My Catalina's Rochester 210 Dualjet carburetor has thrown me a curveball in learning how to adjust the mixture screws. Are these pin-like things within the recessions at the base of the carburetor the adjustment screws? They don't seem to be easily adjustable, like with a screwdriver, unless you're supposed to use a thin pair of needle-nose pliers. The fast idle cam adjusting screw is a flathead screw, as is the base idle speed screw on the throttle, but these pins don't have screwdriver heads for adjusting.
There is a special tool for this idle mixture screw that fits on a socket at the end of a flexible shaft. It comes with several types of little sockets stored in the handle. It's called the Lisle 5520 Carb Adjusting tool and is available here and other places -
https://www.lislecorp.com/specialty-tools/carburetor-adjusting-tool
I got mine about a hundred years ago. They still make them! Good luck.
I believe the weird idle mixture screw heads were originally designed to be factory set and "tamper proof." Some of the screws even had metal plugs over them so you couldn't mess with them without drilling them out.. Of course, it took ICE enthusiasts about 5 minutes to get around this. This was the good old days of early smog control.
Thanks for the info! It doesn't really seem like it'd be that difficult to monkey around with, even using needle nose pliers. An older coworker of mine had never seen mixture screws like that before. Considering you need special tools to adjust, and with this being a rebuilt carb, do they set them at more or less the right mixture points, or do they still need to be tuned to the engine? Obviously the car runs as is, but I wouldn't expect it to pass exhaust emissions. Haha. Thankfully Ohio couldn't care less about that.
You need to tune the carb to your engine. Remember, there's no feedback, the carb will just do what it's going to do regardless of actual engine requirements.
That's what I thought. But I got the impression with the "tamper-proof" design that it may have been set to something specific for emissions.
Hi Justin. Sounds like you have it working good. I just wanted to mention that we used to set the idle mixture screws using a vacuum gauge instead of "by ear" or a tachometer.
It seems like adjusting carburetors is becoming a "lost art" even though there really wasn't any art to it.
But I searched youtube for, adjusting a carburetor with a vacuum gauge, and this guy's video came up.
It's a pretty good explanation on how it's done. I thought your post was a good place to include it because of its relevance to "adjusting idle mixture screws"
https://www.youtube.com/watch?v=aBbD_JlKBGY
I've been looking around for a vacuum gauge like that to really fine tune it, but none of the auto parts stores around me carry them. They must be completely out of style in my area. My girlfriend's dad may have one somewhere in his garage. He's the former owner of the car, and he has an old school timing gun. I drove the car 50-60 miles on the highway today and went through a quarter tank going 60-65 in top gear on a 3-speed, which seems about right. My truck's driveshaft won't fit in my Mustang with the seats down so I had to drive the Catalina to drop it off at the shop. Of course those old trunks swallow anything under 6 feet long.
I always start at around 2 1/2 to 3 turn out from closed. I'll turn the screws in (clockwise) until I get the highest and smoothest RPM, then unscrew (counter clockwise) 1/8 turn and leave it. Then, I set the idle speed screw to the RPM I want. For a manual trans, I'll set the RPM to about 750 and 800 for an automatic in Park. It will drop about 50 RPM when you put it in Drive.
I'll snag the timing light from my girlfriend's dad and check what it's at. It's running higher now after messing with the mixture., so I'll need to back off the base idle.
I saw it a few days after he made the video. I bought the tool he mentioned and got the carb pretty well set up. He did a good job explaining that. I followed his video on tuning the mixture screws from several years ago too.
Last time I saw mixture screws like this was on an '84 Chevy K5 Blazer with a Quadrajet carburetor. Your co-worker friend isn't old enough to remember these. I have used a lot of rebuilt carbs over the years ( I usually rebuild them myself) and I never trust ANYBODY'S adjustments except my own. You really want the most power and efficiency from your engine and that means fine tuning the idle mixture screws properly.
