rough idle and backfire

My 70 GS 350 idles rough and too slowly and hesitates from a stop under light acceleration. If I accelerate hard the engine seems to run and sound normal. When let off of the throttle, the engine backfires and the exhaust smells like its running too rich. what do u guys think could b the cause of this?

 

vacuum leak or 2 sparkplug wires flipped. is the carb tight to the manifold?

 

Nate, Thanks alot, ill go check all of that.

 

Nate, Thanks alot, ill go check all of that.

 

also check if the choke is working properly

 

Good point Mike. I keep forgetting that. I repaired mine early on

 

Sorry to pepper you with questions but we can help more if you answer these questions:

-Is this a new problem?
-Has the car sat for a while but was fine before?
-Have you done any repairs or modifications to the car?

If so please detail them so we can help figuer out what may have changed...

I agree that you may have a choke sticking part way closed and causing a rich condition. Run the car till warm then remove the air cleaner to see if the choke flap on the front of the carb is open all the way.

 

also...to add on to that...if you swtich it from drive to nuetral does it idle better?..

 

Yes, the idle gets a bit smoother.

 

If you can answer my questions we can help more...

 

The car ran fine until I parked it, went into a store came back out and it started to run/idle rough.

 

Most likely a sticking choke. Have you tried starting it since and letting it come off the choke? The engine will run rich while warming up but then return to normal unless it sticks part way closed like I suspect happened.

After the engine is warmed up, remove the top of the air cleaner and make sure the front flap is straight up (fully open) if it is part way closed the choke is sticking.... Try this and let us know.

 

I agree with others about the choke. I ran mine one day and the next it sputtered and had less power, etc...popped off the air cleaner and the electirc choke wasn't working so it was halfway closed. propped open the choke and viola!

 

The "backfiring" could also very likely be an exhaust leak at your header flange, I just had a nasty one and as I would accelerate it would be fine but after I let off it would make a nice little POP POP POP, like a handgun going off. This occurs because of the zero exhaust backpressure at the leak, generally fine when accelerating unless the leak is REALLY big, but on deceleration the unburnt fuel re-ignites instead of being forced down the header tubes like during acceleration.

Somebody please correct me if i'm wrong about this, im no expert.

I'd check the tightness of your header or manifold bolts and listen for leaks.

-Ryan

 

turned out that the choke was stuck, im going to change plugs tonight, what is the proper gap?

 

Per autozone's site .030"