Help me get the off-idle stumble out of my QF 750-AN Double Pumper

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goldduster318

goldduster318

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Hello all,

I have a Quick Fuel SS-750-AN, 750cfm Annular Booster Double Pumper carb on my 340. This carb seems to run great in all areas except right off of idle and if you get into the throttle a little bit at low RPM in 4th gear.

The car is a '70 Duster, full weight, 4-speed, and 3.23 gear.

Primary Jet: 68
Secondary Jet: 78
Idle Air Bleed: 69/69
High Speed Air Bleed: 29/29
Primary Squirter: 31
Secondary Squirter: 31
Power Valve: 8.5

Fuel Pressure is 6 psi with a regulator and the fuel level in the bowls is just slightly below the center of the window.

I have a wideband on it, and I'm satisfied with the A/F at any rpm above 2300, below that it seems to go lean/rich a bit depending on RPM/load

At idle, the A/F ratio is between 13.5 and 14:1 (950rpm)
When I pop the clutch, the engine stumbles and leans out on the wideband into the 16-17:1 range for a second then recovers. In many cases it becomes a bit rich after this.

Any ideas on how to get rid of this stumble? I've tried several accellerator pump cams and haven't really gotten rid of it. It seems to be best with either the brown or green pump cam.

Thanks for any help!
 
Carb too big, made for racing. Annular discharge carbs are made to idle barely or work at wide open throttle. Aren't the designed for engines with big cams that won't idle off the primaries because the butterflies are opened too far? tmm
 
If anything, I've read that annular boosters should improve low end response and fuel signal. Other than during the clutch pop/getting it rolling, it works fantastic everywhere else.

I have a friend who has a car set up which is close... 340 with [email protected] cam/4-speed/3.23's and a 750 DP and his works without this issue.
 
Annulars were used in ford trucks originally I think for better low end, not at all a race thing. My 750 annular works nice. I would try a tubed squirter in the mid 30's your PV seems hi what's your vacuum at idle?
 
I have a quick fuel super street series 680cfm vaccum secondary carburetor and I had the same problem, maybe I can help you out.
First thing to do is make sure there is no slack in your accelerator pump arm, (the bolt and nut) At wide open throttle make sure there is at least .015 clearance, you can use a feeler gauge to check, this way the diaphragm does not bottom out.
Next thing to do would to be increase your accelerator pump discharge nozzle size, you said it has a .031 on it now, try a .033 and a .035, I run a .035 in mine with the hi-flow hollow screw. Next thing you can try is changing the pump cam, But without knowing what color pump cam you have now I can't give you a recommendation.
Mine likes the white pump in the number 1 hole, The white and orange and red pump cams have a more aggressive shot of fuel right off idle.
Another thing you can do is decrease the size of your idle air bleeds, you said it has .069 iab's in it now, you could try some .065's and re-set your mixture screws and see what happens. Another important question I forgot to ask... Where is your float level? These are some different things you can do to ry and tune out a bo, but it's not everything. There are idle feed restrictors in the metering block that can be increased as well. but we need to determine if this is an accelerator pump problem or a transision circuit problem.
Try the nozzles and the pump cams first, if that don't work let me know and i'll do my best to walk you thru this.
 
Does the motor have a big cam? and is the carb a 4 corner idle circuit? If so You may be idling off the primaries and you should be idling off the secondaries.

If the primaries are open to far at idle it will stumble because you are all ready into the transition ports to far. Before setting the carb on you should always square the transfer ports on the primaries. If it idles to low you turn the secondaries up. Not the primaries.

This gives the transition ports full travel to get into the emulsion jets. My friends car was doing what you are describing. It was because he didn't have a 4 corner carb and he couldn't idle off the secondaries..

Then you have to play with pump and jets. And you will never get the afr correct at all rpms as you want. Try it if you have the 4 corner carb.

I have a QF 1050 on my 340 now. It hits like a off and on switch and goes right to 8000 in low with stabbing it.

All 4 corners work this way. Why do you think they have a secondary idle screw. and 4 air mixture.

If you need to resquare the primaries pm me for my number
. I started with a 850 went to a 950 then a 1000 and now a 1050 . Six packs were 1300. So your carb should work good on a mild 340. If you know how to let the carb do the work as it should.

Good motors set right will pull like a diesel and twist up like a chainsaw with out the chain.
 
