Zenith 30VIG-6 Carburetor Tech Help Sought

This is my first post on this forum. I joined with the hope that this user base is sufficiently diverse that I might find technical resources for the obscure Euro carburetors which occasionally come through my shop.

Today's problem involves a Zenith 30VIG-6 single-barrel downdraft carburetor for a 1953 Allard 6-cyl. which I have restored. The customer returned the carb to me with a report that it runs well enough, but that it dribbles fuel from between the bowl and the vertical part of the main body to which the bowl mounts. He is in New Hampshire, I am in North Carolina . . . so I can't watch this happen for myself. We have exhausted the "too much fuel pressure" topic; the electric pump is producing only 2.5 PSI.

I could not get the carburetor to leak on my bench without completely over-filling the bowl . . . and I honestly don't think that is what is going on here since the Viton-tip needle and its seat are new and tight and the float is leak-free.

What is going on is this: when I fill the bowl with test fluid (low-odor mineral spirits) it fills with no leakage. However, if I lower the fluid level in the bowl just a bit (to the point where the needle beins to open) and them blow through the fuel inlet (mouth pressure only), plenty of fluid flows into the carb's throat. Might this be a clue as to why the carb dribbles at idle?

I have done a number of Zenith 30VIGs over my 25+ years in this profession but I have never had one "come back". Does anyone have any suggestions? Please don't suggest that "it might be the float level"; I am 'way past such basics in my diagnostic process. :banghead:

Jeff Dreibus
The Old Carb Doctor

Carbs are, for the most part, before my time. With that in mind, maybe my ignorance can help you think outside the box. My only carb experience comes from a few nitro-powered R/C cars and small engine devices (snow blowers, power washers, push lawn mowers).

You mention that if the bowl is full, the car doesn't leak. But if you have the bowl less than full and provide positive pressrure to the fuel inlet, the car leaks. You also mention that the pump is providing 2.5 psi of pressure.

So blowing through the inlet is similar to the vacuum that is created after the venturi when the engine is idling. The 2.5 psi of fuel pressure seems a bit low to me, but I'm not sure what the specification is. I'm used to fuel injection that works under significantly higher pressure. Could this be a situation of a jet that is too large for the fuel pressure such that as the engine is idling, it's draining the bowl at a volume larger than what the fuel pump can supply? Is there a fuel filter after where you took the fuel pressure measurement that might be further restricting the fuel pressure/volume?

-Rod

Rod,

Thanks.

There's no fuel filter here at my shop, just the carb; I don't know what the customer is using as a fuel filter.

I have no specs on this carb so I have no idea whether the jets are too large or not. The customer has no service literature or specs (very hard to come by, so I'm told) . . . so I'm left in the dark there.

It has been my experience that too large a main jet doesn't affect the idle characteristics (which is what we are dealing with here), though too large an idle jet certainly will. However, this carb drips gas rather than runs rich (as one would expect with too large an idle jet); this is what is so mystifying to me.

Jeff

Most of what I'm keying off of is that things seem to be fine as long as the bowl remains full. On a properly running engine with a correctly functioning carb, would the bowl stay full or is it expected that it can run without issue at the level where it starts to drip? Again, please pardon my ignorance in this area.

-Rod

Rod,

Yes, the bowl should indeed stay full. I have asked the customer to run some fuel from the vehicle's fuel pump through a clear hose to see whether it is running "full-pipe" or pumping air bubbles along with the fuel (which could explain the problem). He has not yet responded.

Jeff

If you disassemble a carb you generally need a Carb Kit with new gaskets and a set of instructions to tell you how to set up the carb--give you float height and such, and it always includes an exploded view of the carb.
The float height relates to the manufacturers desired fuel level in the bowl. This determines when various fuel circuits 'turn on" in response to engine vacuum and load, etc.
Too high and the engine runs rich, and/ or fuel bubbles out of the carb itself. Too low and the engine runs too lean or not at all.
So, here are some guesses---
One of the surfaces is warped and fuel leaks out?
Fuel level is too high. You set the float level to specs?
Maybe there needs to be an assist spring that helps the carb's float to raise and shut the needle and seat?
Still a bit of dirt between the needle and seat and its flooding?
Float has a tiny leak and has absorbed some fuel and now is heavier than spec?
I've worked on Holleys Carters and Rochesters, but not Zenith, so that's MY disclaimer.

i've workd ona lotus also a Stromberg, (not the carb tho) but i'll come to my point..

it's possible you're bench tesing it upright totally vertical, the one i seen mounted on the lotus wasn't it a totally upright position. (altho it's possible this doesnt matter) but it's possible