just rebuilt 17059272 and having some issue

[Frank400]

I just rebuilt a 17059272 with Cliff's kit (this is the second carb I rebuild using since I read Cliff's book cover to cover about 10 times) and this time, I'm having an issue and I sure hope someone will be able to help.

application: 67 Firebird with stock 326 HO 285 HP.  Had a factory Carter, installed a 67 Pontiac Q-Jet intake and the above carb.  Exhaust holes going at base of carb were blocked and proper gaskets and steel plate were used, all installed where they should.

SITUATION: idle is perfect, starts perfect hot or cold, part throttle is marvelous, crisp, perfect.
                   from a 25 mph roll, when I press the pedal to like 2/3, it instantly goes pow pow pow in the exhaust, enough that I'm afraid a muffler is going to split open.  If I accelerate slowly, I can go all the way let's say to 70 mph and it goes like a champ.  It's under heavy load that there is a problem.  It goes pow pow pow (in the exhaust) without stopping until I back off to accelerate more slowly.  The secondaries are not involved in any of this. 

THE RECIPE: idle tubes: .033, stock
                    channel restrictions: .044, stock
                    discharge ports: increased from .082 to .086
                    idle air bleeds: .070 upper, .077 lower (left stock)
             
                    main jets: stock were 72, tried 72 (Cliff's jets) first then 73, which was worse.
                    metering rods: stock were 52, installed Cliff's 44
                    main air bleeds: .074 in air horn, .040 in main body (left stock)
                    power piston spring: dark blue (strongest) engine pulls 22 inches.
                    can't remember the apt setting, but did set it up as per Cliff's method in his book.

  Note: thought I was too lean on acceleration so I moved the accelerator pump lever from the outside where it first was to the inside and it was worse ! 

   I know I will be told to be on the look out for ignition issues but car accelerated fine before carb and intake change.  I'm out of ideas. 

[Cliff Ruggles]

Primary side only?  What does it do when you go onto the secondaries?.....Cliff

[Frank400]

I'm pretty sure it was better but I'm not 100% sure...I nailed it not to see what would happen, I nailed it because I was mad at the situation.  Been chasing this problem all day Sunday and now it's raining so can't try anything else for now. 

   

[Cliff Ruggles]

In most cases, the "pow-pow-pow" is an ignition problem caused by miss-fire at heavy load, then the unburnt mixtures are ignited in the exhaust system.

Could also be a worn cam lobe, bent pushrod, or other problem in the valvetrain.

The carb being too rich or too lean at heavy part throttle would cause lack of power, flat on accelleration, or even surging.

Backfired up thru the carburetor can be cause by no accellerator pump shot, but I've never to date observed a rich or lean mixture cause explosion in the exhaust system.....Cliff

[Frank400]

Replaced the spark plugs today and gave it a try...works like a champ !  I'm sooooo happy !  Thanks again Cliff for your great parts and wonderful info. 
 
   On another note, I think I read somewhere you were planning on a coming out with an updated version of your book, is that true and if so, when ?

            Frank.

[Cliff Ruggles]

I have a LOT of info to put into a revision when the publishing company is ready for it.  Not sure when that will be?.....Cliff