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 <a href="http://www.microsquirt.info/">microsquirt</a> 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 <a href="http://www.microsquirt.info/">microsquirt</a> 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 efficientWell 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. :done:

[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.

[kevie]

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. :done:

Yep the site I found that are producing them is KJS probably same company, http://www.kjsmotorcycleworks.com/kjs_manifold.htm they seem to just bolt strait on the engine without rubber buffering of any kind and the carbs the only part with a rubber flange. they using the Mikuni TM going by the picture on their site same as the Harleys are running.

[kevie]

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

The TM Mikuni according to Anza in Palmy will run a Vtwin Ok, as they are what the Harleys are running as a single into twin cylinders. and the pics on http://www.kjsmotorcycleworks.com/kjs_manifold.htm show a single Mikuni carb with the conversion, in their pic it looks relatively simple setup, on one of the other pages it shows all the parts that become obsolete after converting the fuel intake lol

[doc]

Google the "Virago mailing list" and search for the KJS manifold

[SS90]

I am really interested to see that there seems to be no problem with having a solid mounted manifold like that....Harley pissed around with manifold mounting systems for years to get the expanding type design they ended up with.

I guess the Virago may well run substantually cooler than "american muscle", and, as such not expand as much.

I wouldn't have thought so, but there you are!

[kevie]

Google the "Virago mailing list" and search for the KJS manifold

Came across the KJS advert on their website after I started the thread .... they dont have a flexable manifold and a single 40mm Mikuni Carb ... the pic you posted looks good eh .....

[kevie]

I am really interested to see that there seems to be no problem with having a solid mounted manifold like that....Harley pissed around with manifold mounting systems for years to get the expanding type design they ended up with.

I guess the Virago may well run substantually cooler than "american muscle", and, as such not expand as much.

I wouldn't have thought so, but there you are!

Yeh I would have thought there would be problems with joining a V-twin with a 'non-flexable' manifold which is why I started the post, but as mentioned I found the advert by KJS and going by that they seem to suggest a fixed manifold is not a problem.
My original thought was to run the 40mm manilold into the existing rubber manifold flanges to give it flexability ..... (still think that is the better idea), altho will the weight of the manifold and a Mikuni Carb be ok in flexi joints weightwise ..... guess only one way to find out LMAO just looking for a Mikuni (36mm or 40mm) for around $200 or so but also found the KEIHIN FCR which I understand is OK too and found a site that advertised KUSO carbs (never heard of them) that look similar altho they seem to have 32mm as largest and dont think that would be big enough.

[Motu]

At the last Claudlands swap meet there was a brand new Blue Magnum 38mm,still in the box @ $50.That would be perfect,best bolt on and run carb I've ever used....and easy to tune too.

[Subike]

what about a twin choke side draught?
A delorto or a webber, heaps of jets availabile for tuneing
heaps of different styles of air filters, ram tubes etc
are already used on HD's with success
easy as to make a manifold for and not a complex carb either.
just a thought

[doc]

I am really interested to see that there seems to be no problem with having a solid mounted manifold like that....Harley pissed around with manifold mounting systems for years to get the expanding type design they ended up with.


Aint no expanding type manifold on my sporty and its never been any problem.

[kevie]

At the last Claudlands swap meet there was a brand new Blue Magnum 38mm,still in the box @ $50.That would be perfect,best bolt on and run carb I've ever used....and easy to tune too.
came across a website last night with all the different swopmeets .... saw the Hammie one and they had one not far out of Wanganui too ....... bugger eh seeing them too late to attend lol

[kevie]

what about a twin choke side draught?
A delorto or a webber, heaps of jets availabile for tuneing
heaps of different styles of air filters, ram tubes etc
are already used on HD's with success
easy as to make a manifold for and not a complex carb either.
just a thought

yeh the mechanic I use said to think about that too.... he recons my bike will go like snot LMAO havent told him im a nana rider LMAO

[porky]

Got an electronic copy of the 98 sportster workshop manual. Has some good pics and exploded drawings of carb (keihan cv) and manifold. More than happy to flick you a copy if it helps??

[rogson]

I presume you are looking to install the carb between the cylinders? If so, I have a down-draft 36mm Mikuni CV carb (BDS36SS) from a Suzuki VX800/VS800. It is designed to mount between the cylinders where space is limited - so is quite compact.
PM me if you are interested in it.

