Exhaust backfire and Gasoline smell

[dzwestwindsor]

Hello, this is my first post here.


Last weekend I bought a 71 C20 with a 350 that is original to the truck.

This is the first carbureted vehicle I've ever owned, so I'm unfamiliar with how a normal carb'd engine should run vs how mine runs... I'm hoping to use this as a learning experience, so any help would be greatly appreciated.

So this is my situation: The engine seems to run strong, but occasionally (every 40 seconds or so) when cruising, I hear a loud bang come out of the back of the truck, out the exhaust. It's definitely backfiring, but what could be causing it?

From the moment the truck starts up, I can smell gasoline. The fuel hoses seem fine, so I'm guessing the carburetor is running too rich? Then the unburned fuel is actually igniting in the exhaust, causing the backfire? Is that a possibility?

If that's the case, should adjusting the mixture screws be a solution to the problem?

I'm new to carbureted vehicles so it would be great if anyone can point me in the basic direction of what things I should take care of.

Thanks!

[buckskins4ever]

Welcome to the forum.
first off, you are on the right track. The carb is running VERY rich. I dont think your mixture screws will help you out a whole lot. It most likely needs to be jetted down alot. You will want to find out what size the carb is. What size engine are you running and does it have any mods? You will want to make sure you have all plugs good. Running that rich will foul out plugs real quick......then you will keep dumping fuel in the exhaust.

[TBONE1964]

Before I would do anything with the carb, I would make sure the plugs, plug wires, distributor cap/rotor are all in good shape and replace them if they are not. Make sure the timing is set correctly and make sure there are no vacuum leaks.

If all of the above is good, then look at the carb.

Remember you now own a truck that will never see any better than 12 mpg going down hill with a tail wind. It will never smell as pretty as a new truck. Although it should not make your eyes burn, it is not the most efficient vehicle to begin with.

[Longhorn 70]

any dripping from the fuel pump? There is a vent hole that keeps the fuel from getting into the oil.

I may be mistaken on this but I thought most backfires were from LOSS of fuel or temporary loss of ignition.

Watching to see what goes.

[Jayschevy]

If you're smelling gas in the cab it maybe the in cab tank. When I got my 71 I smelled gas as well. Just make sure all the gas lines inside the cab are in good shape and not cracked.

[dzwestwindsor]

Hi guys, thanks for the replies.

I've heard that an exhaust leak could be a cause of backfire... (anyone know why?)

But anyway, I noticed that there is a pipe coming up out of the exhaust manifold (driver's side) that directs air back into the intake before the carburetor.

Is that the heat riser?

It is loose and rusted out, so I'm not sure that could be a source of the backfiring problem. I'm thinking of just getting rid of it completely. Have any of you done that before?

And also, can someone explain to me why an exhaust leak could cause a backfire? I've tried searching around and I couldn't find a good explanation.

Thanks for the help.

[anglebinder]

Back pressure, gap is sucking cold air into head changing air/fuel requirement for your carb. You can run without it but you need to patch the hole or replace the manifold pipe.