Rough Running 350

[72chevylwbmo]

Well, I have spent countless hours trying to get this truck running smooth. I thought I had it here a couple of weeks ago. Now it is surging. Rode with my son last night to town and when he come to a stop the truck was fluctuating between 1300 and 600 RPM's. What could be causing this? 72 chevy, 350/400 transmission. Any help is appreciated.

[bigmac73]

I can tell you right now, that the guys on here are going to want more info,
Like type of carb, timing setting, motor specs if you got them,

did you check for a vac leak?

[72chevylwbmo]

Ok. As far as motor, I believe it is stock. Just replaced head gaskets because they were leaking. Stock intake, Holley 600 or 650 carb, reman about 8 months ago. Ask whatever you need.

[bigmac73]

was it running rough before you did the head gaskets?

what is the timing set at?
did you adjust the idle mixture screws?

[72chevylwbmo]

It was running rough because of a head gasket leak. It was not surging like this. It didn't surge after I was done. It only started surging this week according to my son. I haven't with the carb because I don't know much about them. I have set the timing at about 18* before to keep it from popping back through the carb.

[Lee H]

Sounds like a vacuum leak to me. Was it surging before the gasket swap? Please explain the surging in detail, when, what speed, in gear or out, etc. what type of distributor?

[72chevylwbmo]

It was not surging before or after the gasket swap. It only started surging this week. It only seems to do it at low RPM's. Seems to do it no matter if in gear or not. The distributor is a HEI but that is all I know. It looks to be a factory swap not aftermarket.

[geezer#99]

Originally Posted by Lee H

Sounds like a vacuum leak to me. Was it surging before the gasket swap? Please explain the surging in detail, when, what speed, in gear or out, etc. what type of distributor?

You forgot to ask for pics Lee!!
Burn us off a pic of your motor set up. Lee or me or anybody else might see something obvious.

[Lee H]

Good point Geezer,

Lets do a cursory vacuum leak check. Check all the vacuum hoses for deterioration. With the truck idling, and surging, take a spray bottle of water (some folks like using starting fluid) and spray the base of the carb, you will be looking for a change, maybe slight, in the idle, it should stop surging and rise a little for a second or two. If nothing there shoot all the vacuum hoses, and around the intake. Power brakes, if yes disconnect and block hose, maybe the booster is leaking. Is there any oil in the radiator? Water in the oil? If no leaks are found we'll move to the ignition system.

[Coley]

Ditto on the Vacuum leak probability!! Check it out carefully.
A guy I know recently brought his newly rebuilt 350/truck over due to rough running and he wasn't sure what was up.
Well, as it turns out he had connected his brake vacuum supply to his intake manifold instead of the carb back vacuum port for the brakes which was completely uncorked and open....wow, that was some vacuum leak.
Again.....check for vacuum leaks at the intake manifold, then the carb.
Don't forget to plug or path all the ports on the carb and if you can....put up some pics of the upper engine, left side, right side and it will help us help you.
My two bits.
Coley

[72chevylwbmo]

Thanks guys. I will post picks if I am smart enough to figure it out.