I need some advice from someone who knows their stuff about a motorcraft model 2150 2bbl. carburetor on a 351M. The carb. is the original one and as far as I can tell it has never been re-built. I have put a power valve and diaphragm in it but the rest is untouched. My question is concerning the air/fuel adjustment screws (or mixture screws). I have very little experience with carburetors so I'm going to need a little help determining how to properly adjust these. As far as I can tell the idle adjustment must be fine tuned when adjusting the mixture screws. In other words, just because my idle is ok now does not mean that the air/fuel ratio is correct. I have run the screws in all the way and when I do, I have to hold the throttle to keep it from cutting off (which is normal). I have found that in order to achieve the proper idle, I had to back each screw out five full turns. However, this was without making any adjustments to my idle screw. It seems to run fine now and it idles perfect but I want to make sure the air/fuel mixture is safe. How do I properly adjust these screws to achieve the right mixture of air and fuel? What tools should I need to properly do this?

dentside79 said:

I need some advice from someone who knows their stuff about a motorcraft model 2150 2bbl. carburetor on a 351M. The carb. is the original one and as far as I can tell it has never been re-built. I have put a power valve and diaphragm in it but the rest is untouched. My question is concerning the air/fuel adjustment screws (or mixture screws). I have very little experience with carburetors so I'm going to need a little help determining how to properly adjust these. As far as I can tell the idle adjustment must be fine tuned when adjusting the mixture screws. In other words, just because my idle is ok now does not mean that the air/fuel ratio is correct. I have run the screws in all the way and when I do, I have to hold the throttle to keep it from cutting off (which is normal). I have found that in order to achieve the proper idle, I had to back each screw out five full turns. However, this was without making any adjustments to my idle screw. It seems to run fine now and it idles perfect but I want to make sure the air/fuel mixture is safe. How do I properly adjust these screws to achieve the right mixture of air and fuel? What tools should I need to properly do this?

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Hook a vacuum gauge to a good manifold vacuum source. Tune your carb to the maximum vacuum.

Somebody will correct me if I'm wrong but I think the standard rough adj. is to turn the screws all the way in then back them out 2 1/4 turns. Then make small adjustments until it picks up as much idle as possible. Or runs the smoothest. Then adj. idle as needed.

I was told turn the screws in all the way and then back em out 1 and a half or so. then turn your idle where ya want it and it should run.

blacksnapon said:

Hook a vacuum gauge to a good manifold vacuum source. Tune your carb to the maximum vacuum.

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Sounds like a good plan. I will use the front main vacuum port on the front of the intake. Is seems like the vacuum would be the best with the screws all the way out, but your saying as I back them out the vacuum pressure will increase and once I reach the maximum pressure (whenever that may be) if I keep backing the screws out it will start to drop off again? If this is the case, it sounds pretty straight forward to me.

There actually is no standard way to adjust a carburetor (this is why mechanics aren't sorry they're gone). Differences in wear on butterfly shafts and the carburetor body they go through from one carb to another make standardizing impossible. Thats why the vacuum gauge is the preferred method. That adjusts each carb to maximum efficiency no matter how the wear.

You're both wrong.


Turn both screws out 2 turns from stop (don't screw them in too tight or you will damage the needle point.


Set idle at 600 rpm

starting with one screw turn to the left until you feel/hear the engine begin to run rough. STOP. now counting the turns screw that screw IN until the engine starts to runs rough STOP. Turn the screw back half the amount of turns it ook rough to rough....If you counted 2 1/2 turns ..turn it back out 1 1 /4 turns.
Now do the other screw..

this spot is where the highest vacuum reading it as well

A/T ?
Now set your idle 650 rpm in Drive with Brakes on


M/T 600 rpm


now take an unlit propane torch with the valve open and point it at the throttle shaft on each side of the carb looking for leaks

Hey!! I was close!

chuck101st said:

Hey!! I was close!

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HAHAHA...I woulda been too, that's the same info I woulda given. But I guess that's why Dennis is the man, or 1 in a million if you prefer.

I just tune mine by ear works great

By the way, thank you all for the advice. I appreciate everything!

dentside79 said:

By the way, thank you all for the advice. I appreciate everything!

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Well we do what we can. BTW...just took a look at your truck's videos, that's a sweet rig for being all original...and I'm not too quick to compliment 2by's, but yours is pretty sweet. I know you said you wanted to restore it, what else do you have to do? Or are you just gonna clean it up and repaint?

Yes TN, Me too. 45+ years of it.

But people that have never done it before need parameters to learn by

Of course all the mechanical has to be set correct FIRST before any fine tuning.My procedure is fool proof and it is a reality tune,then once done,you check the vacuum related parts that could be defective or worn.


I'm going to post and article on how Ford says to do it

Mil1ion said:

I'm going to post and article on how Ford says to do it

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Thanks...I'd really like to see that. While I'm confident in getting my carbs running, I'm not so confident in my tuning skills.

I'm no carb master but I've always managed to keep mine running well. Totally by listening to how she runs in relation to whatever changes I implement. Between Edelbrock and Holley, I'd pick a Holley anyday.

