454 with dual tanks cutting out

[John House]

My '74 C20 has been dying on me out of the blue. Once while getting on it pulling out on a 4-lane while pulling a trailer, about 5 seconds into it, maybe 55-60 MPH. Second time in town pulling away from a stop, going maybe 25 MPH. It just suddenly and completely dies. Both times I pull over and crank it, it restarts after cranking a few seconds, runs a couple seconds, then dies, then crank again and after a few seconds it catches and it's OK. Under normal circumstances a restart fires immediately. It has the factory 454 (mildly modified with cam, head work, intake, double pumper) and dual tanks (also HEI), and I'm wondering if that factory tank switching valve might have a problem? It seems to me to be losing fuel prime or something, because it restarts after a short cranking period then cuts out again, then repeat and it restarts and is fine, does not seem electrical, same pattern both times). I think each time it's happened on a different tank, the first one was when it was very low on gas, so I switched over, and after it ran thought I had just run out of gas and it was OK. But the second time I was running on the other tank and it was 3/4 full.

 

[1987 GMC Jimmy]

I’m not an authority on the valve, but I feel like if it was messing up, it would just stick to one tank, but I think it’s possible for them to stick mid-switch and/or get clogged up with trash. I’m not discounting it, but I’d check other things first. I don’t think it’s ignition related, and if you have a good vacuum reading, then I think it’s safe to isolate it to fuel. The only 454 I ever had had vacuum issues and would do a worse version of yours. I’d check accelerator pump function by looking down the carb, moving the throttle, and watching the two streams of gas. They should be strong and begin the moment you move the throttle. If they’re sloppy to react or drippy, the diaphragm needs replacing. You mentioned getting on it one time. Did you get on it again the other one? It could be the secondary circuit acting up. You’ve also got the fuel pump. You could try a volume test. I’ve been told it should fill a 20oz bottle in 8 seconds. If the stream looks weak, I’d try it out of a can, and if it still looks that way, I’d replace the pump. Fuel filter good? All that leaves is the valve and a potentially dirty pickup, but I don’t think it’s a dirty sock.

 

[John House]

Also, it does have an aftermarket HEI installed by a speed shop when they upgraded heads & cam, and they made one electrical mistake by hooking a column tach up to battery 12V and it drained the battery, so now I'm wondering if the HEI was hooked up to the old resistor wire feeding the factory points distributor, could that cause these symptoms?

 

[John House]

I just checked ignition voltage and it's good, 12+ volts right off the fuse block IGN source, so that's not it.


The second time I was accelerating lightly pulling away from a stop in town, first time was accelerating heavy. In fact it's possible both times I was just letting up when it happened. couldn't say for sure. But the carb is new, internal vacuum 4150 Holley 750 double-pumper. Has an electric choke but it's doing OK other than having trouble finding the sweet spot on high idle. Vacuum is good, had a leak previously but not now, all vacuum modules tested OK (distributor vacuum advance, TH400 vacuum modulator, vacuum canister for internal climate controls, and master brake booster).

Only thing I haven't checked is fuel filter, it's a metal one you can't see into, so I need to replace it. I'm not even sure how old it is since I just bought the truck last year. But this is the first I've had problems with it, although I didn't drive it but a couple times over winter and then only in good weather.

 

[4WDKC]

could it be the floats adjusted to low in the carb? It would allow a low level of fuel in the carb and when on it with a double pumper suck the bowls dry faster than the pump can make up for?

 

[John House]

Float level has been checked, both bowls are correct. Only once would I say I was getting on it, the other time was just barely accelerating in town, going maybe 25 MPH.

Actually happened again yesterday. Wasn't accelerating, actually slightly decelerating, in town, going only 20 MPH. Bother of the last two times, in town, it happened very shortly after starting it up after it sat 20-30 minutes.

And I'm having issues with the choke, I installed an electric choke kit on the carb which was a manual choke carb. It doesn't want to adjust and act properly. In any case, it's like the fuel flow just stops, and I have to crank it a few seconds before it will fire again. Not sure where to take this, maybe check the mechanical fuel pump flow or something.

 

[4WDKC]

Float level has been checked, both bowls are correct. Only once would I say I was getting on it, the other time was just barely accelerating in town, going maybe 25 MPH.

Actually happened again yesterday. Wasn't accelerating, actually slightly decelerating, in town, going only 20 MPH. Bother of the last two times, in town, it happened very shortly after starting it up after it sat 20-30 minutes.

And I'm having issues with the choke, I installed an electric choke kit on the carb which was a manual choke carb. It doesn't want to adjust and act properly. In any case, it's like the fuel flow just stops, and I have to crank it a few seconds before it will fire again. Not sure where to take this, maybe check the mechanical fuel pump flow or something.

That was going to be my next question, followed by have you checked the fuel pressure?

 

[Derrick]

it could be a pump or filter if it is not the carb and it has low fuel pressure.

 

[John House]

My climate control head unit seems to be leaking vacuum when set on A/C, Heater, and Defrost. My guess is someone has swapped the vacuum lines on the manifold. I need to pull the instrument panel and check it out.