I have a Motorcraft 2150 on a fairly healthy 351w in my 70 Galaxie. The carb is not stock, it was set up for a dirt track car, polished out to ~450cfm according to a flowbench, de-choked, etc. The only intake I have for this engine is the 2BBL that's on it, and this is the best carb out of my collection right now.
The problem is that the cam is a bit large for this carb, it gives the engine a rather thumpy idle, and I am having a very hard time trying to dial in the idle. With the screws all the way in, it won't idle. Back them out a half a turn, and it's rich at idle. I've pulled and checked the carb, changed the jets out (I'm only running 57s in it), checked the power valve (6.5), and still can't get the idle mix right.
I'm thinking it needs too much throttle at idle, due to the large cam, and can't properly use the idle jets. On a 4BBL, I'd fix this by adjusting the secondary stop and adjusting the primary idle screws as needed, but not really doable on this thing.
I have heard of people drilling holes in their throttle blades to allow a bit of air bypass, but where is a good place to start? Should I just start with a 1/8" hole, and go from there?
If I do massively butcher this thing, I do have a couple other Motorcraft/Autolite carbs I can raid the throttle blades out of.
Anybody have experience with this?

 

I would try 1/16" first. It's easier to go bigger than to try to go back once you go too far

 

I'm having the same trouble with a 2100 on a 360FE in my '75 F150.

In my limited experience... 1/8" might be as big as you'll need to go.

Recently figured out where most (all?) my extra richness is coming from tho...
fuel is slowly dripping off the booster venturis! :/

I've drilled the throttle plates (7/64") and got the idle RPM down to 360-380 which is scary slow to me, even tho it idled great! :) There was no fuel dripping off the boosters at that RPM is the reason.

My case is odd?? ...I've got a booster assembly from another carb in it and figuring that's where my trouble is. I've got several booster assemblies and need to experiment with them as a way of trying the various emulsion tube types.

I need to "lighten up the signal" by drilling a small hole (adding one at a time and/or reaming out existing holes) near the top of the emulsion tubes until i just barely stop the fuel from dripping, then start over with my throttle plate drilling experiments.

Let us know whatever you figure out -right or wrong- ok? :)

Any other carburetor experimenters on here?

Alvin in AZ
ps- to fill a hole just stuff a hunk of copper in it and squeeze it from both sides with some pliers, don't worry about making it look pretty until you are done experimenting? :)
pps- another experiment in the works is lengthening the low speed circuit slots.

 

This is a tell-tale sign of your float height being off. Don't go drilling anything - you just need to adjust your float. With the height off, there is too much fuel in the bowl and it overflows to the venturis. It is also possible your needle valve is sticking, which is preventing the float from closing the needle. This causes the same effect.

 

Cool post, thanks, and I couldn't agree with you more ...except that ain't it. ;)
I've experimented with various fuel levels.
The signal is just too strong with a certain air flow with that booster assembly.
(which ain't the original remember?;)
Now that you got me to thinking about it again...
Air correction hole size experimentation should be my next step?

I've got booster assemblies I can experiment with,
even letter stamped for identification and all the hole sizes written down
(in pencil;).

I have a full set of jet reamers and .0001" reading micrometers too.

Any other carburetor experimenters on here? :)

Alvin in AZ

 

Hi Alvin, please do a "wet check" on the fuel level in the carb. You'll have to have the engine running, to operating temp, and pull the air horn off of the carburetor to expose the fuel bowl. Be very careful of the fan and be aware of the open fuel source. If you bump the float, pressurized fuel will squirt out of the needle like a fountain. Once you have the fuel bowl exposed, measure the level of fuel from the middle of the fluid surface, straight up to the top machined surface of the throttle body assembly. Write back with that measurement. Sorry to keep beating this down, but unatomized fuel in the throat of the carb is almost always a float problem, so I'd feel better about having a measurement before advising anything else. If you come back with a measurement that sounds good then we can eliminate that. I'm a numbers guy, so I like to go off precise measurements instead of experimentation and eye-balling. With these carbs, 1/8's of inches can make all the difference. If someone else chimes in with a solution that fixes your problem then that's great, too.

