[glhs#116]

So, I've learned that for best results I need to use the "B" rods that are appropriate for my 1971 QJet. I've got a "tuneup" set of the later rods (non-"B") from Holley (they sold the kits to go with their QJet clone carb they used to sell) and they just don't seem to transition as nicely.

I've tuned for WOT and I know my primary jets like to be 73 (not the 71 that is "correct" for the carb stock) and so I need bigger rods to get correct idle and cruise. I'm on 45B and I feel it's close but needs to go a bit leaner. Probably 46B or 47B. But Carbs Unlimited (where I got the last ones) don't have these sizes available. Not many people seem to sell them. Any good sources for these early 70s rods? I'm guessing those sizes are hard to find because they are popular..

Sam

[DOC]

Try Cliff Ruggles https://cliffshighperformance.com/

[4dblnkldude]

Pretty sure I got them, I'd have to check. When I bought my 71 looking through boxes there was a little pill container from HO with them in it. They would be out of a 7041268 carb. Not well versed in q jets but the are the rods that go through the secondaries?

[4dblnkldude]

Duh, it says primaries I'll look through if ya need em.

[BLUE TA]

x2 post 2, cliff will have what you need BLUE TA

[65 Lamnas]

I don't usually have a need to set carbs quite that lean unless it's an altitude thing. Are you trying to achieve stoich at cruise and idle with a meter or something else. I thought I had some 49Bs around here....I'll have to look, but that's the closest I've got

[Held for Ransom]

OE PN's are listed here....
https://forums.maxperformanceinc.com...d.php?t=413003

[Kenth]

1968-74 primary rods with "B" suffix are used only in Qjets with the APT adjustment located in the throttle body from factory.

Primary rods w/o suffix are used in all 1965-67 Qjets.

And, 1966-74 Pontiac Qjets never used the rods with the "B" suffix from factory.

HTH

[steve25]

If your Cam’ed different then stock ( less idle vacuum ) you likely need a lighter tension power piston spring also.

Just the simple fact that your E heads have 25% more port volume then the stock iron heads means that you have less idle vacuum then stock.

[glhs#116]

My carb is 7041262 which would be correct for 455 and 1971 and automatic but not HO. My compression is a little higher and displacement a little larger thanks to a .030 over rebuild with the SpeedPro pistons that are just about zero deck. Heads are round port Edelbrock 87cc. Thin head gaskets. 60916 041 type cam.

I dunno. I tune by feel and by plugs. I just feel it needs a step or two leaner at cruise. I am sure the rods are down at idle. Already went through the primary spring tension and secondary flap adjustment. No stumbled or flat spots anywhere. It is responsive and drives great. Just feel it could do with a step or two less idle and cruise fuel.

Sam

[steve25]

The metering rods do not come into the air to fuel ratio picture until the throttle blades are passed the transition slot when opening the throttle, assuming as you say the power piston IS seated at idle.

With the motor fully warmed up you should not see any fuel dribbling out of the booster ring(s) like you do when opening up the throttle.

If you want a leaner idle then that’s done with the mixture screws .
If your mixture screws can change your idle quality when moved in or out from a decent idle position then the carbs idle feed tube size in not too bad.

Once a cam gets to about 220 intake duration @ .050” ( depending on motor cid) then changes may need to be made in this size.

Also keep in mind that as with any duel plane intake manifold your air to fuel mixture especially at full throttle needs to be such that the cylinders that tend to run the leanest are still in a safe range, and this means that the other cylinders will be a tad on the fat side.

There’s just no way around this fact with a duel plane!

[glhs#116]

Steve,

So, I've got the idle dialed in for best vacuum but it's at around 3/4 of a turn out which makes me feel that circuit is a bit fat. The old combo was the same 73 jets with 42B rods and it drove pretty well but the plugs looked fat. I ordered rods 73B, 74B, 75B. 75B was best of them. Plugs still look a little fat that's all.

No dribbling or anything. The carb has a Cliff kit in it with new idle tubes and the idle channel restriction got drilled out to .0625 per the kit. The idle bypass air got drilled out to .1015 per the kit but I had to reverse this move with some cut up roll pin pieces because with the idle bypass that big I literally couldn't pull down the idle enough even with the throttle closed. Now, this was with the old 197 heads. I haven't tried opening those back up with the Edelbrock heads. But I don't feel it was a big change in idle throttle position when I changed heads...

Carbs Unlimited was where I got the other rods. They seem to have a gap between 45B and 48B. My feeling is I want a slight change and the 48B is going to be too lean. That's where this question was coming from.

Sam

[Kenth]

The idle/low speed circuit is a separate circuit independent of the main jetting.
If the main jetting were to affect the idle/low speed circuit, the engine would not run at all.
So it doesn't help to change the main jetting to lean out the idle/low speed mixture.
Then smaller idle tubes or/and larger idle air bleeds must be used.
To reduce the amount of this mixture down to the lower idle air bleeds and mixture screw tips, smaller idle channels must be used.

For your 1971 Pontiac Qjet carburetor with the primary pull-over system, I would use correct style primary rods (without the B suffix) stamped 73. (EDL-1944).
The adjustable main airbleed under the triangular retainer seated to start with.
For the idle/low speed circuit, I would start with .038"-.040" idle tubes and .057" idle channels for a 455 w/RAIV cam.

To check if your efforts are close to optimum use the "tip-in" test at idle speed an at 2200 rpms. If good and no hesitation while driving you´re fine.

FWIW

[steve25]

This guide is what I have always found works well.

[Held for Ransom]

Originally Posted by steve25

This guide is what I have always found works well.

I flipped it...

[glhs#116]

Originally Posted by Kenth

The idle/low speed circuit is a separate circuit independent of the main jetting.
If the main jetting were to affect the idle/low speed circuit, the engine would not run at all.
So it doesn't help to change the main jetting to lean out the idle/low speed mixture.
Then smaller idle tubes or/and larger idle air bleeds must be used.
To reduce the amount of this mixture down to the lower idle air bleeds and mixture screw tips, smaller idle channels must be used.

For your 1971 Pontiac Qjet carburetor with the primary pull-over system, I would use correct style primary rods (without the B suffix) stamped 73. (EDL-1944).
The adjustable main airbleed under the triangular retainer seated to start with.
For the idle/low speed circuit, I would start with .038"-.040" idle tubes and .057" idle channels for a 455 w/RAIV cam.

To check if your efforts are close to optimum use the "tip-in" test at idle speed an at 2200 rpms. If good and no hesitation while driving you´re fine.

FWIW

OK. I think then I know what is up with the idle screws. My idle channels are opened more than that and it was probably intended to accompany the larger idle air bypass. As I said, I basically ended up re-restricting the larger bypass after having trouble getting the idle speed down with the larger holes. It doesn't bug me, and I can get a good idle mixture. It's just with the screws further in than would be normal.

My real concern was the cruise mixture. That's the bit I was trying to tune in. I did like the B rods after I tried them. I had a straight 42 before and there was a little bit of a flat spot just around the top of the primaries (just before you're applying enough throttle for the secondaries to start engaging). I didn't find the same behaviour with the 43B nor the 45B. But I probably have some other "straight" rods and maybe I'll just try one of them if I happen to have a 46 or 47. I don't think I do. But I'll look again at what I have..

I do appreciate all the insights.

Sam

[65 Lamnas]

Originally Posted by steve25

This guide is what I have always found works well.

That's fairly well detailed....what book is that?

[steve25]

Thanks for the flip and the enlargement!

[steve25]

It’s a Carb book from David Vizard.

[gto4ben]

Carburetors and intake manifolds
by
Vizard, David 1996, page 87