Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor

Primary jets and rods for AFR?

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lrust:
The APT is 4 turns from fully seated.  69 jets, primary rods are .029 at bottom and next taper is .040

Cliff Ruggles:
I don't remember the carb build but likely it was recalibrated with smaller MAB's.

3.5 to 4 turns up is where I like the APT to end up.

I've only done a couple of Pontiac builds with dished pistons on them and they didn't make the power I thought they should have, but still made decent power.

Quench area is not your friend with iron heads.  Adding more quench area may not get the results you are looking for even though it lowers the static compression ratio some.

For tuning, in any and all cases set total timing first for no ping, even if you end up a little shy of "normal", as detonation with destroy the best prepared engines......

lrust:
In summary sounds like you are suggesting that I NOT change the primary jetting?

Cliff Ruggles:
Do two simple tests.

First, do some heavy primary side pulls w/o the secondaries.  Lug the engine, climb some grades, and speed up in high gear to make sure it has PLENTY of jet in it.  If it pulls well and not nosing over, lackluster, flat, lacking power etc the jet size is fine.

Next set the APT at 3.5 turns and do some light load testing.  Evaluate transition right off idle, speeding up with minimal throttle opening, etc.

Raise/lower the APT if/as needed for best throttle response, power, etc at the leanest setting.

It may take quite a bit of driving and a couple tanks of fuel to nail down the best APT setting. 

IF you find yourself backing out the APT much more than 4-4.5 full turns you may need to go to a smaller primary rod.  I'm not sure I had the full taper rods back then but a good option for the later carbs that used the shorter rods with .026" tips.

During all of this you can gander at your A/F meter but tune for best results, not any particular reading(s).

A well thought out engine build with optimum compression, tight quench, well chosen cam, adequate timing from the VA, etc will not want a lot of fuel to be happy.

Pontiac engines are very sensitive to quench distance, especially with factory iron heads on them.  I've seen plenty of them run hot, overheat, and ping on pump gas when they shouldn't be simply because they had the pistons way down in the holes at TDC and/or thick head gaskets. 

I will not assemble one here with more than .040" quench for any reason, even it tightening things up with decking/squaring and thinner head gaskets raises the compression.

Not paying attention to these things, decking, squaring, establishing tight quench, can not only cost the end user a LOT of power they can end up with other issues like running hot/overheating.

Take the average 350 SBC build.  I get at least a dozen calls a week from folks who have just finished one up and wanting carb parts or carb work.

So I start asking questions, like what heads did you use?

What pistons and deck height?

Head gasket thickness?...etc...etc.

Most will say "stock" iron heads that were "ported", but no idea the casting numbers and 9 times out of 10 they will be 882 (door stops) with large 76cc chambers.

Deck height....not a CLUE, but likely .030" or more in the holes if they used a "rebuilder" set of pistons.

Head gasket thickness, not a CLUE, but for sure it was NOT the OEM .020" shim gaskets and probably .040-.060" thick instead.

So their engine instead of being 9.5 to 1 compression as their machinist told them which it would have been with .035" or so quench, flat top pistons and a little cut off the heads, it's probably closer to 8 to 8.5 to 1 compression......and they wonder why it doesn't idle well with their Comp XE or Thump-ya-muther cam in it and doesn't make chit for power either.

....continued

Cliff Ruggles:
Compression is a BIG deal as is quench distance and quench area.  Higher compression also offsets the negatives from the tight LSA camshaft the cam companies are pushing these days.  If you don't think so look at where the factory ended up, high compression, wide LSA camshafts and super-efficient combustion chamber shapes vs the "bathtub" chambers we started seeing in and after 1968 clear till the late 1980's.

One has to wonder how and why anyone ever though that deal could be more efficient and put out less pollutants?  Matters not, they figured it out and we should learn from it.......FWIW.......Cliff

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