Running rough and stumbling

[Dmax21]

I have a k20 that's been stumbling pretty bad when I get to around 60mph and yesterday it started doing it at any mph and I barely made it home, but the truck starts and idles fine and revs pretty good, it seems like the problem is while under load. The truck has a 350 with a Holley carb and been updated with a hei distributor and I recently just changed the plugs and plug wires. I've only owned the truck for about 2 months. What do you guys suggest I start with or maybe someone knows exactly what it is.

[Gregski]

same thing happened to me also after plug and wire change, so I know you say it idles fine, but please, double and tripple check your firing order, ie do the plug wires go to the right plugs, I had two wires flip flopped

[geezer#99]

Your fuel pump might be almost dead.
Pull the line off the carb and put it in a jar. It should pump and fill a jar quickly.

[Dmax21]

I will check the firing order but I'm sure that's not it because I changed them 1 wire at a time.

[Zach079xxx]

Mines doing the same thing. Check your timing or you may just need a carb adjustment

[Gregski]

Originally Posted by Dmax21

I will check the firing order but I'm sure that's not it because I changed them 1 wire at a time.

So did I, or I thought I did. ha ha

[LBJ]

I just went thru some of this on my 77 Bonanza. Check the rubber fuel line between the chassis and the fuel pump to see if it has a tear or hole. It can suck air and the engine will stumble. Perhaps a dirty fuel filter as well. As a last check, see if your gas cap is allowing air to enter. It has a one way valve that lets air in, but not the fuel from sloshing out. If it won't let air in, you eventually will build up a vacuum that the fuel pump won't be able to overcome, usually after driving a while and under acceleration or more demand on the fuel pump. The more gas in the tank, the faster the vacuum will build up. Worth a look. But since you just worked on the distributor, sounds like a crack in the distributor cap. Double check the things you "fixed" first, as that's your most likely point of failure. ha. You can put a load on the engine in the driveway by putting it in gear, standing on the brakes or using the parking brake, and giving it some throttle pedal. If it's going to miss, it will usually do it like that. If it's a standard shift, guess you'll have to get out on the road to test. Ha. One more thing that happened 30 years ago was that the centrifugal advance weights got rusted and would barely move. Truck would barely run and accelerate. This is fairly unlikely to most as I was in the habit of washing my engine down every time I washed the truck. Rusted right up. Ha

[Dmax21]

Originally Posted by LBJ

I just went thru some of this on my 77 Bonanza. Check the rubber fuel line between the chassis and the fuel pump to see if it has a tear or hole. It can suck air and the engine will stumble. Perhaps a dirty fuel filter as well. As a last check, see if your gas cap is allowing air to enter. It has a one way valve that lets air in, but not the fuel from sloshing out. If it won't let air in, you eventually will build up a vacuum that the fuel pump won't be able to overcome, usually after driving a while and under acceleration or more demand on the fuel pump. The more gas in the tank, the faster the vacuum will build up. Worth a look. But since you just worked on the distributor, sounds like a crack in the distributor cap. Double check the things you "fixed" first, as that's your most likely point of failure. ha. You can put a load on the engine in the driveway by putting it in gear, standing on the brakes or using the parking brake, and giving it some throttle pedal. If it's going to miss, it will usually do it like that. If it's a standard shift, guess you'll have to get out on the road to test. Ha. One more thing that happened 30 years ago was that the centrifugal advance weights got rusted and would barely move. Truck would barely run and accelerate. This is fairly unlikely to most as I was in the habit of washing my engine down every time I washed the truck. Rusted right up. Ha

The gas cap sounds more like it cause I had just filled the tank up and also when it did it before it seemed like the more I drove it the worse it get, what do I need to check? Do I loosen the gas cap and drive it or what?

[LBJ]

I tested my old locking (STANT brand) gas cap by testing it with a vacuum pump to see if it allowed air to come into the rubber diaphragm/valve. You can "suck" on it and see real fast if that is working. Or, remove the old gas cap, get some real heavy duct tape and seal off the fuel nozzle opening and punch a small vent hole in it and then drive. Try not to make too many fast corners or you'll get spillage. But, you will be able to see if it runs or not. Mine took about 10 minutes to build up enough vacuum to kill the gas flow, with an almost full tank, with the old gas cap. Or, just buy a new gas cap and test it out. Gas caps aren't too costly and won't hurt to replace it anyway. That's the least trouble and most likely source of the problem. I start with the easiest solutions first, usually, but when it happened to me, I never thought of the gas cap being faulty. ha. Wasted LOTS of time and money on a new fuel pump, gas filters, fuel line, and a carb kit (never installed it though) until a friend suggested that fix.

[Dmax21]

Originally Posted by LBJ

I tested my old locking (STANT brand) gas cap by testing it with a vacuum pump to see if it allowed air to come into the rubber diaphragm/valve. You can "suck" on it and see real fast if that is working. Or, remove the old gas cap, get some real heavy duct tape and seal off the fuel nozzle opening and punch a small vent hole in it and then drive. Try not to make too many fast corners or you'll get spillage. But, you will be able to see if it runs or not. Mine took about 10 minutes to build up enough vacuum to kill the gas flow, with an almost full tank, with the old gas cap. Or, just buy a new gas cap and test it out. Gas caps aren't too costly and won't hurt to replace it anyway. That's the least trouble and most likely source of the problem. I start with the easiest solutions first, usually, but when it happened to me, I never thought of the gas cap being faulty. ha. Wasted LOTS of time and money on a new fuel pump, gas filters, fuel line, and a carb kit (never installed it though) until a friend suggested that fix.

thanks for the help and I will try this out and see if it fixes and report back. I would have never guessed that a fuel cap could cause such a problem.

[geezer#99]

Easiest way to check if it's the cap is take it for a longish drive, get out and open the cap slowly.
If you hear the sound of sucking air, you've found the culprit.

[LBJ]

Well? Fix it? Give us a report! haha