I have a 86 F-150 with a 300 in it. When I bought the pickup it had the feedback carb on it that was not working correctly. I removed the carb and installed a non-feedback carter and a Duraspark Ignition setup. After I installed the new carb, I could tell that it was running a little lean as I would get a slight misfire. I didn't think much of it at the time as It had old vacuum lines and a manifold that was warped badly. I ran it for about a year and then decided to do a rebuild. I rebuilt the engine and also put a kit in the carb. I also installed a new set of manifolds that were true and replaced the vacuum lines. I even threw on a new fuel pump while I was at it. I have checked everything for leaks and everything is good.
but... my lean misfire condition is still there. I ran it for awhile and checked my plugs and they indeed look lean. I pulled the top off the carb and adjusted the metering rod up. This seemed to help maybe a little. I pulled the carb top again and adjusted it some more. This helped a little more. I then adjusted it just about all the way up. but it still is running a little lean. My idle is fine but about 1400 rpm on up I get the misfire. My plugs are also very light yet. There is maybe just a hint of tan to them. I've also took the carb apart again thinking maybe something was stuck in the jet. Everything is fine. I have three other 300's with this same carb so I'm fairly familiar with them, but this one's got me baffled. Any suggestions??? Do they make different needles and jets for these carbs? The only difference between this pickup and my others is this has a single 2 1/2" exhaust with an 18" glasspack where the others have 2" exhaust with mufflers.

 

Just a few ideas. First off, is all the emissions stuff in place, like the EGR valve and AIR pump? The carb is calibrated for those in place and will run lean without them.

You can also change the main jet that is inside the carb to adjust for altitude and such. I don't remember which type to use since when I did mine, a guy at the carb shop just gave me one to try. It's more than just the metering rod.

Lastly, check for play in the throttle shaft. If the shaft is worn, air will seep in around it, and no amount if rebuilds will ever cure it. You'll have to have bushings put in the base plate.

Just a few things to check.

 

When I swapped out the feedback carb for non feedback, I romoved the AIR pump and EGR valve. Where the EGR valve was, I fabricated a plate and bolted it on with a new gasket. I put pipe plugs in where the tubes connected to the exhaust and intake spacer. The bushings are fine. This carb has less wear than on any of my other 300's. I guess I'll maybe see if I can find some jets for this carb. If I remember right, when I rebuilt it it had the same main jet and metering rod as my other 300's I have so i thought it would be fine. I guess maybe my freeier flowing exhaust is just allowing more air to flow through. This engine also had to be bored out 60 thousanths where the others are standard yet. Not sure if that would make much difference or not.

 

If you don't have an EGR and AIR pump, that's probably your issue right there. The carb is calibrated for those in place and will run lean without them. I fought this issue when I swapped in EFI manifolds because I couldn't hook the emissions equipment back up (they both tie into the stock exhaust manifold).
The metering rod adjusts how much fuel the carb uses under different situations, but if the main jet is only 90% of what you need, a small amount of 90% and a large amount of 90% (as set by the metering rod) is still only 90% of what your engine is requiring and you'll always run lean.

It's easy to change out, and when I went to a larger one it made a LOT of difference.

 

any mods made to the engine you must readjust the carb to make up for. like the exhaust and ridding of the B.S. just step up the mid and high jet and you will be back in biz.

 

i have a 84 ford f150 300 inline in it and i had the same trouble i was told buy a ford specialist that the carboraters they put on 80 model trucks and up were no good he sujested to me to get a carborater from a 71 model truck and i did and it works perfect

 

That's another good option, since they're not calibrated for emissions.

 

Do you still have the stock exhaust manifold with the O2 sensor still installed?

If so, read the output of the O2 sensor to tune the carb for the proper mixture.

 

Well, this is probably my problem... Where did you find your jets and needles? Did you go through a dealership??? Also, is there a chart or something to go off of to find the correct jets and needles for a persons altitude and such???

 

Yes I do still have the O2 sensor installed and a wire into the cab for this exact purpose. I checked my mixture this way on one of my other 300's this way. Its been awhile and I can't remember what reading is about right?? If my memory is correct I believe it was around 700mv or something. Am i correct on this?? What reading should a guy have?

Ben

 

I found them at a local carb shop. The guy had an array of jets and gave me one to try. I don't know what style jets they were, or much more about them. I wish I could give more info. Maybe call around to some carb shops and ask if anyone's dealt with tuning 1bbl ford carbs? There's also quite a few places on line that deal with them. Might give this guy an email: Carter Y, YF, YFA, YH, YS - The Carburetor Doctor
Wish i could give you more info, I only did it to limp me through an emissions test before I put my 4bbl on, so I never looked deeply into it.

Generally, you want the jet to be leaner when you raise in elevation and richer when you drop in elevation.

And yeah, 600 - 700mv is right where you want to be. A touch rich (800) when you're accelerating and right around there for cruising.

 

OXYGEN SENSORS

This site has some good info, charts and graphs.

Jim

 

I had the afternoon to myself yesterday and decided to tinker with my carb. I took the top off and removed the jets. I have two other carbs for parts and decided to compare jets. I found out that my two parts carbs have the same jets as the carb on my engine. I was hoping I might find some bigger jets in the parts carbs but no luck. I pondered for awhile and then decided I would try drilling my main jet out some. I figured if I screwed it up I had two spares. I ended up drilling it out to 1/8". I adjusted my metering rod 1 turn past bottoming. It was a little lean so I kept adjusting until it was right. I ended up about 3 1/4 turns past bottoming. I still need to find a bigger low speed jet as my idle screw is definately open more than 1 1/2 turns. I'm not sure how many but I know its too much. I connected my multimeter to the O2 wire and had a range from about 7-750 mv when driving. I think its about right. It might be a touch lean yet, but not much. It still will ping a little when I get on it hard. I might need to check my timing and back it off a touch I'll drive it for awhile and then check the plugs. It sure has alot more power.

As far as the low speed jet goes, does going up a number mean a bigger jet???

Also, thanks to everyone for the info and for the great wealth of knowledge on this site.

Ben

 

If it's correct during cruising, and doesn't massively lean out when you accelerate, I'd say you have it right on. 700 - 750 is about right where you want to be. If it's pinging, it could be that your timing is adjusted for the emissions equipment too. It's all interconnected and changing one thing affects another. Is your distributor's vacuum advance adjustable?

 

My timing is set for whatever a 83 is supposed to be set to. I can't remember what it is off the top of my head. I'm guessing around 10 deg. but not sure. I'm not sure if my advance is adjustable or not? How do I tell?

Ben