refurbished carburetor - wild ride

TomInCologne

TomInCologne

Senior Member
hi guys, i need your expertise


i swapped my carburettor on my model a (1931, stock, Zenith) against a refurbished one from ebay.


with the 'new' carburettor:


- the engine is idling fine, 450rpm and very stable
- as soon as i hit the throttle, the engine is very rough, it's more like a wild ride than a smooth acceleration. The engine will not go above 1.900rpm
- choke screw is turned completely in, turning it out up to 1 rotation does not make the engine run better
- the engine runs rough on both 1/2 throttle and full throttle
- after a while of running, the spark plugs look black with soot


I assume that my engine is running very fat/too much fuel. Correct me if i am wrong, but this means either that the float level is too high or the main jet is too big.



my next step:
- reduce float level
- if that doesn't do the trick, swap/check main jet


any more info or advice?
 
I have had nothing but trouble with Zeniths bought off ebay 3 in a row bad. Chief problem seems to be that the “rebuilder” only test them at idle (if they test them at all) the don’t get road tested. If you are dealing with “Hole in the Wall” I have received Zenit’s from him that were no good unfortunately.
 
I experienced the same problem last weekend with a Zenith I had completely rebuilt, including drilling out the passageway plugs and cleaning the passageways of accumulated gunk. I installed NOS original jets I had been saving. After reassembly, the engine idled just great. But when I revved up the engine or attempted to drive the car, it popped, banged and backfired horribly and couldn't reach enough RPMs to move. I went through the carb again, but found nothing obviously wrong. All the jet orifices were clear and I know the passageways were, too. Finally, I dug into my old carb parts bag and came up with four used jets, which I used to replace the NOS jets one at a time, testing for RPM improvement each time. The first three jets (idle, compensator, and cap) made no difference, as expected. Finally, I reached for a dirty, disgusting looking main jet, which I cleaned up and installed. Eureka! The engine suddenly had the power it needed above idle. The NOS main jet I had carefully saved all these years must have been defective from the manufacturer, which is probably why it was never installed.
If you have a spare main jet handy - even if it hasn't been flow tested - try that. Make sure the little gasket is in place beneath the jet's head with NO extra ones. You've got nothing to lose by swapping main jets. It sure solved my frustrating problem.
Marshall
 
For years I had Zenith Carbs that did not perform good on my Model A. It wasn't till I read Steve Pargeter's booklet "Zenith Model A Carburetor Restoration Guidelines" that I realized it was necessary to have a cleaned carburetor with flow tested jets. Steve also publishes the required flow data for each jet. I proceeded to build Steve's flow test tool (a 36 in. water column) to provide pressure into a jet under test. The construction of the tool was easy and low cost using PVC pipe. I would never consider rebuilding a carburetor with jets of unknown flow data. Steve published data from his experience on how many jets he found that were out of spec. The point he made was that both original jets and reproduction jets are not worth installing if you have not tested them. Note, that you cannot just use the correct drill size to correct a jet. The way a drill leaves the edges of the hole, will affect how flow will pass through the jet. I use the drills like a round file to slowly increase the flow rate until the jet is with specifications.

In summation, with a clean carburetor and flow tested jets you can't miss, the car will run perfectly everytime. Ed
 
Tom,
PM me your address and I will send you a set of jets that have been flow tested
 
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I tend to go only with "original numbered" jets.

A couple of carbs I did early I took out all the original jets (ugh - these are old and must be defective by now) and replaced them with the equivalent jet from the then commonly bought "rebuild kit." No welch plugs, but did include the gaskets and the plug for the bottom.

All those original jets were saved - the carburetor they came from was used for a VERY short while, and then replaced with a Tillotson X, which worked a LOT better.

But the moral of the story is USE ORIGINAL JETS - or calibrate/re-orifice the jets if necessary.

Someday I hope to set up the column and determine the "calibration" for a bunch of aftermarket/not marked jets.

The best description of the jet numbering/application is found at your friend and mine Vince Falter at his dedicated web page https://www.fordgarage.com/pages/jetflowrates.htm


Joe K
 
David Renner is a wonderful guy who not only flow tests his jets but he makes them himself to insure quality. Look him up @ Renner Corner.
 
Thanks all for the advice. I have no time to work on the problem until middle of next week. First thing i will try is to lower the float level, if that does not help i will swap the main jet.


I will keep you informed.
 
I had a problem with a different carburetor but with the same problem with jet sizes. An idle jet, labeled as size 60, measured to be size 50. I drilled out another jet to size 65 which solved a problem I was having. If you have a set of small drills, you can use the drill shanks to measure the size of the holes in the jets. Or use the drills to drill the jets out to the correct size. If the holes are too large, you can carefully solder closed the holes and re drill them. The Zenith main jet needs a dimple in the top so if the soldering fills in the dimple, you can use a larger drill to clean it out.

A helpful site for the Zenith carburetor is https://model-a.org/default.html

All about the main jet is here: https://model-a.org/main_jet.html
 
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Just to tout goodness for the Zenith carburetor, I have one on my Coupe that has been running without maintenance since the 1980s (thousands of miles), and another on my Victoria since 1993. These carburetors do not have air filters, and the fuel supply systems are 100% stock.
 
TomInCologne said:
Thanks all for the advice. I have no time to work on the problem until middle of next week. First thing i will try is to lower the float level, if that does not help i will swap the main jet.
Click to expand...


As per a recent experience, I suggest placing the bottom half of the carb on the bench, shimmed to a level position, then fill the bowl with fuel. Let the part stand for a while to see if the jets in the throat of the carb are leaking at their bases before proceeding with new jets etc.


I had to use the black, plastic??? gaskets from Bratton's rebuild kit to resolve this problem. The red ones would not seem to seal.
 
Rob Doe said:
As per a recent experience, I suggest placing the bottom half of the carb on the bench, shimmed to a level position, then fill the bowl with fuel. Let the part stand for a while to see if the jets in the throat of the carb are leaking at their bases before proceeding with new jets etc.

I had to use the black, plastic??? gaskets from Bratton's rebuild kit to resolve this problem. The red ones would not seem to seal.
Click to expand...


I was thinking about using 'Hylomar universal blue' which is fuel resistant. Obviously just a bit so it doesn't go into any ports and stays where it should. In my professional life i use fluid gaskets all the time and i have a good degree of experience with them, albeit in gas systems not fluid systems.
 
Okay, first experiment...
The calibrated jets, which a very generous Barner offered to me, have not yet arrived but i was thinking along these lines:


- the engines runs bad at half throttle and at full throttle, so in both cases the fuel-air mixture seems to be too rich. If i reduce the float level that will make the mixture leaner in both half- and full throttle.


- i took the carburettor out, took it apart and put a second gasket under the needle valve for the float.


- put everything back together and did a test drive.


- Car still runs fine on idle, half and full throttle seem to be a bit better but not by a lot.


next step?
 
sure you don't have a manifold leak/crack,or a valve problem?I've rebuilt many carbs. the ones on my vehicles have been in use for many years no problems with my flow tested jets.
 
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CA Victoria said:
Hoe does the GAV opening/closing affect the carburetor response?
Click to expand...

Gary WA said:
sure you don't have a manifold leak/crack,or a valve problem?I've rebuilt many carbs. the ones on my vehicles have been in use for many years no problems with my flow tested jets.
Click to expand...


90% sure because it was okay with the old carburettor. I swapped because i had a constant misfire going down hill. I don't know whether the jets on the new carburettor are flow tested, supposedly yes, but i am not 100% sure.
 
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