aaron871 : Quadrajet is Idling too Rich? Basic adjustments...

Quadrajet is Idling too Rich? Basic adjustments...
Post by aaron871 on Apr 22, 2006, 9:13pm

I am NOT good at working on carbs. I have proven this in the past many times.

I have a remanned Q-jet that was suppose to be "pre-tuned" from the factory. It seems to run great, idle is good too.

But if I let it idle for an extended period of time, it has a loss of power when I need to use it. It misses and won't rev over 3500 rpm in neutral. Not over 2000 under load. Every time, the same thing, I pull the plugs and they are all the same. An even film of black, dry, carbon. I replace the plugs and it runs and rev's great, awsome power. It only takes 1/2 hour of running slow on the trail and I have fouled the plugs again.

I have replaced everything in the ignition (HEI) so it is all new. I have new 8mm copper, double silicone insulated plug wires, rated for 600 degrees ($125), and I have installed another layer of insulation over them to insure that I'm not leaking spark.

I think that I'm just idling too rich.
How do I adjust this thing?

I have the manuel bookmarked and it's too much information.

I'm looking for some real simple, 1. 2. 3. style instructions on how to adjust the idle mix screws w/o screwing the thing up.

Re: Quadrajet is Idling too Rich? Basic adjustment
Post by fsjguy on Apr 22, 2006, 9:17pm

Do you have a fuel return line? Does it work? <g> The self adjusting fuel pumps often don't self adjust.

Re: Quadrajet is Idling too Rich? Basic adjustment
Post by aaron871 on Apr 22, 2006, 9:21pm

I have a return line from the mechanical pump back to the tank.
Do I need another one?

Re: Quadrajet is Idling too Rich? Basic adjustment
Post by fsjguy on Apr 23, 2006, 11:12am

One should be good enough. Once it's back up, surf over to ww.ifsja.org and ask on that forum. Lots of Q-jet knowledge over there, too. I run TBI, and have forgotten what little I thought I knew about tuning carbs.

Re: Quadrajet is Idling too Rich? Basic adjustment
Post by brute4c on Apr 23, 2006, 8:20pm

Can you see the mixture screws or are they under limiting caps? If they are under limiting caps, you can usually grind away at the caps and get rid of them if you want to invade the carb like that...take it off of course to do so...

Once you can see the mixture screws, put a tach on it of some kind that you can see while under the hood and that is sensitive in the low range....bump your idle up to about 800-1000 rpm and then turn the mixture screws in for lean until the rpm drops and then out toward rich until you achieve the highest RPM on the tach...do this for each mixture screw and bump the throttle every so often, then wait for the motor to settle into its rhythm again before continuing...might need to go back and forth a couple times to get it all tuned.

Re: Quadrajet is Idling too Rich? Basic adjustment
Post by wrecker on Apr 23, 2006, 10:40pm

Jon's suggestion is right on. Often those screws are an oblong type of head and if you buy an adjusting tool, it should have a flex shaft and a bunch of interchangable bits stored inside the grip. Also, if you do a lot of low speed idling, I would retard the timing just a bit and back the idle speed down a little too low. This will require you to give it a little more throttle to keep the rpm's up on a consistant basis. This will cause some slightly irregular throttle inputs since no one can maintain an exact input with their foot. That may be enough to keep the carb from loading up at low rpm crawling. The fuel return line may be blocked as well or as was mentioned above, it may not be working correctly. Check to see if your in carb filter is in place even if you have an in line filter. As a last resort, I would consider a fuel pressure regulator of some sort, but it really shouldn't be neccessary.

Re: Quadrajet is Idling too Rich? Basic adjustment
Post by osut362 on Apr 25, 2006, 8:53pm

problem may not be with an external adjustment. have you replaced your cam with a cam that has lowered your manifold vacume at idle? (more duration than stock) if so your vac signal to the primary metering rods may not be strong enough to fully retract the metering rods causing the engine to "load up". if the cam has been changed and the carb is still at stock specs. this may cause the your problem. just use a softer spring or cut a coil off of the spring. if you take the top off the carb. be sure and check the opperation of the metering rods to make sure they will retract. could be junk in there holding the piston up just a little. quadrajets ARE good carbs. just a little confusing to set-up.