Any help for this novice noob would be appreciated.

I have a 72 Cutlass with a 2bbl 350 with no mods other than exhaust - she's fairly stock. She's been running fine until just recently. She starts up fine, idles fine. When the motor is cold it runs great, normal acceleration, etc. But once the engines is warmed up to operating temperature (after about 4 -5 miles of driving) the engine starts to stutter, putter, stumble and have mini backfires - the engine wants to die but is barely limping along. It feels like the one time I ran out of gas on the freeway. Once I put it in park and let it idle it seems like she seems like she wants to die, rough idle, stumbling, etc. I've checked plugs, hoses, cleaned the carb, checked fuel filter - all of the basic novice stuff.

Coincidentally my problems started after my last refill of gas. But I got gas from the same place I always get gas and the tank was probably 1/3 full when I topped it off.

If anyone has any ideas I am all ears. Otherwise I will limp it along to a local service station for a diagnosis.

thanks

 

Are you positive choke is holding wide open after warm-up?

 

Adjust the carb with a vacuum guage, after warm-up. You'd be surprised as to how many problems it'll solve.
Also check for a 'hissing' under the hood, while running.
Could be a aged vacuum line hardened and cracked.
Trim for now, but replace later.

 

I'll check both the choke and the carb with a vacuum gauge. hopefully I'll uncover something

 

It couldn't hurt to fill the tank and add some fuel line antifreeze, you might have picked up some water.

 

Stock points distributor? I had similar symptoms on a 69 GTO in the early 70s. The primary wire (coil to the points) had gotten pinched and most of the wire strands were severed, but the plactic insulation was in tact on the back side. When the 2 ends of the wire strands made good contact, it ran fine. When they warmed up or vibrated and lost conact it did just what you described. The dealer checked the wires for continuity, and when they checked them they were fine. After I spent $500 in 1971 dollars on repairs, I discovered the problem and replaced the 80 cent wire.

 

Make sure the thermac valve isn't open when its warmed up. This is the valve on the air cleaner housing that allows heated air from the exhaust manifold stove to the carb.

 

Check your coil, your coil-to-distributor wire, you spark plug wires, and your condenser.

- Eric

 

I bought atruck that behaved as described backfire and fart sometimes engine dies somtimes no syptoms would appear for miles....eventually drained the tank and found water in it, when it got sloshed nto the pickup the truck would sputter or die if it didnt it ran OK ususally would idle all day w no troubles too

 

Still got the brass filter in the carb??

 

Remember guys, it starts and runs fine cold - the problem only comes up when warm.

- Eric

 

And it started the last time he bought fuel.

 

Could it be a temperature operated vacuum switch on the intake? It'll operate once it gets warm and if there is leakage in the circuit could lead to the problems at idle. Can't remember if the 69 has that switch on the front end of the intake.

 

Any updates on this?

 

if you got gas shortly after a fuel delivery ,the dirt and water in the bottom of the gas stations tank got mixed up and pumped into your cars tank.when you start it up after sitting and the dirt and water is settled it will run ok, untill you start driving and mixing it around then it gets sucked up to your carb and will spit,sputter and yes backfire. ask me how I know. your only solution to bad fuel is to either drain the tank or keep getting fresh fuel and dilute it and burn it out .it's possible it is an electrical issue but check your fuel first.maybe disconnect the fuel line after the fuel pump and pump some into a clear glass container to see if it is dirty. good luck

 

Focus on things which could malfunction with a change with temperature: choke operation, hot air intake, ignition electrics from battery ->coil -> distributor -> wire -> plug.
If it runs fine when cold then it probably is not the fuel.

 

sorry for the long delay in posting - had to go overseas for a bit and access to Internet was limited.

So, I took a lot of your collective advice and of course it was the classic "while I'm here I might as well fix that too" syndrome. Yes, a couple of the plug wires were frayed and required replacement but that wasn't the problem. And yep, filter in the Rochester needed replacement as did the inline filter. Carb needed some adjusting although vacuum was good. Even put some fuel line freeze in to get rid of any water.

Of course none of this helped but all needed to be done.

Finally, I did what I dreaded (although it wasn't that bad); I dropped the gas tank. I'm proud that my car is pretty much all original but some things do begin to wear - like all the lines and fittings getting fuel from the fuel tank to the engine. I don't know what the mechanical/scientific term would be but I think the tank was simply not feeding enough fuel to the engine. I replaced all the lines and fittings plus the gas cap. I can't tell which fix did the trick but she's running like a champ once again.

Thanks again to everyone. Your help was invaluable.

 

Glad you got it fixed.