[tremo]

I'm considering changing the primary metering rods on my rebuilt '72 Chevy #7042210 650cfm QuadraJet on my 400ci and I'm wondering what the stock metering rod number is for my carb?

The secondaries are DA rods (which are stock) - easier to pull out and check vs. the primaries. I'd like to know the stock primary rods as I want to order a few new ones before I pull the top off to see how they perform at my altitude (~5,800 ft).

Thanks!

Jim.

[77 TRASHCAN]

There was never a 650 Quadrajet. Most were 750 or 800. Yours is likely a 750? Not trying to sound like a Beavis...

You need to call Cliff, if you don't have any parts on hand. Www.cliffshighperformance.com he cal sell you any parts you may need, AND sell you the parts needed for today's blend of gasoline. Tell him I said hi!!!

[Kenth]

From factory the 1972 Chevy Truck 350 #7042210 Quadrajet has #74 primary jets and 43B primary rods.

[shaker455]

Originally Posted by tremo

I'm considering changing the primary metering rods on my rebuilt '72 Chevy #7042210 650cfm QuadraJet on my 400ci and I'm wondering what the stock metering rod number is for my carb?

The secondaries are DA rods (which are stock) - easier to pull out and check vs. the primaries. I'd like to know the stock primary rods as I want to order a few new ones before I pull the top off to see how they perform at my altitude (~5,800 ft).

Thanks!

Jim.

Jim, With Kenth info provided, (Thanks Kenth) rule of thumb is drop a jet size for every 1,500 ft in elevation.
Big thanks to Mr. Tom Vaught!
Jeff

[25stevem]

Yes, but that formula assumes that the carb was jetted right for the motor it’s on, but at a lower altitude.

We don’t even know what cid motor the OP has this carb on no less.

Also if the OP has this carb on a 350 ( actually a 354 cid stock) it has to be taken into account that the stroke of a Pontiac 350 is a heck of a lot more then a Chevy 350 and this produces a much earlier demand for air and in turn fuel for any given rpm .

I might be more inclined to first change out the power piston spring to a lighter one then to change rods.

If it where my car I would get a set up in the exh system to check out what the air to fuel ratio is now then to potentially put the motor into a pinging of Knock condition while checking out metering rods.

[tremo]

Thanks everyone, good feedback! Note that the specs for my 400ci are in my signature, fyi. In my research years ago on my #7042210, I read it was 650cfm. It's actually good to know it's most likely a 750cfm.

My mechanic built the carb back in 2008 and it's been installed since and has performed very well over the years, but I'm investigating if I can improve the performance even more since I don't believe it's ever been set up for altitude.

I confirmed the secondaries are indeed DA rods (stamped and I measured them), but I have not yet confirmed the primary rods nor the jet size currently installed as I wanted to gather some details first to see what to order and have on hand before I opened up the carb.

I'm guessing that since the secondary rods where stock DA rods, the primaries will also be stock (43B.)

Next I'll check out Cliff's site to see what's available...

[Kenth]

Very few Chevy smallblock Quadrajets flows full 750 cfm, most of them are restricted by the reduced air valve opening degree.
Only some HP units got the full 750 cfm flow.

I wouldn't count on there being original jets and rods if the mechanic who fixed the carburettor was knowledgeable.
These carburetors have .120" main air bleeds in air horn and floatbowl and the mixture would be too lean with the original jetting.
One way to still use the original jetting is to reduce the main air nozzles with .050" inserts.
Otherwise, #77-#78 jets paired with 48B-49B primary needles may suit your engine's needs with this carburetor.
There's only one way to know what's inside (or two if the mechanic has notes).

FWIW

[77 TRASHCAN]

Keith, my thinking was that the carb repair person has ground down the air flap arm to allow it to open fully.

In my salvage yard endeavors, I bought a couple of 301 Q-Jets. The secondary air flaps on those, barely opened at all!!!

[tremo]

Originally Posted by Kenth

Very few Chevy smallblock Quadrajets flows full 750 cfm, most of them are restricted by the reduced air valve opening degree.
Only some HP units got the full 750 cfm flow.

I wouldn't count on there being original jets and rods if the mechanic who fixed the carburettor was knowledgeable.
These carburetors have .120" main air bleeds in air horn and floatbowl and the mixture would be too lean with the original jetting.
One way to still use the original jetting is to reduce the main air nozzles with .050" inserts.
Otherwise, #77-#78 jets paired with 48B-49B primary needles may suit your engine's needs with this carburetor.
There's only one way to know what's inside (or two if the mechanic has notes).

FWIW

Thanks Kenth!

- If most don't flow at 750cfm, what do you think they actually flow at (approx)? ~700cfm?

- You make a good point not to assume what the primary metering rods and jets are. My mechanic was a Pontiac-guy, so he may very well have set it up for my altitude? I was also thinking they might be stock because it's always run very rich at idle even after adjusting the idle mixture screws properly. I will see about opening the carb to check.

- Thanks for the recommendation to use #77-#78 jets paired with 48B-49B primary needles. This input is what I'm ultimately after! FYI that the car is primarily a street car, but over the last couple of years I've started to race the full season for my class (about 7 times/year) so I'd rather tune it for racing vs. street.

