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CARB THEORY

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nightwatchman59
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CARB THEORY

Post #1 by nightwatchman59 » Wed Jan 08, 2014 2:43 pm

I've been learning a great deal on this site for a while now. THANK YOU ALL for sharing. At the risk of revealing myself as the true shadetree mechanic I am, I respectfully submit the following: For the cost of an Offy-C and rebuilt 2bbl or 4bbl, MIKUNI makes a downdraft 48mm 1bbl for Yamaha that supposedly flows over 400cfm (model BN 48mm).
Has anyone tried anything like this? My current YF is OK... but that's boring. THANKS AGAIN!

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Re: CARB THEORY

Post #2 by Harte3 » Wed Jan 08, 2014 7:45 pm

Well, try it and let us know how it works. You never know when you might hit on something.
'83 F150 300, 0.030 over, Offy DP, Holley 4160/1848-1 465 cfm, Comp Cam 260H. P/P head, EFI exhaust manifolds, Walker Y Pipe, Super Cat, Turbo muffler, Recurved DSII, Mallory HyFire 6a, ACCEL Super Stock Coil, Taylor 8mm Wires, EFI plugs.

Nashtooth
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Re: CARB THEORY

Post #3 by Nashtooth » Wed Jan 08, 2014 7:53 pm

What is the Yamaha application?

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80broncoman
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Re: CARB THEORY

Post #4 by 80broncoman » Wed Jan 08, 2014 8:02 pm

I remember a few years ago on e of the 300 truck guys had a sled pulling truck in a stock class and he hada aircraft 1bbl carb on a aluminum intake.
Is that what this carb is for?
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When it come to engines If its .001 loose nobody knows, But if its .001 too tight EVERYBODY KNOWS!!
80 bronco FUV (farm utility vehicle)300 T-18 3.50s EFI head, offy C dual plenum, 500 edel carb, 1.7 roller rockers, Crane 260 cam EFI Exh

nightwatchman59
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Re: CARB THEORY

Post #5 by nightwatchman59 » Wed Jan 08, 2014 8:27 pm

The BN is a performance replacement / upgrade for waverunners / snowmobiles. Chad in the small six section adressed Mikunis awhile back. My observation is that it appears bigger than a YF, the design isn't 50 years old, parts seem plentiful, and although I doubt it flows 400 cfm, other sources rate it over 200 cfm. It is a newer performance oriented design. I was hoping someone had tried it.

nightwatchman59
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Re: CARB THEORY

Post #6 by nightwatchman59 » Thu Jan 09, 2014 9:49 pm

Thank you 80broncoman, that's kinda where I' m coming from. The veterans over at SPEED TALK , and forums relating to GM products, have a lot to say about carb flow / theory/ manifolds, etc... If there is a higher flowing 1v carb that allows me to retain the stump pulling characteristics of the stock intake, I'd want to try it. I'd like to know why Ford made a HD exhaust manifold, and no corresponding intake /carb... and forged cranks matched to cheap pistons.... my learning curve continues....

motzingg
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Re: CARB THEORY

Post #7 by motzingg » Fri Jan 10, 2014 11:05 am

I think the HD mods were made to run at extended periods of 3000-4000, like driving down the highway for 3 or 4 hours wound the heck out.

It didn't need to make much power at that RPM (because it was just running down the road, not accelerating) so it didn't need the BMEP.

These were in trucks with 6:1 gears, so they were meant to cruise at a high RPM and accelerate through a lower rpm. The name of the game being mechanical advantage, and back in the day. Even if torque drops off 30-40% from 2000-4000 rpm, you have twice the reduction to double your torque.


Most crank and rod failures come from running at high rpm and fatiguing the metal, they get pulled apart at the top of the exhaust stroke- forgings resist fatigue much much better than castings- and the manifold either pulls the powerband up slightly, or reduces the backpressure and the temp at the backside of the exhaust valve, or a combination of the two.




as for the carb, if its a two stroke carb forget it. two strokes require a totally different fuel delivery curve than 4 strokes do. Especially if they have a pipe. The atomizer/emulsion tube and needle (if it has one) profile will be. Some carb mfg's use the same carb for both so you can buy the parts to convert them, a VM mikuni for example can go on a 2 or 4 stroke by changing the needle, emulsion tube, slide cutaway, etc.

