Hello all, I have a high milage 302ci, 2bbl carb that gets about 10mpg. The exhaust wreaks of unburned gas and the motor knocks when under acceleration. Is this just a matter of engine timing and a carb that needs adjustment? Any info would be greatly appreciated. Thanks!
Mark

 

I have the same motor , I tune my engine air fuel mix by the plugs , pull a spark plug , if it is dark in color and wet you are running to rich , the fix is to take a screwdriver and turn your two air fuel mix screws in , to the right , both the same , try starting the truck and then 1 at a time screw in until almost dies and back out 1 and 1/2 turns and do other side . Run it like that for a day and pull plug , if to rich turn in , to lean white turn out , you want your plugs greyish brown . Get it timed also , I am getting 15-17 miles pg with a 95k motor , perfectly timed and perfect air fuel mix , this is where you are losing your gas mileage , it will run great when you dial that in , might want to change plugs after you get the air fuel dialed in also , check plug wires , cap and rotor also . Good luck , always tune by your plugs , they tell everything about your air fuel mix , remember brownish grey . Have you driven a flord today ? Ken

 

Ken,. what differences do you see between your procedure and mine?


Before you do anything.

with engine cold.. push accelerator to activate choke..

inspect choke plate for correct setting.
1/8" from completely closed

Now the mixture screws:

They are only for idle mixture...... you know that right ?

Turn them into just about snug (be careful: turning them in too far will damage needle-seat area)

Now, back both of them out 1 1/2 turns

start engine and get engine to normal operating temp with choke fully off.

Set idle at 550 rpm

Turn ONE screw only in until engine starts to stumble

Now, while counting turns.. turn counter clockwise until engine starts to stumble.

Now divide the amounts of turns from stumble to stumble in HALF and turn the screw back in that amount.

Reset idle to 55 rpm and do the other idle mixture screw.

Next cold start up inspect choke plate again

__________________

 

I always thought the 2 screws were for air fuel , when I turn in they lean it out and turn out they richen it up . Learn something everyday , I have a stock 2 bbl . Where are the air fuel mixtures ? I never could get the engine to idle down to 550 , it idles about 900 when warm on my tach and if I turn the idle screws in to lessen the idle it runs to lean , Im confused now , been tuning like this for 10 years now and my engine purs but a tad to high on the idle and idle screw on linkage turned all the way off . Thanks for all input , this is a great web site to learn from .

 

They only effect the idle of the engine in regards to how much fuel goes to the intake when the throttle plates are closed. Dennis hit a home run in regards to setting the screws for idle fuel.

In regards to the RPM being to high at warm/hot idle? Remove your air cleaner and examine the linkage the throttle cable attaches to. You'll probably see a couple of screws that are used for adjustment. One screw should be mated (for lack of a better word) to work with a "cam" that has a few steps on it. The way that works is via the choke plate linkage. Depending on how shut or open your choke is? It will have that screw resting on 1 of a few cam steps. Now this is the important part, that screw is actually working with the linkage to hold your throttle plates open to control idle speed at cold engine temps.

Now when the engine is nice and warmed up? The choke is wide open--And that adjustment screw if I'm remembering this right? Won't be resting against the 'cam step'. So that can't control your idle speed. There will be another adjustment screw probably at the base of the carb which points towards a small bit of metal attached to your throttle plates. That's the hot/warm/choke fully open idle speed adjustment. That's the one that sets the RPM when the choke is fully open.

If you back off that adjustment when the choke is fully open? You should see the idle speed go either up or down depending on how it's mounted. All it's doing is just holding your throttle open a touch.

I'm working off memory here, and without having photos to post it stinks. But if you pull your air cleaner and really examine the linkage on the carb? I think you'll see what's what.

As for where the air fuel mixtures are? This is not like working with fuel injection. The carb has a very well thought out set of passages that are designed to meter out the fuel...and the inner workings use needle valves, metering rods, 'jets'..etc..Using what's known as "Venturi Downdraft" to feed fuel into the engine depending on air flow.

It's very hard to describe, and if someone has photos of the linkages? It should become clear. (Haynes manual has some excellent photos and how this works)

S-

 

For the knocking you should check your initial timing, vacuum and centrifigal advance to make sure they work and aren't sticking.

 

Ooops!!!!!!!!!!!!!!

 

Double oops!

 

Hey Mil1ion, I finally was able to try tuning tips you suggested on my truck. I gotta tell ya, thanks a million! I put new plugs & wires on it, set the choke, set the idle mix screws and that raw gas odor is mostly gone now. I just filled my tank yesterday, so I won't know my MPG's for awhile, but I have a feeling that it'll be better than 9 or 10mpg. I counted the number of turns (to seat) my idle mix screws and one of them was almost 7 turns to gentle seat!! That just seemed like a lot to me. The other was only 3 turns. Thanks again.

 

My glad everything "Turned Out" fine.
Thanks for reporting back to us