[kevie]

Got an electronic copy of the 98 sportster workshop manual. Has some good pics and exploded drawings of carb (keihan cv) and manifold. More than happy to flick you a copy if it helps??

Thanks for the offer, my mates a motorcycle mechanic and I went over and had a look at a couple of Harleys in his workshop with the setup im thinking of and chatted to him, so have a good idea now what to do, the manifild will have to fit into the rubber flanges I have to allow for movement and I recon also make it easier lining them all up as they will give me a mil or 2 give and take on the measurements...... so ..... first off im just chasing a person who is selling a Mikuni TM 38mm carb at the moment :) mechanic said minimum of 36mm and max of 40mm carb will do fine and he will sort the jetting out for me , he also recomended I avoid the CV carb as what I plan will be easier with the TM :) sort of gobblygook to me LMAO dont know too much about the differences as I have only compared them off pics on the net.

[kevie]

:shit:LMAO sheesh ..... talk to experts and they all have differing opinions LOL told NOT TO use the CV carb but use the flatside by a friend that builds custom Harleys and repairs bikes, and talked to a couple Harley shops and they said not to bother with the flatside carb as the CV is a far better carb , but both Harley dealers had the same reasoning for that (they did agree on something ) that the CV is self compensating and adjusting and handles the fuel needs better than the flatsides and both of them cant understand why so many Harley owners rip out the CV and go for the flatside....... *****wonders whose right**** .... but have just bought a second hand CV carb off a 1200 sportster and will see if it all works out,,,,, then have the manifold to build and the task of rejetting the carb to suit the bike. will let you know later if I got things right of stuffed it all up:shutup:

[doc]

Dont know anything about flatside carbs. But from what I understand with Harleys if your going to modify anything stick with the factory designed addons or theme. The bad press about HD reliability really comes from experts adding their bit, but not looking at the whole picture. I think CV's are simple therefore fine IMOA.
Good luck

[kevie]

yeh my theory is ... if the flatsides are a far better carb ....... why is Harley Davidson putting CVs on instead of the flatside as standard........ but guess over the next couple weeks I will find out if im as clever (or crazy) as my wife thinks I am

[TripleZee Dyno]

yeh my theory is ... if the flatsides are a far better carb ....... why is Harley Davidson putting CVs on instead of the flatside as standard........ but guess over the next couple weeks I will find out if im as clever (or crazy) as my wife thinks I am

my 2 bobs worth
Stick to a CV.
Flatsides are all well and good but setup in a non-standard install like this will be a pain.
Flatslides are performance orientated carbs and are only worth using on a highly modified motor.
btw TM isnt a Harley CV.
Get a later CV with a pumper, if you dont need it you can always disconnect it.
As some one has already said CV's are 'self compensating" and its hard to overcarb an engine with a CV. Real easy to overdo it with slide carbs and 38 or 40mm flatslide would probably be too big for this engine in stock form.
Same with putting a car carb on, probably run like crap because the carb will be too big.
You could always try one of the stock carbs off the bike.
good luck

[kevie]

my 2 bobs worth
Stick to a CV.
Flatsides are all well and good but setup in a non-standard install like this will be a pain.
Flatslides are performance orientated carbs and are only worth using on a highly modified motor.
btw TM isnt a Harley CV.
Get a later CV with a pumper, if you dont need it you can always disconnect it.
As some one has already said CV's are 'self compensating" and its hard to overcarb an engine with a CV. Real easy to overdo it with slide carbs and 38 or 40mm flatslide would probably be too big for this engine in stock form.
Same with putting a car carb on, probably run like crap because the carb will be too big.
You could always try one of the stock carbs off the bike.
good luck

Thanks for the comments ... its what I was looking for as Im going into this a bit blind and was looking for feedback on how to do it, what traps not to get into (one of them being the wrong carb) and wether a manifold strait onto the block is OK .

