AFB Carb (C1 and C2) Runs Dry

[Christopher R.]

After sitting for a few days, there's no gas in the bowl of my AFB carb. I know this because no gas squirts into the carb throat when the accelerator pump is worked. After cranking for a several seconds, the bowl fills; the accelerator pump squirts; and, the car starts. Don't know exactly how many days. It'll start after 1 day. But not after sitting all week. I don't know where the gas goes. I'm hoping it evaporates.

1. Did people put up with this condition in 1962 to 1965? I would think there'd be lots of dealer complaints. It would not be uncommon for a sports car to sit Monday through Friday, and only get used on weekends. My car always starts. But it takes a fair amount of cranking before the fuel pump puts enough gas in the bowl. This condition would concern a customer. Heck, I know why, and I'm still concerned.

2. There's no gas on the manifold. So I'm assuming the gas evaporates, because the design of the AFB is such that the gas can't leak down into the engine. Is this correct?

[Clem Z.]

Re: AFB Carb (C1 and C2) Runs Dry

when you park the car after a run quickly remove the carb top and see what is happening in the float bowls,see if the fuel is boiling. the heat can cause fuel to be pushed out the discharge nozzles or cause it to evaporate but once it cools down it should lose no more fuel thru evaporation so what happens after the first day i have no idea.

[Joe R.]

Re: AFB Carb (C1 and C2) Runs Dry

Chris,

Clem has the idea but why is it happening? I am going to assume you have a 62 300 or 340 327 that is stock in every way. Stock means it should not have a boiling problem. The 300 had a phenolic block and stainless gasket to keep the cast iron intake from heat soaking the carb. The 340 had the aluminum intake and was assumed by chevy that the intake would insulate the carb from the heat of the heads. Normally it does.

Somehow, you or a previous owner has defeated the system designed by GM engineers that works very well.

Please let us know which HP version you have and let's work from there.

Regards,

JR

[Mike McKown]

Well, JR. I have a '63 300 hp

It does the same thing. If you park it overnight, it will start quickly the next morning or even two-three days later, instantly. If you let it sit for a couple weeks, you will crank the engine until the fuel pump pumps the bowl full enough get get the accelerator pump to work before it will start. Takes a fair amount of cranking to start.

I never worried too much about it. Just figured the fuel siphoned back through the line to the pump. Am I wrong?

Actually, I don't know what causes this. Just know it does it. I have seen several inquiries on this forum for this problem but no solutions.

[Stephen W.]

Re: AFB Carb (C1 and C2) Runs Dry

For whatever its worth... I have a 64 c2 with l-75 and when it sits I have to crank it to fill the bowls up with fuel.
I really didn't think too much about it because I remember my dads 1966 Buick GS with a carter AFB. If it sat for more than a day or two you had to crank it to fill up the bowls with fuel. He bought the car brand new. The dealer told him it was normal.

[Gary Bishop]

My `62 is the same. *NM*

[Joe R.]

Re: Well, JR. I have a '63 300 hp

Mike,

OK, just thought he was talking C-1. Must not have read the subject very well but anywho----

Had a 300HP 63 also and had NO problem with starting days later. My 63 had a phenolic spacer between the intake and the carb insulating it from heat soak. I suspect that is going on with your 63. Do you have the spacer and the stainless gasket?

Regards,

JR

[Bill B.]

Re: Well, JR. I have a '63 300 hp

I have a 65, 327, 300 HP coupe that does exactly the same thing. Mine has the phoenalic block and aluminum gasket installed. 1 or 2 days after driving, it starts right up but let it sit for the better part of a week and it has to have the fuel bowl filled by cranking and pumping the accelerator. I always assumed that was normal for that type carb. I'd like to know if it's not and what I could do to fix it. Thank you in advance, This board is great.

Bill Bonnichsen

[Mike McKown]

Re: Well, JR. I have a '63 300 hp

Joe Ray:

I don't have the car handy so I can't tell you what is under the carb but I don't think it has the phenolic block. I don't think '63's had this. In any case, I have seen several posts on this subject and no one has come up with an answer. In any case, if the carb percolating after shutdown was the problem, it probably wouldn't start two days later, let alone a week later.

I don't know what the problem is. Always wondered. But, to me, it's just a minor inconvenience.

[William C.]

Re: Well, JR. I have a '63 300 hp

All AFB 327 chevy's with a cast iron manifold used the insulator and baffle. Lack of a baffle will cause the aluminum bottom of the carb flange to eat away, VERY not good.

[Joe R.]

Re: Well, JR. I have a '63 300 hp

Mike,

Yes, all 300HP had the phenolic block to insulate the carb from the cast iron intake along with the stainless steel shim (gasket). If the block is not there that could cause the problem. If the block is in place I would block the heat riser passage to further keep the heat away from the base of the carb. This should help or eliminate the problem of boiling the gas out of the carb.

Regards,

JR

[RICK CHARLES]

63 327/340

i'am owner of a 1963 327/340 with aluminum ( 129 ) intake doing the same thing as the ( 300 hp. ) with cast iron intakes. would putting a cool can or an electric fuel pump solve this problem?

[Timothy B.]

Re: Well, JR. I have a '63 300 hp

My 1963 300 HP does the same thing and the gasket,insulator and shield are in place. I am wondering if there are lead plugs in the carburetor bowls and from age and repeated hot cold cycles are seeping gasoline into the manifold. I have decided to plug the small heat passage holes in the top of the intake and see if that helps.

I just did this on my 1967 327/300 cast manifold and upon inspection of the heat shield there was a burn or rust through spot and the engine had started to suck the hot exhast into the intake. There was a black soot tel-tale left behind for me to see. I decided not to change the operation or flow of the heat riser as the cast manifold and carburetor needs this heat to function properly.

[Christopher R.]

Re: AFB Carb (C1 and C2) Runs Dry

Heat's not the problem. Heat generated by running the engine would cause the problem to manifest itself right away - right after running the engine. No, this problem is a function of time. The car will run a day or 2 after operation. But it won't run after a week. This problem seems to only affect AFBs. It doesn't seem to matter whether the intake is aluminum or steel or whether the phenolic gasket/spacer/insulator is in place.

What could happen to the fuel in the bowl over time? It could only leak out or evaporate. If it evaporates, you don't see it. But you would get that gas smell, especially in the garage, that people complain about with these cars. If it leaks out, it could only be inside or outside. If outside, the manifold, you'd see it. So, that leaves inside, draining down into the engine.

Is there a passage from the fuel bowl to the manifold that could leak?

[Clem Z.]

Re: AFB Carb (C1 and C2) Runs Dry

when the engine is running the cool gasoline is comming into the carb to keep down the temp. the only cure i have found for gasoline boiling in the carb after shut down is the block the heat riser crossovers in the aluminum intake and to plug the 2 holes in the cross over under the carb on the cast iron intake. GM elimiated these 2 holes on later cast iron intake. to check the carb for leaks.remove it from the engine,remove the top and set the carb base on a sheet of clean white paper and fill the float bowls with lacquer thinner. let the cab set for a while and then check the paper for wet spots.