If its a 4 stroke carb you might have something, especially if its a CV (constant velocity) carb. They use a plunger to maintain a reduced venturii size so atomization and pressure signal are always maintained. That allows you to use a bigger venturii size carb on a smaller engine without being a pain to tune at the bottom end.

Post more info on this carb, sounds exciting!

Baron Von Ottomatic
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Re: CARB THEORY

Post #8 by Baron Von Ottomatic » Fri Jan 10, 2014 5:34 pm

Predator carbs are variable venturi 930cfm one barrels. I've got one I'm gonna try on the 300 one of these days. Gotta fab up a throttle linkage/bell crank first, though.

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THE FRENCHTOWN FLYER
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Re: CARB THEORY

Post #9 by THE FRENCHTOWN FLYER » Fri Jan 10, 2014 7:20 pm

Why not a single SU. They are available in many sizes.

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nightwatchman59
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Re: CARB THEORY

Post #10 by nightwatchman59 » Sat Jan 11, 2014 10:53 am

I called Mikuni, spoke to a recording, and left my number. Having received your responses, I plan on contacting a good friend, who is a Mercedes expert and classic car restorer to get his blunt force opinion- and steer me in the direction of one of these carbs,,,, he put a Mikuni on his Harley back in '78 and was very impressed.
I'll dutifully report my findings. I think outside the box because it's not my box. :idea:

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Re: CARB THEORY

Post #11 by Seattle Smitty » Sat Jan 11, 2014 1:10 pm

Now THAT is a cool idea, Flyer!! Is this a new project for you? Vizard put three SUs on a 292 Chev six and said it pulled like a steam engine. Your split intake idea looks even more interesting.

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Re: CARB THEORY

Post #12 by BIG 6 farmer » Sat Jan 11, 2014 4:00 pm

As far as the 1 V Mikuni... Im sure its a fine Carb. :thumbup: Thing about Carb. CFM ratings, you have to know @ what pressure drop they are tested at to compare... I know Holley 2V & 4V Carbs. are tests @ 1.5 & 3.0 ? :hmmm: Working in the Sports Car Shops gave me a lot of experience with British SUs, Jap. SUs, Stromberg CV Carbs. They are easy to tune, and perform well :nod: An early 60s Jaguar 3.8 inline six, with 3 SU Carbs. Had the same or better performance as a same year Corvette, with the higher output Engines... No B.S.
83 F 150 SB 4x4 300 six NP 4speed - - 1950 IHC L162 (1&1/2 ton?) - 87 & 88 T-Bird Turbo Coupes - 2000 Triumph Tiger , 76 Honda GL 1000 , & other toys and parts (& junk) -

nightwatchman59
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Re: CARB THEORY

Post #13 by nightwatchman59 » Mon Jan 13, 2014 4:25 pm

UPDATE as of 4pm 1/13/14- A young person named Chad from Zebco Distributers returned my phone call to Mikuni. Chad informed me that the Super BN 48 carb is a down draft, variable venturi carb designed to work on 4 stroke applications. The carb would need re-jetting for the application. He also informed me that Mikuni does not provide cfm flow numbers, and that Mikuni quit making carbs for autos 15 years ago.
The hunt is ON. 8)

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Re: CARB THEORY

Post #14 by woodbutcher » Mon Jan 13, 2014 6:53 pm

:twisted: Whats that ole Sherlock always said?Ohhhhhhhhhh yeah."The games afoot".Sounds like a very interesting project.Please keep us informed.Thanks.
Good luck.Have fun.Be safe.
Leo
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nightwatchman59
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Re: CARB THEORY

Post #15 by nightwatchman59 » Tue Jan 14, 2014 12:18 am

Thanks again,everyone. The plan is to obtain one, measure everything, see if it can be adjusted to handle the (stock) 300. If it matches the yf it's a win unless mounting it envokes Murphy's Law. The best engine deserves the best options.....

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THE FRENCHTOWN FLYER
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Re: CARB THEORY

Post #16 by THE FRENCHTOWN FLYER » Tue Jan 14, 2014 9:54 am

Seattle Smitty wrote:Now THAT is a cool idea, Flyer!! Is this a new project for you...


Nope Smitty, Its the intake for my champ car project. I will replace the HS-3s shown with larger HS-6s eventually.