I would also recomend you check the ignition is set and working correctly (mechanical and vacuum advance, etc).
Also this article is worth reading on how to setup the idle on your carb. About 2/3 down the page it mentions the correct butterfly position to have for a smooth transition from the idle circuit to the main circuit.
http://www.circletrack.com/techarticles/ctrp_0707_carburetor_tuning/viewall.html
 
Ok, I'll try a few of the things. To answer what some of you are asking.

The cam is a 227/231 .530/.530 hydraulic roller 110 LSA
Accel pump cam is the green one
Idle vacuum is 13-14 in Hg in neutral
Float level puts the fuel just under halfway up the clear sight plug. I can try raising it slightly.
It is a 4-corner idle carb
Ignition is 14 degrees @ idle and 35 total. When I hooked up the vacuum advance, it ran worse.

I have cranked the rear throttle blades open some more, I'd say they are similar in their position, but I'll pull the carb off and check to see if the primary transfer slot is a square, if not, I'll lower it and raise the rear blades.

I have some 0.035 tubed squirters, I'll put one on the primary side. I can also try the air bleeds as I have all sizes between 65 and 75. I'll check the adjustment on the accelerator pump as well.

I had a lean spot on the wideband between 8.5 and 6.5 in Hg when loaded, so I used the 8.5 powervalve and it cleaned it up nicely. It still stays shut at idle and any time where there is not a heavy load as this engine pulls easily 15-20 in Hg during any steady speed operation.
 
Is the squirter hitting as soon as you move the throttle? If it's got any delay, you'll get a spike.

I'd bump the initial to 17ish and if it's an MSD distributor, put the black 18* bushing on it.
 
I put the "35" shooter in and that didn't seem to help so much. It did increase the time where it was rich after tip-in when shifting though.

I then switched to the 65 air bleeds and reset the mixture screws. That helped a lot. The stumble was almost gone, but not totally.

I tried raising the floats and it made it worse, so I dropped it back down to about where it was.

I do seem to be getting accelerator pump right away.

The distributor is a stock style one with a reset advance curve. I've got an FBO box on it (the A668 I think). I can try a few degrees in it as it won't be too far off on total. I'll get to that tomorrow.
 
ok since the air bleeds helped a lot that tells me it is not the accelerator pump circuit it's the transistion circuit. Next thing you need to do is determine what size idle feed restrictors are in your carburetor. My super street 680cfm came with .031's, you can call quick fuel and ask them what size are in your carburetor.
There are 2 idle feed restrictors in each metering block, they are up top and they can be removed with a small flat head screw driver, go up .002 on the idle feed restrictors, so say you have .031 try the .033's. After putting bigger idle feed restrictors in the carburetor re-set your idle mixture screws to the highest vaccum reading and see if that takes care of your problem, I'm about 80 percent sure this will cure it.
 
Based on the measurements on my calipers (not the best way, I know), I believe they are 0.031's as well. I'll order some 0.033's, and probably 0.032's and 0.034's just because I can't get these locally and they're relatively cheap.
 
If you have an .061-.063 IAB, try that and see if it improves again.
 
Yall are all full of it. More nitrous.
 
I think going to the .033 idle feed restrictors will be the last thing you will need to do, But lets say you put the .033's in and it still has slight stuble, then try the .061 idle air bleeds. But I think your problem will be gone with the .033 idle feed restrictors and the .065 idle air bleeds.
I had the same exact problem with my carburetor, my carburetor out of the box had .070 idle air bleeds and .031 idle feed restrictors and .031 squirter with a pink pump cam. I ended up running .033 idle feed restrictors and .065 idle air bleeds a .035 squirter with a hi-flow screw and the white pump cam on position number 1 and each step took the stumble away a little at a time, the last thing I did was change the idle feed restrictors and that took it completely away.
 
Let us know when you install the .033 idle feed restrictors and let us know what happens.
 
Turn your initial timing up to 20 reset your total if that works I think it will Im running 22 initial on mine 35 total and its great at all speeds, you wont believe the diffrence.
 
Maybe I missed it, but what intake is on that? I found my single plane M1 with my Holley 750 really liked a smaller shooter (31) and 50cc pump kit with the large brown cam... Before that I had a bad stumble off idle like you described I couldnt get rid of.
 
The intake is an Edelbrock Performer RPM Air Gap

No nitrous or Edelbrocks...from my experience you're more likely to get an off idle stumble with an edelbrock than a Holley type carb regardless.
 
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