1971 Pontiac Lemans 350 engine running a bit rough


Asked by Sep 12, 2015 at 10:06 PM about the 1971 Pontiac Le Mans

Question type: Maintenance & Repair

Hello, again fairly new at this so any advise would be helpful. My 350 starts great first time in am, runs strong and comes to idle speed quickly but start it again after that, say 10 minutes to 2 or 3 hours later and it runs kind of rough and wants to stall if I don't play the gas pedal for a minute or so and also doesn't turn over quickly like the first start of the day. I have recently converted to petronix electronic ignition,changed the plugs and wires to try to correct the rough running but it is still happening. When the car warms up after a few moments it runs out fine its just the initial rough idle. again thanks for any suggestions.

18 Answers

It's sounds like it's the choke. Could u post a pic of your ride? I like them old Pontiacs..

What kind of carb is on it?


Rochester 2bbl

Have u put a kit in it. ?


No sir, it ran fine up till a week or so ago.

OK. It's gonna be fine. Theres a tag on it. Right that down and call or go to parts store. Get a $15.00 kit and let me know when u get it..I'll help u. This is common with them. Buy a can of good carb cleaner as well..

1 of 1 people found this helpful.

There should be a metal tag. It's the carb number. If not if the carb is original. Just tell them what it's on. They'll get the kit for you..


So its a carb rebuild kit or cleaning kit or what kind of kit will I be asking for?

Just call part store as for a carb kit for a 71 Pontiac LeMans 350. They'll hiok u up..

They're just called carb kits..


for give me for asking but when did this problem occure. before or after putting in the electronic parts in the dist. if it happend after this it sounds like you have it out of time just alittle. I would start there check your timing and I'm sure there is where the problem is it sounds like it if to far retarded and there for when cold the idel is higher then when it is warm if you try to start it warm the timing is not compinsating for the fuel to air ratio and there for giving you a slow start. there is some thing else for every one that has a carb. take a rag or hand towl wet it down ring it out till it is still damp but not really wet. start the car rev it from under the hood to about 3000rpm to 3500 rpm and try to choke it out with the rag over the inlet in the carb. just before it dies take the rag off let it rev again and choke it out again with the rag. this will clean any gum or dirt out of the jets. the mixture of the wet from the towel and the vacuum that is produced by doing this sucks what might be causing you problem out of the carb. still though I believe its timing

1 of 1 people found this helpful.

Interesting, I will try the rag trick this week. The electronic ignition went in a couple of days ago and the rough running may have increased a bit. The electronic is the lobe sensor variety and I had read that the timing would not have to be adjusted as it is with a magnetic type .


Check the heat riser valve on the right (passenger's side) exhaust manifold. It should move freely. This valve is closed when the engine is cold to direct hot exhaust gases through a passage in the intake manifold. The automatic choke coil sits in a well next to the carburetor. The hot exhaust gases heat the choke coil and open the choke plate on the top of the carburetor. Make sure your choke plate is fully open unless the engine is cold or cool. Also, check that your thermostatically controlled air cleaner assembly is hooked up and working correctly. If your car is very original you have an emission control device called Transmission Controlled Spark (TCS). There is no vacuum advance to the distributor until the transmission shifts into 3rd or when the engine coolant temperature is between 85°F and 220°F. This is normal for your car. Finally, whatever you do, DO NOT buy a rebuilt carburetor and turn the old one in for the core charge. I did that with my '70 Catalina and regretted it dearly!! It took me 3 years to find the correct carburetor for it again. It wouldn't run correctly until I got the exact correct carburetor for it. Nothing newer will interchange! HTH. -Jim

Needs a carb kit. That's all..Good luck..


Thanks for more good info, I checked the heat riser and the choke flap on carb and both seem to be fine. Curious about the TCS , would the small black piece inside the air cleaner pass side that is on the other end of the pipe coming out of the valve cover the piece you are referring to? And if so what would I be looking for to know its functioning properly. thanks again

1 of 1 people found this helpful.

No, that piece inside the air cleaner is the vacuum valve for the damper door inside the snorkel of the air cleaner. You should have a vacuum hose going from the intake manifold to it. Then another vacuum hose from the vacuum valve to the vacuum motor (round disk) on the air cleaner snorkel. With the TCS system you have a solenoid in the vacuum line going to the distributor. If it's still there you'll see it near the back of the engine with a 2 wire electrical connector plugged into it. To test the system, hook up a vacuum gauge to the vacuum line going to the distributor. Start the engine. If the engine is between 85°F and 220°F you should have no vacuum. Shift the transmission into reverse. You should have vacuum. HTH. -Jim

Best Answer Mark helpful

Got it, check it tomorrow, thanks

1 of 1 people found this helpful.

You're welcome. Glad to help. I had my '70 Catalina for 16 years. I'm very familiar with Pontiacs of this era. -Jim

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