Best method to remove APT screw in baseplate?

"For stock applications, moving the APT adjustment to raise the primary metering rods to correct a symptom is usually just covering-up something else that’s wrong. But whatever works, works. After 50+ years, I wonder if the power piston spring has had enough?"

ALWAYS replace the PP spring even though it is NOT a player with light engine load/high vacuum or A/F at cruise.

I have found that with this new fuel we need to increase delivery about 5-7 percent (idle/off idle) vs what fuel those carbs were tuned with back in the early 1970's. Most likely because of the lack of BTU's and slower burning rates from adding ethanol. I employ the APT on every carb built here and find that I have to raise them just a tad to make things happy on the test engine.

The few times I've returned carbs to customers because they insisted on leaving them stone stock they were complaints. I love stories with these things so here ya go:

The last one was for a 1978 Chevy 350 Corvette engine (L-82). The owner was a die hard purist, owned the car since new, and absolutely insisted that I return his fully restored carb to him completely stock in every respect.

So instead of opening up the idle tubes .002" (those carbs are really lean idle/off idle) and using my full tapered primary rods in it instead of the "K" series rods I did EXACTLY what the owner requested. He didn't have the carb on the engine 20 minutes and called up here whining about it not idling smooth with minimal control of the mixture screws plus it felt "flat" and surging slightly at light part throttle cruising. His "guru" mechanic/engine builder was flaming me as well, said when he tipped in the choke flap the engine sped up and smoothed out (no chit Sherlock it's TOO LEAN just like I told you it would be), and he had to turn the mixture screws till they were falling out to get it to idle halfway decent. Even then it wasn't up to his standards. Then he took a propane torch and went around the carb and got a slight speed increase when he got around the secondary throttle shaft. This led to him throwing some BS at me about not bushing the secondaries and he claimed ALL the issues were from me leaving out that step in the rebuilding process.......you've got to love these "gurus" because they are completely CLUELESS when it comes to these things and very quick to put all the blame elsewhere.

Anyhow, I was thinking to myself that if he knew so much he should have built the f#@king thing himself, but I kept my cool and politely told the owner to return the carb and I would correct the leak at the secondary shaft that was causing all his issues per his "guru" that had been doing this since Moby Dick thought he was a minnow.....whatever.

I got it back, opened up the idle tubes .002", threw the stock "K" rods in the trash can, installed my full tapered .045" primary rods, fine tuned the APT on my test engine using the "tip-in" procedure and sent it back. The owner called a few days later thanking me for correcting ALL the issues with the carburetor and also mentioned that he never remembered it running that well in all the years he'd owned it.

A story with some food for thought........Cliff

 

Before you make any modifications get it fully warmed up, adjust the idle speed, mixture, timing so it runs the best that you can manage. Gently "tip in" the choke flap or place your hand over the choke area to richen thing up just a tad. If you get a positive response, engine smooths out, RPM's increase, etc, then adding .001-.002" on the idle tube size will show improvement......

 

Its not a multi port sequential fuel injection. The car runs great and smooth as can be. It doesn't idle like it has a window rattler in it, barely has a lope to it. Let's not all forget that pretty hard to get color on the plugs on cruise with total timing at 46 degrees!!!! Timing keeps the plugs clean. Its not surging, added fuel on mixture screws, starts better and runs smoother. Jason do you need to reread the timing thread and the effects of it ?

 

LOL!!

 

I can see where this is going. The carb should get a complete overhaul, and the next thing you know the engine is out and the body is on a rotisserie...

 

Who did all that and didn’t dig the APT screw out?

 

That is my work. I don't fool with the 4M baseplate apt screws. It requires a lot of time and has a low success rate in my experience.

 

Always love your stories Cliff! I had similar experiences in my business. Also appreciate you sharing your knowledge on here.

 

Nice! If the 246 one breaks off, just put the cap back on

 

"Popped the plug off and just put a little pressure on it and the damn thing just screwed right out! Here I was all ready for a fight!"

About 1 in 4 will come out with minimal effort. THE key to success is taking the time to make a custom fitting screwdriver. If you aren't smart enough to do that when you attempt to remove the screw and start applying force to it the "ears" spread out LOCKING the screw into the baseplate instead of having it turn easily in the threads and back right out. Once you've spread one out the result we be the ears breaking off, the screw is NOT coming out at that point despite your best efforts.

I actually have a custom screwdriver in my tool box that I made specifically for that purpose and even took the extra time to harden it on the end. I've remove two APT screws in 455 Olds carburetors since this thread started and both of them came right out, one didn't need any heat at all and the other started to turn before the propane torch was on it 5 seconds. Even with all that said you'll run into one once in a while that isn't coming out no matter what you do. I have the skill sets to remove them, but it is NOT easy so don't put me on your list to send them to when this happens......I'm retired from carb building and for sure NOT wanting any more work, it's a LOT more fun to throw sticks for my dogs, play with the grand kids, and come on these Forums once in a while and help out.

Over the years every carb that came thru here that has that feature got external APT if it was possible.......saves COUNTLESS hours of time as I can dial them right in on the test engine and don't have to pull it back off, remove the top, pull the PP out and swap out metering rods. I'll add here that you also get different results simply swapping rods and sometimes if the factory has the APT pretty high you will get no change going to a different rod number as it's already mostly out of the jet and on the tapered section between steps. You are SCREWED trying to dial those in with metering rod changes alone without having a working APT system.....FWIW.....



"Always love your stories Cliff! I had similar experiences in my business. Also appreciate you sharing your knowledge on here."

Thanks, stories reflect real life experiences and there is usually a lesson folks can learn from in there someplace. One thing you will find with me is that NOTHING I put up here is copy/pasted from the work of others. I do NOT Google either. Using real life experiences typically keep me out of trouble as well, as 2nd, 3rd and 4th hand information looses part of the message and will have flaws by the time it gets to print......IMHO......