Adjusting Carb

[Guest]

I own a 62 Invicta (Go Figure Huh?) and need to adjust the fuel/air mixture.

The carb in question is a Rochester 4GC. I know which screws are for the

fuel/air mixture but in all the resources that I have, none say which is

to adjust fuel and which is to adjust air. Thanks everybody, hope to hear

from you all soon .

[Smartin]

Both are the same, just for different sides of the carb.

To adjust them properly, you need a vacuum guage...you can buy one at the parts store - a MUST-HAVE tool for every one of us.

Start by turning them all the way in until they barely seat. Then back them out 2 full turns. This is a good starting point for adjustment. Now, hook up your vac guage to a full manifold vacuum port. You should be reading at least 15 inches on the guage. Turn each screw until you get the highest vacuum reading. Most of the time, it is right at the point where you turn the screws in just enough to make the motor run a little rough. Then back the screw out until it smooths back out. You may be able to do this without the aid of a vacuum guage if you can get a "feel" for it.

It takes a few tries, but you'll get it after you do it a couple times.

Good luck!

[Guest imported_NikeAjax]

Beware, once you hook up your vacuum guage and get the motor running, you may find you have other engine problems too; like bad valves! If you're the type that gets fixated, you may find that you'll want to make that thing, needle, stay rock-solid, so you'll adjust, and readjust, etc... It can make you crazy! Make sure, if you can, that you motor is hot, not just warm, this will help in making finer adjustments. Also be sure that you have a good source for vacuum, like off the manifold, and not on the carburetor.

Hope this was a little help,

Jaybird

[NTX5467]

A vacuum gauge and a tach are good tools to have in adjusting the carbs on earlier vehicles. Typically, they were adjusted to the highest manifold vacuum for the smoothest idle.

A "shade tree" method I developed in the later 1960s is a little different. I did the prototyping of this procedure on Dad's '69 Chevy pickup with a 350 4bbl and then later expanded it to our other cars.

Basically what I did was this. I first set the idle speed to specs with the dwell tach. Then, I'd tweak the mixture screws for the max rpm (also highest manifold vacuum) and then go back and forth until the speed and mixture adjustments were fine-tuned to the correct rpm. After getting that baseline, I went to Plan B which further fine tunes the settings for the particular vehicle.

On the earlier vehicles with the more basic emission controls, the factory procedure was to get max rpm and then lean the idle to obtain a 20 rpm drop per mixture screw. With other vehicles, you set the idle to a particular idle speed, maxed the mixture screws out for that speed, maybe having to tweak the idle speed screw to keep the specified idle speed, and when everything was completed, then turn the idle screws "lean" to drop the speed to the final idle speed. If some roughness came in, then you could tweak the mixture to hopefully smooth things out at the final idle speed. Basically, this was called "lean best idle".

In Plan B, which is an extension of the first procedure, I would FIRST put the parking brake on all the way. Then, at base idle, I'd put the trans in "D" (which means it might not be useable with a manual trans at this point) with the foot brake also applied, but then ease it off to see if the car would stay stationary. With the vehicle sitting there in "D" against the brake, I'd then check the pulses from the rear exhaust pipe. The aim was to just get them to smooth out from perceptible individual pulses. When that rpm was fine-tuned, then I'd tweak the mixture a little with the aim to not have any hydrocarbon smell out the rear pipe. This might result in some additional tweaking with the trans in "Park" also. End result, smooth pulses out the rear pipe and no smell. That usually coincided with a smooooth idle too. When done, I'd recheck the idle speed in "Park" to see if it might be a little too high to not stop when the ignition was turned off and would then restart easily and quickly. The deviation from factory specs might not have ended up that far off, but the extra care to baseline these settings made things just a little bit better in many cases. By the way, the "smell" part might not work on vehicles with working catalytic converters.

Also, in the days of the old Sun diagnostic machines that many dealerships used to have, with the Air/Fuel Ratio meters, you could adjust the idle mixture by the a/f meter. In many cases, you could lean it out to the desired 14.2 a/f ratio with the normal carb. Many times, you could get by doing that without loosing smoothness, but heading into the middle 13.0 area would usually smooth things out. All depended on how good the other engine components were at the time.

Probably too much information. These things worked for my, your results might vary. Proceed at your own risk.

Enjoy!

NTX5467