Idle adjustment

[Paul]

When you adjust the idle on a quadrajet, #17057266 do you want the primarys completely closed? so that the idle speed is adjusted solely by the idle mixture screws?

[omaha]

   I would not recommend that kind of set up. I think it is possible to run the carb that way, (and some carbs do run like this, VW Solex PICT 34 is an example) BUT, I think you would have to have some kind of variable air adjustment under the throttle plates. The point is you are mixing a fuel and air "emulsion" with a body of more air. To get it to mix right and have rpm adjustment at idle requires both the air to be adjusted AND the emulsion mixture to be adjustable. Bypass air is  not set up to be adjustable on the Q-jet. (although I have been thinking of this idea??) And, the other problem is what happens to the transfer slots. Usually the portion of the lower slot is exposed while idling and the slot make s a nice somewhat linear transition into the off idle circuit. It all overlaps in kind of a very ingenious design. With the plates closed this would not happen, at least with the stock design.  (long post , sorry it's late. haha) Good luck!

[von]

omaha you mentioned the possibilty of adjustable bypass air. IMO that could be very do-able if some sort of small adjustable air valve could be attached to a large manifold vacuum port on the carb. There might be a difficulty in getting a large enough port in some situations, plus it would be unfiltered air. Now if some sort of tiny air filter and air valve could be rigged up it might be a good tuning aid.

[TommyK]

omaha you mentioned the possibilty of adjustable bypass air. IMO that could be very do-able if some sort of small adjustable air valve could be attached to a large manifold vacuum port on the carb. There might be a difficulty in getting a large enough port in some situations, plus it would be unfiltered air. Now if some sort of tiny air filter and air valve could be rigged up it might be a good tuning aid.

This is the essence of the Idle-Eze feature on Demon carbs.

[omaha]

Yeah, I was thinking of using a ball valve (easy to find and adjustable I would think) with some hose barb fittings and tapping a port right into the manifold somewhere then on the other side of the valve I would run the hose to a fitting on the air filter housing (into the filtered air obviously). The only problems I could think of is how to keep the ball valve from maybe moving on its own. Maybe try and put some kind of set screw in there somewhere after you get it all adjusted up and then when you needed to readjust it, loosen the set screw and reset. Hmmm........? Might just work.

[Paul]

Thanks for the reply. It seems like ive read either on this site, or somewhere that you want to have the engine idle with the throttle blades closed, using only the fuel mixtures screws. because if the throttle blades are cracked open durring idle, doesnt that cause nozzle drip? Maybe I dont understand and need to re-read Cliffs book,

Paul

[Paul]

in the diagnose a carburator problem section, beertracker is having a high idle issue, some of the respones are telling beertracker that the throttle plates should be closed at idle. so now im a little confused.  If the throttle plates are closed all the way, the air/fuel has to be coming in from somewhere? And I was under the impression it was through the idle screw circuits.

Paul

[omaha]

I think what was being discussed was the fact that the rear throttle plates were not closing or they were hanging up, this would cause a very erratic idle as the rear plates need to be 100% closed AND cannot be hanging up for any reason during the idle on the Q-jet. The rear plates are soo big that just them being open a little would cause either a fast idle or a very lean condition. Or, if the spring tension were weak on the rear plates, the plates might not close with a good result. The air flaps on top of the rear throttle plates would not  keep the air from passing into the engine at least during idle. Even though the rear throttle shaft rarely wear out, it is always a good idea to check them just for the reason of making sure the rear plates are closing "nicely."
   So, in the end, the front plates (primary) will be  open a bit to be able to adjust the idle speed, just not too much. This will not cause nozzle drip (for all intents and purposes). Remember, the primary plates control the AIRFLOW at idle, not the fuel mixture (emulsion) coming out of the idle port. I think that the air has more of an effect on the idle speed than the mixture screws but both are important. Hope this helps.

[Paul]

so it sounds like you want the primarys to open just a little, but not enough to cause the ported vacume to be activated or to cause nozzle drip?

[omaha]

Exactly....now you got it. The primaries are open a bit to let the air in for the idle needs of the engine. Now if you have a big engine and you have to open the blades too far(to achieve your desired idle rpm), then you will have the nozzle drip. This is why a lot (but not all) of the factory carbs had what is called bypass air. This is just some drilled holes in the casting and base plate that would let some AIR go into the engine WITHOUT having to go through the carbs venturi or throttle blade area. Having this feature will let the engine idle at the desired idle rpm without the throttle blades needing to be opened too far (causing the nozzle drip). You would still have the adjustability of the idle screw adjustment to vary the idle rpm but a portion of the air will be routed through the bypass passages. The size of the bypass holes would vary depending on what the engine would require for that particular engine set-up. Yes it gets a bit complicated but this design makes for a very responsive throttle feel. Once you get it set up correctly, it is the best carb for a street driven car and for best miles per gallon. (Long post I know, good luck!!)

[1967CHEVYII]

This thread helped me. I'm trying to pass emissions with the throttle blades TOTALLY CLOSED.


I am getting high HC.

Idle at 500 RPM.

Is an emulsfied mixture of air and fuel getting dumped from the idle ports?

Obviously the idle mixture screws control the volume of (whatever) is emitted from the idle ports.

I assume I need more air (via cracking the primary blades) to "lean" the emulsified mixture being emitted.

The "sweet spot" i presume, is to open the IMS as little as possible and open the primary blades wide enough via the curb idle screw, while not activating the timed vaccum port AND accomplishing desired idle RPM?

Am I on the right track?

-Adam