Idle no, acceleration yes. If it is jetted too lean you can get an off idle acceleration bog especially with slower throttle actuation , As soon as there there is enough air flow to create a vacuum in the venturi's the main circuit (main jets) are discharging fuel. With annular boosters in the Autolite/Motorcraft carbs the vacuum required is way lower than say in a Holley, and they will start to flow fuel as soon as you open the throttle even a little.

As soon as the throttle is opened it starts the transition to the main circuit the accel pump is only needed when the throttle is open quickly to cover the sudden drop in manifold pressure till enough vacuum is built back up for the boosters to flow fuel.

The main circuit is airflow dependent and has nothing at all to do with vehicle speed.

If the carb is jetted too lean this can lead to an off idle bog as the engine will be starved for fuel when the boosters start to flow.

The idle mixture screws just set the total volume of fuel going in to the engine at idle not the actual idle circuit mixture.

We have no pics of the carb unfortunately, and now with the lean running on the mains off idle bog and the 3 plus turns on the idle mixture screws to get it idle correctly I am suspecting this carb is off a 302. And starving the 351 for fuel.

It may be also be missing the air distribution plate that was found on some applications which will also affect acceleration.

The throttle plate bore diameter of the 302 Carbs is 1.56" 351 and up should be 1.67". This is different than the venturi size stamped on the side of the carb and is measured across the throttle bore on the bottom of the carb. (The face the mates to the engine).

 

Well I think we're getting way off in the weeds here. The odds of jetting to cause a flat spot stumble or "falling on its face" on acceleration and backfire through carb are pretty much zero. Jets are for steady highway cruise at low load conditions.

Backfire through the carb points to a timing problem, this is what you want to focus on. Also the vacuum gauge fluctuations of 1/2" mentioned earlier, the gauge should be rock steady. There is a lot of information that can be gleaned with a gauge. Study the vacuum gauge charts. Did you rebuild the carb? That means complete disassembly, cleaning everything out with a good solvent and blowing out all those tiny orifice with shop air, new gaskets.

You'd have removed the jets at that time. What size are in there? Reset the float, verified fuel height. In an old carb it's good practice to replace needle and seat, and float. If the fuel level isn't right it can be difficult if not impossible to tune. The fuel level in the bowl is the baseline for carb tuning. Distributors too can have problems other than the vacuum advance, the mechanical breaker plate can get sticky and or erratic. How many miles are on the distributor?

Also have to verify that the timing marks on the damper haven't shifted, one thing that stuck out was this 20" initial timing business. Without any other changes to there would be way too much total advance with stock springs and advance plate, it would rattle like a can of marbles. If the advance plate is stuck or sticky - that would explain a lot.

You can disconnect and plug the vacuum advance and run the engine up in neutral and observe the timing marks with a light. Spool it up through 2 to 3k (carefully!, stay away from the plane of the fan) and see that the mechanical advance operates smoothly up and down. The marks should stay sharp and focused, and not get erratic or fuzzy and should snap back smartly. What is the total advance observed? You mentioned "running out of room" to adjust the distributor. This points to it being installed off a tooth.

Sent from a pay phone

 

Actually back firing through the carb on acceleration is symptomatic of a lean AFR that is too lean. As well as of several other items valve timing, ignition timing, egr problems, vacuum leaks (to lean AFR) etc.

The chance of carb back fire with too lean a AFR on acceleration is not zero but in fact is a near 100%.

What can cause acceleration carb back fires, Accelerator pump not delivering enough fuel, vacuum leaks, to lean main jets, idle mixture lean or not delivering enough fuel.

The main jets are used to meter fuel at all engine RPM except idle and just off idle with low manifold vacuum. And even at idle the fuel is still going through the main jets to the idle circuit it is just that flow is so low they have no impact on fuel metering.

The power valve delivers it's fuel through the boosters so the boosters have to have enough vacuum to pull fuel and in turn the main circuit would be active.


Given the fact that the plugs are showing a lean mix, the engine is back firing through the carb, there is an off idle stumble, and the idle mix screws are out pretty far this all points to a carb calibration/adjustment issue.

No other issue would show all these symptoms. Providing the plugs are being read correctly. Again tough to tell with out pics.

If the distributor cap or wires were cross firing this would not explain the lean mix shown on the plugs, and actually may show the opposite due to unburnt or incompletely burnt fuel.

