The Stovebolt Site Forums The Shop Area Engine Shop Zenith Carburetor issue

I apologize up front for this post. There’s been a ton of discussion on my subject and I think I’ve reviewed it all. Yet here I am!

Back in 2017 I bought a nos Zenith O-10355 for a 1937 ~ 1942 Chevrolet 216 from Carbking. It’s been a Great carb until very recently. It won’t idle below around 750 rpm. The low speed adjustment screw has no effect. Try to adjust the throttle plate down more and it dies.

All I’ve done so far is to remove the low speed screw and spray carburetor cleaner through the passageway from both directions. It’s open,

Second thing was to hold my hand over the carb throat with the engine at 2,000 or so rpm and repeatedly choke it down till it almost died.

The above easy steps have not helped.

Before I pull the carburetor and disassemble, blowing out all passages and looking for problems, anyone have advice on what to expect/look for.

Timing is advanced just past the ball, vacuum advance works, points are good and set on 18. Carb has only had non-ethanol gas through it.

Thanks in advance.

RonR

Have you verified that fuel isn't leaking down the throat and flooding the engine at low idle? Needle valve not closed all the way, bowl over-topping , etc.

When it conks out, look down the carb throat and see if fuel is puddling on the intake or dribbling from the orifice

Otto, no I haven’t looked specifically for flooding signs. But while fiddling with it this morning, it cranks almost instantly when I give it a bit of throttle after it dries from adjusting the throttle plate closed (while attempting to get it down to the mid 500 rpm range.

I will be sure to check for signs of flooding though.

Thanks

RonR

OK, I think I know what to do? I’m currently waiting on the wood’s to settle down after taking a couple squirrels out of a hickory tree. Yesterday when I sprayed carb cleaner through the low speed adjustment port I was whizzing in the wind.

After considerable pondering while hunting I realized I should have backed the low speed screw out several turns, then sprayed cleaner through the low speed intake port located in the carb throat. If my theory is correct, that should clean out the low speed passageway that ends below the throttle plate. If cleaner fails, a short shot of high pressure air should do the trick.

Added in edit: Well, seems my speculation on how the low speed circuit works is wrong. With the idle screw backed out several turns, had no luck spraying carb cleaner and air into what I believe is the intake port for the idle circuit.

It would help to have sufficient bandwidth on my cell phone to find a diagram showing the intimate details of the carburetor. But alas, tis not the case.

Time to start turning wrenches.

RonR

Verify that it isn't flooding itself before taking things apart.

It has the symptoms of a vacuum leak.

Not sure what accessories on a stovebolt could leak vacuum, but prior to disassembly of the carb, try snugging the carburetor mounting bolts, and all of the carburetor body screws.

Does this engine have a distributor vacuum advance?

Jon

Jon, thanks for the suggestions. I think you’re the winner! Otto, no flooding.

I had already checked all the fasteners and they were tight. Distributor does have vacuum advance. I believe it’s working properly. What I hadn’t checked was the pcv valve. It did rattle but did feel sticky. A good blast of MAP sensor cleaner fixed that. Next I shut the needle valve controlling vacuum to the wiper motor off.

With the two items above addressed I was able to get the rpm down to the 500/600 rpm range. No 12v battery handy for the timing light tach. But I will check just because inquiring minds need to know.

The wiper control switch may be the primary culprit here. You need to tinker with it a bit to get it totally shut off. Back when I could hear it wasn’t a problem. But that was yesteryear.

I did not check vacuum prior to this morning’s efforts but afterwards at the lowest idle point it was 17”. Not the best but likely better than when I started this project.

RonR

That's good. No carb rebuild needed.

The older updraft carburetors used before 1935 are MUCH more reliable than more modern downdraft carburetors.

WHY is this true?

The downdraft carb sits on top of the engine and the easiest culprit.

The updraft carburetor is much harder to get to; thus other items are checked and the culprit found before looking at the carburetor.

Glad you found the problem!

Jon

Plus it is much harder to flood an updraft carb.

Touché Jon, I earned that! After all, we know it’s never the carburetor.

RonR

Everyone knows that 80% of all carburetor problems are electrical.