Dead Spot

[390xlt]

There's a dead spot in my throttle, and I can't figure it out, though there's every chance I've overlooked something obvious.
What happens is this: Give it a little bit of gas and it starts revving fine, but once the pedal hits a certain spot, it dies; it doesn't stumble or bog, it just stops running. With enough cajoling, it will get through that spot, but with slow to normal application of the pedal, it dies. Naturally, this dead spot occurs right around the point I need to start engaging the clutch, which can get pretty interesting, especially on a hill.
Given the fact that it dies (rather than stumbles), I thought that maybe the ignition is somehow involved, but I'm not sure how it would work fine on either side of the dead spot but not right there in the middle.
This leads me to think fuel delivery issues. Since it revs fine as long as I don't put the pedal down too far, I didn't think it would be the accelerator pump, but I checked it anyhow and it's working as it should be. My only other thought is that something is creating a massive and sudden vacuum leak when the pedal is in that position, but everything at the carb seems tight and has uniform movement throughout the pedal's range of motion.
The problem never completely goes away, but it gets better as the engine warms up.
What should I check next?
Thanks,
Chris

[Ranchero50]

Pull the vacuum advance hose off the carb and draw on it while the engine idles. You should be able to feel the timing advance (idle will climb some). I've seen the diaphragm split and cause a chatter at a certain vacuum causing all sorts of grief. Other check is to remove the air cleaner and watch how the fuel comes out of the venturi ports (wrong term) when you slowly rev it up. Also check for play between the accel pump arm and the cam as well as the duration of the spray. Finally make sure the choke system is working correctly.

Post up some pics of your combo, something obvious might be seen.

[Busboy]

I think Ranchero 50 was right on with the suggestion of checking the vacuum advance, however I'd be more inclined to think the vac advance diaphragm might be rotating the breaker plate and causing the wire to the points to either touch the inside of the distributor or there is a broken spot inside the wire to the points and the rotation of the breaker plate opens contact. I've seen it before. Remember our trucks and wiring is 50 years old.

[390xlt]

Thanks so much. I'll check all those things, starting with the vacuum advance. I am sure the choke is working properly (it's manual), and I have electronic ignition, which I'm pretty sure not the culprit, but I'll double check all the wiring involved in that. Thanks again, and I'm certainly open to other ideas/suggestions. I'll report back when I've checked everything.

[390xlt]

Drawing on the vacuum has no effect on the idle at all. Should I pursue something there?
Accelerator pump and associated linkage is all functioning as it should, but I do wonder if perhaps I should try it in the outboard hole.
I am getting a little pop from the carb right before the dead spot, so I’m hesitant to put my face over it to check the fuel flow while it’s running.
I double checked all ignition wiring, and other than the wire providing power being a little short, everything is in order.
Not sure if it’s worth noting, but when manipulating the throttle by hand, this issue is not nearly as noticeable.
What should I try next?

[Ranchero50]

Advance your timing to 15-18 BTDC and see if it runs better. If it does, you need to figure out the vacuum advance. might be worthwhile to pull the cap and see if the timing plate is moving when you draw vacuum on the can. It should.

[390xlt]

I think the vacuum advance diaphragm is completely shot. The timing plate moves fine when I push on it, but sucking on the tube doesn't do anything. There's no resistance and air seems to be moving through the vacuum hose unimpeded. Can I reasonably deduce that that's my problem?
I was about to try advancing the timing, only to find out my timing light has gone missing. I suppose I could estimate it, but past experiments in guessing at timing have met with, shall we say, mixed results.

[ButtaBean]

That, along with the unmetered air drawing through the diaphragm is a very likely culprit. Excellent detective work! Even if it doesn't resolve your dead spot, still an issue that needs corrected.

[390xlt]

Any advice on part numbers for that vacuum advance? It's available (though not as widely as I thought it would be), but plugging in make/model/engine gives a number of different results. Any ideas?

[Ranchero50]

I'd just pick up a new dizzy and plop it in. They used to be pretty cheap. Just mark the rotor where it's at on the old one and make sure it ends up in the same place on the new one.

[390xlt]

I found a vacuum advance listed for a Thunderbird with a 390, so I went with that one. There's about a $60 difference between the vacuum advance and the cheapest distributor I could find; not a ton of money, but enough that I'm willing to roll the dice and hope that everything else in the distributor is good enough. And to keep the OE instead of whatever Autozone is offering. Also got a new choke pull off, since I know mine's been bad for a while. I'll report back once I've got some results.

[iwhodat]

Make sure you don't have any vacuum leaks. I had a really bad stumble on acceleration until I fixed my vacuum leaks. Try closing the choke on your carb, a little bit, and see if it runs better when you take it for a spin. Good luck, man

[Twisted_Steel]

Any word on if the new diaphragm fixed your off idle problem?
I'm going to check a 302 w/ a 600 Holley today that is having the same problem.

[Ohiotinkerer]

Any advice on part numbers for that vacuum advance? It's available (though not as widely as I thought it would be), but plugging in make/model/engine gives a number of different results. Any ideas?

Just bought one from our local NAPA but you'll need the part # off the distributor because there's 4 different ones and they all have a different letter at the end of the part #.........mine was around $40......NAPA was the only store that could give me the correct part for mine........or could even get it........

[390xlt]

I (finally) found one that fits: B7A12370A. Apparently, it also fits later y blocks. It hasn't fixed the problem entirely, but it has made it considerably better. Well, considerably not as bad, at least. I also found my timing light, so I'm going to check the timing as soon as I can. If the timing is off and setting it fixes the problem, I'll be happy. Otherwise, I guess I'll just keep searching.