Changing a V-twin to a single carb?

[kevie]

My Virago V-Twin 750 has carb problems......... Im thinking of building a manifold out of 40mm tube to link both cylinders and run a singe carb.

Apparently this is a possibility and its been suggested I go for a Mikuni TM type carb (similar to the Harleys).

Before I attempt the modification any suggestions on design, carbs that will handle twin cylinders (constructive ones only please) and has anyone had any experience with this type of conversion?

[CookMySock]

Just do it quick, before someone tells you it can't be done.

Steve

[Motu]

Good idea,no damn tuning problems with a single carbs....apart from the initial setting up.Your carb doesn't need to be twice the size just because it's doing twice the job,the same size will work quite well....but 4 mm bigger is plenty enough.

[kevie]

yeh my present carbs are 40mm Hitachi's, and the Harleys are running a single carb setup with around 34mm Mikuni TM according to the Harley Shop here and they are set up similar to the layout im thinking of and as my mum always said ...... "If youre silly enough to try it , its silly enough to work"

I have since found a site on the net in USA that actually have the conversion available at US$600 so gotta be workable. converting the US$ makes it around NZ$1300 so wont be buying their one.

will see if I can pick up a second hand carb for around $200-$300 and the piping etc I have roughly costed at $120

[98tls]

Interesting thread,keep us posted eh,ive a bitsa race bike with an old Honda motor that was originally a twin carb but converted to single flatslide.

[kevie]

Interesting thread,keep us posted eh,ive a bitsa race bike with an old Honda motor that was originally a twin carb but converted to single flatslide.

Cool, did you make the manifold yourself and what carb did you use and any problems encountered or things you may have done different?

[CookMySock]

as my mum always said ...... "If youre silly enough to try it , its silly enough to work"

reminds me of what my dad used to say, "once you have tried the possible, then try the impossible."

How much money do you want to throw at it? You could make a basic steel manifold from exhaust tubing, slap a car throttle body on it, and microsquirt it.

Steve

[kevie]

How much money do you want to throw at it? You could make a basic steel manifold from exhaust tubing, slap a car throttle body on it, and microsquirt it.

Steve

Sounds similar to the idea a mechanic that works on my bike suggesting, have to go see him after Easter about his ideas.

Dont have too much money to throw at the bike and the alternative I have is to send the carbs to England to be rebuilt so this conversion is an alternative im looking at.

Just want something simple .... make the manifold, slap on a suitable carb and trust it will be efficient

[CookMySock]

Just want something simple .... make the manifold, slap on a suitable carb and trust it will be efficient

Well the mikunis et al are simple enough, but you will have some setting up for sure - and that probably means dyno time or at least a fuel-air ratio gauge on your bars while you tune it - still not cheap. Then there is the problem of finding a single mikuni that large. Why not put two of them on it? There's often carbs on trademe for late-ish model 600-1100cc bikes.

Steve


Steve

[SS90]

Well the mikunis et al are simple enough, but you will have some setting up for sure - and that probably means dyno time or at least a fuel-air ratio gauge on your bars while you tune it - still not cheap. Then there is the problem of finding a single mikuni that large. Why not put two of them on it? There's often carbs on trademe for late-ish model 600-1100cc bikes.

Steve


Steve

Yea, but running two carbs will bring it's dramas..... syncronising them is just one.

The big problem with the single carb feeding a vtwin is getting the inlet manifold to seal correctly.

Have a look at the evolutional changes of inlet manifold from Harley Davidson over the last 40 years, and you will see what I mean.

The main drama is that an aircooled engine expands from hot to cold, on some aircooled vtwins this can meet 20 thou....that's 20 thou more in height, and 20 thou more in width....and considering the carb is mounted in the top of a "V", something has to give!

Think of it as a tug of war between the cylinders as they expand, and the manifold trying to keep them together!

That's quite a bit, and if you think about an inlet manifold (which, by comparison to the hot cylinder heads it is feeding, stays quite cool, and, as such, does not expand to the same rate as the engine), and all this will either crack the inlet manifold, or, at the very least, make it leak air.

In this type of problem, it may well be worth trying to adapt an exsisting rubber manifold from another vtwin (Japanese one), or, have a steel manifold, with rubber flanges between the head, and a rubber flange between the carb.

All this will allow for the engine expansion.

[doc]

I did this with an XV1100 10 years ago. There was a guy in Canada making a manifold and the most popular carb was a CV off a hardley. It worked well improved the torque low down, I think it was called KJS. Will find out for you.

[CookMySock]

Yeah to sync them you need to bracket them together properly, and rubber mounting them will be a must.

The hard parts will be making the manifold(s), and getting the jetting right. Mind you, I don't think that will be hard if you are methodical and just keep going. Paniccing and stopping doing anything will kill the project before any technical problem does.

Decide what will do it, and then just start on it and do it.

Steve

[Motu]

An 1 1/4 SU from a Mini will work perfectly.A manifold with O rings for slip joints would be the way to go,but harder to make at home.

[kevie]

In this type of problem, it may well be worth trying to adapt an exsisting rubber manifold from another vtwin (Japanese one), or, have a steel manifold, with rubber flanges between the head, and a rubber flange between the carb.

All this will allow for the engine expansion.

Thanks for that..... food for thought.......I will ask the mechanic when I see him regarding using the rubber flanges that are on it and if the manifold would be better into them, would save me a bit of welding.

[kevie]

LOL my mechanic said in brief chat that I should slap a small car carb in and it will go like snot LOL but space is also a problem trying to get a carb and air cleaner tucked onto the side of the motor as the manifold will take up the space between the V cylinders by my calculation and I can afford to come out about 100mm from the engine.