with Tygon 4040. Fuel filter cleaned and reused. The crankcase up to the carb heat insulator block passes pressure and vacuum (at least when cold).

The main symptom are that this appears to happen after prolonged (several minutes) WOT operation as you would expect from a blower. Today I actually caught some of this foolishness as it ran out of gas (at idle) after a several minute run at WOT. It got leaner and leaner and finally stalled. I pushed the purge bulb and it actually emptied out before refilling! After refilling it started and ran normally.

Any thoughts on what might be causing this weird behavior?

Thanks,
Steve

 

[Vintage Engine Repairs]

Hello,

I have a Walbro WT-875 on a small 25cc blower that is driving me nuts! The carb has been rebuilt with an OEM kit. Fuel line has been replaced with Tygon 4040. Fuel filter cleaned and reused. The crankcase up to the carb heat insulator block passes pressure and vacuum (at least when cold).

The main symptom are that this appears to happen after prolonged (several minutes) WOT operation as you would expect from a blower. Today I actually caught some of this foolishness as it ran out of gas (at idle) after a several minute run at WOT. It got leaner and leaner and finally stalled. I pushed the purge bulb and it actually emptied out before refilling! After refilling it started and ran normally.

Any thoughts on what might be causing this weird behavior?

Thanks,
Steve

Check the things Hotshot has mentioned first, also check the accelerator pump o-ring and the main nozzle and if present the part load nozzle check valves. When they fail they can suck air and run lean. When they work intermittently they will switch to run lean and then run rich a few seconds apart.

 

[SteveSr]

What happened to @hotshot 's post? I was going to award him the grand prize (or at least bragging rights). I am pretty sure that "tank vent" is the correct diagnosis.

But wait... Theres more!... This thing has a "duck bill" valve in the gas cap. I all my years of working on small engines I have NEVER seen one get stopped up. Usually on old Stihls they tend to leak gas out of them.

So I got out the Mity-Vac and tried to pressurize the vent from the top of the cap with the cap sitting in a drill press vise for stability. I was surprised to find that I couldn't force air through the vent even with 5-10psi.

The duckbill was found under 2 layers of one-way press nuts. The first layer held the cap retainer and the second the actual duckbill valve. Much to my surprise upon removing the duckbill I discovered one of those sintered brass filters which was apparently molded into the cap.

A #65 drill bit fixed the "stoppage". I haven't had a chance to run the blower again but I suspect that it will be much better behaved now that the tank can breathe!

Below is a photo of the cap after modification.

 

[SteveSr]

Check the things Hotshot has mentioned first, also check the accelerator pump o-ring and the main nozzle and if present the part load nozzle check valves. When they fail they can suck air and run lean. When they work intermittently they will switch to run lean and then run rich a few seconds apart.

Are you intimately familiar with this specific (875) model? Does it have an accelerator pump? It doesn't appear to have the regular main nozzle and check valve. Instead it has some form of check valve in the base of the metering chamber.

 

[SteveSr]

Posts seem to be magically disappearing from this thread...

In any case, the new hole in the fuel cap seems to have fixed the issue. It now runs a lot more consistently from startup to shutdown. Tuning is also now predictable and consistent.