OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Trimmers and Brushcutters McCulloch Trimmers & Brushcutters Diaphragm, Fuel Pump

Can someone please advise if the fuel pump diaphragm on a Zama C1u m29d carby is a clear plastic moulding with a "bubble" in it. I suspect that what I have is not right. Does a standard repair kit include a new fuel pump diaphragm?. Thanks

After much Googleing, I found this: Fuel Pump Diaphragms: The tan Teflon coated pump diaphragm has been superseded by the clear mylar diaphragm. If your carburetor does not have a machined fuel pump surface you cannot use either the tan or the clear diaphragm. You must use the black buna diaphragm also found in the kits. All three diaphragms found in the Zama carburetor kits offer resistance to alcohol fuels. My question now is...can I use the black buna diaphragm instaed of the clear mylar is I suspect problems with the mylar?. I assume that is the thin black one in the kit. Thanks

Pedro, I recommend doing what the instructions say. From what you have reported, that means using the mylar diaphragm if you have a machined fuel pump surface.

"Improving" on the instructions is sometimes possible, but is unlikely to be achieved except by someone who knows more about this process than the people who wrote the instructions. That certainly lets me out - how about you?

Those instructions were part of Zama's official guide. Where can I then get a mylar diaphragm then, it seems that repairs kits only contain the black ones?. I certainly do not know much about these carbs, which must be obvious from my posts. Appreciate your help Grumpy, but getting to the point of throwing out this Mac 2816 and continue using my 40 year old Hanimex whipper, at least that still runs well. Regards.

Pedro, quite a bit of what goes on in the way of service part changes on carburetors is driven by reliability difficulties in the US. The underlying problem is keeping flexible parts such as diaphragms alive when they add substantial amounts of oxygenates to most of the fuel. The oxygenated fuel, sometimes described as "reformulated", consists of normal petrol plus ethanol and various ethers such as MTBE. Those additives tend to be slightly aggressive against gaskets, diaphragms and hoses, especially those based on the older elastomers from before about 1986 when the US began encouraging reformulated petrol. My guess is that the mylar diaphragm you are talking about is mainly a defence against those additives. However the additives can be avoided in Australia since oxygenated fuels are not mandatory here, and those that do have ethanol in them are usually labelled at the pump. Hopefully most Outdoorking members are aware of the perils of ethanol and do not buy petrol containing it.

If a reputable repair kit only contains one diaphragm, and it is one of the traditional black ones, I think it is likely to stand up pretty well to Australian fuel that does not have ethanol in it. It is after all, the same as the original equipment diaphragm that came with the carburetor when the machine was new.