I am wondering if anyone has any experience with high flow (500cfm) 2bbl carbs. I recently built the 351M in my 78 F150 4*4 with a towing cam and 2.25 dual exaust (manifolds, not headers). I am building this truck for the trail so I am after low RPM torque. I am wondering if replacing the Motorcraft 2bbl with a higher flow Holley 2bbl would help with drivability. It feels as if there is a big cork stuck in the intake right now. Is the major restriction in the carb or the manifold or both? If anyone has tried this I would really like to hear how it worked. Thanks.

 

In theory, a 4bbl would have better low end response and torque, especially if you got a small 4bbl. The front two barrels are way smaller then the two barrels on a 2bbl carb.

 

I see your point. If the primaries on a 4bbl where smaller than the 2bbl then low RPM drivability would be improved. Then the exra flow is gained when the pedal goes to the floor and the secondaries wake up. So would a small 4bbl mated to the stock intake be favorable to a larger 2bbl? and would there really be much gain realized in switching to an after market 4bbl intake given that I am using the stock exaust manifolds and a relatively small exaust tube (2.25)? I would like this engine to feel a little less plugged up though while maintaining that brute off idle torque. I never intend for it to turn over 4000 RPM. But now it struggles anywhere above 3000.

 

Well if it was up to me , I'd get the aluminum 4bbl. intake out of summit or jeg's for the 400/351M bolt it on and get a 650 Vacume secondary holley to put on top.After you tool around with it for a while and you get the need for more opfafa you could always go to a 750 Dp ..


ShaZam

 

I don't think you would be happy with a 4bbl adapted to a 2bbl manifold. I would get the 4bbl manifold. You can get a low rise dual plane that will fit. And you could get a 600 cfm holley or a edlebrock 600 to bolt on.

 

highcountry,

If you're building your truck for the trail, stick with the Motorcraft 2150 2V carb. That carb is widely regarded as one of the best off-road carbs available. Its bowl vents are well isolated from the throttle bores and it is very resistant to off-camber flooding. You can get a spring to load the float/needle valve to prevent problems with jostling.

The Holley 2V carb is like the front half of a 41xx carb, with of all the usual Holley difficulties.

The stock M-block 2V carb and intake setup does seem to run out of poop by about 3-3.5K rpm. You can extend the mid-range by another 1-1.5K rpm just by installing the Ford 5/8" phenolic spacer under the Motorcraft 2150 2V carb. The Ford part number is E5SZ-9A589-A (or Motorcraft CM-3545A) and it only costs $11.

What cam are you running?

 

Thanks for the input all. I am glad to hear that the 2150 is a good off road carb. I would love to retain it instead of going aftermarket. Like I said I am not interested in a race motor but a reliable tractor motor. I will order that phenolithic spacer for sure. Are there any other tricks that improve this carb aside from careful tuning? BTW I am running a Summit cam (204/214@.050" .484"/.510") Seems like a good low duration dual pattern torque cam. I have heard that the M heads like a dual pattern cam. Also the heads are shaved about .020". Probably not much compression difference but a guy can always hope. I live in SW Colorado so altitude sensitivity is important, this truck can go from 5000ft to 12000ft above sea level in a day. The aftermarket carbs (holley in perticular) don't like this. How about the 2150, is there an altitude compensating model or are the standard ones OK? I am new to the world of the M series engines. Any tips would be appreciated.

 

Sorry for the double post but I just checked out Bubbas website. That is going to be a great resource. One more question what does the spring to preload the float look like. I recently rebuilt the carb (after it had a three year vacation). It had a spring that went around the pivot pin and pressed against the bowl wall. I don't remember seeing this spring on the other 2150s that I have worked with (mustangs mostly). Could that be the one you are refering to? Thanks again.

 

highcountry,

There is a later model 2150-A feedback carb that uses a solenoid valve (called an "aneroid") on the back of the main carb body to regulate an air bleed and adjust the main metering circuit fuel/air mixture. I would avoid it because of its complexity. I have not had any major problems with my plain old 2150 at altitudes up to 12K feet.

One of my favorite improvements with the stock M-block intake and 2V carb is to disable the EGR and retune the carb to enrich the mixture.

All you need to do to disable the EGR is remove the vacuum source hose from the EGR valve diaphragm and plug it off. I also put a plug on the diaphragm nipple to keep contaminants from damaging it, in case I need to re-activate it later.

Since the carb is tuned excessively lean to work with EGR, you'll need to install larger jets to enrich the main metering circuit. I'd start with two sets at 2-step increments above what you have now. For example, if you have #57 jets now, get a set of #59 and a set of #61. Try the #59 first and if it's not too rich, go up to the #61 set. Ideally, you want to get up to just the point where it's too rich, then back down one step.

Disabling EGR and enriching the mixture will improve overall power and driveability.

I'm over on the front range, but I get up in the hills as much as I can. I'm running #63 jets in my 2150 at about 5.4K feet now. It's a little bit rich, especially over 9.5-10K feet. Plugs are running just a little black, but I'm not puffing any smoke out the tailpipe.

Yes, the little spring that goes around the float pivot pin is what I was referring to. The long straight end should be against the wall of the bowl and the short bent end should be against the bottom of the float arm.

IMO, your cam looks like a good cam for a mostly stock 351M. If you enrich your mixture some, it should really liven up your engine.

 

Bubba,

Thanks a bunch for the info. Hard to find someone who has messed with the stock stuff. Most people use it as a door stop. EGR is there but disconnected. It is way too lean. I noticed that right away when I drove it. A little elbow grease and I'll have it sussed out. My truck and I both really appreciate your input. I look forward to seeing your website get going. Thanks

 

i have one of those holley 2bbl's and it is somewhat better than the motorcrafts. not much though. i'm probably going to get a 4bbl intake and carb anyway so just go with the 4bbl

 

While messing with this 2150 I noticed a 1.21 stamped in the side. I think that is refering to the Venturi diameter. Is that correct? I have also seen that a 1.08 carb body flows about 300cfm which would (by my calculation) make a 1.21 body flow at around 390cfm. Is that possible? I never realized they made different sizes of this carb. I think my mixture and the resulting lack of timing advance (it runs poorly on acceleration with timing much over 6degrees) is causing the corked up feeling. Where have people managed to find jets?

 

[updated:LAST EDITED ON 11-Mar-02 AT 04:07 PM (EST)]highcountry,

The 1.21 on the side of your carb body indicates the diameter of the throttle bores. The Motorcraft 2150 was made with 1.12" and 1.21" throttle bore sizes. Earlier 2100 carbs may have been made with the 1.08" throttle bore size, I'm not sure.

Most truck 2150 carbs were rated about 300-325 cfm. The highest factory rating of a 2150 carb was 369 cfm for the 1982 Mustang GT carb (#E2ZE). My tweaked Mustang GT carb benched at 399 cfm.

Here are a couple of places to get 2150 jets:

Parker Carburetion http://www.holleypartsforsale.com

Carbs Unlimited http://www.carbs.net