afb carb problems

[Phil A.]

After my 340/327 warms up, and not until then, the primary venture cluster on the starboard or right side dribbles fuel. Does anyone know what the cause could be? I sure could use the help. Thanks Phil

[Mark E.]

Re: afb carb problems

This describes nozzle drip, which is fuel coming down from the main nozzles at idle speed. I've never worked with AFBs, but this is a common issue with Quadrajets.

Another symptom of nozzle drip is little or no change in vacuum or RPM when the idle mixture screws are turned in (leaner).

The direct cause is the primary throttle plates are open too far, activating the main fuel circuit. Some root causes can be:

- Idle speed too high
- Problem with the idle circuit
- Problem with the idle air bypass system
- Aftermarket cam that requires a different idle circuit calibration (due to low vacuum)

What happens is that if the idle circuit isn't working properly, the idle speed screw needs to be adjusted up to compensate. This engages the main fuel circuit at low rpm resulting in nozzle drip. It isn't noticed as much when the engine is cold because the choke is in fast idle mode.

There might be other causes with an AFB, but these are the common ones for Q-Jets.

[Mark E.]

Re: afb carb problems

A couple more things to check with nozzle drip:

- Check your float level, and for a saturated or leaking float. A too high fuel level in the bowl might make the main circuit too rich.
- Check your power system. If it is stuck in the rich mode, that also might make the main circuit too rich.

[Edward J.]

Re: afb carb problems

Phil, a lot of the root causes of AFB fuel dribble is a float just a bit to high, I have found that with ethanol fuels the fuel level will rise slightly when shut down when hot. I have been through this with quite a few 63 AFB carbs. just lower the float level in Carburetor about 2/32ths. If the car runs fine other wise this should fix the problem.

There can also be a problem that can happen and that is long crank times when hot (without the fuel drip issue), and its a vapor lock kind of issue. I have not found any quick fixes for this issue, My 63 has had both issues and fixed the fuel drip with lowering the float. vapor lock issues I cannot find a fix. I have tried wiring the heat riser- did not fix. I will some day try blocking the heated cross over on the intake manifold.

[Frank D.]

Re: afb carb problems

Wiring the heat riser open, blocking the intake heat runners with an F/I gasket and adding a 1/4" phenolic spacer under the carb will fix the percolation issue in 90% of the cases -- it did for me...

[Steve G.]

Re: afb carb problems

Carbs have a "spillover" level, a point higher than the level of the fuel in the bowl that fuel must reach to enter the throat. Pressure differentials between the nozzles and the top of the fuel in the bowl cause fuel to flow above the level in the bowl. If this weren't the case gravity would cause fuel to run out the bowl all the time.

Gasoline, with or without ethanol, expands as it heats. Just like antifreeze, bridges, railway tracks.... It always has and the rate of expansion of today's fuels isn't much, if any, different than fuels of yesteryear. For that reason carbs were designed with a "cushion" space between the fuel level in the bowl and the spillover height. When fuel starts to spill it is because the level of the fuel in the bowl has crept up, usually due to float and/or needle and seat issues, and is encroaching into this "cushion" area. When you shut the engine off, or when it heats up, the fuel expands enough to reach the spillover point and flows down the throat of it's own accord.

So you can fix it by either correcting the fuel level or reducing the the amount of expansion of the fuel. To reduce the amount of expansion you would reduce the amount of heat to which the fuel is exposed. Wire back the heat riser and plug the cross over. Of course this adversely effects the vaporization of the fuel and the consistency of the air density which those items were designed to address.

Or, you could fix the problem and correct the fuel level and the cause of the incorrect fuel level.

Percolation is something totally different and exists primarily on this board. Percolation is boiling of the fuel in the bowl and manifests itself as white fuel vapour exiting the bowl through the vent and richening the mixture at idle as it enters the air stream in the carb throat. This was known to happen on sweltering hot days when the mercury would approach 100F. According to data from the fuel industry and those of us that have stared down the throats of these carbs for the last 40 years, it is far less of a problem today than it was when it was at it's worst in the 80's. And it didn't have to do with engine temp, it had to do with the temp the fuel was leaving the tank.

Likewise with vapour lock. This, again, was something that happened on sweltering hot days when the low pressure on the fuel caused by the fuel pump pulling it from the tank would cause the fuel to vapourize rather than pump through as liquid fuel. This resulted in fuel starvation and a no start condition. Also almost never seen today. The fuel industry sites fuel temps of 200 F as the threshold for percolation and vapour lock to become a problem.

If your float level is set accurately you want to look at the reasons that a correct float setting is yielding a high fuel level.

Steve

[William F.]

Re: afb carb problems

Steve,
Nice to the point, "just the facts, maam," discussion, dissection of various carb problems.