Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor

Mods on a 7041262 for a RAIV 455 engine. Are they OK?

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MattK2:
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.

Kenth:
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.

MattK2:
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.

Kenth:
Does your new coil get proper current/voltage for the ignition system used?

Cliff Ruggles:
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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