i have a 355, 268 duration 454 cam, weiand intake, edelbrock 750 man carb, headers, jegs dist, jegs wires, it ran ok for a while then strating running rough, would miss at cold idle then would get better, thought it was the auto choke on the 600 i had on it, so i got the 750 from a buddy, will barely start and misses, im totally clueless and am sicking of working on something that never runs, any help would be appreciated.  

Check and make sure the ignition module in your distributor hasn't gone bad, and hook a timing light up to the motor and see if the timing's where it should be. From there, check your plugs and plug wires. Also check the hold down on the distributor and make sure it isn't loose.  

i have put 2 dist in it, they run for a little bit, and then dont work anymore!  im very very confused and dont know what to do, the one was a msd dist, then a accel, now a jegs one and they have done the same thing  

Ok, when you say the distributor doesn't work anymore, what specifically isn't working? No spark going to some of the cylinders? The timing goes out of adjustment?
What do the plugs look like when you pull them out of the motor?

Also, when you pull the distributor out, what does the distributor gear look like? Any badle worn teeth?

I need some more specifics to help you diagnose the problem.  

the gear looks fine, and the plugs are too bad, some blackness thats about it, and i think that it could be going out of time but i dont think so, i think it just burns up the modules or something, maybe tonight ill pull the cap off and see if its out of time. but ur ideas would be apreciated  

If the plugs are black that means it's running too rich. Sounds like the ignition's not advancing enough, so it's fouling the spark plugs and that's why it'll run fine for a little while then start missing.

What heat range plugs are you using? This could be as simple a problem as too cold a plug in the motor. Do you have iron or aluminum heads?

Check and see if you're getting the right amount of voltage to the ignition module. If there's too much it might be frying the module.

You really need to do a full timing check on the motor. Check your base timing, then check advance timing and see what's happening. If you have vacuum advance, make sure it's working properly also, adn the vacuum line powering it actually has vacuum.  

the truck ran fine, then all the sudden it didnt, i put the new distributor in it, ran for a bit(cpl drives) then did the same thing as before i put the new distributor in, i bought a new distributor and the same thing happend. it has iron heads on it, and like i said it ran fine with the plugs that were in it and then all the sudden this starting happening, i dont see how it could be the plugs due to the fact that it runs after a new dist. then dies.!!!! HELP  

And the carb is what? How about fuel pump pressure?

What year is the engine and how old is it (miles). Is this a points distributor chassis with a HEI distributor, or an HEI chassis with a points distributor? Either way they wire the primary voltage differently and will have problems if simply mixed across generations. In the former the points voltage is too low for the HEI and they just get weaker and weaker till they quit; in the latter the full 12 volts of the HEI input frys the points.

This could be ignition or fuel. The suddeness makes me think fuel, especially with the black plugs. Holley's, and Edlebrock Q-jets or Carters each have their unique leaking modes as well as a float could be sinking, or line pressure too high.

The ignition timing could be off, but usually you can't get a light with a failure inside the distributor.

Cam could jump time that would put things running retarted which will foul plugs, engine may start eaisly but has no power.

Cam lobe or lobes wiped out, a fresh flat tappet engine without proper assembly lube on the cam will eat the shaft pretty fast, plugs on affected cylinders will be dry and sooty black, or wet with fuel and black, or oily black depending on how bad the cam is gone and how leaky the rings are.

Bogie  

like i said it ran fine for the longest time and now this, its an edelbrock carb, it wont start at all now, it used too, but if i put a new dist in it then i bet it would fire right up, its been rebuilt within the last 1000 miles and the cam was broke in properly, like i said it will run for a little bit then this will happen...so not sure, tell me what to look for. please and thanks  

and as for the dist its a HEI  

My thought is, if every time you install a new distributor and the truck runs fine for a little while then starts acting up again, something is frying the distributor.

If you've got one of the toasted units laying around, pull the ignition module and take it to the local parts store to be tested. If it tests bad, you're getting some sort of power spike or surge that's frying the unit, or the constant stream of voltage is just too much.

One more thing. You said it ran fine for a long time, no problems, then started doing this. Did you change anything, I mean ANYTHING, at the same time this problem started?  

i didnt change anthing like i said it just started happening.  well how do i test the module?  

OK let me go back to a basic question, how old is the vehicle, year of manufacture? Did it originally have points or HEI? Have you done any rewiring of the ignition switch or anything that would have changed where or how the distributor gets its primary battery voltage?

Speaking of voltage do you monitor it and is it in the range of 12.8 to about 14.5 volts. Higher or lower will indicate a regulator problem which eventually will take out the battery but in the short term be very hard on the HEI.

What about engine to chassis to battery ground. A loose or corroded ground will cause weird voltage surges which are hard on the electronics.

Polarity, probably not a problem but if the battery or the alternator are reversed power thru the HEI will be backwards and hard on it.

The suddenness points to a failure of something common to the vehicle in general and the HEI is taking the brunt of the consequences. This could be regulator as previously stated or a wiring problem either ground or power that's causing an over or under voltage problem with the HEI or a bad/intermittent ground which will act as if a switch is constantly being thrown and power will surge and electronic components really don't like that.

It's hard to tell you what to look for as diagnostics by long distance is difficult. The best we can do is to make suggestions and you get to go test them. Maybe we'll hit on something you haven't already tried, maybe not.

The more we communicate on this the more we can discard from the possibility list as you check things out. The more info you're giving us the more I think we're looking at some sort of electrical problem. The issue will be to isolate it and then correct it. As I said in an earlier reply, if there's an issue with the voltage source the HEI will be unhappy, too little, too much or surges really play havoc with electronic stuff. Same goes for too much heat, too much cold, or water dripping from the firewall onto the distributor, oil vapor getting around the distributor shaft and into the HEI, stuff like that just kills these things. Even incorrectly gapped plugs or too much or too little resistance in the secondary wires, a growing failure in the coil can be the source of the problems you’re experiencing. If you're reusing parts from an earlier HEI in the replacement HEI like the coil, cap, rotor, or timing module you could inadvertently be reloading the source of the problem into the new distributor. You know these things and need to do a data dump to us. Sometimes the source of the problem is inside the most obscure piece of information.

Bogie  

ok, its a 1972 Chevy C-10, 2x4, its a 355 with iron heads, when i got it it was set up for HEI, it ran fine up until now, there had been some rewiring done a while back but ran fine after the rewiring, could i run a independent wire to the dist giving it power from the battery and see if this eliminates the problem temporarily, this way i could pin point if it was the dist going out on it?  like i said it has ran fine until now, HELP!! ask what u need to know or give me something, VERY MUCH APPRECIATED  

OK a 1972 most likely started life with a point’s distributor. In which case the wiring is a bit complex thru the ignition switch; points don't like 12 volts, so the industry reduced operating voltage after the change to 12 volt systems back in 1955/56 with a resistor of some sort to bring cruising voltage thru the points down to 9 VDC. But the larger high compression engines evolving in that era didn’t crank and light off well with only 9 volts across the coil and the battery trying to run the starter motor at the same time so manufacturers modified the start switch so that when cranking a full 12 volts was fed to the coil. The points providing switching on the ground side of the coil in that arraignment. Once the engine fired and the key switch was released to the “RUN” position, the 12 volt input going thru the switch to the coil was rerouted thru either a resistor or a resistive wire which reduced operating voltage to the coil to 9 volts.

If all this is still hooked up on your truck, it might be that you’ve got a 9 volt power source going into the HEI. So you need to check this out with a volt meter. The HEI needs 12 volts all the time, whether cranking or running.

Bogie