Strange issue - APT does not contact the power piston pin. Carb won't idle.

[corellian corvette]

Hey everyone!

I have Cliff's book and have successfully rebuilt several Qjets for my cars, but this problem has me stumped.

Got a nice core Q-jet, #17056288, which is going on a built 327 from an early Camaro. Unknown cam, headers, 4-speed car. Manual brakes, so no vacuum accessories other than PCV valve and Vac Advance.

Looks like the carb had been rebuilt previously, but poorly. It had 47k rods with 44 jets and was running lean. It would idle @ 1200 rpm, but if I slightly closed the air-horn RPM would increase, so I determined it was lean.

Putting new jets #48 jets (stock was #47), now the carb won't idle. Doesn't appear to have any leaks or dribbling into the venturi.

I did add an electric choke but was sure to plug the vacuum hole with a set screw. Got some new idle mixture screws, 4 turns out from base.

This really has me stumped. I can't figure out why the carb won't idle.

The only thing I noticed, strangely, is that the APT was screwed in all the way down, and wasn't contacting the power-piston pin. I turned it out 2.5 turns, but it STILL doesn't contact the pin. It appears the rods are actually what's preventing the carb from bottoming out.

At this point I noticed that the Rod hangars were mis-matched (one side was lower), and I evened them out. This gave me some play in the piston (apparently before it was barely moving) - but still no contact with the pin.

Is this normal? Does anyone have a good picture of an original power piston so I can see the measurements for the rod hangers?

[Cliff Ruggles]

They do not make 48 jets for the Quadrajet.

Most were from .063" to .077".

It is also important to use the correct jets, they MUST be for a Quadrajet, not for a 2GC or 4GC carburetor or they will not work correctly.

We have the correct jets for the Quadrajet, most others stock and supply the wrong ones......Cliff

[429bbf]

i believe that takes m rods i can't remember  some are .040 longer . i think the older carbs are longer . i try to stay away from 75 and 76 carbs. I've found all kinds a different stuff in them. fwiw

[corellian corvette]

Sorry I meant *78* jets - this gets me near the .30 difference per your book (stock was 48/78 I'm at 47/78)

I also measured and I have the K rods, which should be the slightly shorter ones correct for the later carb. .26 power tips.

My understanding is that the 75/76 carbs are really good cores - this is for only a slightly modified SBC. Cliff?

I did some measurements on the idle tubes and idle air bleeds and they are all on the HUGE side.

Idle Mixture: .086
Idle Tube: .042
Idle Chan. Restrictors: .046
Idle Bypass Air: .104
Main Air Bleeds: .120
Upper Air Bleeds: .120
Lower Air Bleeds: .076

This is all unmodified by me. There is no evidence of tampering, so I'm assuming this is stock for this carb.

I found some other measurements for the APT online (measuring the upper plunger depth from the seat collar) and had to turn the APT screw up about 3.5 turns to hit that. Visually, this looks correct as the Rods aren't bottoming out first, and it puts the Rods in a good position on the jets (so that the bottom of the body of the rod - above the 2nd step - is level with the top of the jet)

This looks more correct relative to other carbs I've built, I was just surprised I had to turn the APT so far out.

Will run the carb this weekend and hope for the best.

[Cliff Ruggles]

Where did you get the jets?  They MUST be for the q-jet, or they will not work correctly.

APT and jets/metering rods have nothing to do with idle or idle quality, unless they parts are so restrictive that they are limiting fuel to the idle tubes......Cliff