Rochester Quadrajet set up help

[Don W.]

I have a '69 427/390 auto. Correct carb is the #7029204. I'm currently running a remanufactured Edelbrock from summit racing (SUM-210210) until I get the $'s to locate a correct unit. The Summit rebuilt unit has 0.069 main jet; 36B primary rod and DR secondary rod. On acceleration I get a hesitation.

According to this article I found: http://www.corvettefaq.com/c3/QJetTuningPaper.doc
The original set up should be a 0.071 main jet, 0.047 main rod, GM #7036019 spring and AX secondary rod.



Any idea where I find these parts and does this look like a good starting point?

[Thomas H.]

Re: Rochester Quadrajet set up help

I have a '69 427/390 auto. Correct carb is the #7029204. I'm currently running a remanufactured Edelbrock from summit racing (SUM-210210) until I get the $'s to locate a correct unit. The Summit rebuilt unit has 0.069 main jet; 36B primary rod and DR secondary rod. On acceleration I get a hesitation.

According to this article I found: http://www.corvettefaq.com/c3/QJetTuningPaper.doc
The original set up should be a 0.071 main jet, 0.047 main rod, GM #7036019 spring and AX secondary rod.



Any idea where I find these parts and does this look like a good starting point?

Don,

I'm not familiar with that particular carb but, usually hesitation on acceleration can be traced to accelerator pump issues.

Check the pump is functioning properly and you have the mixture set right (don't forget about the dwell and timing) before tearing into the carb.

Does the hesitation occur from a standing stop, during acceleration from a steady speed or when the secondaries open?

Tom

[Jack H.]

Re: Rochester Quadrajet set up help

Guess I agree here, problems of hesitation or 'stumbling' seldom flow from raw jetting. The purpose of the carb's 'shooter' is to provide that extra fuel needed to smoothly complete the transition from one stage of carb operation to another (idle circuit to primary jets + exclusive primary jet operation to primary & secondardy jet operation)...

[Don W.]

Re: Rochester Quadrajet set up help

I understand that jetting that is too lean or too rich will also cause hesitation, that is why I'm trying to find the correct parts as a starting point.

So far; I have not found any place that has any of the metering rods or secondary jets. Any thoughts on where to look?

What type of local specialty tuning shop should I look for to evaluate this car's performance and fuel system? I'm in the Mesa/Phoenix, AZ area.

[Patrick H.]

Re: Rochester Quadrajet set up help

Don,

A couple of questions...

When you get hesitation, where in the acceleration cycle do you note it? Is it when you first hit the gas? At a certain RPM? Can you tell us more? How about if you accelerate at a "moderate" pace - what happens then?
Have these symptoms always been present?

Also, did you install the jets/rods in this carb or did you leave it as-is?

How about the distributor - what do you have, and which vacuum canister is installed?

If you can answer these, we can go from there.

Patrick

[Jack H.]

Re: Rochester Quadrajet set up help

Where have you looked? I used to be able to get Rochester rods and jets through independent AC distributors as well as my local United Delco dealer. Expect to find little from GM/Chevy...

Plus, there are independent carb shops (fire up your Google search engine) and one who's name I can't remember out of the LA basin had just about anything that others didn't have...

[William V.]

Re: Rochester Quadrajet set up help

Don

I would check the plugs to see if you are running too lean. If the plugs are white then the main jet may be lean. For a proper mixture, there should be a light to medium tan deposit on the plugs.

If you have a stumble or bog is it the problem off throttle from idol (could be accelerator pump) or when you punch it. (then the secondary butterfly spring needs to be tightened). I adjust the quadrajet carb on acceleration by loosing the secondary butterfly spring until the engine bogs when punched and then tighten the spring until the bog goes away.

Good Luck

[Jack H.]

Re: Rochester Quadrajet set up help

Checking the plugs for burn time telltale is an EXCELLENT suggestion!!!

[Don W.]

Re: Rochester Quadrajet set up help

Patrick,

The car bogs or hesitates when ever you hit it; weather at stop, moving up a freeway ramp, etc. Then everything catches up and goes. It happens only on the initial part of every acceleration.

When I start the car from cold, I need to pump it slightly once maybe twice and then it fires up. If I start it anytime later that same day, including after sitting for 8 hours; it's turn the key only to start. If I pump it--it floods.

The carb is as is from Summit with the jets, etc as stated above; remanufactured replacement for this application. The distributor is a used replacement completely rebuilt and bench tested by Reno racing in Mesa, AZ. The vacuum curve was also set to GM spects. They work only on vettes and Joe Reno ownes 6 '69 vettes. He did not know the correct starting config for the carb jets, metering rod etc. and since there are 20 + options for each jet, rod, etc, that's 1,000's of possible combinations. So the suggestion is to set it up as Chevy originally did and go from there. We also did not want to dump a bunch of money into a non-matching carb.

Jack,

I've looked everywhere I can think of and emailed a few carb places. Checked paragon, pacific corvettes, summit racing, Dr Rebuild. I've used google on quadrajet and edelbrock carb parts, quadrajet AX metering rod, GM part 7036019, etc, etc all with no results.

Don

[Thomas H.]

Re: Rochester Quadrajet set up help

Patrick,

The car bogs or hesitates when ever you hit it; weather at stop, moving up a freeway ramp, etc. Then everything catches up and goes. It happens only on the initial part of every acceleration.

Don

Don,

From the pdf available on Summits web site based on your part number.
Maybe this will help a bit.............

