Q-Jet Retuning 400 -> 350
 

I'm wondering what to expect from the 7028268 carb I put on top of the 350 in our Ventura. I had this carb running on my GTO when it was a 400 and built it to one of Cliff's recipes years ago, likely the most conservative. It starts and idle great, just like it did when it was on my GTO. I haven't had a chance to get the car out on the road yet since we're waiting on seats, but have the carb off to install a new secondary air door spring so I figured I might as well ask.

I left the primary combo of 72/42B and it has a set of of BE secondary rods in place. I can't remember for sure, but I think they may have been modified with smaller tips? Since it's a smaller cube engine with a smaller cam (9777254 269/277/113 @ 0.377”/0.413 vs. Crower 90619) should I change anything?

https://forums.maxperformanceinc.com...1&d=1759337359

CH for the win .. The carb knows not 400 or 350 just air flow the CT is just to lean anywhere BG never had any luck with short tips just to lean early air flap opening

I'd do the tip in test, and see how that goes. Not sure you need any mods except for post 2. I like Cliff's custom primary rods...
Don't listen to anything I say, I'm no expert...

Here’s some info to allow you to check your BE rods.

Ken, I think the current jetting is a good place to start. Drive it for a while to see if it lean surges at steady state/read the plugs, do some full throttle blasts and read the plugs, then jet accordingly. It's good to have a good array of jets, primary and secondary rods, and hangers to swap out.

I wouldn't be surprised if you had to richen both sides of the carb a bit.

Dennis

Richen up the carb to go from a bigger cam’ed 400 to a smaller cam’ed 350!?

Yep, we've found that low compression engines like more fuel than their high compression brothers. Now, the 46 cube difference may alter that a bit, but the carb is close enough to start testing. Who knows, the current jetting may be perfect, but every engine has unique jetting needs.

Dennis

I do have a pretty good selection of primary rods and jets to choose from. The 2bbl always ran lean and I band-aided it with bigger jets. I can already tell the engine is a lot happier with the 4bbl, I just thought it might be rich with the carb set up for the 400. I guess we'll see what it likes.

A whooping 13% less CID and it might need to be jetted richer, I anti going for that I gotta say, but where both entitled to our opinions.

My former '74 GTO's 359 (.030" over 354) had pretty rich jetting that I recall...73/44/BE from my notes. It did have a CC XE256H cam and a true 7.9 to 1 compression though. My 455HO couldn't get rich enough on the dyno, same for my RAIII with the original 068 cam. However, my small valve 400 with 10.0 compression and a 2801 cam I couldn't lean out enough on the dyno, which was identical to the RAIII except for the small valve #16 heads and log exhaust manifolds. So, every engine will tell you what it truly needs.

Dennis

Nothing is really to me apples to apples in what you’re talking about here.

I think the differences you have saw where more due to the many details in a running motor (of a given CID ) that make for the reversion pulses the motor must deal with in terms of leaning out a new incoming fresh air and fuel charge before it lights off.

Very little of that can I see having an effect even with full point and a half increase in compression.

I'm just sharing what we've seen/experienced, so, your actual results may vary, like the info-mercials say. All the engines were dyno'd with their original carbs and were 'pure stock' style builds. I'd be interested seeing what both '74 350 4-bbl engines had jetting-wise from the factory (with and without the shaker). Might be the same...Kenth???

Dennis

Recognizing that the carbs are different, I was wondering what they came jetted from factory as well. I couldn't find that info anywhere.

If you're wanting original 'primary side' jetting, you're already very close with your 72/42B, as original jets for the 7028268 carb was 73/42.

FYI, the 350HO carb from '68 was an entirely different carb number, 7028266, but with similar primary jetting as the 7028268

In getting back to the original question that started this post string I would like to offer this thought.

When you look at the large list of secondary rods that Rochester made for the Q-jet that I posted, one of the 3 main differences you see is the rate of change in the taper from the main start of the metering body to the power tip .