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Seattle Smitty

Re: CARB THEORY

Post #17 by Seattle Smitty » Tue Jan 14, 2014 1:23 pm

And that's a weekend good-weather street-toy? Awwww, maaannn!!! :thumbup:

Obviously the problem with pretty much any carbureted intake manifold you fabricate yourself is getting it to feed all cylinders roughly the same amount of (and quality of) mixture, and do this over the working rpm range. Flyer, I'd guess you aren't a stranger to this advanced tuning. I can easily fabricate a manifold welded up from steel tube, and have made little stub "manifolds" for I.R. systems for boats. But for a deal like yours, with runners of various lengths and shapes, is is better to fabricate a manifold that splits horizontally, so that if you find big differences in flow at the runners you can unbolt the top of the manifold so you can tack-weld in some little dams and diverters in the floor of your manifold for further testing??

(Now don't all you fuel-injected guys start telling me to get out of the Stone Age. I grew up in the Stone Age, my brains and abilities are all Stone Age, I Like the Stone Age!!!)

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Re: CARB THEORY

Post #18 by Harte3 » Tue Jan 14, 2014 1:44 pm

" I grew up in the Stone Age, my brains and abilities are all Stone Age, I Like the Stone Age!!!)" :mrgreen:

Me too!
'83 F150 300, 0.030 over, Offy DP, Holley 4160/1848-1 465 cfm, Comp Cam 260H. P/P head, EFI exhaust manifolds, Walker Y Pipe, Super Cat, Turbo muffler, Recurved DSII, Mallory HyFire 6a, ACCEL Super Stock Coil, Taylor 8mm Wires, EFI plugs.

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Re: CARB THEORY

Post #19 by motzingg » Tue Jan 14, 2014 5:32 pm

nightwatchman59 wrote:UPDATE as of 4pm 1/13/14- A young person named Chad from Zebco Distributers returned my phone call to Mikuni. Chad informed me that the Super BN 48 carb is a down draft, variable venturi carb designed to work on 4 stroke applications. The carb would need re-jetting for the application. He also informed me that Mikuni does not provide cfm flow numbers, and that Mikuni quit making carbs for autos 15 years ago.
The hunt is ON. 8)



Sounds perfect!

Whats the stock carb bore, 1.75? Thats 45 mm, so you'll be a little bigger on the bore and the CV will be a huge benefit.

Pretty exciting proposition, cant wait to see how it goes!

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THE FRENCHTOWN FLYER
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Re: CARB THEORY

Post #20 by THE FRENCHTOWN FLYER » Wed Jan 15, 2014 10:15 am

Seattle Smitty wrote:..is is better to fabricate a manifold that splits horizontally, so that if you find big differences in flow at the runners you can unbolt the top of the manifold so you can tack-weld in some little dams and diverters in the floor of your manifold for further testing??


I've never tried to build a split manifold like that, but I don't see why it would not work. I've done a split "pancake head" like that though, with welded up exaust floor runners.

I don't mean to hijack this thread. Sorry
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1986F150six
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Re: CARB THEORY

Post #21 by 1986F150six » Thu Jan 16, 2014 1:55 pm

After reading this thread while it centered around the Mikuni BN48 as an alternative to the Carter, I searched and found something which was new to me. I wrote about it on Ford Truck Enthusiasts Forums and decided to post a link.

The information is about a Zenith model #228 carburetor which is advertized as being a replacement for the Carter on Ford 300 six cylinder engines.

http://www.ford-trucks.com/forums/12916 ... st13961816

nightwatchman59
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Re: CARB THEORY

Post #22 by nightwatchman59 » Thu Jan 16, 2014 10:09 pm

[b]FTF[b], please don't apologize for sharing information. I obtained my current YF from an abandoned truck back in '07 and put a $14.00 generic rebuild kit in it. My theory was If I couldn't master the Yf, I've got no business moving up. The thread about using a Qjet was awesome... I've had good luck w/ them. I was all set to make a spacer / adapter for a 2150 / Holley 2v when I came across the Mikuni BN, I was compelled to see if anyone had tried it. My expert friend tells me that if I can mount it and jet it, it should be a noticeable improvement- a set it and forget it deal, like a 2150.
I looked at ZENITH carb's website a while back but couldn't get specs. The Mikunis are showing up new for around $200-250 and seem to be the replacement carb of choice for a variety of cars (4's mainly).
I'd like to know if anyone's tried a Holley PRO-JECTION set up.

