v twin single carb querrys

hmmmnz · Super Spammer Joined: 22 Aug 2006 Karma :

ok say i was to go from a twin carb to single carb,
would i need to get a carb that would work or the equiv single cylinder
eg if the bike was a 650cc each cylinder is 325cc
would i just need a carb that would power the single 325cc as they are sucking in at different times,
or would i need to go for a larger carb??
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Kickstart · The Oracle Joined: 04 Feb 2002 Karma :

Hi

Useless answer coming up. Somewhere between the 2 I suspect (and think there would be some overlap on a proper V twin depending on exact firing order, but don't know with a Honda style dual crankpin). But the useful range of carb sizes is pretty large anyway.

Single carb on a V twin is likely to result in a quite awful intake manifold though.

All the best

Keith
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Robby · Dirty Old Man Joined: 16 May 2002 Karma :

Depends which way round the cylinders are for inlet manifold complexity.

From my own looking around in this area, the CB250RSA is a 250cc single that runs a 26mm carb. The CB two-fifty is a 250cc twin that runs the same (or very similar) 26mm carb feeding both cylinders.

The CB250RSA is quoted at 27hp. The CB two-fifty is quoted at around 20hp.

So I would do the same as Keith suggested and go for something in the middle. For simplicity, you could find a single carb Harley with the same torque output you're after and use a similar carb. Should help estimate the jetting as well.

I have heard of a guy fitting a carb from a mini to a 550/4 with a homemade inlet manifold. He achieved similar performance, but far better economy. No pissing around with balancing either.

Ichy · World Chat Champion Joined: 15 Jul 2005 Karma :

You should be able to use one of the standard carbs but your going to have to mess with the jetting and needle height.

All my experience is based around the '87 XV1000 I owned. OEM was a hitachi and it was next to impossible to get spares. If you changed the filter or exhaust then you would need to drill the original pilot jet and bang some washers in there, not very practical.

Originally I fitted one of the OEM carbs and it ran fine with a bit of tweaking, more torque low down but less top end. Changed it to a 42mm Mikuni with an accelerator pump and it was much better.

Might be worth you searching for P901 on viragotech.com. May not be specific to your bike but there is lots of discussions around manifold design including making them out of PVC pipe.

hmmmnz · Super Spammer Joined: 22 Aug 2006 Karma :

ok:D
the bike is a xz400 it runs extremely oddball 34mm down draft carbs
with no floats and an accelerator pump,
the 550 version also has 34mm carbs with slightly different jetting
and i don't think the size of these carbs relate to other types of downdrafts fitted to other v twins ect

i want to get rid of these insults to the gods, and chuck a single cv type downdraft in its place, if all else fails, 2 cv downdrafts
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cb1rocket · World Chat Champion Joined: 30 Jan 2010 Karma :

Going from a twin to a single carb would result in loss of power UNLESS you had a massive carb on it!

Why on earth would you want to that anyway?

Gotta be a reason for that surely?

Ichy · World Chat Champion Joined: 15 Jul 2005 Karma :

hmmmnz · Super Spammer Joined: 22 Aug 2006 Karma :

my mz500 only has a 34mm carb, and guys reckon the rotax lump runs better with a 32mm carb (obviously venturi effect)
i might see if i can get some carbs off a vfr
and see how i get on,
cheers for the advice lads,
most appreciated Thumbs Up

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temeluchus · World Chat Champion Joined: 01 Oct 2008 Karma :

the xz400/550 runs solex carbs just sp ypu know.

If you want to run a gingle carb setup, assuming there is no intake overlap (like most japanese vtwins, XZ included) you are correct, the choke size of the carb should suit the size of one cylinider.

for example on a friends mt11, we made a single carb manifold using one of the original two carbs. The bike ran fine. Someone in the US makes manifolds for old Z750/1000/1100s and suzuki GS.

Single carbing wont be a good idea on high revving high horsepower engines but on lower power engines it should work ok.

The trick is getting the inlets to each cylinder the same length.
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Ichy · World Chat Champion Joined: 15 Jul 2005 Karma :

I wouldn't mind trying it on my GS500, just for the hell of it. Got to be easier than the XV.

hmmmnz · Super Spammer Joined: 22 Aug 2006 Karma :

temeluchus · World Chat Champion Joined: 01 Oct 2008 Karma :

my bad. Copies of solex carbs. Ahem.
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hondapartsman · Renault 5 Driver Joined: 13 May 2010 Karma :

Not on v twins but I've heard of people converting something like a Norton Commando to a single carb and just using one of the original carbs. I would say it was a good point to start from then if you need to get something a bit bigger you could.
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