This chart list the dimensional change / taper in 6 steps across what might be no more then .250” of the total metering rod length that effects fuel delivery.

If we assume that on a stock 400 cid motor a 750 cfm Q-jet would at full throttle have its air flap open enough to meter fuel to the motor by having its power tip exposed by whatever length in the jet well, then that same carb on 350 would without question have less of those rods power tip length length exposed at full throttle.

This would be just due to the simple fact that the air valve flap would not be open as far as it would be on a larger CID motor which would be calling for a higher level of air flow for any given rpm.

The rods proper rate of taper reduction for a given CID motor to me is hugely important to the air to fuel ratio and how the motor performs on its way to arriving to the point of having the rods power tip exposed in the jet.

This need for fine tuning, besides that long list of secondary rods that where made to assist in that is also why Rochester made a good number of secondary metering rod hangers.

These were made with different height holes in them to hang these secondary rods from.

Making the secondary jetting progress richer as the rods lift up is easy since you can place shims under the rod hanger arm.

If you need to go leaner these days with the limited different size rods we can get our hands on this might be an issue.

You then might have to resort to drilling new lower positioned holes in the metering rod arm to get the needed fuel curve the motor wants thru the transition before reaching the point where the metering rod tip is being called upon.

Another way to go leaner if you have spare parts is to file down the bottom of the Cam that the secondary arm rides on.

This is all very fine tuning that might have to only be done on a chassis dyno, as I am sure the factory did back then to gather empirical details .
but one thing is for sure, once a motor has reached it rpm of peak torque with the secondary fuel metering of the rods being used it’s far better / safer to loose 5 or 8 hp of what might be the peak HP number by having the motor running a tad too fat.

It’s very interesting to me that it’s been found that some Q-jets when reworking them for full on high HP production race use that the secondary metering rods would never lift to the point of exposing the power tip at all!

Was this a true defect, or was this perhaps only needed to be done on the carbs used for example on the OHC 6 with the Q-jet ?

F.e in 1969 all 350 HO, 400HO AND 428 HO w/AT uses the exakt same #7029268 Ojet, and
In 1970 all 400 RAIII/IV and 455 RA w/AT uses the exakt same #7040270.
How can that be?
It can be done because a larger engine creates a larger difference between the atmosphere and the void (vacuum) under the throttle bladet and pushes a larger amount of A/F mixture thru the carb circuits into the intake than a smaller engine.

I appreciate the detailed info. Interesting that the carbs were the same between a 350 and a 400. I reinstalled the carb tonight and hooked up my wide band. The a/f ratios are definitely in the ballpark - at least with the minimal testing I can do in the driveway. We'll see how it does on the road. Hoping we can get our seats back in time to do some shakedown drives before our drive to Arizona.

Just 3 Q-jets for the 350-4bbl. 1 CA unit and 2 49-state carbs 1-AT and 1-MT. There was no different or special carb for the "shaker 350s" in '74, probably cuz the shakers were non-functional...?? Pretty sure the only 4-Bbl. equipped 350s in 1974 were on the GTO (or maybe optional on the Ventura??), cuz all the f-body and b-body 350 cars had the 2GC.

The 49-state AT Ventura/GTO's 350 Qjet was nearly identical to the CA emission unit, they shared the same pri/sec. jetting according to my book. But some of these older publications have errors, so I try not to say, "never" and "always" when talking about Pontiac. Plus there's just a lot of sh*t I either don't know or else have forgotten.....LOL

Thanks for the info! The shaker was functional on the '74 GTO's, but not the TA's and SD455 Formula's. Can you post the jetting for the '74 350 4-bbl carbs?

Dennis

In 1974 as I recall the only way you got a 4bbl on a 350 was in the GTO package with the 200 hp motor and the 200 hp motor was not available in the Ventura as a production model.

But who knows what the engineering guys might have been testing out that 200 hp motor in ( in 73 ) before the body details where nailed down for the GTO package.