Motorcycle Mikuni carbs on 3 short intake pipes

[Bryan]

Ok, just a crazy thought, I won't be doing this. Given the outer original intakes are longer than the center, and 3 straight pipes could be adapted to any size of simple flat slide Mikuni carbs,  wonder why nobody has attempted this?  I was looking at calculations that suggest a 230 ci motor at 4000  rpm and 100% (I know it's not) volumetric efficiency would be about 260 CFM.  CFM Calculator (summitracing.com)  Flatheads are probably at 70% VE so say a guess is 186 CFM.  186/3 is 62 CFM..  Was looking at Mikuni carbs (a lot of looking since they don't give CFM on their specs, you have to check on forums for people that have measured flow) and it seems like 22 -24 mm models would be about the right size.  Even if someone bashes me and says you could have 6000 rpm, okay, you just recalculate for larger carbs.  I think you could get a solid tube linkage to pull up on 3 cables.   Maybe rig something to pull/push the chokes in.   Plus if you have vacuum advance & modified PCV you'd have to tap in somewhere.  Then it's a matter of the right pipe lengths, inside diameter would be the same as the block hole.    Only other problem, does the rear pipe/carb have clearance to the firewall? 

[Plymouthy Adams]

it's been done.....but not as effective on the larger car engines.....this is where the larger side drafts designed for cars are the better players...

[Bryan]

Even if they  were  regular carbs, I wonder why it's always a joined system when 3 upturned elbows would have equal lengths and directly feed the ports without any side currents from carbs on each end.

[Plymouthy Adams]

cycle carbs...multi carbs single manifold PLATE....each carb dedicated and the common air box...where is the air currents?

[Bryan]

9 minutes ago, Plymouthy Adams said:

it's been done.....but not as effective on the larger car engines.....this is where the larger side drafts designed for cars are the better players...

I saw the old Webers and other older carbs but Webers seemed pretty complicated.   The Mikunis are relatively cheap, have idle & main jets,  a mixture needle that can be adjusted, etc. They come in sizes from below 20 mm up to 48 mm..

[Bryan]

1 minute ago, Plymouthy Adams said:

cycle carbs...multi carbs single manifold PLATE....each carb dedicated and the common air box...where is the air currents?

On some motorcycles, yes.  But 2 or 3 carbs on a common (open internally) log manifold, wouldn't you have a flow from the sides (or center) as each piston (or 2) pulled down?   And it changing direction depending on which piston is intaking?   Am I misunderstanding the process?

[Plymouthy Adams]

there are different style webers...to me and my opinion only is that anything progressive in the weber line is a headache.....do not care for them...do not buy them...

[Plymouthy Adams]

of the many multi carb cycles I have experienced do not use a log....the better webers say dual and triple in duals and triplets are basically common shaft unit only....these are so easily balanced off the engine it is not funny

[Plymouthy Adams]

3 minutes ago, Bryan said:

On some motorcycles, yes.  But 2 or 3 carbs on a common (open internally) log manifold, wouldn't you have a flow from the sides (or center) as each piston (or 2) pulled down?   And it changing direction depending on which piston is intaking?   Am I misunderstanding the process?

 

well made manifold utilizing multi carbs will have a balance tube and as such this condition does not exist....even without the balance tube the condition is not a problem but a more delicate process to balance is all...

[Bryan]

Yep, had an older motorcycle..getting all the slides to lift at the same time is a chore when there are independent cables. A one cable pulling an adjustable bar system would be the best.  I don't know what a balance tube is, but it seems like the intake ports pulling the flow in different directions might be an issue. Single pipes would be pulled in a pulse, but not experience any side flow.

[Sniper]

A common plenum after the carb is ideal.  Some thoughts on this here.

 

http://victorylibrary.com/mopar/log-c.htm

 

 

[Bryan]

That was a long read, but see what you mean. The common manifold has to be designed properly to work correctly... -> quotes "Volume is a rather imprecise terms when analyzing a plenum. As interior space increases (vs. cylinder volume) the buffer or anti-reversion effect increases, but this isn’t linear and probably decays to a minimum value with some multiple of cylinder size." ......."Ideally, each runner entry should also have unoccupied floor radially with OD about 2.5 × runner ID. This insures separation of the wave signals, and allows maximum wave energy recovery since the runner wave sees this as “open”. "

[Bryan]

Ah ha..an individual pipe separate from others would have  a pull, stop, pull, stop pulse.  Multiple ports pulling on a carb would have a continuous pull if it was designed correctly..

[DJ194950]

Harley CV carbs are cheap used and work well on many apps.

 

Not sure if 2 or 3 would be best.  American made, parts, rebuild kits available easily..

 

Have seen done on some Street rods, etc.  Mostly 4 cyls. though.

 

Just another possibility.