HESITATION: LACK OF PROPER ENGINE TIMING, CAN BE THE CULPRIT. ENSURE TIMING IS SET TO VEHICLE MFGR. AND/OR CAM MFGR. SPECS. IS THE EGR VALVE OPERATIONAL? DOES THE VEHICLE HAVE A GOOD FUEL PUMP? FUEL STARVATION WILL CAUSE HESITATION, STALLING, AND AN EVENTUAL INABILITY TO START THE VEHICLE, SHOULD THE FUEL PUMP BECOME INOPERABLE. CHECK ALL VACUUM LINE AND POWER BRAKE BOOSTER CONNECTIONS FOR POTENTIAL VACUUM LEAKS. ACCELERATOR "PUMP SHOT": OFF IDLE HESITATION MAY BE ELIMINATED BY ADJUSTMENT @ THE ACCELERATOR PUMP ARM. ALTERNATE HOLE IN PUMP ARM PROVIDES ADDITIONAL ARM TRAVEL AND "PUMP SHOT".

I would verify the accelerator pump is working properly and that the float level is set to specs.

Tom

[Patrick H.]

Re: Rochester Quadrajet set up help

Patrick,

The car bogs or hesitates when ever you hit it; weather at stop, moving up a freeway ramp, etc. Then everything catches up and goes. It happens only on the initial part of every acceleration.

When I start the car from cold, I need to pump it slightly once maybe twice and then it fires up. If I start it anytime later that same day, including after sitting for 8 hours; it's turn the key only to start. If I pump it--it floods.

The carb is as is from Summit with the jets, etc as stated above; re manufactured replacement for this application. The distributor is a used replacement completely rebuilt and bench tested by Reno racing in Mesa, AZ. The vacuum curve was also set to GM specs. They work only on vettes and Joe Reno owns 6 '69 vettes. He did not know the correct starting config for the carb jets, metering rod etc. and since there are 20 + options for each jet, rod, etc, that's 1,000's of possible combinations. So the suggestion is to set it up as Chevy originally did and go from there. We also did not want to dump a bunch of money into a non-matching carb.

Jack,

I've looked everywhere I can think of and emailed a few carb places. Checked paragon, pacific corvettes, summit racing, Dr Rebuild. I've used google on quadrajet and edelbrock carb parts, quadrajet AX metering rod, GM part 7036019, etc, etc all with no results.

Don

Checking the accelerator pump is easy:
Take off the air cleaner. Now, be sure the choke is open (just hold it open with your hand if you need to do so). Open and close the throttle a couple of times. You should see gas squirting down each of the two front venturis. If so, then you know your accelerator pump is good.
Based on your starting history and your ability to flood it I'll say that your accelerator pump is probably good.

If the carb is set up with the exact same jets and rods as it would have had in 1969, that will be close enough that it should not cause any issues.

My bet is that it's the distributor. You did not state which vacuum advance can is on the distributor. This info is needed to finish answering your question. I suspect that what is really happening is that your advance can loses vacuum when you tap the throttle, your timing falls off, and the car stumbles. Why? Because the distributor can is working "all the time" whereas the accelerator pump does not really affect part throttle acceleration to any significant degree. Please check the can and report back. FYI your next assignment will probably be to supply us with distributor advance curve info, so if you have that from your rebuilder...
Also, some "reproduction" or even off-the-shelf vacuum cans are not at spec, so if you have a chance to even check the can function and report back, you'll be two steps ahead.

Less likely than any of the above? Missing power piston spring. I only know this because my 72 was missing one when I bought the car. It kept "falling on its face" when I drove it. Believe it or not the previous owner never drove it "hard enough" to notice.

For parts, check Summit but look under the Edelbrock Quadrajet parts. I just checked - there are plenty. They are the same parts as GM (or at least used to be).

I'm no expert, but I've played with Q-jets a lot and also with distributors enough to learn some things about them. Please let us know what you find out.

Patrick

[Stephen B.]

Re: Rochester Quadrajet set up help

I believe a bad vacum advance could give you hesitation problems also some one can correct me if i am wrong Steve

[Patrick H.]

Re: Rochester Quadrajet set up help

I believe a bad vacum advance could give you hesitation problems also some one can correct me if i am wrong Steve

I think it could, but we're still waiting for him to check out his car and bring back some answers.

[Don W.]

Re: Rochester Quadrajet set up help

All,

I removed the air cleaner and found a small amount of gas leaking in the front and on the left side. All of screws were loose and were tightened down. The accelartor pump is working fine. That seem to take care of the issue. A vac leak from the carb.

The car is going back to shop on Wed and they are going to run it on the dyno and dial everything in and check the carb performance, advance curve, etc.

Thanks,

Don

[Don W.]

Re: Rochester Quadrajet set up help

All...thanks for your help. Here are the results...Don

Carb top hold down screws were loose--all of them; ran smooth without hestitation after screws tightened

Run on dyno, 1st run showed running at 16.01 air/fuel ratio; timing at 4 degrees, 205 hp and 293 ft lbs torque at rear wheels

Carb primary jets changed from 69 to 72, float level adjust up ~1/4" to correct level; larger needle and seat installed;

2nd dyno run, timing adjusted to 16 degrees btdc. Tuner reports likely low compression because no issues at 16 dtdc; maybe 8.5:1. air fuel at 12.76. run up to 135 mph on dyno. 221 HP (+ 16) and 310 (+ 17) Ft lbs torque at rear wheels.

14.43 mpg highway at ~70 mph with no hard accelleration; 3.08 rear T-400 trans.