Tuning Primary Circuit

[67RedC10]

I just finished rebuilding my Quadrajet.  I need some advice on selecting the proper rods for the primary circuit.  Here's what I've got:

1971 Chevy 350 from a Chevelle rated at 270 hp.  The engine is pretty much all original with cast iron intake, original points distributor, original cam, etc.  The only changes are ram horn exhaust manifolds and single exhaust into Flowmaster muffler with 2-1/2" inlet and dual 2" outlets.  This engine is in a Toyota Land Cruiser FJ40 with manual tranny, 4.11 gears, and 31" tires.  Idle vacuum is 15" @ 650 rpm and altitude is about 3300 ft.  The carb is from a 1977 Chevy truck.

Carb number is 17057229
Installed bushings on Primary Shaft
Idle tubes - .038"
Idle down channels - .055"
Upper idle air bleeds - .075"
Lower idle air bleeds - .063"
Idle bypass air - .139"  (now plugged)
Mixture screw holes - .060"  (now .086")
Air Horn primary air bleeds - .052"
Fuel inlet seat diameter - .130"
Acc. Pump discharge holes - .075"
Primary jets were 64's (now drilled to .066")
Primary rods - 398 - I measure them at .026" and .039"
Original power piston spring

Before rebuilding the carb (but after installing the primary shaft bushings) I did a little tuning and hooked up my wideband AF ratio meter.  It was running quite well and was happiest with an idle AF ratio of about 13:1.  I've always had a problem of my idle being too high (850 or so) and the only way I could get it to slow down was to plug the bypass air passages in the baseplate.  After doing that and readjusting the idle AF ratio to it's happy setting of 13:1 then the idle is fantastic at 650 and can go a little lower if I want.  When I opened up the primary's going up hill in 4th gear my AF ratio was running a bit lean at 15:1.  My cruise AF ratio was about 14.5:1 (4th gear 2000 to 3000 rpm's was the same). 

I purchased a rebuild kit from Cliff and just finished installing it.  I decided to drill my jets out to .066" (from .064") to richen up the primary side on heavy acceleration.  Now my AF ratio on heavy acceleration is running about 13:1 (Idle is set at 13.2:1), but my cruise is about 12 to 12.5.  When the engine is fully warmed up and I rev it up from an idle it dies out for a split second then smooths back out.  I noticed the AF ratio meter goes to below 10 during that time.  If I pull a small vacuum hose it doesn't die out quite as bad when rev'ed from idle.

So with all that, what would you recommend?  I do think I will order a new power piston spring since my old one is a little rusty and questionable.  What would you recommend for primary rods?  Before I drilled out my #64 jets I did notice a little flat spot when accelerating heavy from 4th gear (primaries only)

Sorry to be so wordy.  I wanted to make sure you have all the info. you might possibly need.  If this is too much to read I'll call you.  Thanks much!       Dave

[Shark Racer]

I'd order correct 66 jets from Cliff. My understanding is that drilling out the jets actually ruins them as there's a specific taper to the entryway of the jet.

I'd probably order an 040 and 041 rod set too.

I'm only a newb at this, though... so my advice is worth $0.01 rather than $0.02.

[Cliff Ruggles]

Correct on the jets.  Jets are stamped and flowed, drilling them doesn't provide the same result and is not recommended.

We always install a new power piston spring, as the old ones get weak, rusty, etc.

If possible, free up the APT screw in the baseplate for fine control of the part throttl A/F.  We remove the factory APT screw and install an external screw instead.

With the correct jet/rod relationship, the tuner will have full control of part throttle A/F from lean to rich, with quick/easy on the vehicle adjustments without having to change parts.....Cliff

[67RedC10]

Cliff.  Thanks for the information.  I'll call you to order the replacement jets and power piston spring.  Can you describe how to install an external APT screw?  You don't talk about this in your book for the older Quadrajets that come without the adjustable APT.  Is it that little metal plate on the baseplate between the primaries?  I see a plug installed in the front side of the baseplate. 

I hope you have a revised edition of your book coming out soon.  That would make a fantastic Christmas present from my wife!        Dave

[67RedC10]

Okay, now the photo you included just came up on my screen.  Sure enough it's what I thought you must be talking about.  I hope I don't mess it up.     Dave

[Zac Agee]

dave make sure you get the atp screw well heated before you attempt to move it.. I heated mine several times before i was able to remove it.

[67RedC10]

I received the new jets, rods, power piston spring and APT screw from Cliff.  I installed it all and spent the evening tuning it.  My engine is running like a top now!  Thanks Cliff!!    Dave

[Cliff Ruggles]

Good news!

That APT system works great, but the screw can be difficult to get out of the baseplate.

It is imperative as mentioned above to heat it up some first.  Apply heat to the underside of the airhorn with a propane torch and use some penetrating oil on the screw.  In most cases they will come right out. 

If you attempt to remove it before heating it up, often the screw will break off and it becomes nearly impossible to remove.....Cliff

[Frank400]

  We remove the factory APT screw and install an external screw instead.

  do you provide this service if one sends you the baseplate ?  If so, how much and how long is the turnaround time ?  Bought a few carb kits from you in the last year and was very impressed with my results (followed your receipes, read your book many many times).  Frank.

[Cliff Ruggles]

Yes, we completely rebuild them and free up the APT/install external screw.  Turn around time 1-2 days....Cliff