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V Twin Dual Carbs

54K views 73 replies 41 participants last post by  cpsdeni 
#1 ·
Whats up fellas. I've heard on more than one occasion that a v twin can greatly benefit from running dual carbs. I guess its something to do with uneven vacuum or something like that. I was wondering what everyone's thoughts were on the subject.

-Chris
 
#4 ·
Whats up fellas. I've heard on more than one occasion that a v twin can greatly benefit from running dual carbs. I guess its something to do with uneven vacuum or something like that. I was wondering what everyone's thoughts were on the subject.

-Chris
There's alot of information about dual carb setups in "The Linkert Book" from Victory Library.
http://victorylibrary.com/L-BK.htm
 
#10 ·
I am running this 74" motor with a pair of 34mm Mikuni's. They hang from an old Kosman intake manifold, and the bike runs great. It takes a few primer kicks to get the bike to fire cold, but it runs nice with them.

I'm surprised to hear any probs with Cycle Exchange. Kenny has always been good to me, and his kits are nice too. I got the tuning info from Doug at Doug's Custom Cycles in GA, and they were easy to set up.
 

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#12 ·
I am running this 74" motor with a pair of 34mm Mikuni's. They hang from an old Kosman intake manifold, and the bike runs great. It takes a few primer kicks to get the bike to fire cold, but it runs nice with them.

I'm surprised to hear any probs with Cycle Exchange. Kenny has always been good to me, and his kits are nice too. I got the tuning info from Doug at Doug's Custom Cycles in GA, and they were easy to set up.
i love cycle x. i think the problem was me!the kit was super nice,and they are nice to deal with.the only thing i would have liked to see with the kit was longer threads on the cable fittings at the top of the carbs,but thats neither here nor there....
 
#13 ·
Thanks for the great responses fellas. It seems that every time I see a dual carb manifold for a Harley it always has a port of some kind that joins the two runners together. It seems like this port would defeat the purpose of a dual carb setup. Is there a reason that opening is there on most dual carb manifolds?

-Chris
 
#18 ·
I have these dual-carb Panheads, set up for a matched pair of Amal 932's. They were started over 40 years ago by Pat Leahy, when he was still at B&O Cycles. Guess what? They still aren't finished. I spend money on them when I can. They too have two 1/8NPT plugs in the intake spigots. The plugs are supposed to come out, and a balance tube is supposed to connect the two spigots together. It's supposed to work like the balance tube between two exhaust pipes does, that's what I understand. They'll go on my "keeper" Pan when they're finished.

As a sidenote, I got the new Custom Cycle Engineering catalog about a week ago. There's a couple pictures of Pat when he was a "kid" on the inside cover. One is Pat sitting on Randy Smith's 45 Magnum when they were both building it, and the other one is the "crew" right before they helped Randy with his shoot for the famous picture of Randy holding up the 45 Magnum.

 
#25 ·
Thanks for the pictures and info fellas. I do have a question though. How important is it to have a balance/crossover tube between the two carbs. Did the XR bikes have any kind of balance tube fitted from the factory? I noticed that the KR earlier in this thread had a crossover tube between the two runners on the manifold. Thanks guys.

-Chris
 
#26 ·
On the bikes with an XR/Vincent style setup (each cylinder has intake on rear and exhaust on front), what do you do about cam timing? Do you reverse the pushrods on the rear cylinder (so the exhaust rod is opening the exhaust valve). Jock-E-Shift posted the video of that beautiful dual carb pan, but I couldn't see any closeups of the pushrod tubes.

I don't understand how it works on a single cam bike. Four cam is another story, where each valve has its own personal cam (XR750, for example). Can someone please school me? Seriously, I don't get it.
 
#29 ·
Here is an interesting read.

The combination of partial vacuum, intake pulses, and acoustic wave pulses within the intake tract leads to another counterproductive phenomena called reversion, or fuel "standoff." Reversion is the process where airflow in the intake tract reverses direction momentarily during the valve overlap period. Typically, reversion is accompanied by a fog of fuel or fuel standoff seen at the carburetor's entryway. Reversion negatively affects the carburetor's ability to accurately meter fuel. It also increases the tendency of some V-Twin motors to puke oil from the air-cleaner housing. Sometimes engine builders install a full or partial cover over the end of a velocity stack. The cover creates a plenum, which calms the airflow and minimizes the undesirable vacuum effect caused by stack shrouding.
The Harley V-Twin engine's uneven firing pulses exacerbate the reversion phenomena even more. The V-Twin's 45-degree cylinder angle dictates that the time between the intake pulses for each cylinder is different. For example, the rear cylinder fires 315 degrees after the front, and the front fires an additional 405 degrees after the rear, completing the 720 degrees of crankshaft rotation required for the four engine strokes. This sequence typically results in the front cylinder achieving greater cylinder fill. Moreover, much of the charge drawn into the V-Twin's intake tract passes the carburetor venturi three times. Firstly, the rear cylinder pulls air past the venturi on the intake stroke. Then the air left in the intake tract reverses direction because of the pulse generated when the intake valve closes. Finally, that same air passes the venturi a third time during the front cylinder's intake stroke. The fundamental design and harmonics of the V-Twin engine create a constantly changing fuel mixture throughout a wide rpm range. And the larger the displacement, the greater influence these pulses have on carburetion.
Source
http://www.hotrodsbikeworks.com/tech/0705_hrbp_carburetor_velocity_stacks/air_distribution.html
 
#31 ·
White Nightmare:
How important is it to have a balance/crossover tube between the two carbs. Did the XR bikes have any kind of balance tube fitted from the factory? I noticed that the KR earlier in this thread had a crossover tube between the two runners on the manifold.
Balance pipe is just there to help the iddle side of things, not strictly necessary. Mine on this 54 KHK (NOT KR sadly!!!) is actually a bit too small, carbs work fine so I've never made it any bigger.

XRs never had a balance pipe...

Patrick
 
#34 ·
White Nightmare:

Balance pipe is just there to help the iddle side of things, not strictly necessary. Mine on this 54 KHK (NOT KR sadly!!!) is actually a bit too small, carbs work fine so I've never made it any bigger.

XRs never had a balance pipe...

Patrick
Thanks for the confirmation and info Patrick.

This thread is pretty awesome so far. Definitely some nice setups in here.

-Chris
 
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