I was just on one of my Mustang sites and saw this.


This is for an autolite 1100 but the process is almost the same. The point being Vacuum leaks cause problems and gauges can't really be that useful

http://forums.mustangmonthly.com/70...-autolite-1100-with-a-vacuum-gauge/index.html

soaring3
Enthusiast | Posts: 440 | Joined: 03/08


The three screws, two on left side of carb are idle screws. Fast and slow idle. One is the idle speed of engine while warming up, the other idle speed once the engine has warmed up. The other screw is the idle circuit air fuel mixture. I adjust this mixture, once car is warmed up, by ear so to speak. Turning the screw a little and see if the car idles smoother. The proper way is to use a vacuum gauge, and adjusting for the most vacuum. Back to idle once warmed up, if you have an automatic tranny the car should be in gear. So you will need a helper. Turn idle screw until the engine is at 600 RPM's. The choke also has an adjustment by turning the big black cover on the side of the carb. Do a search for fordsix.com tons of info for these six cylinders. Good luck




mechanicalguy48 mechanicalguy48
Enthusiast | Posts: 477 | Joined: 12/07
Posted: 06/08/08
05:41 PM


Actually the proper way is the way you describe. At the factory we never set idle based on vacuum. It would be too unreliable as it has so many things that influence it. I have heard this method before and I think its sort of an urban legend. Maybe some auto maker used it I dont know, but to my knowledge we never used that method at Ford. Turn the idle screw in (lean) until it starts to miss or lose speed then out until it does the same thing and set it between those two. Generally speaking I used the leanest setting possible, in other words turn it in until it starts to miss and then out a quarter turn. The engine should be set hot out of gear first ,,then if its an automatic reset it to the spec in gear afterwards. Depending on your initial advance setting this may or may not be the highest vacuum,,,so ignore Pony carbs advice.

UTfball68 said:

Well we do what we can. BTW...just took a look at your truck's videos, that's a sweet rig for being all original...and I'm not too quick to compliment 2by's, but yours is pretty sweet. I know you said you wanted to restore it, what else do you have to do? Or are you just gonna clean it up and repaint?

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Thanks for the compliment UTfball68. Your trucks are amazing as well, I LOVE that 77'! My Father has a 77' F150, but it too is a 4x2 By the way, is that the back of a Shelby GT500 in one of your pics in front of your Bronco? Is that yours? I have a stang too '06 GT, I almost bought a 09 GT500 for an amazing prices about two months ago but decided not to because of the economy, plus mines paid for and I want to use my money from now on towards my truck. Answering your question, I have done most of the work already, I just need to re-paint it and re-paint the engine bay, valve covers, oil pan, intake, block, and pulleys, etc... Almost everything else is done. I'm still going to check out Sudden Impact Auto Body, which you recommended by the way, by this coming summer. I just cannot justify spending another $10k for paint and body work right now so hopefully by this summer I will be able to make this happen. Thanks again everyone for the help!

TexasRho83 said:

I'm no carb master but I've always managed to keep mine running well. Totally by listening to how she runs in relation to whatever changes I implement. Between Edelbrock and Holley, I'd pick a Holley anyday.

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I'd go for the holley too. That's what'd be on my rig if I hadn't got this Eddy for free...

dentside79 said:

By the way, is that the back of a Shelby GT500 in one of your pics in front of your Bronco? Is that yours? I have a stang too '06 GT, I almost bought a 09 GT500 for an amazing prices about two months ago but decided not to because of the economy, plus mines paid for and I want to use my money from now on towards my truck. Answering your question, I have done most of the work already, I just need to re-paint it and re-paint the engine bay, valve covers, oil pan, intake, block, and pulleys, etc... Almost everything else is done. I'm still going to check out Sudden Impact Auto Body, which you recommended by the way, by this coming summer. I just cannot justify spending another $10k for paint and body work right now so hopefully by this summer I will be able to make this happen. Thanks again everyone for the help!

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Yes it was a Shelby..and no it wasn't mine, it was a buddy's, but luckily he's a good buddy and lets me drive it every now and again. It sounds like you're well on your way on your restore. I know I always get antsy when all that is left is paint. You definitely need to check out Sudden Impact, they do greeeeeeeeeeeeeeeat work. Their best painter, and the guy who does/did all my stuff is Rob Roberson...I haven't been there in about a year, so I dunno if the economy has had an impact on them yet, but the owner is Danny Shaw and is a really great guy, funny as hell and just a good ol' boy with loooooooots of toys.

SuperCab said:

I'd go for the holley too. That's what'd be on my rig if I hadn't got this Eddy for free...

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As I've stated before...Holley FTW. Only carbs I use with my preference towards the 4150's...I believe the part number of the one I try to stick with is 80-783C...it's a 650cfm e-choke and vac. secondaries. Extremely street friendly and holds well on angles offroad. However, this is a general application carb, so you have to buy the Ford linkages for about $20.

Or a Rochester Quadrajet. I'd take one of them - I hear the're good carbs...