Cliff's Quadrajet Parts and Rebuild Kits
Quadrajet Problem Solving => Dialing in your rebuilt Quadrajet carburetor => Topic started by: MattK2 on September 17, 2023, 07:37:15 PM
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I modified a 7042262 to the "3rd recipe" for my motor, and it worked great when I first got it going last fall, but it dripped. Ended up having a micro crack that would open when hot.
I sent my carb out to be serviced, he confirmed the crack, and found a donor 7041262 body and air horn. It was a 11 day turnaround with shipping. This "newer" carb has slightly different guts than the one I built.
My engine is a 455, 0.030" over, RAIV cam (adv.4*), 9.3 CR, 6X-8 heads. 400 hp/ 500 ft-lbs
The first carb: jets 74, rods 37, tubes 38<-------cracked carb
the second carb: jets 77, rods 43, tubes 36 <---------current setup.
It idles at about 13" vacuum, cruises at about 19"
Total timing is 37*, initial 15*, vac advance 15+*, curve is very generic and being addressed.
I'm still experimenting between ported and manifold vacuum signal to the vac adv. can. At the moment I'm using manifold.
Idle is poor when cold, seems like a choke issue, better when hot.
Restart is weak when hot, better when cold or warm.
At a 20 mph roll, if I drop the hammer in 1st, the tires break hard and it goes like hell, so that's good.
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77 main jets are HUGE in one of those carburetors. I'd go back to 73's or at most 74's with 42 or 43 primary rods.
Idle tubes will need to be .038" for the RAIV cam at that compression ratio...
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A 73/43 combo. OK. I need to purchase some jets and tubes.
Any thoughts on why a hot start seems overloaded with a weak vacuum signal? That’s my assessment of it. It starts soon enough, but doesn’t come alive like you would expect from a hot engine. No quick pop off. I could set the idle up to 900-1000 and it’d start better but then it could develop runon at shut down.
And, it would be great to get the idle around 700 and not be so rich. I lean out the mix screws and it’s not helping much. The. I start messing with vac advance signal to help. Going in circles.
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Check to see if there is enough vacuum at start-up to fully employ the vacuum advance.
In many cases to effectively use MVA you'll need a VA that fully applies much lower or all of the timing may not be in at start-up or even at idle speed.....
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Soon I’ll be swapping dist. Current one has a long curve. Generic. Going to try 1000-3000 range. Letting the builder decide on the rate.
Swapping jets and idle tubes once they arrive. Going to measure everything when it’s apart.
Given that carbs were spec’d to specific engines, does it make sense to modify a carb similar to a 69-70 RAIV spec’d carb. those carbs are around. Rare, but those specs are out there, right? It’d be a good starting point. Small tweaks from there.
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Distributors are easy to modify, provided it's an OEM unit. No need to replace it otherwise as you can put any curve you want into any unit.
Low compression with a big cam likes more initial timing and quicker curve starting right off idle and all-in by 2800rpm's or so.
No need to look for another carb, then one you have will work once it's dialed in for what you are doing.....
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I may know why it starts poorly when hot. The choke isn’t staying wide open, the choke rod is wrong. Just got it out of storage. It won’t come close to the index notches. I admit I never noticed them before as a way to calibrate the air. It was sold as a Pontiac part. The one with the hairpin bend in it. Old Service Manuals from 1970 show it with single kink. The Rochester parts diagram is the same. So all this time I’ve had a choke rod 1/2” too long. Cold starts should improve given improved full closure on a cold start. I’ve got the vacuum break at 0.330+/-. Now I just need to find the correct one and see if things greatly improve.
Still need to change out the rods, jets, and tubes too.
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I stock everything you would need including high performance quick release choke pull-offs, links, jets, metering rods and the correct power piston spring for the RAIV cammed 455......
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I have new jets rods tubes and pull off I bought last year from you. Correction, not the “choke rod” ,but the “choke stove” rod. Sorry.
Since I’m mixing 68 intake with a 71/72 QJet. The choke stove rod is too long in this setup. Was the cam follower different pre1970? I have 70-74 Delco Bulletin for Pontiac carbs so I can see the stat/rod setup won’t attach to my intake, it’s rotated 90d.
Still researching old part numbers to see if I can find one that works.
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Pontiac used several different arrangements for the divorced choke carbs from 1967-72. They had three different choke pull-offs and several different fast idle cams. Mixing and matching parts typically doesn't work well.
I don't stock or sell the choke coil to carburetor links, only the choke pull-off to secondary airflap shaft links. So not sure where that came from or if it even the right part.
In any case they are easy to make from an aluminum TIG rod, much easier to bend and modify that steel rods.....
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Thanks Cliff. I’ve done a full history of manifold stat coils from 67-72. The 67-70 and the 71-2 which sat on a diff intake design. Found a Delco Bulletin from ‘71 with all the part numbers.
