Hello gents,

I am in dire need of some assistance. I recently inherited a 1972 Ford F250, manual, 2wd with a 360. I have gone through the engine and replaced all rubber hoses, belt, fluids, fuel filter, spark plugs, added a Pertronix unit in the distributor, and new spark plug wires. I am running into a fueling issue with my the reman carb that was just put on the truck about a year ago. It is a factory 2100 carb. I have pulled it completely apart twice now and cleaned it in its entirety and made sure it is set to factory specs. As a bit of a forewarning I am somewhat new to carbs however I am quick to learn and have studied more info on this carb than I care to admit. The truck will start and idle perfectly without flaw. Where I run into an issue is once it is warmed up to operating temp and I attempt to step on the throttle it will sound like it is inhaling a ton of air and will choke. I have tried to mess around with the two idle circuit screws on the front bottom of the carb to no avail. When I turn them all the way in the truck will want to die so I know they are working as they should. I have a vacuum gauge hooked up to pure manifold vacuum and the reading is steadily in the green. What could possibly be causing this issue? I am completely stumped and have been messing with this issue for many weeks.

 

Are you getting a good squirt of gas from the accelerator pump when you move the throttle?

Any vacuum leaks, especially the carb base, and any spacer gaskets?

What about timing? About 10 degrees seems to work well for these. (Factory spec probably 6 degrees)

Set your idle with timing and the screws, then look elsewhere for the stumble.

How is your PCV connected? Got the right valve, in good condtion?

 

When I look down in the carb I appear to be getting a good squirt of fuel. I sprayed some carb cleaner around the carb base plate and around the brake booster and do not have any leaks that I know of. I set the initial timing to 10 degrees as well since I read on here that that is where most of these motors like to be. My PCV and PCV grommet are both brand new as well and run to a base plate under the carb. Kind of a dumb question but would trying to rev the engine without the air cleaner cause a lean condition? I ask because while I have been working on the truck I have not had an air cleaner on it as I have been waiting for the one I ordered online to come in.

One other thing to note is when I had the carb apart again this past weekend I did notice that the jets say "50" on them but I was under the impression that the 2100 needed jets that were .053. Could it be possible that the vendor installed too small of jets? When I initially started working on the truck weeks ago I did note that I had the idle screws turned almost 3.5 to 4 full turns out and it was still not running rich and basically acted the same as it does now with them out about 2.75 turns.

 

What hole did you set the accelerator pump linkage arm ? What you describe is a overly lean condition at idle and not enough fuel to overcome the sudden butterfly going WOT.

 

I am using the outermost hole (the one at the end of the arm). I have also tried moving it to the other hole closer to the accelerator pump and have the same issues. Which hole is suggested for use during the summer? Or do you ever have to adjust it? Sorry for all the seemingly dense questions.

 

Take a rag and partially cover the carb. If you give it gas and it doesn't stumble, then you have a lean fuel condition

 

It is definitely running lean as that is typically one of the conditions of the carb backfiring from what I gather. I did take a screwdriver and push the choke plate about half way closed and then used my other hand to give it a little throttle and it responded a little better. I have made sure the timing at idle is as close as I can get to 10 degrees. I am going crazy trying to figure out why it is going lean on me though since I don't have any apparent vacuum leaks and it is squirting a good stream of fuel in when I open up the throttle. I have done a little more digging and think that the jets might be a little too small for the application even though those are the ones that came in it from the rebuilder the first time.

 

The jets have nothing to do with off-idle fueling. They control how much fuel you get while cruising. Your accelerator pump controls fuel enrichment. So are either not getting enough fuel, or you have a vacuum leak. If when you step on step on the gas while cruising and it does this, it could be a bad power valve.

 

Where or who did you buy the carb from? If they bench flowed it the jets installed should be close to perfect. Very misunderstood topic. Keep in mind a lot of the info out there is either by or for drag racing. Nothing wrong with that, but normal driving on the street is different.

The jet size is what determines the steady flat ground cruise air fuel ratio above 35 or 45 mph. The jets don't factor for idle. You can install larger jets to remedy an off idle bog, and jet size does affect the wide open throttle AFR, but that is exactly the wrong way to go about tuning, will run pig rich at cruise and waste amazing amounts of fuel. Once the correct jetting is found (do this first) it's always a matter of tuning the other circuits - transition, power valve, accelerator "pump shot" etc.

 

Thank you for the info on the jets. I thought they assisted in the initial on-throttle blip and then continued to work while cruising. I originally bought the carb a few years ago online for my grandfather. He just installed the carb about a year ago or so and was getting around to tuning it when he had a pretty bad accident involving the truck (see other thread for more info). I live about 45 mins away from where I am currently keeping the truck or I would supply you guys with some pictures of the carb. I do know that it has a "reman in Canada" tag on it. With me rebuilding it twice I have made sure all the gaskets/seals, accelerator pump diaphragm, and power valve are in good working condition. I am able to start the truck just fine and it idles great. I am also getting a very good/steady vacuum reading as well so that leads me to believe that I do not have any vacuum leaks. I have also went through almost an entire can of carb cleaner looking for any leaks and cannot find any. At this point I am completely stumped.

 

I think I may have found a potential problem. I was looking through a few schematics this morning and think I have the spring in the accelerator pump backwards. It is smaller at one end than the other and I have been installing it with the small end pressing on the little red plug on the carb and the larger end butted up to the accelerator pump diaphragm. In a couple rebuild videos and some exploded diagrams I have found it shows the larger end of the spring has to touch the carb and the smaller end is in contact with the diaphragm.

 

The jet size will most definitely affect the operating points and the AFR in the other circuits, particularly at wide open throttle, but the objective generally in carb tuning or calibration is the jetting is optomized first for a clean running economical cruise at highway speeds. This is the point just short of misfire or "lean surge". Then the other carb circuits are fattened up (or leaned out) as required. A mistuned carburetor will waste an amazing amount of fuel straight out the tail pipe without necessarily running badly.

 

​​​​Don't know if this is your problem, but the small end does go against the diaphragm

 

Looking at a few pictures and videos it seems like there is a little hole that the red plug covers and that is a relief of some sort (someone correct me if I am wrong). The way I have the spring oriented it does not allow for that relief to work. I guess that is the first thing to correct on my list.