 

DJ

[Andydodge]

Years ago......lol......maybe close to 45 yrs actually...a local car yard had a speed boat of some sort........clinker hull with a side valve mopar six in it.......I saw it after the yard had closed for the day & it was parked behind all their cars with no one around to ask details but it had a set of triple sidedraft Weber DCOE's and a weird exhaust setup which I later realised was some sort of water cooled manifold arrangement ..........didn't notice what their size was(its cast into the body) but each carby had a small 2 into one manifold that bolted up to the block with a tube running between each "manifold".............back then, around 1975 the 250 cube mopar six was a common install in Oz speedboats but this was the only one I've seen with side draft Webers......no idea how it went nor who owned it as I didn't check with the car yard afterwards but from memory it appeared to have certainly been used so I assume it worked..........lol..........andyd   

[Andydodge]

Here in Oz the GM designed OHV Holden 6 in capacities from 132/138/149/161/173/179/186 to 202 have had the single barrel downdraft factory stock Stromberg replaced with triple English SU carbies ranging in size from 1 &1/4" to 2", these were a common aftermarket "hot up" usage and worked well using cast aftermarket intake manifolds, also the LJ series Holden Torana XU1 used a 202 cube six with triple Stromberg CD side draft carbies from the factory & was/is the hot setup for these Holden sixes which use a triple intake port arrangement similar to the Mopar six, albeit into the OHV cylinder head so these type of carburetors may also be suitable on the Mopar six.....

..............in fact the Holden intake manifolds can be used on the 23/25 inch mopar six except that as they are used on the opposite side to that on their original Holden engine the carbies need an angled adaptor to bring the carbies back to the vertical.......the Holden sixes were in 2 distinct series, the early 132/138 series and the 149-202 series, intake manifolds to suit the early engine have the same port measurements as the 23" mopar........the 149-202 intakes have the port measurement that works on the 25" mopar engine...............

...............intake manifolds to suit the Holden sixes include twin & triple downdraft Strombergs, twin & triple side draft SU, Stromberg CD & DCOE Webers, two barrel WW Strombergs and 2 & 4 barrel Holleys have all been made over the years............andyd  

[49D-24BusCpe]

 

Bryan,

 

    THIS WORKS! It isn't guessing, and it isn't theory. IT WORKS!

 

    The A-o-K intake and Tom Langdon (Stovebolt) cast exhaust manifolds combination on my 25" DeVal (Chrysler Industrial IND-32 clone), has been in my D-24 Business Coupe for over two years. It has very good power and excellent drivability. It is teamed up to a Paul Curtis T-5 (Mustang/S-10 hybrid) transmission and a  '65 Mopar 8 3/4 rear end.

 

    In passing, I want to say that I'm a long-time friend of all the principals involved here; George Asche Jr., Paul Curtis and Tom Langdon. The intake, the exhaust manifolds, and the trans adaptor are still available!

 

    Just do a "DeVal" search in this site, and you'll find some more particulars! If you're going to continue with the 23" 230, the same gentlemen can assist you on that!

 

Walt

 

 

 

 

[Bryan]

7 hours ago, 49D-24BusCpe said:

 

Bryan,

 

    THIS WORKS! It isn't guessing, and it isn't theory. IT WORKS!

 

  Walt

 

 

 

 

Oh, I understand that, and after other's posts I see why a  combined manifold works well.  It was more just wondering if anyone had tried 3 separate side draft carbs on straight pipes or normal carbs on elbows.  It would be very cheap to do..maybe not as great as a $400 - 500 manifold but better than a single carb stock intake. Length of pipes probably critical with engine's RPM range.

[moose]

This is what I did about 30 years ago in a welding class.

[Bryan]

So I'm not crazy...?.  How did it work? Were there any issues at  certain RPM ranges?    You made my day..?

[Plymouthy Adams]

go back to your first reply and my first response.....it's been done....!!!!!!!!!!!

[moose]

11 hours ago, Bryan said:

So I'm not crazy...?.  How did it work? Were there any issues at  certain RPM ranges?    You made my day..?

I know nothing of your level of sanity.

 

That was back in college.  A guy down the hall from me in my dorm gave me a bank of four Mikunis that came from an early 80's 750 Suzuki.  The throats on them looked about the same size as the ports on my freshly rebuilt(cammed and ported) flathead in my 54.  Three ports. three carbs, no problem.  I built the manifold then installed it.  Started right up, idled great but wouldn't rev at all.  I knew nothing about carburetors, so I limped it to my hot rod mentor's house.  He looked at me funny, then ran a drill bit through each of the main jets.  Ran like fuel injected after that.  Made a lot more power.  Smooth power through the range. Lacked a little in the low range, but easy to deal with. 

 

[moose]

This is what I'm currently running.  One of the float towers broke on one of the original three carbs.  I couldn't come up with a set that exactly matched, so I decided to upgrade.  It runs and idles great, but it won't pull in the low range now.  Once it gets to around 1800rpm it pulls hard with full throttle.  I would love to have more low end, but on the highway in overdrive at 75mph, sitting at 2100rpm, it has so much smooth power!

[Plymouthy Adams]

If I were to consider the multiport MTC carbs I think I would hold out for a set of inline 6 carbs as found on say the Honda CBX1000  that was an exciting bike to crack the throttle on at 60MPH  bike manifold to the engine will always be flow issue due to the choked down ports at the block.  Have you tweaked the needles for this portion of the fuel delivery...you maybe be a bit fat here....

[Bryan]

25 minutes ago, Plymouthy Adams said:

.  Have you tweaked the needles for this portion of the fuel delivery...you maybe be a bit fat here....

Yep, when I was messing with motorcycle carbs it was for a Kawasaki H1.  Idle jet for idle and off idle, needle height and taper for lower & mid range, then main jet for 3/4 to full throttle.  You could get needles with different tapers.