I recently rebuilt the carb on my 77 f100 I6. It has the original carter carb. Well, I am having trouble with a high idle and exhaust backfiring issue. The idle screw will only adjust it so far, even if it's all the way in. I tried messing with the timing but that made it back fire worse... Any suggestions?

 

Well, don't want to scare you, but when I had what sounds to be a very similar issue on my 300 I6, it turned out to be a big intake manifold leak. I had the rearmost tab of the manifold break off so I was getting a big leak on the back cylinder. When I screwed the idle all the way in it wouldn't kill the engine. Now, it also exhibited some weird knocking and pinging... reminded me of the sound of a diesel!

What was the reasoning behind the rebuild, and how did it run before the rebuild?

By 'messing with the timing', did you actually get a timing light onto it? If so, what was your timing at idle?

 

I rebuilt the carb because I had put new plugs and wires in the truck and it was running very well, but after about 20 miles the plugs were fouled so bad it was hesitating and backfiring. I tried adjusting the idle screw before the rebuild and it nothing then as well, so I decided a carb kit was the next step. There is a tic coming from the motor that I haven't located yet, not really any pinging though. I will spray some carb cleaner around the intake and carb tomorrow to see if I can locate any vacuum leaks

 

I checked for vacuum leaks with carb cleaner, there is no change in idle. Also disconnected the vacuume advance line and it, seems to run closer to the way it should with it off. The only other idea I have is that the float setting got messed up during the carb rebuild.

 

Definitely double or triple check everything (i.e. float, mixture screws, timing and idle).


There is a possibility that the vacuum advance canister has an internal leak giving you a vacuum leak when it is hooked up.

 

I had similar issues, rebuilt the carb and regular tuneup but had issues with it running well after it warmed up. Could smell gas in the oil and it ended up bieng the diaphragm on the fuel pump leaking.

 

Yeah, if the plugs fouled after that short time, something's up with the fuel delivery and ignition - either too much fuel, which an incorrectly set float might explain - or something effecting the air/fuel ratio (maybe a vacuum leak on the vacuum advance? Super easy way to check - pop the hose off wherever it connects at the carb and use a vacuum pump or even suck on it (just don't want to accidentally inhale dirt or something) and it should hold vacuum.)

 

Found the problem... When I was reassembling the carb I didn't have the throttle plate positioned correctly and it was getting hung up, preventing the throttle cable from fully extending. On top of that, I was relying on where the throttle cable stop screw was set before the rebuild, the previous owner must have messed with it to keep it idling? I had to back the throttle screw way out. I also reluctantly used the old (thicker) base gasket instead of the thin one that came with the kit. I still have to set the timing with a light instead of by ear, but it runs ten times better now, than before the carb rebuild.

 

Wow, that's great! hopefully that fixes some of your plug fouling issues. Gotta love it when you figure something out

 

I already put more than 20 miles on it and a no problems yet. It does feel good to get a carburetor right for once, they tend to frustrate me more than anything