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November 2010 Techline

Willie Worthy | Writer
Posted November 1, 2010

Jeep Quadra-Trac, Jeep Wrangler Rust and More!

'70s GM K-Truck Aspiration Woes
Q I'm inquiring about the age-old problem of carbureted vehicles dying on steep hills. I have a '77 Chevrolet K5 Blazer with the Carter AFB sitting on top of a 350, and I was recently wheelin' up a steep and winding trail when my truck died. I had heard about this happening, but it had never happened to me personally. I would like to know what I can do to my truck to get it to the top of the hill. What are the expensive and less-expensive options I have, and what are the advantages and disadvantages of each option? I have to get to the top of that hill!
Jon Crawford
Mesa, AZ

A Having an engine die when going up a steep hill is not fun. Even less so when you have to back down without power steering and power brakes.

As you found out, the Carter AFB is not one of the better carburetors for off-road use. It was, and still is, a fairly popular street performance carburetor. I wonder why this is on your engine and not the original Rochester four-barrel Quadrajet that all 1977 350 V-8s came with. The Rochester four-barrel is what's commonly referred to as a "spread-bore" design, with a large amount of difference in size between the primary and secondary throttle bores. The AFB is what is referred to as a "square-bore" carb, in that all four throttle bores are approximately the same size. If I remember right, the bolt pattern is different, so that means you also must have an aftermarket intake manifold; but then again, as I think about it, they may have dual bolt-pattern holes.

So let's start with making your AFB a bit better. The first thing you need to do is figure out why the engine died. Was it from too much fuel or too little? If it blew black smoke out of the tailpipe when you got the motor restarted, that's a pretty good indication that the motor died from too much fuel. There are a couple of things that you can do. Make sure that the fuel pump pressure is between 3.5 and 5 psi. Yes, you're going to need some type of a fuel pump pressure tester to do this. I suggest you obtain some type of a repair manual and read how to do this. You may want to do an Internet search under something like "AFB carburetors" for information, too. Used bookstores are a good source, as is your public library.

Excessive fuel pump pressure can force the float's needle off the seat and cause too much fuel to enter. You also need to check the float level. This will give you a chance to check the condition of the needle and seat assembly. If there is any question as to its seating ability, then replace it. If the float is set too high or the fuel pressure is too high, then when the vehicle is on a steep hill, the fuel can flow back through the vent tube and flood the engine. You might even want to slightly lower the fuel level. We're talking increments of 1/16 inch here. The float is kind of a pain to adjust due the metering rod design.

Could it be that you actually ran out of fuel when on the steep hill? Either from the fuel being a bit too low and the pick-up tube uncovered, or the fuel pressure from the pump too low? In this case, the engine should have restarted without the black smoke.

If none of those measures help, then I really suggest you sell the AFB for what ever you can get for it to some street rodder and go back to the original-style Quadrajet. These are quite common on any of the '70s GM vehicles, so it doesn't have to come from another pickup. Just match the cubic inches of the motor as a general guideline. There may be some minor differences in throttle linkage. Keep in mind that the carb is not the easiest to rebuild and there are several updates that need to be made, so again I'd suggest taking a look on the Internet for these tips or using a rebuilt unit. They work quite well off-road, and in fact I use one on my own 4x4, as it works almost as good as a fuel injection unit. (Keep in mind that I said almost as good.) I believe the fuel pressure for it should be somewhere around 7 psi.

As an alternative, Holley's Truck Avenger, the 670cfm version (p/n 0-93679), is an excellent off-road carb if you want to buy something brand-new. I have used one, and can say I have been impressed with its performance. You will need a new intake manifold with the proper bolt pattern or an adapter. You will also need to make some changes to the fuel lines and linkage.

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