Need help with mixture adjustment
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Freshman User
Joined: Jul 2005
Posts: 38
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Need help with mixture adjustment
Hey guys. As some of you may know, I just put a new 300 in my 81f150. It's running now, all the vacuum is done, but it's running rich. I need to adjust the mixture on it, but am lost as to how. It's got the carter 1bbl on it. The last time I adjusted a mixture, I seem to remember there being a "mixture screw". The book says to do all kinds of crazy stuff with the vacuum, rpm, etc. Can't I just warm it up and turn the "mixture screw"? If not, Is there an easier way than the one that the book describes?
Lead Driver
Joined: Feb 2002
Posts: 7,970
Likes: 595
From: Greater Austin, Texas
The mixture screw only adjusts the idle mixture, so if you are rich at idle, then this is what you want. If you are rich thru the entire range, ie, your plugs are black or you have black smoke, then you are going to have to rejet.
To adjust the idle mixture, you just need to make sure that you are down on true idle. Some of those motors had electrical and/or vacuum solenoids to bump up the idle. These must be defeated or you may be drawing from the main metering port, or the transfer port, instead of the idle port.
With that done, and idleing as slow as possible, turn the screw in until you hear the engine start to slow down. Then back off about 1/4 - 1/2 turn. If you have one, you can use a vacuum gauge and adjust in until you see the vacuum start to fall, then back off to max vacuum.
Before you adjust in, you should probably back the screw out about 2-3 turns to make sure you are not already lean, although I doubt it since you said you were running rich.
Remember this only works for the idle mixture. Once you get it right, then you will have to determine if your problem is solved or if you are going to have to rejet.
To adjust the idle mixture, you just need to make sure that you are down on true idle. Some of those motors had electrical and/or vacuum solenoids to bump up the idle. These must be defeated or you may be drawing from the main metering port, or the transfer port, instead of the idle port.
With that done, and idleing as slow as possible, turn the screw in until you hear the engine start to slow down. Then back off about 1/4 - 1/2 turn. If you have one, you can use a vacuum gauge and adjust in until you see the vacuum start to fall, then back off to max vacuum.
Before you adjust in, you should probably back the screw out about 2-3 turns to make sure you are not already lean, although I doubt it since you said you were running rich.
Remember this only works for the idle mixture. Once you get it right, then you will have to determine if your problem is solved or if you are going to have to rejet.
Fleet Owner
Joined: Sep 2005
Posts: 28,609
Likes: 18
From: Lake Tahoe, Nevada
I second jas88's advice, but if the float level is too high it can make it run rich at all rpm's. Make sure the float is good, not saturated and/or not leaking fuel into it making it too heavy. Also, if you do open the carb replace the needle/seat valve in case it leaks.
