Can someone explain what problems the pumps are causing? Are they just wearing out?
The fuel for acceleration has to come from somewhere. Without an accelerator pump you make the L richer than it needs to be, and that provides the fuel for acceleration. With a pump you set the idle for the correct mixture and the pump shot provides the extra fuel. I'd rather be able to set the idle mixture correctly compared to having to set it rich.
My GZ4000-based saws all have pumps, and looking through spare carbs recently I saw that even some old Craftsman/Poulan carbs had them. I have not had an issue with them and was curious what happened on these.
Thanks - I never really liked the design that both Walbro and ZAMA use. The cheap soft metal of the casting makes a poor wear surface, and the cut-back throttle shaft probably puts a fair amount of side thrust on the piston. Some of the ones I have do not have a brass cover over the throttle shaft/linkage portion, and I oil them regularly.The accelerator pump piston is a brass piston with a rubber O-ring. It rides in the aluminum carb housing. After time and wear, the O-ring wears, the tiny brass piston gets some wear and what looks like light scoring marks from rubbing the aluminum housing, which is more or less the pump cylinder. In my mind, when that piston and bore gets some wear, it allows tiny air bubbles to enter the carburetor fuel metering system...making the carbs act like they do.
On the one's I've done, I remove the welch plug on the back of the carb first. I remove the accelerator pump piston and spring. I put a small amount of epoxy in the accelerator pump bore then put just the brass piston back in the bore and mash it down. That will squirt the excess epoxy out the welch plug you removed filing the whole passage as well. Then just epoxy up the welch plug hole and let it set up good. Just like Preston done.
I noticed no difference in acceleration at all.
True story!there are several models of the aftermarket carbs by different builders, unmarked for easy resale, beware of the ones that wont tune or keep a tune, I see them being "closed out" by ebay merchants very cheaply, they might work or they might suck as bad as the one you re trying to replace
We have had a lot better luck with the Zama sourced carbs than the Hunai model