If the ignition timing was out far enough to cause a pre ignition stumble there would be pinging under acceleration increasing with engine load. This era of SBF combustion chambers are not exactly pre detonation resistant.

If the Timing was so late to cause a stumble this would be due to incomplete combustion on the power stroke and the plugs would be showing a rich mixture due to incomplete combustion.

If this was a valve timing issue it would be across the whole engine RPM and the plugs would not show a lean mix but the opposite.


All the symptoms point to too lean a carb, And it would it not surprise me one bit if the carb is calibrated for an emission equipped 302.

 

Backfiring through carb can be caused by a lean condition - a vacuum leak - but it's almost always due to a timing problem, or a stuck valve etc. We can only make diagnosis by the symptoms he describes. He says the vacuum is steady 18" in a later post, so we can conclude (I guess) there's no vacuum leaks. But "falling on its face" acceleration wise with 20 degrees initial timing doesn't add up, and he mentioned "running out of room" to adjust the distributor.

It is tough to troubleshoot from 2nd hand info!

Sent from a pay phone

 

Amen to that, and the lack of pics does not help either, least with those we would know right off the hop if it is even the correct carb to begin with.

 

I've been off camping for a few days. The carb does have the original ID tag on it, and it corresponds to what Napa says that truck should have for a stock 351. I have no reason to believe it's anything other than an OEM carb, but after 40 years, that can be hard to tell.


Timing:
The truck initially had nearly 20 degrees of base timing - 750-800RPM with vacuum advance off and the line plugged. I tried again at about 10 degrees, and currently have it at about 15-16 degrees. There is no detonation at any of these settings which led me to believe the pointer was off a bit, as I'd mentioned in earlier posts. Still, timing by ear and to get the smoothest running, I'm up around that 16 degrees mark.


I have been using a vacuum guage for mixture setting. It's not rock solid but only wiggles by less than 1/2" or so. The truck barely runs at 2 turns out on the screws. It idles nicely at 3.5 turns out. The hesitation comes when you stab the gas in the 1k-2k RPM range. If I ease into it, it'll rev and pull up to redline, but if I floor it, the truck bogs down, and I can get an occasional backfire.


I have not torn this carb down completely, though it has obviously recently been done, because it's nice and clean and all the gaskets appear new. I did replace the power valve and accelerator pump. Not sure if you guys saw but I was able to get the idle to stumble by spraying carb cleaner directly on the throttle shaft going into the carb body, so I think I have a vacuum leak there, and that would cause a lean condition and all the symptoms I've mentioned. A rebuild isn't going to help that. I do have a remanufactured carb that just showed up that I'll try this weekend.


Plugs are whiter than they should be, adding further credence to the lean condition. They're also pretty old (indicated by surface rust on the plug body), so I'll be replacing those, along with wires, cap, and rotor just for good measure since I have no idea how old those parts are.


Vacuum advance on the dizzy works. I put a vac pump to it with the cap off and can see the arm moving in there.


Fuel pressure is a constant 8.5lbs from the mechanical pump, and both filters are new.


PCV and the hose going to it are also new.


Will report back once I slap on that new carb and tune it to see if that did the job.

 

Welp, turns out my carb was in fact a piece of Sh#&. The new carb has solved ALL the driveability problems. I was also able to compare the two and the choke linkage on the old one is all broken.


When I bolted it down I again have a small vac leak along the base gasket, so I'll be pulling it off, cleaning the surface, and reseating and hopefully it seals better. Even with that, it drives 100% better and I actually have some power now when I step down the gas.


A NOS speedo and new cable are on the way now too to hopefully solve that once and for all. This will be the 3rd speedo I've put in the truck, but the first new one. (and it wasn't cheap). Looking forward to having that working, and then I can turn my attention to installing the lift next. Got that planned a couple weekends away. Need to start soaking 40 year old bolts with PB blaster this week. Fortunately this is a rust free truck and I'm optimistic I'll be able to get everything broken loose.


Thanks everyone for the tips in this thread. While I suspected the carb from the start, I took a methodical approach to address everything else that it might be and learned a LOT about fine tuning this thing in the process. I can now remove/replace, and retune a carb in about 15-20 minutes, and also did diagnostics on nearly every system including fuel pressure, vacuum, checking vac advance on the distributor, adjusting timing....you name it! This is all good knowledge to have, as I plan to own this truck for a good long time.