VCCA Home Forums TECHNICAL - 1945 AND NEWER 1971-1979 troubles w rough running 350 chevy

Guys I have a 350 chevy in my old coupe I have had issues with it running rough after about 3 or 4 miles. I just put new fuel lines on it today and a new filter. I sprayed carb cleaner through the new hoses and blew them out with air before installing them. The carb was just rebuilt two days ago and set and adjusted it runs and idles great sitting here even at running temps and driving for around 3 miles, it's after it is driven 3 or 4 miles that it starts acting up every time this has gone for a short while now. It acts almost like a plug wire has slipped off a plug and it has not done so. I had that happen with a previous set of wires.

This is the third set of wires on the car now(The original ones were old looking, the second new set got burnt on the headers so I built a nicer thicker custom fit set for it now) keep in mind I have driven this car less then 250 miles total since owning it. It has been doing this with the last two sets of wires on it. I have replaced the fuel pump, fuel lines, filter, rebuilt the carb Holley 600, new plugs, wires, cap, rotor, coil, had timing checked,The whole car has been rewired I had a parts store check module they said it checked ok.No vacuum leaks.... What else can be going on after the car has been run and driven for say 5 or 10 minutes? I have owned the car for around 6 or 7 months now it hasn't always run like this. Thanks for any help.

You might want to check the voltage to the distributor, sounds like you have an HEI system as you mentioned you had the module checked. You should have a good 12 volts to the coil all the time. You may also want to check the wires on the pole piece in the distributor.

Where does the power actually come from to the dist.... the starter? I had the car rewired not long ago and also a new starter and new alternator plus the new coil has new wires with it if that is the ones you are talking about. I will get a friend to come check the voltage to the dist and see what he comes up with.

What would cause it to only lose voltage after so many miles of driving? I just came home from supper with the wife and started it up to pull it in the garage after sitting from earlier and as usual she ran fine and revs up nice. She was cold though and hadn't been run in hours. Thankds for the reply!

When an engine begins to run poorly after it heats the most likely cause is electrical. It can be a loss of good ground, something expands to produce a short, resistance goes up or something similar. The problem can be anywhere from battery terminal, through: ammeter, ignition switch, coil, condenser, points, distributor cap, rotor. Finding the actual problem can be a challenge. Fuel can also have heat related problems though not as likely. Good luck finding the problem.

The coil should get voltage from the ingition switch, and if you do have HEI make sure there is not a resistor in the wire to the positive side of the coil, it needs full battery voltage. A good way to find out if it is a fuel or ingition system problem is to give it extra fuel when it is running bad. Use a propane cylinder,( like for a small torch) rig up a hose and run to the air cleaner and drive. When it acts up open the valve and if it runs better look at the fuel system, no change the go to the ingition system.

If you have hei check the plug that goes in the distributer cap they are known to lose contact with temperature rise and cause poor running and at times total failure.
Al

Thanks for the replies guys. I replaced the dist with a whole new dist and that solved the problem. It had to either be the module was checking good but messing up when it got hot or the coil pick up I think it's called in the dist going bad. Everything else was new. Anyway it ran great on the test run this time. Thanks for all the ideas.

I had MANY issues with my HEI modules going bad. The car would run fine and once in a while the car would simply die. It happened so much that I could change the module in 5 minutes and then be back on the road. I installed a ballast resistor but then changed the entire distributor.

I've had many HEI OEM distributors.NEVER had an issue with any of them.The reason the ign modules would fail on HEI's in mid 70's is not enough di-electric grease was applied under the module.Heat is the biggest enemy of the module.I currently own 3 B-Body HEI Chevy's,never had/have any dist issues.Every 2 years I clean off and simply re-apply fresh di-electric grease under the module and under the coil.My experiences. Jim