 

You also may have a damaged gasket for the squirter assembly, or the screw gasket may be missing. I have had this happen a couple times when rebuilding carbs.

 

Hey Im a Chevy guy but I was crusin the web and came across your problem, one thing that I think people missed was to check the vacuum at idle, if it is bouncing or lower then your power valve in the carb this will cause fuel to dribble out of the boosters, but as noted above I would check my float level, needle and seat, saturated float (heavy) but I find the best tuning tool for carburators and engines is the vacuum gage if you know how to read these they can tell you alot about your engine. Good luck I hope that I have helped.

 

As for drilling holes in your throttle plates this is a viable fix for your idle problem, but only after you have confirmed that all the other parameters are right. I have alot of experience with Holleys and I think that the Motorcraft carb is close to a holley, if im wronf I still would confirm the vacuum reading at idle with what power valve is in the car. If you do drill holes in the plates start in the .085/.093 range. Start small and don't bend the shaft while drilling. Again good luck

 

A Ford friend found that by drilling 1 or 2 1/16" holes at the 9:00 and 3:00 position of the throttle plate produced noticeable performance, while letting the car start and warm up normally in damp or cold weather.

This was on a 1971 Boss 351 Mustang, automatic.

Something about the airflow into and around the carb, even when fully open (choke) at the normal engine operating temperature proved to be beneficial at the 9 and 3 positions.

It's a 60s era street and drag racing tip. Also, people seem to think that the "more air" the better, but it sometimes not in certain weather conditions.

Plus, the drilling of the choke plate, you keep the safety factor in place, there is no fire hazard in the event of a backfire, ect.

Good luck.

 

I hear you! isn't it fun messing with things most people don't get? did your cluster reposition the booster venturi? is the venturi a little smaller? that would increase the pull on the fuel(signal), do you still need the squirt pump or will it pick up right away?

 

Yes. :) Modifying stuff is cool. It's what I do. I'm a tinkering junky. ;)

"if it ain't broke, don't fix it"
The worst signal maintainers on the railroad often repeated that. :/
And they didn't learn how to wire signals like I did, neither. :/

From what i can tell...
all the 2100/2150/4100/4300 "booster assemblies" have the same size "booster venturis" and "booster venturi holes" inside them and are the same height too.

Bowtie, at one point I had the full power circuit (power valve) defeated and it was still -slowly- dripping gas off the booster venturis. :/

At this point I know I have a vacuum leak (whistles to beat heck;) and narrowed it down to a cracked intake manifold runner or intake manifold gasket. :/ That needs to be addressed and so haven't been messing with it lately.

FMC400, I'm going to again experiment with fuel levels, at this point I know it's below what Ford calls for. BTW, I've ran it that way for ~20 years (climbing hills wasabitch).

Ford_6, (the OP ;) drilled any holes yet? Ed's idea sounds pretty cool. :)
Do you have fuel dripping off your booster venturis too?
Wondering if it ain't "just something they do". :/

Alvin in AZ

 

Haven't drilled yet, haven't had down time, good weather, and health at the same time just yet.
The only time I had a gas drip off the venturis was when I pinched the double holed gasket that the screw goes through.
This carb has been de-choked, polished out a bit, and gone through a few times. I used it on this car because it worked well on two or three other vehicles beforehand. I put a quick gasket kit in it, new power valve and accelerator pump, checked the throttle shaft for play, etc-
You can see the throttle is open more than it should be at idle, it's a hair above the idle fuel ports, more than about half a turn out on the screws and it's massively rich at idle, so it's pulling manifold vacuum against those ports.
Funny thing is, it started idling really stable on the way to work this morning. Popped the hood at lunch, and found a vacuum cap popped off. I guess this confirms that a couple holes would help-

 

How about making a calibrated vacuum leak? Rig it up so high vacuum opens it and low vacuum closes it. (Just as an alternative to drilling.)