73smith

I installed a brand new GM crate motor, the most basic one. added a new summit HEI, previously in use edelbrock carb+intake, and fuel pump. Have a few hundred trouble free miles on it. one day while driving home from work, about 20mi in it started running real rough and surging about a minute then died. Wouldn't start. Checked under the hood, all looked good, banged on the float area of the carb, about 5 mins later tried to start and it stumbled a bit then started, drove another 10miles trouble free then it happened again. Whent through the same process of banging on the carb and waiting and it started and finally got home.
Took the carb off, tore it down and cleaned out some junk that found in the fuel bowles. Reset the floats and put it back on the motor. flushed the fuel filter for good measure and the engine started up fine and ran smooth in the driveway.
Didn't get a chance to road test. Today i needed to take it to work and the same exact thing happened. Only this time i had to repeat the process a few more times to get back home. Engine just surges up and down, need to pump the pedal hard to keep it alive as it starts to die. once it dies, i can wait 10min or so and it will start and run for a few more miles.

Any ideas???

anthony7996

could be a bad distributor module or timing could be off slightly i got a 70 c10 shortbedstepside did same thing and timing was of just a bit redid timing hasnt done it since.but could also be float in carb not working right as well id try timing first.

cnorton

Does the exhaust puff black smoke as the problem begins? A stuck float would cause the mixture to go dead rich and "flooding" would occur.

Conversely, you MIGHT be experiencing a vapor lock situation due to a line running too close to an exhaust pipe or other heat source. In that case, surging might be a symptom but the engine would ultimately die due to a lean condition and not run correctly until the hot spot cooled.

It's a long shot but worth considering.

c

Max-Z-Payne

That is odd. Check the timing. If that isn't the problem then see if gas is leaking out of your venturis(I am pretty sure that is what they are called anyways) while the engine is running. If gas isn't leaking out of there then the floats aren't hanging open so you don't need to tear it apart for that. Check the plugs and see if they are fouling. I don't know why your HEI would be having problems since it is practically brand new. Maybe a problem with the coil? It's really hard to identify the problem without being there but just go through the process of elimination starting with the cheapest problem and work your way up. Good luck.

73smith

replaced the fuesl pump and filter. Filter was nearly impossible to blow through. Didn't help. But in my test drive i noticed that the engine temp never got higher then 130-140 or so. This time i was able to do some work on it while it was running so badly. found a definite issue, a few of the 3 of the cylinders header pipes were not nearly as hot as the others, hooked up a timing light to those plug wires and you could tell those cylinders were mis-firing when the engine was stumbling. The coil/cap/ignition cylinder/distributor are all brand new. Plugs are nearly new. Is it definatly something electrical? could i have wiped out a few cam lobes and have that cause the issue? I suppose a conpression test would diagnose a cam lobe issue?
-Mike

cnorton

1. Have you compared the resistance in the plug wires between the dead cylinders and the hot ones with an OHM meter? I've seen new plug wires with unacceptably high resistance.

2. Plugs have not been firing may be potentially so badly fouled that they will never conduct correctly again.

3. Remove the valve covers and visually compare movement of the individual rockers. If you've scrubbed a cam lobe those valves will only open part way.

4. If you suspect that you have a bad cam lobe, remove the oil filter, cut it open, and inspect the pleats in the filter medium for debris that may have been trapped there.

73smith

thanks for the ideas, the plug wires are new also, but i'm not counting any 'new' parts out of the equation just yet.
Let it sit for about 15-20 mins and it ran fine again, firing on all cylinders. So i highly doubt it's anything mechanical (like a wiped lobe). Latest theory is the ignition module, i guess they can start acting quite funny when hot and on their way out for good but return to normal once cooled. I have a spare or two I can swap in there for test.
-mike

daveloveless

i think you are on the right track with the module because if the timing was off it would likely run bad all the time or start hard when the engine is hot and the carb would run like crap as well.change the module or the coil and you shouldnt have any problems

Z28Freak86

This may be a dumb question, but is the distributor hold down bolt tight? Swear to god ive seen it done! Just though id throw that in

73smith

solved!
Wasn't the module, swapped in a new one, no change, replaced the coil with a known good one just for fun and still no better. But started to notice more things. The cylinders that were misfiring we're all feed essentially from one side of the carb (dual plane edelbrock manifold) and when it would stall, it would spit a lot of fuel up and out of the carb. So i took the carb off again and when i took the top off it (edelbrock performer carb) you could see fuel filling up the space around the secondary barrels (it shouldn't be there). There are openings in this space that is open right to the manifold. The top gasket had no signs of wear, tear, holes or anything. My guess is either the top or base peice was somewhat warped and as the engine warmed up, the two peices would seperate enough to allow fuel to spill past the gasket and directly into this open area that would flood the engine.
Put another carb on it and ran just fine...
There are a 6-8 screws that hold the top onto the base of the carb, they hadn't been touched in years, and they were all properly secured when i tore the carb down the first time to chase this issue. So i don't think they were the issue nor the gasket. hence the warping theory.

thanks for the ideas and feedback.

Z28Freak86

Glad to hear you solved the problem! And thanks for replying with the solution.