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Stock '49 216
Rochester B carb.
Daily driver
My old truck had been starting and running poorly and developed a dead spot off idle. I wasn't surprised because I hadn't done a tune up in over a year.
The other day I changed the points, cap, rotor, and plugs. She now starts up and runs great, but I still have the flat spot. It's bad enough that if I crack the throttle open quick and hold it it dies.
Hold on, this is where I get confused.
If I'm driving down the road at any speed and I quickly change the position of the accelerator pedal (about 1/4") in either direction, open or closed, I get the same flat spot loss of power. In my mind that eliminates the accelerator pump.
I've verified that the distributor is rotating when I open the throttle.
Is this electrical? Carb? Timing?

I know it sounds simple but double check your points gap. Lost a drag race many years ago because the points had closed up just enough to cause a stumble upon acceleration.🛠

Easy enough to check the accelerator pump, look down the carburetor with engine off and crack open the throttle.

I've double checked the points gap = .020

Joe, checking the accelerator pump was one of the first things I did. Will a bad accelerator pump cause the symptoms listed above?

"If I'm driving down the road at any speed and I quickly change the position of the accelerator pedal (about 1/4") in either direction, open or closed, I get the same flat spot loss of power.

I'd check the fuel lines, fuel filter are clean also tank, my 53 ran but line from tank was almost totally clogged. Had a bad flooding problem with my Rochester, had to add an adjustable regulator to fuel line, getting pressure down to about 2-3 lbs did the trick.
Best of luck

Had a similar problem last spring. It was a bad vacuum advance. Make sure the entire distributor rotates when you give it throttle from idle.

OK, missed that LOL. I've reread your OP and didn't see where you checked the timing. Point gap effects timing.
If you've got a dwell meter make sure it falls in between 30-38.
I've also got a 49 216 and I've found that setting the timing to the BB at 500-550 rpm worked best. I've tried setting by max vacuum and advancing more due to better fuel but in my particular engine setting by original spec gives best all around performance.

If all that is good, take a $1 bill or similar and hold it over the exhaust pipe while idling. If it sucks in at any point you've got either a tight, sticking or burnt exhaust valve.

If it doesn't I wouldn't rule out checking the Roch B's throttle shaft for being a little loose causing a vacuum leak.
Good luck

Volfandt, Thanks for the good info. I don't have a dwell meter. Looks like it's time I get one. I set my timing to the BB at about 550 rpm, but that was last time I adjusted the timing about 5 years ago.

I'll try the $1 bill test for my own piece of mind, but also because it should mess with the nosy neighbors head

Accelerator pumps mess with the opening of the throttle, not closing. When you close the throttle, the vacuum goes way up, so the vacuum advance should pull all the way in. When you open the throttle, the vacuum drops to near zero (depending on how quickly it's opened) so the vacuum advance returns to home position. The accelerator pump also operates when opening the throttle, without it, the engine runs lean for a split second till the air flow and fuel flow come back together. You should see fuel spray about anytime you open the throttle. With a quick stab, it should really spray, a slow open might just dribble out, but either way, you should see some fuel.

Since it's acting up opening and closing, look to the distributor and timing. Dwell meters are cheap, no need for Snap-on here!

Joe

When you turn the idle/mixture screw in or out with the engine at idle does the RPM go up or down at a steady pace?
If not, you may have developed a vacuum leak. The idle/mixture screw won't do much when you have a vacuum leak.

That’s interesting. When I was messing around with my 235 last night, I was surprised at how little difference the idle mixture screw made.

However, vac gauge was a pretty steady 19-21 inches depending on how I adjusted. Some intermittent, minor needle bounce when I would adjust the timing.

Now you need to hone your vacuum leak detecting skills. Turn the idle screw out to lower the RPM as low as you can so that it barely stays running. This will reduce the amount of wind caused by the fan so that you can use an unlit propane torch to find a vacuum leak. RPM will increase when propane gets sucked into the leak. To illustrate what the RPM increase sounds like, place the propane over the top of the carburetor for a test.
Carl

Propane check was on my list, but never thought about the fan interfering. Good point.

UPDATE:
In addition to the new points(.020), cap, rotor, and plugs(.045), I replaced both fuel filters (fuel lines are only a couple years old), and checked for vacuum leaks (none).
I adjusted the valves. I also did as Volfandt suggested and held a piece of paper over the exhaust pipe for about 30 seconds as it idled. Not only was there no sign of a tight or sticking valve, but the paper came away totally clean except for sprinkles of water.
I even replaced the carb with a known good rebuilt one. It ran the same, so I addressed the timing.
Timing and dwell were off so I adjusted dwell to 35-36 (needle steady as a rock) and timing on the BB at 550 rpm. I guess I'm confused as to how you're supposed to set the dwell at 30-38 and yet keep the points gap at .018
Anyway, the truck ran like a champ the rest of the day. But the following morning, even though it started up and purred, I could hardly give it gas without killing it. This morning (33° outside) she started right up and is running great.
If you have any ideas, I'm listening, but I think this is something I'll have to figure out myself.

Did you try Carl's unlit propane-torch test (at idle)?

Are you sure this is not a choke issue? Since trucks don't usually run "good" on a cold morning, maybe you choked it and kept choke on awhile. Play with the choke and see if you can leave it partial and if truck runs better all day. That will give a clue to narrow down trouble. It could mean a lean issue. Fuel issue. Not electrics.

I thought I posted on here already. Was my post deleted?

Pretty sure you had posted. That’s weird.

The propane test was one of the first things I did. Then I adjusted the idle mixture screw as Carl suggested and it definitely has an effect on the engine.

Bartamos, The choke is operating correctly and the truck runs better with the choke in/open when warmed up. If I pull the choke out a bit I can tell it's running rich.

I drove it around all day yesterday running errands and it ran fine. It's hard to diagnose an issue that appears and dissapears of its own accord.

With intermittent issues like what you have can best be solved by driving it until it gets worse.
I have had an issue with a '53 235 with a loud hydraulic lifter until the engine reaches operating temperature. Finally it is now clacking even after fully warmed up. I can now pull the valve cover and find out exactly which lifter is making the noise and replace it.
Carl