Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

Idleproblems on my Cadillac 500 stroker

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77cruiser:
One of the nice things about doing that mod, if if you miss the calibration a bit you can just change the air bleed to get it where you want.

MonteCarlo:

--- Quote from: Cliff Ruggles on September 19, 2017, 03:29:01 AM ---That vacuum reading seems low to me for the CID, compression and cam specs.

Anyhow, did you try tipping in the choke flap or gently placing your hand over the choke housing to richen it up some at idle?  If the engine responds well to this it may simply need more idle fuel instead of a bunch of idle bypass air, and may like a little of both.

With .096" holes under the mixture screws you're probably at or close to maxed out on idle fuel delivery with the screws backed out 6 turns (metric threads).

I'd do the test mentioned above before blocking off the upper idle bleeds and moving them to the airhorn as it's always best to determine if the engine actually needs more fuel, or more bypass air before drastic changes to the original set-up.......Cliff

Thanks for your tips. I have not tried tipping the choke. Should do that. I think the vacuum is a bit low to. I have checked cylinderpressure and camtiming and Find nothing wrong. Maybe get some more vacuum if I get the idle right? Your suggestions seems logic now when I think about it. I was lost in the wrong loop  ;D. Another thing, do you ship spareparts to Sweden or is it to far away?

Thanks

Erik

--- End quote ---

MonteCarlo:

--- Quote from: 77cruiser on September 19, 2017, 04:39:19 AM ---One of the nice things about doing that mod, if if you miss the calibration a bit you can just change the air bleed to get it where you want.

--- End quote ---

You have a point there. Maybe try it later after trying Cliffs suggestions.

Thanks
Erik

Cliff Ruggles:
One has to keep in mind that this is 540cid, so it's going to require a significant amount of idle bypass air right to start with simply to feed the required cfm to sustain a "normal" idle speed.

I do a LOT of carburetors for big block Chevy engines, and many of them are over 500cid.  The factory used HUGE idle bypass air on the carbs used for the 454 engines in the 1970's due to lower compression and "weak" idle signal.

We've found that many are completely happy with generous amounts of bypass air to keep the throttle angle low and not expose too much transfer slot.  This fact applies as long as idle fuel delivery to the mixture screws is adequate. 

When you don't have enough fuel to the mixture screws you never find out if the engine really wants/needs more bypass air because it refuses to idle well at low throttle angles and the shear size of the engine has it pulling hard enough on the carb that it quickly/easily pulls from the boosters (nozzle drip) to sustain itself.

To diagnose this condition the tuner can use a manifold vacuum leak to lower the throttle angle.  We do this all the time here in the shop by removing the small hose going to the pull-off unless there is another one that is easier to access such as the front of the main casting above the pull-off source or rear of the baseplate.

Once you add some bypass air, re-adjust the idle speed and then take a gander to see if the nozzle drip has stopped and if you have more control with the mixture screws.  Bypass air is an EXCELLENT feature in a Quadrajet and allows the tuner to get the carb off the main system and reduce fuel flow from the exposed transfer slots at the same time.

I also recommend to experiment with initial timing as well when idle tuning.  Sometimes just a few additional degrees of timing will increase vacuum and signal to the idle system without having to go into the carb and open things up. 

Most engines with well chosen cam/compression for the CID will NOT require a lot of timing at idle speed so adding a lot of timing is not a cure-all for inadequate idle fuel, but it is still very important as a tool for idle tuning.  Basically we don't want to go in and open up things too much as idle fuel also becomes transition fuel as the throttle angle is increased.......Cliff

MonteCarlo:
Hi.

A lot of good information here. This will certainly help me get along with the idle tuning. As you say it is a big engine so it was hard for me knowing how much I should open things up without overdoing it.
The idle chanels are allready in the size of recipy 3 in your book but maybe I can go a little further then if I found out that it needs more fuel.
Should give it a try next weekend. Hopefully my next message is "problem solved" or something like that.

Thanks for information.

Erik Olsson Stockholm Sweden

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