I'm in a bind and need some help. I'm no expert but do know enough to be dangerous, unfortunately most of my knowledge is with efi. I know to use the three screws to tune it, but how do you know how much to turn each. I'm familiar with chainsaws in which you can just do it to the sound, but I figured there had to be a better way for vehicles. My truck is a 1978 with the 400 and stock carb. If someone could tell me how to tune it I would really appreciate it.

 

The three screws you're talking about are most likely idle mixture (2) and curb idle speed. Both of these must be adusted with the engine at operating temperature (at which time the choke should already be all the way open and the fast idle linkage dropped out).

Set the curb idle speed to the RPM specified on the valve cover sticker. It should be in the ballpark of 700 RPM. If you have an idle stop solenoid, adjust the curb idle speed by moving the solenoid bracket using the long screw, instead of the standard curb idle screw that threads into the throttle body. Because the idle stop solenoid is not strong enough to push the throttle back on its own, you'll have to bump the throttle each time the solenoid is moved forward to let the throttle rest at the new position. In the case of an idle stop solenoid, the standard curb idle screw is only used to keep the throttle plates from binding when the solenoid retracts and the throttle plates try to close when the engine is shut off. If the carburetor didn't come equipped with an idle stop solenoid, then the curb idle speed is adjusted using the standard curb idle screw. You can use a tachometer to measure RPM. However, you can also go by ear or by "feel" just like you would with a chain saw to get an idea of what's right. The engine should idle high enough that it can run smooth, but not so high that you have to mash the pedal to stay stopped at a light if you have an auto trans.

Once the idle speed is set, shut the engine off and gentlly turn both idle mixture screws all the way in. Gently back both out 1.5 turns. Connect a vacuum gauge to a manifold vacuum source (usually available at the vacuum tree on the firewall) and start the engine. Slowly turn both screws out simultaneously until the vacuum reading is maximized and steady. Ideally the needle should rest between 19 and 21 inches of mercury and should not move very much. Both mixture screws should be the same distance out, but you may find that both need to be tweaked individually to maximize the vacuum reading.

After the vacuum reading has been maximized and the engine idles at peak efficiency, the idle speed may have increased. In that case, you'll need to re-adjust the curb idle speed as before.

It's also good to know how to set up the choke and fast idle linkage, explained here: https://www.ford-trucks.com/forums/7...ml#post6422219

Here is some more useful information on choke operation: https://www.ford-trucks.com/forums/8...ml#post6939116

Hope that helps.

 

Couldn't have asked for better information. Thank you so much.

 

Great info FMC 400, thanks for the step by step directions.

 

Used your instructions to adjust the chose, idle mixture and idle speed on the brown truck today.

Fixed some of my problems, but still occasionally stalls when rolling up on stop signs. I noticed once that the fuel filter was empty when that happened. Probably 35 years of crap in the bottom of the tank.

 

I would check the fuel pump they get weak when they get older and can't keep up with the fuel bowl level when you running. If there is alot of residue in your tank you might consider putting a inline filter before the pump so you don't get alot of debris in the pump ( more so if your running an electric but I have put the in front of mechanicals to) to keep garbage out of your pump, and then have a second filter before the carb.

 

I changed the fuel pump, but do see residue in the filter before the carb. I'll try adding a good filter ahead of the fuel pump.

Thanks!

 

Just what I was looking for...thanks for the info!