- So the idea is to increase the jet size (more fuel) and decrease the metering rods (less fuel)? I'm still learning how the jets work in conjunction with the the metering rods so bare with me! Note that while I live at 5,800', Bandimere is typically at ~9,000'+ density altitude during the season and even higher during 90+ degree days. Would your recommendation change given the higher density altitude?

Thanks again for all the input, very valuable!

[mchell]

Originally Posted by tremo

Thanks Kenth!

- If most don't flow at 750cfm, what do you think they actually flow at (approx)? ~700cfm?

- You make a good point not to assume what the primary metering rods and jets are. My mechanic was a Pontiac-guy, so he may very well have set it up for my altitude? I was also thinking they might be stock because it's always run very rich at idle even after adjusting the idle mixture screws properly. I will see about opening the carb to check.

- Thanks for the recommendation to use #77-#78 jets paired with 48B-49B primary needles. This input is what I'm ultimately after! FYI that the car is primarily a street car, but over the last couple of years I've started to race the full season for my class (about 7 times/year) so I'd rather tune it for racing vs. street.

- So the idea is to increase the jet size (more fuel) and decrease the metering rods (less fuel)? I'm still learning how the jets work in conjunction with the the metering rods so bare with me! Note that while I live at 5,800', Bandimere is typically at ~9,000'+ density altitude during the season and even higher during 90+ degree days. Would your recommendation change given the higher density altitude?

Thanks again for all the input, very valuable!

Unless your carb is waaaaay off, changing jet/rod combo’s is not really going to move the chains …..At those altitudes I would think you’d want it a tad lean….. I would play with timing if anything….and nitrous…. Lol

[Cliff R]

Before you make any big changes to the carb contact me at the shop. I've tuned thousands of Q-jets for high altitude use. On that unit it is best to replace the factory APT screw with one of my external screws for easy access. Teamed up with the correct jets and metering rods you will find it easy to dial in for your application. The factory APT screws in those can a bit difficult to remove but it is LIGHT YEARS better to use that system than to pull the top on and off the carb to make metering changes for part throttle A/F.

You actually got lucky with the carb choice. It's too lean right off the bench with the stock jetting, plus we typically add 5-7 percent more fuel when using this currently avalable fuel. So if you factor in that plus the altitude you really aren't off that much. It's probably a little "fat" at WOT with DA rods in it, but being too rich is ALWAYS better than too lean and never results in putting a few pistons in the oil pan!...........


https://cliffshighperformance.com/

[tremo]

Thanks Cliff!

I popped the top off the carb today and learned I have .042 primary rods and #74 jets. As per Kenth the stock set up for my carb is:

Originally Posted by Kenth

From factory the 1972 Chevy Truck 350 #7042210 Quadrajet has #74 primary jets and 43B primary rods.

So, it looks like my mechanic dropped one rod size down to .042 (42B = more rich) and kept the jet size the same at #74. (Or he left the same primary rod that was installed when he rebuilt the carb?!)

Perhaps this is too rich for my altitude (+9,000' density altitude)? Would Kenth's recommendation be something to try:

Originally Posted by Kenth

Otherwise, #77-#78 jets paired with 48B-49B primary needles may suit your engine's needs with this carburetor. FWIW

Maybe I try #77 jets and 48B rods?

Thanks for any advice!

[tremo]

Originally Posted by Kenth

Otherwise, #77-#78 jets paired with 48B-49B primary needles may suit your engine's needs with this carburetor. FWIW

So if I'm trying to lean out the carb, why would the jet size increase from my current #74 to #77-#78 which have larger diameter holes and therefore provide more fuel?

Wouldn't I want to decrease the jet size like to a #71-#73 while also increasing the rod diameter from .043 to .048-.049 to lean it out?

Thanks!

[Cliff R]

The best way to tune that carb for what you are doing is to employ the APT system in the baseplate. I make an external screw for those to replace the factory one. Then you can simply fine tune the part throttle A/F for altitude without having to take the top off the carb and change parts......

[tremo]

Originally Posted by Cliff R

The best way to tune that carb for what you are doing is to employ the APT system in the baseplate. I make an external screw for those to replace the factory one. Then you can simply fine tune the part throttle A/F for altitude without having to take the top off the carb and change parts......

Thanks Cliff, that looks like a great option! I don't see the APT system on your web site. What is the cost for the conversion and can I do this myself with parts sourced from you?

For now, can you make a base recommendation for a primary metering rod and jet size for my altitude (+9,000')?

I was thinking of going from .042 to .045 (45B) and jet size from #74 down to #72. Secondary rods would remain at .0443 (DA). This seems to me like a good place to start (without going too lean) and would give me something to try in the interim.

Thanks again!

[Cliff R]

The APT set-up is easy but in some cases that original APT screw can be difficult to remove. I'm able to get most of them out, but once in a while run into one that's not coming out despite your best efforts.

I'd start there. Remove the cap over it, use plenty of penetrating oil and some heat from underneath from a propane torch. Once it's out contact me at the shop and we'll cut right to the chase for tuning parts.....