I may be dealing with a choke rod that’s over bent and not close to the 2.5” end to end length. Will confirm. If it was over bent the other part of the choke setup would not be able to compensate since the cam follower would clock differently. More CW than it should be which would make the stat rod appear to be much longer than it should be at full choke. Just a theory at this point.
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Rod was overbent. Had to make it straight to be at 2.5”. The outer rod now sets in R notch. So it’s getting better.
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Installed the new jets and tubes I bought from Cliff last fall.
To recap it’s a 7041262
Jets 73, Rods 43, Tubes 38
Idles nice down to 650 RPM
No bog.
But I have a new issue. It bucks at low speed off idle. Not sure what that’s telling me. Too lean?
It wasn’t bucking prior to swapping out the jets and rods per Cliff’s recommendation.
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Is the upper adjustable main airbleed screw under the triangular retainer seated?
Do you have at least .052" down channel restrictions (may need .055")?
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Not sure about the down channel restriction. I’m assuming they’re fine. As for the bleed screw under the triangle. My understanding was that it didn’t do much on the 71-2 carbs.
There was zero bucking until I after I swapped jets and tubes yesterday. So frankly I don’t know what improved. Yes the jetting is leaner than before but low speed driving has diminished.
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Did the tip-in test. Too lean now. APT screw is at max rich. Increase of RPM went from 2020 to 2280. So an increase of 260. Not the preferred 50-100. My understanding of the mods I made yesterday increased air slightly (tubes) and reduced jetted fuel 77 to 73 jets. Guessing a 75 jet may get it closer. The bucking is clearly a symptom of lean air fuel mix.
I’m not sure what down channel restriction does. Can’t find a description on Cliffs book or where it’s located. I’m sure it there somewhere.
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The bucking may also be an indicator of a weak ignition system.
The downchannels delivers idle/lowspeed A/F mixture to the lower idle airbleeds, off-idle slots and finally to the idle needle screws.
Note: the lower idle airbleeds changes to deliver A/F mixture when throttle blades are opened past the holes.
Too small downchannels may in some cases restricts A/F mixture to the off-idle circuit to the point of hesitation.
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The ignition coil is new, has been working fine. Let’s assume for now the ignition is working as it should. I don’t want to be chasing 2 things at once.
The tip in test done to analyze A/F mix is quite telling. At 2000 RPM, the APT screw was in all the way, so at max rich. It leaned out further when turned out with measurable drop in RPM and even rougher idle. As it should. I was surprised that it actually affected the mix to that degree. So in theory if my initial A/F mix is close, this screw can fine tune it from there.
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Does your new coil get proper current/voltage for the ignition system used?
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Just a note on tuning with jets. Changing jet size tunes both heavy and light part throttle.
Tuning with primary metering rods only effects light part throttle or the "normal" driving range.
Any heavy or full throttle rods are up and you are on the jets plus the .026" end of the rods.
It is best to tune for jet size first by doing some heavy part throttle driving without the secondaries. Once you have found the ideal jet size then tune light load/light throttle driving with the metering rods. Idle system mods have an impact there as well since some of the fuel comes from the transfer slots.
The "tip in" test is used to tell you when you are close but final testing is always done by looking for the best results. Timing is a player for part throttle tuning as well as the vacuum advance is applied when engine load is light and engine vacuum is high.
It's really not all that complicated but for tuning in the normal driving range you need to do both timing and fuel to get the best results. What makes it not so complicated is that all N/A engines run pretty much in the same range for A/F. I also know what needs to be done for any particular engine combo having spent nearly half a Century figuring all that out.
What I've seen so many times over the years is that folks want to yank the top off these carbs and start throwing bigger jets in them, when more times than not the jet size was adequate and they just needed to change to slightly smaller primary metering rods.......
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Coil gets proper voltage.
Jets went down in size from 77 to 73. Rods remained at 43.
APT at max rich.
The way I read your tip-in procedure, anything over 100 rev increase was out of range. You used 50-100 revs. I had nearly 260+ rev increase at tip in. Used the choke valve. Went from 2000 to 2260. That’s not “close” from what I read. Conclusion, it needs a little more fuel. Already increased the air by bumping the tubes from 36 to 38.
I’m willing to try other parts. I don’t have much to choose from so I’ll source what I think I need and buy it. Just looking for a suggestion on what a possible part change could be. Smaller rod? Perhaps a 75 jet. I know what you mean about the jets controlling higher rev and how the rods control lower revs when party raised.
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In any and all cases tune for best results actually driving the vehicle.
Tune for jet size first. If the 73 jets are fine leave them alone. If it needs more fuel at light engine load go to smaller primary metering rods.
It may be fine as-is, do some testing before making any more changes.....
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I have a nice variety of jets and rods to work with now. Will block off the secondaries to assess primary jet performance. Once that’s good the rods will be dialed in.