Need Carb Help Please

[Tony 69]

I have a quick fuel 850 DP mechanical secondary. Always ran rich but other than that real strong, no hesitation, no problems. I took it off to replace the valley pan and decided to adjust the transition slot. It was way open and I adjusted it to a little “square “. I also ran the idle screws out 2 turns. They had always been out about 1/2 turn and didn’t make that much of a difference. Get it all back together and it won’t idle unless I barely touch the gas or screw the idle screw in which opens the transition back up. I pulled the idle air bleeds and they are #70. My question is do I go to a 72 or 74? Or is it a fuel problem? Also the idle air mixture screws are back to the original 1/2 turn out to keep it running. Thanks.

 

[AR67GTX]

You probably need to flip the carb over and adjust the secondary throttle blades a little more open. When you close down the primary blades to correct the transition slot, you generally need to open the resting position of the secondary throttle blades to make up for the loss of air flow necessary to support idle speed. Some recommend the best way to adjust idle speed on Holley-type carbs is to adjust the primary throttle position so the transition slot that’s exposed is square. Mount the carb, start the engine and to adjust idle speed, leaving the idle speed screw alone, and instead adjust the secondary plate position to set the idle speed.

On Holleys the secondary throttle plate screw is a small, straight slot, interference fit screw in a small well. Some people replace that with a long Allen screw and lock nut to ease adjustment w/o removing the carb. A DP may already have an Allen head screw in there or something different from a Holley. The only real precaution is that there are transition slots in the secondary bores too and you do not want any of the slot exposed below the blade or your rich idle will get richer. If you have a wild cam and you can’t get enough air on the secondary blades w/o exposing the transition slots, then you could be faced with drilling a hole in each primary blades to allow more air to pass.

 

[Tony 69]

I will give that a try tomorrow. Thanks for the feeback

 

[hunt2elk]

If you have a Q-850, the secondary blades are adjustable without removing the carb.

 

[jimbosride]

While you guys are here....

What would be the correct method to adjust the 4 mixture screws on a car with a cam. Vacuum Gage ( don't think it will work)

QFT-H2-780 VS 383

 

[Geoff 2]

I presume this carb has 4 idle mixture screws? If so, opening the secs is just transferring the problem to the secs.......will still be rich because extra fuel coming from the now exposed sec T slot.

Correct fix is more idle air. Some of these carbs had an adjustable air screw. If not, the t/blades should be drilled to let more air in...because that is what the engine wants. Exposed t/slot & rich idle is the casualty. I would start with a 3/32" hole in each pri blade near the t/slot. If that does not close the blades enough [ 0.020-0.040" slot showing below blades is ideal ], drill the sec blades & re-check. Go up in 1/64" increments. Nothing to with air bleeds.

 

[hunt2elk]

I wouldn't jump the gun and drill holes yet. I have run several Q series Quick Fuel carbs, and have them on my cars now. I have never had to drill the blades. I will say that every one I have had is set up pig rich for a street car from the factory though. You need to start with smaller idle feed restrictors. Don't mess around with the air bleeds until way later in the tuning process, and they typically don't need much of a change.

 

[Tony 69]

Idle feed restrictor? Is that in the metering block? Sorry I don’t know what that is.

 

[hunt2elk]

Yes, in the metering blocks. There are 4 of them. You should pick up a Holley tuning book to get aquatinted with all the different circuits.

 

[AR67GTX]

While you guys are here....

What would be the correct method to adjust the 4 mixture screws on a car with a cam. Vacuum Gage ( don't think it will work)

QFT-H2-780 VS 383

Good question. Just for clarity my comments above pertain to vacuum secondary Holleys. I’ve never messed with a dual pumper Holley.

 

[Tony 69]

the carb has 76 in the front and 80 in the secondary
Any suggestions?
Thanks.

 

[BSB67]

the carb has 76 in the front and 80 in the rear

I have no idea what you are saying here.

The things you need to do have been touched upon. You just need to step through them. Open secondaries, doesn’t work, drill t-plates. Then reduce the IFR.

Any idea what cam is in it?

 

[Geoff 2]

The IFR is not sized for.....idle!!!! There is always enough fuel for idle.

The IFR is sized for the t-slot, for transition to the main cct....

Changing the idle air bleed will not change the idle mixture. The idle mixture screws do that....

The IAB will change where idle cct [ which includes the T slot ] stops fuel flowing from the idle cct. The idea is to size the air bleeds so that the idle cct dies off & the main system takes over in smooth transition with no overlap of the ccts..

 

[451Mopar]

I would leave the air bleeds alone, and set the front and rear throttle plates to expose the transfer slots square. Adjust idle screws, and if engine is idling too slow, then likely have to drill a small hole(s) in the throttle plates to where idle speed increases, and then you can slightly close the throttle blades for a lower idle speed.
If the off-idle transsion / just off idle driving in the area of the transfer slot mixture is wrong, check the Idle Feed Restrictor size in the metering block. Usually larger than 0.032" is on the rich side from what i have seen.