Mechanics

TRUCK CUTTING OUT

1981 GMC Other

A few months ago, I purchased an old truck that had recently went through an engine swap. From a diesel engine to an oldsmobile 350 5.7 Liter I believe.

After driving it the first 2-3 months it ran great. Over the past month it started developing a problem that when the engine was at normal operating temperature, the truck would start stalling and cutting out. First thought I had was the plugs were fouled.

So after replacing the plugs (gapped to.040), wires, distributor cap, rotor, ignition coil, fuel filter and fuel pump I still have the problem. The other day I was on my way home from work and about 10 miles into my drive it cut out completely and died. After fighting it for 20 minutes and not getting it to start, I had someone haul me and my recently embarassed truck home.

Right now, when it basically starts cutting out, I mean its acting like it's being fuel starved, then a surge of fuel will pick up and the truck will resume, but after 1-2 seconds, it will cut out again. This seems to only happen if the truck reaches RPM's in the 3,000-4,000 range. If I drove to the store down the street, it more than likely would not do this.

I'm starting to suspect that the carberator may be to blame here, and thats only because I think I've eliminated any other possibilities. I've read that something called the needles or float may be to blame, but I have no idea what this is. I do know that a rebuild kit costs roughly $20 for a quadra-jet carb, and I need to do something about this ASAP.

My question is. Is there any way to know for sure? I'm looking at rebuilding the carb, but I've never done this so I know it will take some time for me to do it. I just want to make sure that this will more than likely take care of the problem. Or am I missing something thats not been looked at yet?

Thanks!
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MDewell
April 3, 2007.




Just an update here.

I think I figured out the problem. The carb.

I went and purchased a 20.00 rebuild kit for the rochester quadrajet, and only after breaking 2 needles did I realize I should have just bought a rebuilt carb.

So, after running back and forth tracking down a carb, I replaced the entire thing. Roughly 120 bucks for a rebuilt rochester at your local autozone. Not bad.

I got the carb installed, adjusted the throttle cable and suprisingly, the truck fired right up. I was impressed.

I took it for a test drive and noticed a few things.

1) The carb seems to load up on heavy acceleration, in other words, you can tell it's getting ALOT of fuel. I haven't adjusted any screws as of yet, as the autozone guy said that it's not needed. I think he might have been wrong here. Is there somewhere I can go to see how to go about adjusting the mixture screws? I know in the back of my head it feels as if it's running either too rich/too lean and needs to be adjusted. When I floor it, you can tell the truck wants to move, but the engine won't let it, almost like it's getting too much fuel. When the gas pedal is floored, It almost feels like it's bogging down with fuel and then catching up and surging ahead. If that makes sense.

2) Choke: The old carb had a thermatic choke control on it. The newly replaced carb does not. The autozone guy said I could either attach a choke cable like old trucks, or in the summer time just zip tie it so that it's not operational and in the winter just cut the zip tie? Does that sound about right?

Thanks for the help. If I can get some replies, I'll donate : -)

I've spent the better half of the past week reading on some articles and this is a great place to learn something. Keep it up.
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Tiny
MDewell
Apr 7, 2007.
Hello, most carbs will run pretty good right out of the box but some fine tuning could help. You could start by getting the idle mixture set right by using a vacuum gauge hooked up to full vacuum. Adjust your idle screws- there should be one on each front lower corner of carb- to get the highest reading. Then re-set idle speed to specs. Bogging down when you hit the throttle may be caused by a few things, timing being not advanced enough often does this, Myself I would get the choke cable and hook it up, don't think they are very costly, you can leave open in the summer but will thank yourself for doing it come winter. If motor is stock you should be able to get the specs GM dealer and go from there. Good luck.

Tiny
Burrbrian
Apr 8, 2007.

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