Low RPM, Shallow Throttle Stumbling

Looking for some advice from some carb experts out there, I'm having an issue which I can't understand or trace. Installed a new Quick Fuel 680cfm VS carb on my stock 1987 F-250 with a 460, got it all set up and running properly, but now it has suddenly developed a horrible dead zone / stumble from 1,000 to 1,500 RPM and up to 2,000 RPM at little to no throttle angle (resting foot on accelerator). The engine idles perfectly and runs perfectly from 1,500 RPM upwards. I can richen or lean out the idle circuit and it has no effect, I jetted up and down multiple sizes again with no effect. It isn't idling too far into the transition slots, it's a stock motor with a stock cam. If I rev the engine in neutral it still exhibits this extreme dead zone in the same RPM range. I tested the vacuum source to the distributor, and it pulls full manifold vacuum as it should. What is even stranger is when I first installed the carb it ran absolutely perfectly and had lots of torque 1,000 to 1,500 rpm. With my old 750DP carb it also ran great in this range.

I have no idea why this is happening or what I can do to fix it. Any tips or ideas?

Thank you in advance,

Ian

Did you change locations for the vacume advance it should not be pulling full vacume unless you are at wot

No. I had my distributor recurved years ago specifically for a full vacuum source, not ported. Truck has run great for ages with it.

Accelerator pump adjustment?

LivermoreDave wrote:Accelerator pump adjustment?

x2 that what I would check might be little late or stopped up some of the bowl gaskets only how the pump hole on one side something you might wont to check if not already

I had dismissed the thought of it being the accelerator pump system since it happens with unchanging throttle position (holding steady), however I had forgotten that I recently checked the accelerator pump nozzle and the lower gasket may not have seated correctly when I replaced it. I will look at that tomorrow. Thanks guys!

Well as much as I hoped it would be just an accelerator pump issue, it is not: it works exactly as it should, squirts fuel right when it is necessary. Engine still runs terrible from 1,000 to 1,500 rpm, stumbling as if wanting to die and backfiring. Above that it sounds like something comes in and it totally changes: it roars to life and freely revs. I'm right back where I started with this. I wonder if some idle fuel passageway is blocked?

Sure sounds like a vacuum leak some where to me. May even be inside the carb. on the metering block gaskets. You guys are way different than me on the vac. advance though. At WOT you should have no vac. to the dist. I have never heard of running direct, manifold vac. to the dist. except on smog motors that used it to help cold start warm up, and then switched to ported, through the vac. switch in the water neck. Manifold vacuum drops under load. You want dist. vacuum do go down under load, to avoid pre ignition, not UP. I would also clean out the carb and blow out all air passages and the air bleeds, primaries and secondaries, check all gaskets, and or, replace them, make sure you don't have any cracked vac. lines also, and go from there. Also check your idle screw settings, with engine warmed up. If they are too lean, they can cause a dead spot, on transition to the main circuit. Don't rule out the power valve either, if you close the idle screws all the way, and the engine does not die, it is likely the powervalve is bad. Pull needle and seats and check for trash under them too. Lots of things to eliminate, but you say this carb worked good once, so it should not be anything to hard to correct.