My coworker is 60, haha. I think the last carbureted vehicle he had was a '79 Ford F-100. He had that truck up until at least the mid late 80s. He poetically hasn't seen the tamper proof design.
Thought I'd update on the improvement in fuel economy, I drove the old barge 125 miles yesterday, primarily on the highway and got around 13 MPG. That's a 100% improvement over the 6 MPG I was getting pre-adjustment. It still seems a little low, but between going 70 MPH in a 3-speed with no OD, and letting it warm up on the defrost fast idle for 10 minutes before driving in 10 degree weather, I think it's pretty close to being right.
@justin-sheperd, that's about the right gas mileage for those old barges driven under those conditions. The EPA mileage estimates back then were even more out in left field versus the real world than they are today.
I had a feeling that was about right. The commercial in my signature from 1979 promoting the "Classy" Catalina and getting MPG -More Pontiac to the Gallon, boasts about getting 18 MPG estimated by the EPA. I highly doubt a 301 V8 with a carburetor could get the same mileage as the fuel injected V6 in my truck, which has 1.9L less displacement. It was probably the 3.8L V6 they were promoting, he never mentions engine size on that rating. Haha. There was a very limited run of these cars with the 6.6L Pontiac V8 that was the powerplant in Burt Reynolds' famous Trans-Am. They had to be absolute gas hogs.
As an alternative to buying the tool ( it's pretty low priced) you could try to remove the idle mixture screws, put them in a padded jaw vise and carefully cut slots in them with a very fine hacksaw blade. Then, you'd only need a fine blade straight slot screw driver to make the adjustments you need.
I found out O'Reilly down the street has the adjusting tool in stick for $21.99, I could find out if I can rent it, my other carbureted cars have normal screws, haha.
Success!
I played with those mixture screws this morning. I let the Catalina idle for 10 minutes or so, then to make sure it was at operating temperature, drove it around 55 mph on a little 4 mile circle and then started messing with the mixture screws. I assumed they were out too far, based on the rich condition, so I turned them inwards. They both twirled like mad, I didn't count the number of rotations, but it was at least 4 or 5 each, maybe more. I heard the RPMs jump a little through a narrow range, then they dropped off again. I tightened each screw individually to where I could feel it start to hit, then backed them out 1.5 turns while listening to the RPMs. They both seemed to have been at their highest speed close to 1.25 rotations out and were dropping after 1.5, so I set them where they seemed to be at the highest. The Lisle adjusting tool doesn't have anything on it to keep track of turns, so I made a heavy mark on one side of the handle with a Sharpie.
I turned the engine off, then started it again right away. It fired right back up without having to touch the gas pedal. I shut it off again and let it sit another half hour, then tried to start it. Same result, it fired right up without having to feather the gas pedal like I was having to do before.
How sensitive are these adjustments? Can you "brick" your carburetor by over-adjusting those screws, meaning the engine will not run if they're bad enough out of adjustment, or will the engine still run, but run poorly? That was probably my biggest concern when I started messing with those, I don't need a 4500 pound paperweight in the driveway. Haha
I don't think you'd "brick" anything, it would just run like crap if way out of adjustment. If that happens you keep at the adjustments until it's right. (The worst you could probably do would be to tighten the mixture screws down so much the tips would be damaged.) During the 1970s there were different methods used to prevent people from adjusting their carbs for better performance. 🙂 GM used those mixture screws that needed a special tool. Others had plastic "limiter" caps over the screws to limit the adjustment range. (Those tended to be "accidentally" broken off which permitted a wider adjustment range.)
I wasn't really sure what those would do, or I would have tried this a long time ago. A few friends seemed to imply that if you mixture screws are wrong, the car will not run. I tend to overthink and over-analyze stuff sometimes, especially if it's unfamiliar. My girlfriend's dad saw me under the hood and asked how it's coming. He could tell when he heard it running it sounds much better than a couple weeks ago.