I'm thinkin' of designing my adapter to fit anything that comes my way.... :hmmm:

Seattle Smitty

Re: CARB THEORY

Post #23 by Seattle Smitty » Fri Jan 17, 2014 2:02 pm

I have two of them, the 670cfm 2-barrel version, in a dusty box. They're older and used, so I'm going to see about shipping them back to Holley to be checked out, esp. for the actual fuel flow of each of the injectors. I'll also get the closed-loop add-on (which came installed on later versions, I believe). Although this is not a carburetor, which needs to be limited in size to get a good vacuum signal at the venturis, I still think 670 cfm is maybe excessive for a modestly hopped-up street 300. So I took an old Clifford single 4bbl intake manifold and put a splitter in the middle of it, to the same effect as the split intake that Flyer has mocked-up in the photo above. I did this years back, but have never got around to finishing the engine I'm going to put under it, for several plausible-sounding reasons. :roll: Anyway, splitting the Clifford intake makes it similar to a dual-plane intake found on any Detroit V-8, so that any cylinder only ever sees half of the carb, or in this case, half of the 2bbl Pro-Jection.

Google "Holley Pro-Jection" and you should find some Jeep/off-road sites that have tuning advice on this unit. One thing they don't say, but that is essential (for any fuel mixer, IMHO) is to take the fuel tank and the hard lines out of your older vehicle and get it steam-cleaned or boiled out, then treated with a rust converter. Put the tank, etc., back with all-new fuel hoses, filters, tank screen, etc.. This will save you a lot of grief, grief that I had in one dramatic instance because I had settled for trying to flush out the tank in-place by just washing some fresh fuel through it. That was NOT NEARLY sufficient. Take it out, boil it out. If you think that's a chore, the consequences of gasohol acting on old fuel tank and hard line deposits make for much worse chores. And the electric fuel pump, injectors, et al, will be much happier.

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Re: CARB THEORY

Post #24 by Oregoon » Fri Jan 17, 2014 3:15 pm

A note on the SU carbs: they're known leakers as they age, but once you get them dialed in, they're hassle free.

I had a set on a Volvo B20 years ago. It had quite a bit of head work, a euro-spec cam, and an exhaust header. A company here in Oregon who specialize in Z-cars did my carbs (to the tune of $500) and they were without a doubt, the nicest and easiest things I've ever dealt with.

As to your Mikuni, good luck and happy wrenching! Sounds like a fun project.

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Re: CARB THEORY

Post #25 by nightwatchman59 » Sun Jan 19, 2014 1:50 am

Thank you Smitty, The Jeep guys, along w/ the small six crowd, are very informative. Had a van w/ curb weight of 5500 lb. v-8, 8-lugs, no overdrive. It had a tbi that looked alot like a PRO JECTION unit on it ,it ran great & got 15mpg. w/ family / pets aboard. I gave my YF a thorough cleaning Friday, and it rewarded me by flooding out when I tried to take it to work tonight. Ol' Red truck does not like to sit for more than a few days or it cops a 'tude.
My friend's Harley always started on the 1st kick after he installed that Mikuni.
Tax return on the way.... then there's that winning Lotto ticket I neeed to purchase......

joe4bch
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Re: CARB THEORY

Post #26 by joe4bch » Sun Jan 19, 2014 8:28 pm

Don't know if this is what's causing your problem, but... I've been wrestling with a YF for a while and you really need to check the float level and fuel line pressure. They don't like much over 2.5 to 3 pounds of pressure on my off topic truck. I eventually got mine to behave itself (more or less) but those two things were the trick

nightwatchman59
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Re: CARB THEORY

Post #27 by nightwatchman59 » Mon Jan 20, 2014 4:43 am

The YF and I are gettin' along. Biggest problem is the ol' truck doesn't get driven alot. That's never good for any vehicle (if it's not in a proper garage). Will revisit ZENITH carbs next time I get the chance. S&S bike carbs are better than Mikunis I'm told- so I'm headed there.
FTF's pics of multi carb set ups did not go unoticed.... 3 YF's on chopped stock log sections could be done cheap and easy... but I'd still want to know how well the BN48 Mikuni might have worked....
A stock looking six hanging w/ the v8's at the mud pit is my idea of a good day. My son wants me to enter the sled pull this spring.... but I learned awhile back "can't afford to break it? can't afford to take it!!"

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