My bronco has a 2150 on the 351M, I am running lean, I know this from pulling the plugs out.

Are the idle mixture jets only take care of idle? what rpm's do they stop and the mains take over?

I know when I had it smogged it was like 55ppm for hydro carbons when normal is 99ppm. This was in the heat of the summer.

I do not want to melt pistons in this thing, I know I need to richen it up but where do I start?

Which jets should I replace? I guess just stepping up 1 size should do the trick.

 

jopey,

The idle mixture needles are the only things that determine the A/F ratio at idle. As soon as the throttle plates move, the main jets start to affect mixture, as will the accelerator pump (temporarily) while the throttle shaft is rotating. There is not a specific rpm at which this occurs, as it is controlled mechanically inside the carb. The transition from the idle circuit to the main metering circuit occurs gradually over about 5-15 degrees of throttle shaft rotation from idle.

The main jets are at the bottom of the fuel bowl. To access the main jets, remove the 6 screws that hold the top of the carb on the body. Remove the screw that attaches the lever to the choke plate shaft, and remove the lever from the shaft. Lift the top off. You'll have to tilt it to the left to get the pull-over tube to clear the fuel bowl, and then rotate it about 90 degrees to get the top off the choke rod.

You can remove the main jets with a large flat-blade screwdriver. You need not drain the bowl to remove and replace the jets. (Make sure your screwdriver is clean so you don't contaminate the fuel in the carb.)

You need to remove the jets to determine what size they are. OEM jets are stamped with a number. I recommend you buy the next two larger jet sizes and experiment with them. Try one size up first and make sure you're not blowing black smoke. If you can get the truck on a gas analyzer, see what a one-size change does to your CO and HC emissions. If you're still within spec and not blowing smoke, try the next size up and test it the same way.

Even with a one-size increase, you should notice more power and improved driveability, assuming everything else is working/tuned correctly.

Good luck.

 

Thanks Bubba.

I will try that when I get into Arizona tommarrow.

Hopefully it will be a easy thing to do.

Happy holidays!