Carter carb problem

[monte]

Hi all ,I have just replaced my 650 Holley on my 302 clevo with a 500 CFM Carter AFB Compitition carb ,the 650 was too big .I fitted the Carter and ran the engine for a while was very happy with it ,throttle response and idle were perfect without any tuning however after leaving it overnight it seems to have no fuel in the bowl as it wont start without winding it over for a long while ,when you pump the throttle nothing comes out the jets until I reckon the bowl fills up then it starts and runs fine ,like you can leave it all day and start it again anytime but overnight the fuel seems to disappear somewhere ,no fuel in the manifold can it be returning back to the tank somehow ,that is my thoughts anyway ,any thoughts please

[enjenjo]

I have had similar problems with Carter and Edelbrock carburetors. this was on various 454,350,351,402 and 302 motors. Outside of this I have had no other problems with them in 30 years. The only real solution I have found is an auxiliary electric pump with a primer switch when using a mechanical pump, or a priming circuit on electric pumps. I used to use a fuel filter with a built in check valve but they have become unavailable in the last few years. They helped but were not a final solution.

[FRANK BASILE]

Must be a design issue? . How are these constructed ? I can only think of the accelerator pump being perhaps higher than the bowl ,fuel evaporating from the bowl will leave nothing for a first start unless you crank the daylights out of it. Unless fuel is actually physically leaking out from the bowl , drain back via the fuel line would be inhibited by the needle and seat.

[46 deluxe]

I had the same issue with twin edelbrock carbs on the nailhead in my 36 coupe .
As enjenjo said a fix is an electric fuel pump , or there is an option of a anti drain back valve in your fuel line .
I never bothered as it was only on the first initial startup of the day , after that it was fine .
If you have a mechanical fuel pump on your car , it may be the valves in the pump as well not functioning properly ?

[ramp]

I had this happening on a 2 barrel Carter on my CM Valiant. Put an inline electric pump just for priming. Operated by a hold for on, push button switch. 20 sec prime and she would start ok. Roger.

[monte]

I just bought a 3/8in inline non return valve so I will fit that and see if it improves ,never had this problem with the holley so it must have something to do with the carb design ,thanks for the replies and i will keep you informed

[FRANK BASILE]

I would differ with the drain back emptying a bowl. If this is an inherent problem then I would go the electric fuel pump. Bowl will fill quick, less cranking required.

[Chrisso]

I just bought a 3/8in inline non return valve so I will fit that and see if it improves ,never had this problem with the Holley so it must have something to do with the carb design ,thanks for the replies and i will keep you informed

I had the same problem with my 500cfm Edelbrock carby Monte and after setting the float level to the proper factory setting(miles to rich) it improved a lot but still did it after a day or two so like you i bought a couple of 3/8" non return valves and the problem is solved have NOT had a problem since I fitted it.

[Harv]

I'm with Frank, and struggle with the idea of backflow though Chrisso's experience seems to confirm it. Usually the cause is vented float bowls, where engine heat starts the evaporation and the bowl dries out over a few days. Less common in closed carbs as the vapour pressure is captured in the air horn/cylinders and inhibits further vapourisation.

Thinking it through, if backflow is the drama then

a) the needle and seat needs to be passing. Is the needle/seat design metal/metal, or viton (less likely to pass)?
b) if the needle and seat is not passing, then it is open. Could be that the float level is set waaaaay high and the fuel pump never gets there. Unlikely, as the carb would flood in normal service.
c) another potential for the needle and seat being open is that the car was run hard then shut down before the pump can catch up. Unlikely, unless the needle/seat was overly restrictive (and that would give dramas during WOT blats under load).
d) the float might be partially full of liquid, or of poor density compared to modern fuel (float sits low in the fuel). Small amounts of vibration during stoarge then make the needle and seat weep. I have seen this behaviour in Stromberg floats - vibration makes a huge difference to seating pressure on the test bench.
e) does the float have a helper spring, which could make the needle/seat waver around at the "full" level?

Cheers,
Harv

[FRANK BASILE]

Good points Harv. One thing that sways me from drain back emptying the bowl is that the inlet is normally pretty much above the max fuel level in the float . If the design of that carby has the pump up high, then this will drain back into a bowl that has dropped due to evaporation. Hence no "squirt" on initial start after a layoff until after some cranking has pumped fuel into the bowl itself and primed the pump. This is obviously corrected by using an electric fuel pump. Can the evaporation be relieved by better insulation spacing of the carby to the manifold?

[monte]

I fitted a 3/8 in non return valve in the suction line before the pump ,took the car for a 100 klm drive yesterday and it went like a bird ,but it seems to use more fuel than the 600cfm Holley anyway I left it overnight and tried to start it this morning but the problem was the same seems no fuel in the bowl as no squirts of fuel when the throttle is worked so after a while turning it over away it went ,Frank I have a 12mm spacer under the carb to keep it cool so I dont see evaporation being the problem but where the fuel is going is just beyond me ,I have ordered another non return valve to put in the pressure side of the pump near the carb ,think I will run it today for a while and in the morning will take the top off the carb and see if there is any fuel in the bowl ,there must be some logical reason for what if happening but its doing my head in

[46 deluxe]

Just as a precaution Monte pull your dipstick out and smell your oil and see if it smells of fuel .
A crook fuel pump diaphragm will let fuel run back into your sump .

[monte]

Just as a precaution Monte pull your dipstick out and smell your oil and see if it smells of fuel .
A crook fuel pump diaphragm will let fuel run back into your sump .

Thanks Scott ,I did that the first thing this morning and only oil in there

[FRANK BASILE]

Good idea to check the actual bowl level in the morning after a run the day/night before. I have a feeling that it will have naturally dropped giving drain back from the accelerator pump. Might be also pay to leave for another day without starting and re check, if the level has not dropped further, it fits the initial evaporation drop/accelerator pump theory and no external leakage. For the latter it would be physically seen evidence outside or somehow down into the inlet manifold . Good fun chasing puzzlers and frustrating Monte.

[Harv]

I've never worked Carters, but am very familiar with Strombergs. They have an idle air bleed, which is a small drilled passage in the carb throat. The idle air bleed mostly provides emulsifying air to the idle circuit, but it also has a secondary purpose. The idle circuit discharge is lower than the float bowl (every other circuit is up a lot higher in the carb throat). When the engine is stopped, the idle circuit will siphon out the float bowl into the carb throat. The idle air bleed stops the idle circuit from pulling a vacuum and siphoning the fuel out of the float bowl when the car is shut down.

How is the idle circuit in a Carter prevented from siphoning? If it is a small drilled passage like a Strommie, is the drilling full of coke/debris? A check of the factory manual and some careful probing with a piece of copper wire strand might check it.

Cheers,
Harv