I have decided on a CV carb, should be here tomorrow (off a 1200 Sportster) and know that I may have to change the jetting on it to suit my 750 (which is 10,000 km on a total motor rebuild where the cams were rebuilt and slightly race machined, heads plained and rebored and oversize rings/pistons)
From feedback I have come to the conclusion its better to keep my rubber manifolds the present carbs are into and run the intake manifold into them to allow for any movements etc. Also I have decided rather than MiG the piping together Im going to tack it in place with MiG and then take the manifold out and gas weld it all up with the theory theres less chance of an air leak through pinholes in the welding.
I cant really use the present carbs as the diaphrams are stuffed in them and I cant get them altho I can send the carbs to England to get them rebuilt which will cost around $500 all up and the conversion to single carb is going to be an estimated $200 not counting the mechanic doing the tuneup and working out any rejetting if needed.
The comments from people in KB have been good and ANZA's workshop in Palmy have been a good help.They allowed me to look at the setup on several models and given their advice.
with Harley strait pipes on my Yamaha and now looking like a Harley carb and air cleaner being added.......does that make my bike a YAMAHArley LMAO

[SS90]

Aint no expanding type manifold on my sporty and its never been any problem.

There are many different evolutions of the HD inlet manifold joints.

Some aftermarket ones where solid mount (and I realise that some people have no problems), but most have dramas when they hang a big carb off it.

The best one (evo/sporty days) was a hex section oring that crushed up when you tightened the manifold down, allowing for a "rubberised" joint, (allowing for cylinder expansion) that still sealed, although it was advisable to replace the orings when ever you took the manifold off.

I think the water cooled Virago may well just not expand too much (like a an HD)

[doc]

Here some pics of HD system of mounting. Pretty simple and it works.

[kevie]

Here some pics of HD system of mounting. Pretty simple and it works.

Thanks for the pics .... I have the manifold made, I sort of followed the theory of the ones available in the states, just been pressure testing it and fixing a couple pinholes then when all sorted will paint it and fit it together .. will know then if I got it right LOL have to get a throttle cable made up for it first before I can try firing it up .... thinking maybe I can adapt the choke cable as I wont need that and its longer than I need so can afford to cut it down and alter it.

[kevie]

UPDATE

Manifold and carb fitted to bike, fired up quite happily (and as expected running too rich) but running :) Have a temporary air cleaner on it at the mo till I score a proper Harley one (dont want to pay 3-7 hundy for a new one) so once the proper air cleaner is on will drop it off to mechanic fella in Palmy and he can work out the re-jetting for me and tune it up :) Its popping a bit occasionally through the carb but possibly cos its running so rich (I hope) so will let Pete sort that out too.

Went up the road and back on it yesterday, was at 20km/h and about 3/4 throttled her and the ass-end tried to pass me LOL even cranking it full open I never got that response with the old carbs on so I think it feels to have about 60% more get up and go. Will see what its like once re-tuned.

Watching an auction for an aircleaner on trademe so hopefully might get that.

Only problem I forsee at the mo is the brake pedal is a bit harder to 'hit' with the amount the aircleaner protrudes so may need to bring the brake leaver out about 3-4cm to line up better as its fairly close to the engine at present.

Will update with a pic or 2 when the final adjustments, rejetting and tuneup are done.

[randel candle]

[QUOTE=kevie;1129703466]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? :eek5: [on bathroom wall: 3 things i hate: 1. graffiti 2. lists 3. irony]

Well i've had the same problem with the '92 xv750 virago I recently acquired. I've worked on motorbikes since I was like 15 and im 57 now. I've restored literally dozens of barn and pasture finds of every brand and type. most were a piece of cake to revive. new battery, carb kits, tube and tires then off to ride the awesome Texas highways. This virago has eaten my lunch with carb problems. ultrasonically cleaned, rebuilt, tweeked etc but no joy. float was especially hard to adjust. so I ordered a 2 into 1 manifold from outfit on ebay for $89.00. it's some kind of either butyl rubber or some compound thereof. fits well. this week I will order an Oko brand 38mm carburetor. it's a D-slide as opposed to round or flat. the only reason I'm going with this one is that they will throw in extra jets in different sizes so that I can tune it. I'm really anxious to get this bike on the road. I had an '82 and it was an absolute jewel. it went over 100k and still had good compression when I sold it. I only sold it because i bought a brand new Shadow. well my oldest son took the Shadow cuz he said he couldn't wait for me to die. lol...he's a good son so it's ok. so i'll get back to you on how well it works and what i had to do to make it work right.