Saturday, April 26, 2014

Weed Eater Featherlite SST -- Zama C1U Carburetor Diaphragms Replacement

The carburetor shown here wouldn't prime at first, even though the primer bulb looked ok. Replacing the primer bulb cured that.

With the new primer bulb in it, the engine would start, but it wouldn't idle, and it wouldn't run off-choke. The next logical move was to replace the two diaphragms in the carburetor -- they're the next weakest links after the primer bulb.

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Body Type And Model Number Identification

Body type and model number are crucial pieces of information when buying parts; without them, your parts dealer can't begin to help you. Even with them, there may still be a problem.

The model number on my carb didn't show up on the parts dealer's listing -- the dealer had to make a 'best guess' as to what would work for me.[1] Had I known, I would have been well advised to have the entire carburetor with me, not just the numbers.

Anyway, here's a view of the body type and model numbers on my carburetor.

"C1U" is the body type. "105H" is the model number. '105H' did not appear on the dealer's listing.

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I won't pretend that I fully understand the operation of this carburetor -- much of it is a mystery to me. What literature I've found was not much help.[2]

To simplify things, I'll refer to the 'primer-side' diaphragm for the diaphragm nearest the primer bulb, and 'main' diaphragm for the more substantial diaphragm at the opposite side of the carburetor.

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Main Diaphragm And Gasket

Two M3x6mm screws fasten the main diaphragm with its cover[3] and gasket in place.

A diaphragm kit will include a new gasket. Replace both the diaphragm and its gasket.

Primer-Side Diaphragm And Gasket

Two M4x15mm screws fasten the primer body and primer-side diaphragm with its gasket in place.

A diaphragm kit may include several different versions of primer-side diaphragm and gasket. Pick out the correct ones for your carburetor and install them.

And that's about it. Reassemble everything and you should be good to go. The above routine got my Weed Eater running fine.[4]

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[1] Zama P/N GND-18 turned out to be the right part.

[2] Zama publishes a 'Technical Guide' on their website. Good luck with that. You may as well read about the care of potted plants, for all the enlightenment you'll get from the Technical Guide. Like much of 'technical writing', it's gibberish embellished with illustrations.

[3] Note the smallest of three holes in the main diaphragm's cover. That hole is there to allow atmospheric pressure to the outer side of the diaphragm. The hole mustn't get plugged up with debris -- if it does, the diaphragm won't function and the engine won't run.

[4] I suspect that most of the trouble was being caused by the old main diaphragm. There was no obvious fault in it -- no punctures or tears -- but it was not as supple as the new diaphragm; there was a perceptible degree of stiffness to it. I suppose that's all it takes for a diaphragm to lose its ability to move fuel adequately.

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