I have a Quadrajet with the following specifications that came on my mostly original 55,000 mile 72, which I purchased in July. Have been having problems with it since purchased, most likely due to the car being run for an unknown period with NO fuel filter.

Specs from carb:
Quadrajet R-4 # 17054927 2811
on back in circle 70456n9 (can't read the n number as it seems to have been purposely center punched)

Main Jets 74
Main Needles 43B
Secondary Needles DA DA
Secondary Hanger 33797 O

I have rebuilt the carb and cleaned it out several times. Original problem was the car would not idle below 1100 RPM. After cleaning out the carb several times, remaining problem was fuel leak at driver side main venturi. I cleared out the idle vent tube with a small paper clip, which seems to have fixed that problem.

I have two questions:

The first is what should the float level be for the carb (new float installed) The older or the newer spec? I currently have it set at .375 in. Should it be this or .410 inch?

The second question. In an effort to stop the venturi leaking (before paper clip cleaning), it appeared that the power piston was set too high, so I screwed with it. Popped out the plug in the front of the carb between the two mixture screws and jacked the height down quite a bit (about 7 turns). I seem to have fixed the Venturi leak (with the paper clip as above) and now introduced an extreme off idle hesitation. Am I correct in assuming that I have most likely set the power piston too low causing an off idle initial lean condition. The power piston is fully seating at idle and rising upon opening the throttle.

Additional information... The engine is a stock base L-48, but with a 327/350 cam installed by the person I bought the car from. It has new points, plugs, wires, and correct dwell setting. The car will idle reasonably well all the way down to 700 RPM now, but the mixture screws don't seem to be doing much in addition to the off idle hesitation.

I haven't tried the disconnect the accelerator pump arm trick to test for lean/rich condition yet, too cold for the past week in NJ. Hope to try this over the weekend.

Thanks in advance for any info...

 

I'm not going to be any help to you, but I have disturbing similarities. My '71 coupe also has the base L-48 minimal options and mine doesn't idle well below eleven hundred rpm. Interested in other's comments. I should thank my lucky stars that it starts well and runs pretty smooth. Still looking for the "umph" and am assuming more horsepower will cure all ills. Haven't modified anything yet but am "in the hunt."

 

petronix,

Check the archives and tech papers for articles by Lars on these carbs. He walks you through it in excellent detail. He even does road tours to tune people's cars... I think your carb is old-spec on the float. I have a newer Edelbrock #1902 and I think that uses the new spec...

 

The float should be .410" to .420". You may want to get a set of .45B main rods. Your cruise metering looks a little fat.

As far as the off idle stumble goes, let me think. I seem to remember that the adjustment screw is exposed to vacuum. If you haven't already done it, it would be a good idea to put a dab of silicone in there after you're done tuning. I don't know, it's been a while since I've looked at one of the 170 carbs. My advise would be to put it back as close as possible to where it was before you moved it then move the adjustment slowly and see if you can make some head way (sense you solved the problem by cleaning the venturi out).

You may look at the accelerator pump to find the source of the stumble. Make sure you're getting the full stroke out of it and make sure the throttle blades are coming wide open when the gas pedal is on the floor. Trust me, this can cause all kinds of strange problems that have you chasing your tail.

With that cam you may need to open up the idle discharge port (DON'T GO OVER .093/.095"). This will give you control of the idle mixture.

I guess that's all. I worked all night, I'm going to bed.

BigBlockk

Later.....

 

Thanks for the info.

I have studied the Lars info, which is how I have gotten as far as I have.

I cranked the power piston adjuster screw back in a few more turns today and the bad off idle hesitation seems to have gone away.

Mixture screws seem to be able to make the car run really bad now, but not able to make it idle real smooth +- 25RPM at 800 idle. At least they do something now though. Occasional carb popping is still happening when mashing the throttle off idle. Probably the best I'll be able to do with the cam, and until rejetting and maybe drilling the idle holes.

I'll probably pop the top of the carb off one more time to reset the float a little lower, and check the main rod height.

Good idea about a dab of sealer on the power piston adjuster screw. Probably do that too.

 

did you replace the float? brass or plastic? they behave differently.

after i rebuilt mine and set everything to 'spec', i had to raise the float a bit to eliminate a high rpm stumble.

 

You may be able to get rid of that popping by going 1/8'th of one (1) turn tighter on the air door preload spring adjustment. ONLY go 1/8'th of one (1) turn. Go easy on this adjustment or you will break the spring.

Also on the idle feed holes. The numbers I gave you are for the most radical camshafts in the world and if you go bigger than .095 you've trashed the base. Your setup should do well with .088 max. Remember, it may be a bigger cam but it is still a stock Chevy grind.

It would be nice if Lars would chime in on this one.

Hope this helps.

BigBlockk

Later.....

 

I replaced the float, it is stock type Phenolic.

I am not sure what is meant by air door spring. I this means the secondary air valve;; it is adjusted tight already. The secondaries don't open until full throttle 3000+ RPM and then only slightly. If anything they are not opening enough.

I think I need smaller primary needles. The suggestion was to go from 43 to 45. Wouldn't this be fatter/leaner rods?

 

I replaced the float, it is stock type Phenolic.

I am not sure what is meant by air door spring. If this means the secondary air valve; it is adjusted tight already. The secondaries don't open until full throttle 3000+ RPM and then only slightly. If anything they are not opening enough.

I think I need smaller/richer primary needles. The suggestion was to go from 43 to 45. Wouldn't this be fatter/leaner rods?

Thanks

 

The 43 or 45 refers to the cruise metering portion of the rod. The power tips of all Quadrajet primary rods is the same .026" except for those funky "M" truck rods that have .036" tips. This is one of the great things about the Quadrajet. You can run the part throttle/cruise on the lean side and still have it richen up when you want the engine to make power. In short, at anything less 90% throttle your metering fuel with the rods. Over 90% you're metering with the main jets.

BigBlockk

Later.....