Another Stage 2 Jet "masterpiece

[Captive8]

An owner of a '69GTO reached out to me, hoping for some help on his new Jet Stage 2 carb. He had Butler do some work to his 400, ported #48 heads and HR cam (230/236, .560/.571, 112LSA. He's at 9 to 1 SCR, iron OEM intake manifold and RA exhaust manifolds. The carb is a 17057274. His main complaint was a bog or hesitation when the secondaries open.

I found the secondary air valve spring wasn't even close to being properly adjusted. I could have just adjusted it and he might've been fine, but I figured if THIS wasn't right, WHAT ELSE wasn't right inside the carb, so I tore it apart.

78 pri jets, with 41J pri rods, no idle bypass air (baseplate wasn't drilled), yet someone thought it beneficial to plug the 1/4" bypass holes in the float bowl with lead plugs. Needle and seat was a small inlet size (my .129" bit would not go), Secondary cam was loose on the shaft. Idle tubes were .046"!!!, yet the idle down channels, I had to open up to .055". Secondary rods were CV, and the pull over restrictions were WAYYY small too. Wow...what a POS.

My question is this....the upper and lower IAB sizes were right on (.067/.070"), but the upper MAB are .070" (about right) while the lower MAB are about .055", which seems small. I'll be jetting it with 73 or 74 and Cliff's 45C pri rods.....should I leave the lower MAB at .055 or open them up to about .070"? I think the latter, but appreciate some more eyes on this.

[Cliff Ruggles]

I'd leave all the airbleeds main and idle at the stock sizes.

That carb is at the top of the pile for factory Q-jets and doesn't need a lot of help anyplace.

I ran the same part number on 5 different engines that powered my Ventura since the mid 1980's and it was FLAWLESS.  I only took it apart to make minor tuning changes for the power levels of the different engines I put under it.

I would get one of my kits with the high flow N/S assembly, small float and HP accl pump.

73 jets are PLENTY in that carb and anything bigger would be "pig" rich everyplace.  CV rods are fine for the secondaries.

It will also need a timed pull-off unless the one on there now is set up correctly, most are not.

I'll add here that over the years my 17057274 carb put my car in the winners circle five times at the Pontiac Tri Power Nat's with two trips to the King Of The Hill Race. 

NOTHING we ever tested against it, dyno and at the track would outrun it......and believe me the Q-jet "haters" at the dyno shop tried EVERYTHING they had against it!.......

[Captive8]

I proceeded with upgrading this unit. Here's what I wound up with:

73 pri jets
45C pri rods
.070" IAB upper and lower
.070" MAB upper
.055" MAB lower
CV sec. rods (.052")
.055" sec POE
.036 sec tube rest
1.35" on the sec air valve (I probably need to shorten this another .050" or so)

This thing does NOT run perfectly at WOT. It "breaks up" and surges a bit. I can bolt on one of my "loaner" 800cfm carbs which pulls very hard at WOT and runs flawlessly at idle and part-throttle.

I'm a bit stumped as to what to do with this thing, other than opening up the secondary air valve another .050"-.070". My loaner carb is built similarly to this 17057274 except it has AX secondary rods.

[Kenth]

Primary rods with suffix C are for the earlier 4M Q-jets and are approx. .040" longer than the correct M4 rods.
CV secondary rods may be a bit lean, i would use DA or CE rods with a G-K hanger and adjust the air valves for full 1.38" opening.

[Cliff Ruggles]

Kenth, I'm pretty sure he is talking about my 45 "C" or Custom rods, or at least I hope so.  They are a full taper rod from .045" to .026" tips and the correct length for the later APT carburetors and a nice upgrade from factory K, J, and L rods.

It's having issues because it's a JET carb.  I used to get a LOT of those in here but the parts mixing/matching and mods done to them not for the better made me decide to stop taking them in. 

It's not that I can't eventually fix all the issues, I just decided that from a business standpoint it's ALWAYS best to start out with a "virgin" or unmolested core and perfect results every single time right off the bench vs having to try to figure out what they modded and butchered up on it, fixing it, then it not working well when placed on the test engine.  Then back apart it comes and oh chit, I missed that stooped modification, etc, etc......FWIW.....

[Jeff K]

Captive8, you have two of the best Quadrajet techs / engineers ever looking at your Quadrajet on this thread..