The Stovebolt Site Forums The Shop Area Engine Shop adjusting idle speed and mixture screws on 283

Hi guys,

I've had several people now tell me that my truck smells like its running rich, and it gets some nasty mileage too, I think (310 kms per 60 liter tank). So I tried to adjust it...I have the shop manual, and it suggests I get an RPM guage and a vaccuum guage. Says the rpms should be at 460 (i think, have to check it again) at idle, and I hook up the vaccuum guage before ajusting screws. Then I adjust mixture screws until I get the correct vaccuum reading ( had this listed too) I don't have either guage, so I winged it by ear. I have a few questions though, the two mixture screws, is right lean and left rich? and where do I hook up the vaccuum guage? I saw a small outlet on the back of the carb, with a rubber plug on it. Is that where? And if I was to get an RPM guage, where would I hook that up? By the way, the engine is a 283, and the carb is a Rochester 2 barrel.

The two screws you are talking about are only for the IDLE system and will not affect you mileage one bit. Sounds like you might be in for a carb rebuild. Is the choke working that is does it open completely when warm?

While a tach and vacuum gauge are nice to have, untold thousands of engines have been tuned "by ear". Turn both idle mixture screws in (clockwise), gently, until they bottom out, then back them out 1 1/2 turns each. That should be a good starting point. Warm up the engine fully, choke all thre way released, and adjust the idle speed screw on the throttle linkage to get a smooth run- - - -speed isn't terribly important right now, but don't overdo it on RPM.

Now- - - -turn the idle mixture screws clockwise, one at a time, until the engine starts to stumble (too lean). Back the screw out again until you get a smooth idle. Repeat with the other screw. It might be necessary to do this several times to get things just right. Finally, adjust the idle speed to whatever curb idle suits your preference.

Once you get the engine speed a little above idle, none of the above adjustments have any effect whatsoever on fuel mileage or rich/lean condition. Cruise or high speed mixture is controlled by the main jets and power valve, and they have no adjustments other than changing jet size. If it's running way rich, check for a stuck power valve.
Jerry

Sounds about like what I did, but will tinker with it again I think. I believe the choke is working as it should, it is manual, so I have to open it myself when the engine is warm.

Took the truck for a burn down the highway tonight after adjusting it, seems to run better and have a bit more power now. Not sure why...But yeah, the fuel mileage, I wouldn't be too suprised if it's normal. Nobody ever said 50 year old trucks were fuel efficient...

Would pretty much agree with Jerry (Hotrod Lincoln) except would start with 1 turn instead of 1 1/2 on pre-smog (1968) carburetors.

And 310kms per 60 liters is about 14 1/2 miles per gallon - not too bad.

Check your tires for proper inflation, and also check to make sure you do not have a brake that is dragging.

Jon.

The ignition must be set before doing any mixture adjustments.

Agreed...when was last tune-up?