Best method to remove APT screw in baseplate?

Tune for best results no matter where it ends up. The height will depend on the combination of parts being used, but not such thing as too high or too low, the final settings just need to be spot on for what you are doing.

If you are using that APT system I still like to "target" the final outcome with the correct parts making sure I have plenty of upper metering rod section in the jets to provide full control from rich to lean. It sure beats removing the top to change parts, and puts those early carbs more on par with the later APT units.......

 

No such thing as too high or too low IF you are tuning for best results.

The higher The PP sits the less movement is required to get on the power tip of the rods, but that's probably more superficial than a real benefit, but how could it hurt anything?

As a general rule it takes about 5-7 percent more of this new fuel to do what the fuel these carbs were calibrated on would do. Anytime you leave one of these carbs absolutely STONE STOCK and the owners set-up is STONE STOCK (very rare), they will work fine but you'll notice the engine will respond nicely to raising up the rods a tad and bringing in some more fuel at light part throttle........

 

Cliff, any idea if the APT on these carbs was used for A/F mixture adjustment when the carbs were being flowed, or were they adjusted/set to place the primary rods at a designed depth in the primary jets to correct for manufacturing tolerances similar to the what the secondary hanger selection accomplishes?

 

The factory had very sophisticated and expensive machines to measure flow and used them to set up the calibrations for the different part numbers/applications.

I've obtained a few factory "test" carburetors, and dissected them noting changes made to them, etc.

One in particular was a 1968 Pontiac carburetor, pretty sure the part number was 7028267 or something very close to that. Whoever was tuning it used a magic marker to make notes on it, plus they plugged, moved/relocated/resized the lower IAB's.

I noted that on my test engine that particular carbs was "spot on" in every area and one of the most responsive carbs I've ever tested. I went on to bolt it to the 455 that was in my Ventura at that time and do some street and track testing with it, nothing short of amazing how quick, "clean" and smooth it was clear across the load/speed range.

I pay very close attention to carburetors that work well, better than others, etc.

The factory "high performance" carbs are the ones to learn from if you are taking notes and trying to improve your tuning skills. In most cases a factory "high performance" carb may look much like it "grocery getter" counterpart, but they have features and mods mad to them FAR beyond just jets and metering rods. I see "builders" and companies catering to the Q-jet that still use "generic" calibrations and try to apply the same basic set-up to every carb they crank out. That's probably what landed them here in the first place, as these days close to 100 percent of the work I take in since retiring was just completely "rebuilt" or "restored" someplace else. The level of "butchering" amazes me, even with all the good information that's out there these days.......FWIW......

 

Yes, I know. I was employed at RPD in the 80's and watched those flow benches work in person. One operator was responsible for MANY test stands. But what I can't recall is whether APT adjustments were made directly for "live" A/F mixture ratios, or to put the primary needles placed in the precise spot in the primary jets to account for manufacturing tolerances, and the "part-throttle" mixtures were set by default so to speak.

One of my college buddies was the son of a large, local Buick dealer's service manager in Rochester, NY. RPD worked with local dealerships for "field issues", and this Buick dealership was their Buick connection. My college buddy's brother was a tech at this dealership and he had a 70 Stage 1 that he raced. At RPD, they had several engine dyno rooms down in the the test garage area. In one of these dyno rooms was a wet flow bench, where they could fiddle with a carb and bolt it on an engine to test their results. So they were able to take his Stage 1 carb and wet-flow test/adjust it, which IIRC they were interested in 12:1 form idle to WOT. This is a long time ago, so who knows if the 12:1 is entirely accurate.

Cliff, you are a credit to the industry. I can't imagine trying to rebuild or correct a carburetor from a mail-in thinking all of their driveability or performance issues were carb-related, let alone working behind the ever-increasing hacks. Good for you taking a step back. Well earned.

 

Thanks for the kind words.

I LOVE being retired, and to be able to pick and choose what I work on these days. I limit work sent here to Marine Q-jets, a few troubled carbs that just got "rebuilt" or "restored", and some local cash & carry stuff. I'm mostly selling parts these days and do that from about 4am till noon Monday thru Thursday. That keeps me busier than I want to be.

The parts business was more or less an evolution of having higher quality parts made, plus parts than are NLA or very difficult if not near impossible to find. I also cater to the "high performance" end with the Q-jet, and supply custom high performance rebuild kits for any part number with the tuning parts in them. This takes the guesswork OUT of the equation so the end user knows he carb is right where it needs to be IF they did they part correctly.

Kind of interesting but at least 80 if not 90 percent of the "carburetor issues" I get involved with on vehicles brought here for custom tuning are with the DISTRIBUTOR. Yes, it's a BIG player with these things. I also see a LOT of folks make piss-poor choices with their engine builds, so find they need to run the timing clear off the scale and really fatten up the carbs idle system to "crutch" things. I blame the cam companies for most of that, pushing really tight LSA with ton of overlap, then you have all these resident Forum "experts" and "guru's" telling everyone you can't run more than 9.5 to 1 compression on pump gas. Between those two things it certainly keeps me a LOT busier than I want to be.

I took engine building, transmission building, setting up diffs out of the equation here. Been selling off most of that equipment and all the spare engine parts from the parts room.

As for the APT thing I'm not sure if they set all of them with test equipment, but the height of the PP is all over the map, even on the exact same part numbers, so for sure they were using a "standard" setting for them. A perfect example were the 1970 Chevy LS5 carburetors part numbers 7040204/7040205. They all came with the 78 jets and 49B rods, but I've seen the PP bottomed clear out on some and raised nearly to the maximum height on others, and NONE of them had ever had the cover off the APT screw......

 

PM or email to cliffshp@embarqmail.com with your name, shipping address and carburetor part number. I'll look in QuickBooks for the invoice....