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Zenith Stromberg Carbs

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martx-5

martx-5

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Over on the Miata forum, someone with a Jag XJ12 was having trouble with two carbs on one bank. Supposedly they were all just rebuilt. The symptoms were that when the engine was running, the pistons would not rise when the gas was goosed. Also, while running, it was difficult to raise the pistons manually. However, when the engine was off, he could raise the pistons with ease. The usual answers were to check the diaphragms, but the part about not being able to raise the pistons when running seemed to point in another direction. I gave him a bunch of suggestions, including that it sounded like the pistons were in backwards. I also mentioned I didn't think this was possible (as it isn't on the SUs) but told him to look anyway.

Well, it turned out that the pistons <span style="font-weight: bold">were in backwards!</span> and the car is now running fine. Is this possible with ZS carbs?
 
If memory serve me right, I thought the diaphragms had tabs to locate them on the pistons and tabs that locate the piston/diaphragm in the carb body so if you align the tabs where they should be, it is not possible to put them in backwards. I did read that when rebuilding carbs, you should not interchange pistons though. Maybe that is what they meant? They had the front piston and rear piston mixed up?
 
TR4 said:
If memory serve me right, I thought the diaphragms had tabs to locate them on the pistons and tabs that locate the piston/diaphragm in the carb body so if you align the tabs where they should be, it is not possible to put them in backwards. I did read that when rebuilding carbs, you should not interchange pistons though. Maybe that is what they meant? They had the front piston and rear piston mixed up?
Click to expand...

The tabs make sense. But, I mentioned the hole(s) in the bottom of the piston, and that they should be facing the back of the carb, and it turns out that they were facing the front. I guess whoever rebuilt them didn't line up the tab properly.
 
That's the most likely explanation, IMO. The tab in the center is easy to overlook, if you don't know it's there. And I've even seen people get the outer tab wrong, although it's a bit more obvious.

The prohibition against mixing pistons only applies to SU carbs, because of the very close tolerance between the piston and dome. On ZS carbs, the diaphragm takes the place of the precision fit, so interchanging pistons won't hurt.
 
I wonder how/if the other two were assembled correctly! I'd be lookin' at those two as well.

The slides in the domes is still of concern as to fit. I'd not mix-n-match Zed-S pistons/domes any more than I would SU's. 'specially on those lumps. 12 cylinders can hide a bunch of issues the four and six lung machines will obviously balk over.
 
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