When the car is driven for awhile and then turned off, there is gas dripping into the secondary venture cluster for 4 or 5 minutes. I've got the floats set at 7/32 and 3/4". Anyone have this to happen and how to fix, thanks
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Re: Gas dripping into the secondary venturi cluster on An AFB carburetor, why
Chet, I don't carb the factory spec.s in front of me but I had to lower the floats on my AFB by about 3/32 from the factory spec.s seems the ethonal will expand slightly after shut down, and overflow onto the blades and then out the shaft onto intake Its not a good thing to have happen on a hot engine.New England chapter member, 63 Convert. 327/340- Chapter/Regional/national Top Flight, 72 coupe- chapter and regional Top Flight. -
Re: Gas dripping into the secondary venturi cluster on An AFB carburetor, why
Chester,
I use a 5/16 drill bit to set the float. Are you sure the gasket on the seat is sealing. No problem when it's cold?Comment
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Re: Gas dripping into the secondary venturi cluster on An AFB carburetor, why
Could be percolation. Drill bits are great ways to set floats I use them on AFBs and WCFBs...Comment
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Re: Gas dripping into the secondary venturi cluster on An AFB carburetor, why
The factory specs, according to the 1963 corvette shop manual, states the primary float level is 7/32" and the secondary is 3/4". Now when you say 3/32" lower what do you mean. Subtract 3/32" from 7/32" and 3/4".Comment
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Re: Gas dripping into the secondary venturi cluster on An AFB carburetor, why
The 7/32" dimension is the level setting of the float measured between the float and the top plate (upside down). The 3/4" dimension is the float drop, also measured between the float and the top plate only with the top plate right side up. Then you knew that. Just testing us, right?
Stu FoxComment
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Re: Gas dripping into the secondary venturi cluster on An AFB carburetor, why
Chester,
Also keep in mind the fuel pressure, It's probably not your problem but excess pressure could hold the needle of it's seat a small amount. Are all the fuel lines routed like the factory so they don't get any hotter then necessary.Comment
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Re: Gas dripping into the secondary venturi cluster on An AFB carburetor, why
The '63 shop manual float level setting of 7/32" is specfied to be measured without the gasket, and modern gaskets may not be the same thickness as original, so that could be an issue. I believe I've also seen slightly different specs from other sources including the instruction sheets that accompany zip kits.
It could be a leaking needle/seat valve. I think Stu Fox has had issues with certain types. You should search the TDB for his AFB related posts. There are a lot of them. Shake the floats next to your ear, and if you hear fuel sloshing around inside the float is leaking and must be replaced.
I recall rebuilding my AFB years ago and the zip kit had a needle with a neoprene tip, which leaked and caused excess fuel to enter the bowl after shutdown from residual fuel pressure. I put the original all metal needles/seats back in and the problem was solved.
It is important to understand that the higher the measured set point, the lower the fuel level in the bowl, so suggest you reset the floats to about 1/4", which should lower the fuel level by an equal amount, assuming the floats and needles/seats are okay. If that doesn't work increase the setting another 32nd.
The only problem that can occur with too low a fuel level in the bowls is lean surge or stumble in the transition from the off idle system to the main venturi system.
Carburetors are rather crude devices and having to fiddle with them has always been part of vintage car service. Be glad that you have an AFB instead of a Holley or, heaven forbid, three Holleys.
DukeComment
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Re: Gas dripping into the secondary venturi cluster on An AFB carburetor, why
I'm going to reset the float using a 5/16" drill bit and see what that does. The shop manual in the carb section states to do that but at the end of the book where the specs are it states to set it at 7/32". All the lines are routed like original and the fuel pump puts out 5 1/2 pounds. According to some guy on ebay you should not have more then 3 pounds. Can you still get the metal needles/seats. Bob Kunz rebuilt my carb so I'm assuming the float valves are good. They are the neoprene tip design. Could I have a heat issue. What about plugging the exhaust crossover, would that help.Comment
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Re: Gas dripping into the secondary venturi cluster on An AFB carburetor, why
What does Bob Kunz recommend? How and to what spec did he set the fuel level?
Wiring open the heat riser valve is a good idea. It will reduce percolation. Blocking it can cause other issues like poor warmup driveability due to lack of sufficient heat to vaporize the fuel, which can cause start off stumble for some time after the engine reaches full operating temperature on the gage.
DukeComment
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Re: Gas dripping into the secondary venturi cluster on An AFB carburetor, why
Chester,
5 1/2lbs pressure is fine, call Bob Kunz and tell him what the problem is and see what he say's. I have spoke to Bob before and he really knows his stuff with these AFB carburetors.Comment
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Re: Gas dripping into the secondary venturi cluster on An AFB carburetor, why
May not make any difference but it's leaking out the primary and not secondary venture.Comment
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Re: Gas dripping into the secondary venturi cluster on An AFB carburetor, why
The correlation between the float setting and the fuel level is dependant on the float having the correct amount of buoyancy. It is not an adequate test that it just float. Older synthetic floats (or unknown age) should always be suspect. A brass float with any visible damage should be suspect.
The dent in the side of this float
SAM_1484.jpg
was all that was visibly wrong with it (no liquid inside), and it floated, yet it had lost sufficient buoyancy that it would overfill the bowl when set to spec. Actually would over fill even when set way under spec.
Keeping up with float adjustments with a float losing buoyancy can be a long drawn out affair.
Lowering the fuel level (not float level) in the bowl leans out the fuel system. That's not good and can lead to multiple driveability problems. It should be at spec.
Reducing the heat in the induction system by elimination of the heat riser and cross over functions also has the effect of leaning out the mixture. The heat is there to ensure full vaporization of the fuel and a constant air density. Fuel that doesn't vapourize doesn't burn and may as well not be there. Colder air requires more fuel. That's why we have chokes. Fuelie cars don't have heat risers because they have a better way of vapourizing fuel. They inject it under pressure which atomizes the fuel much,much better so air heated only to cooling system temp is all that's needed. Unless you've come up with a better way to atomize the fuel, you still need the heat. Others have reported having to raise the fuel levels in the bowls (richen the mixture) to get their engines with no heat to the induction to run half decent.
Heat isn't your problem. Fuel, as with most things, expands when heated. It always has and that was anticipated in the design of the carb. The fuel makes it's way over the spill point when it heats up and expands because it's sitting to close to that spill point. The ethanol may have a higher rate of expansion than gasoline, but in that it makes up less than 10% of the volume, it won't be enough to require a fuel level change from specified.
SteveComment
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