miss, raw gas fuel dump into exhaust after fill up

[firemtbiker13]

Hey guys, new here, just got a 79 CJ5 with a AMC 258 i6 / 4.2l i6 and motorcraft carb a cpl days ago and its running rough. it has a miss and so far I've replaced... battery, alternator, plugs, wires. new distributor. Ive tested compression and running 125-130 on all cylinders. timing may be off. also, I've filled it up twice and its dumping raw fuel into the exhaust manifold and causing crazy gas smelling whitish smoke.... but only after refuel. Any ideas??? feedback would be greatly appreciated. Thanks in advance.

 

[mriplaybass]

Welcome to the forum!
First off, there is no way to dump raw fuel directly into the exhaust manifold. You have to be flooding the engine and the excess fuel from the intake manifold is running into the exhaust via the exhaust valves. There are several reasons that can cause this, but I can't think of any reason why it would happen after filling up. The most likely culprit is that the carburetor float is improperly adjusted and allowing too much fuel into the intake. The other is that the float is shot and not able to rise to shut off the gas flow to the bowl. The float or needle and seat could be stuck. Rather than trying to fix these problems one at a time, I would just rebuild the carburetor. The MC carbs are not complicated. Also, make sure that the choke is opening. If it is improperly adjusted and is staying closed when the engine is warm, excess fuel is going into the engine and this rich condition can cause the missing and white smoke out the tail pipe.
I am assuming that you have an OEM mechanical fuel pump. If you have an electric pump and it is putting out too much pressure for the carburetor, I believe 4.5 to 6 PSI for the MC carb, the fuel can push right past the needle and seat and dump into the intake manifold.

 

[firemtbiker13]

Thanks for the reply. As you can tell im far from a mechanic. I did check the carb and float and seemed to be working properly but I could have wrong. to clarify the engine was missing before I ever fueled it and this happened. I thought I may have overfueled it due to a kink in the fill hose to the tank so the gas pump constantly clicks and don't really know when to stop.

 

[Posi]

I'm with Bass on the carb float or too much fuel pressure. Also thinking head gasket until I saw you posted compression tests that look good.

I did check the carb and float and seemed to be working properly but I could have wrong.

Could you explain what you inspected?
Did you remove the float to check for pinholes?
Did you measure float height?
When was the last time the carb was rebuilt?
How many turns out is the adjustment screw?
What do your plugs look like?
I would suggest taking a vacuum reading as well.

 

[Torxhead]

If gas raw gas is getting into the exhaust manifold like you say it really has quite a way to travel to get in there. It should be easily noticed by looking down the throat of the carb while it is running. Another unlikely method would be by vacuum sucking fuel starting from the fuel tank vents into the carb through the emissions system.

 

[firemtbiker13]

Thank you all for all your help! I broke down and took it to an old mechanic and within a minute he found a return line or something coming from the fuel tank plugged directly into the manifold. did a quick minor adjustment on the carb and Immediatley fixed the issue. For the record i did not mess with that fuel line. thanks again for the responses!

 

[1979bettywhite]

Glad to hear that it's fixed. Sometimes things are nice and simple like this. Sometimes they aren't. But a carb rebuild down the road is not a bad idea. It will teach you a lot. I learned a lot from doing my Carter. It was my first one ever and I only screwed up one thing, lol!

But I would like to have seen where they had the return line plugged into. I can't think of a location on the intake manifold where it would have plugged into....