I just built a motor for a friends '66 Bronco. I went from I-6 to a 302/C-4. Nothing fancy, mild cam, headers, and the factory 2bbl. His intent is to swap out the carb/intake eventually when budget dictates. I believe the motor to be a '74 or '75 and it came out of a Bronco. The carb is an early model 2bbl with a manual choke. I want to say it's off of a '68, but I'm not 100% certain. Anyhow, the thing runs great, except for it has a stumble just off idle. If I'm not careful it will stall. Due to the lack of a tach I guessed on the idle, but it should be around 600-700 RPMs. I'm leaning toward a bad accelerator pump. What do you guys think? It's not leaking externally, could it be plugged up inside? Thanx!

 

Well, you can look inside the carb with a flashlight and work the throttle linkage to check the pump - if you see gas squirt it should be okay.

Maybe the idle mixture is a too lean? Use a vacuum gauge and go 1/4 turns on the idle mixture screw until you get the highest vacuum reading. An air/fuel meter would be a big help as well.

 

I'm still fighting with my off-idle stumble as well. Will a too-lean idle cause this? I set the idle mixture with a vacuum gauge to get the highest vacuum, but it's still doing it.

 

Assuming the ignition system is working well, most stumbles are lean-condition related.

Depending on the carb - I'm an Edelbrock/AFB guy - a lean idle could cause a lean stumble before throttle/air flow transitions to the primaries. Easy to play with on the AFB since you can swap for richer rods with the carb on the engine and test drive it.

A bigger accelerator pump shot might help.

 

I don't really know the condition of the ignition. It starts easy enough, but usually needs to stay on high idle for a long time. It does seem to have a miss at speed, so maybe there is an ignition problem. I have an old MSD 6AL that I can install, as soon as I figure out how to run the wires without turning it into a rat's nest.

I have a pretty generic Holley 1850 vacuum secondary 600 cfm carb. I checked it out today, and oddly, one side was screwed way in, about 1/4 turn out. No wonder it wasn't idling well. Turning it out to 1.5 each side made it idle much better, but the stumble hasn't gotten any better. I will try changing the position of the pump cam tomorrow.

 

Retarded timing and a poorly operating accelerator pump would be the most common causes of this. If both of these are okay, I would suspect a generally lean condition or an issue with the power valve/system.

 

I checked the pump cam today, and it seems like its position is not easily changed. The other available position seems to delay the pump action, so I left it.

I checked the spark plugs, and they all show signs of rich running; covered with black soot.

I readjust the idle mixture to get the best vacuum readings, and I think it's at abut 1-1/4 turns out on each side.

I checked the static timing; it was at about 14, and I set it to about 12 BTDC.

So I test it again. Revving the throttle by hand, and it still has that slight stumble whenever the opening is increased. So I give it a hard goose, and huge orange ball of fire blows through the carb. I think I lost some hair. That can't be good. May be time to get back into the carb yet again.

 

Take off the air cleaner and peer down the carb with engine dead. Blip the throttle and ensure that raw fuel streams into the air stream. It should begin streaming with the first movement of the throttle lever. That's the only way to ensure that the accelerator pump is working correctly.

 

do you have a two bbl or 4bbl..... baronvonautomatic it correct.... its usually lean.... but not enough accel pump.... but it makes a difference on a two or a four bbl.... on a two bbl its usually too lean and as the first poster said check it with a vac guage when idling and watch it while driving.... on a four bbl it could be the secondaries coming in too quickly.... especially when they are vaccum and the engine is under load.... it can also be too little timing as well....

 

I fattened up the mixture a bit and messed with the timing a little and it works great now! I guess I never really thought about the mixture being the cause, but I can see it now. Thanx again!

 

Congratulations Frankenstang. How did you fatten up the mixture? Just the idle screws?

I've been messing with mine over the last week, and found a couple things.

First, I installed the MSD-6AL to make sure I get enough spark. It took a bit of work to find and connect to the correct power source from the ignition switch that supplies the coil. Then dressing the wires so that the new connections don't end up a mess flopping around under the hood. It turns out MSD has these little plastic tabs that bolt onto the coil terminals to anchor all the wires to. It's working, but the tachometer is now reading much lower than it's supposed to. Looking at the circuit, I see the tach is normally connected through the coil. So now that it's connected through the MSD, it's not getting a strong enough signal to show the correct reading. I'll have to do a little more research to figure out how to fix this, or just relearn what the new tach readings mean.

Second, I did a test to see if it's firing on all 8; I successively pulled off one spark plug boot at a time while idling, and found a couple of cylinders not working. I found their plugs to be carbon fouled again, indicating too rich condition. This is after a pretty recent cleaning, after which the engine seemed to run much more smoothly. This time, the engine again, ran much smoother after another cleaning. So the carb is still running rich for some reason. I'll re-check my idle mixtures screws tomorrow. But there is still a stumble on tip-in. I haven't gotten back into the carb yet, so I'll check that tomorrow.

 

I found an old problem...the carb was again clogged with grey mud. This is stuff that got through 2 filters. I pulled the gas tank today, and from what I can see, it looks pretty ugly in there. Lots of rust and congealed remnants of old gasoline. That stuff just continues to flake off and end up in the carb. Anyone know of a good trick to scrub out the inside of a gas tank through a 2.5" opening? Either that, or I buy a new one.

 

ive the same issue, cleaned the carb 3 times and changed the fuel filter twice and it keeps getting dirty, did you end up finding what was the issue

 

Wow, 10 year resurrection.

I pulled the old tank and tried to clean it out, but the amount of rust flakes that came out made me uncomfortable. So I replaced it with a new one. But I also tried flushing the line forward and backward with mineral spirits, which got out a lot more fine crud. But even then, the carb still collected even finer crud after running for a few days, and would stumble and not want to idle. So I ended up cleaning out the carb a couple more times after running a few days each time until I fond no more mud build up.