single carb conversion

I'm sure this has been covered many times before but I can't seem to successfully find previous posts. A friend want to convert his '72 stock roadster to single carb to make a less fussy daily rider. I see manifolds are sold by several people but I'm wondering about the carb. I'm a die-hard mikuni fan so my first thought was a good ol' VM36. Simple, reliable and a good array of brassware available.

I'm wondering if any gurus out there have advice? Especially with a starting setup of needle, jest and cutaway. Or, should I be looking for another carb. Essentially, my friend would like fuss-free setup without losing too many horses. Any ideas?

I had a VM36 on my 850 when I bought the bike. It ran and started fine, so they can work well, unfortunately I have no idea what settings were in it and have since sold it... to fit twin carbs.

A single carb does cost you power, unless you go for something pretty huge and exotic (I think Pete on here has a 42mm pumper Mikni TM on his, or similar).

I seem to be in a minority on this subject, but personally, be they on a Norton, Triumph, BSA, or Vincent, I have never found twin (or triple) carbs to be the maintenance headache that others seek to avoid!

I would agree with this totally. What's so difficult about balancing carbs? On a Norton twin, twin carbs work best especially above 90mph.

I don't understand! If you type 'single carb conversion' into the search box on this page there is enough reading material to keep you busy for hours.

In this case, it is. The carbs refuse to run together in they way they were intended. Every bike is different, wear, age, tolerances etc. That is one of the charms of Brit Bikes. Every rider is different - some quiver in anticipation of tinkering - others wanna kick and go. In this case, a single carb is gonna be the best. While twin carbs (to me) sound best, Burt Munroe had a single carb setup and managed to go pretty quick on a side-valve Indian. It's different for every rider.

When you think about the mechanics, with a twin carb setup, each carb is shooting gas and air into the cylinder at different times with a rest in between because the firing sequences are 2 strokes apart. So, puff, rest, puff A single carb on a Brit bike just keeps pouring air and gas through almost on a continual basis. I'm no guru, just an enthusiastic amateur, but to me, unless you're on a racetrack, a single carb seems more sensible.

And now, I've probably unleashed a huge monster. Look, to each their own. Please help me out - I need advice on setting up a single carb Commando, not an esoteric heated discussion on the benefits of one vs two carbs?

Go for the Mikuni conversion. I have had it on my MK111 for many years. I runs beautifully with no tickling and fuel leaking. no balancing and the perfect idle with a tri-spark ignition. I have the choke on the bars so there is no fiddling under the tank anymore. As far as power loss goes, my Norton is my touring bike with a 22T front sprocket. If, and it's a big if, it looses power over 5000rpm then I don't care. 5000rpm is 150kph which is about as fast as I will ever want to go on the Norton and it has loads of torque.

I have a VM34 with 240 mains. I've tried 260 and 250, but it bogs down a little and it seem happier with the 240.

johno

petejohno said:

Go for the Mikuni conversion. I have had it on my MK111 for many years. I runs beautifully with no tickling and fuel leaking. no balancing and the perfect idle with a tri-spark ignition. I have the choke on the bars so there is no fiddling under the tank anymore. As far as power loss goes, my Norton is my touring bike with a 22T front sprocket. If, and it's a big if, it looses power over 5000rpm then I don't care. 5000rpm is 150kph which is about as fast as I will ever want to go on the Norton and it has loads of torque.

I have a VM34 with 240 mains. I've tried 260 and 250, but it bogs down a little and it seem happier with the 240.

johno

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Johno - from an Aussie in Hawaii to one in Wagga - that sounds like good advice! What about idle jet and needle?

A lot of us are oldish - maybe over 50. We don't have the reaction times or sense of invincibility to go fast anymore. But, one luxury of older age is a financial ability to have several bikes. Its always nice to have a "go-to" one that we know will always start. If I wanted to go real quick - I'd get me a new jap bike. But I prefer the older ones with all their quirks and foibles. Thats just me I guess...

Cas,
I am not sure on the idle jet or slide+needle without taking the carb off and having a look. If you order the mikuni conversion kit for Norton - old britts, british spares NZ,CNW etc. it should come with the correct slides and needle with a selection of mains to try. That's how mine came from british spares and I have only had to try different mains to get the right combination. You won't look back mate. Best 2 things for a commando is this conversion and a good ignition system IMO- just set and forget. Equals one very reliable and first kick start Norton.
If I built a performance Norton I would go with the Kehin FCR or CR carbs. I have 4 CR's on my Rickman CR Honda with a very warm 836cc race built engine. Above 5500rpm it is a rocketship. These modern carbs flow so much better at higher revs than amals ever will.

johno

for your friend's 750 I would get a 34 Mikuni package, comes all properly jetted and with manifold and fasteners, some come also with throttle cable and air cleaner

I too run a Mikuni VM34 on my '72 Combat with great results .... lots of power and torque for me at least .... keeps up with and passes traffic around here .... I am running 2.5 slide and 220 main ... air jet is 25 or 35 I think ... needle jet and needle I am unsure of , without looking .... pretty sure I found and used suggested sizes on this forum ... sorry .... I don't think you will be disappointed with the single Mikuni once set up , though ... Boyer mark 4 ignition , 21 tooth counter sprocket , Uni filter at present but will be changing to K&N for better flow shortly ...
Craig

I kind of fall into the "why" camp.

The Amals are stone axe simple. Balancing should not be scary to someone that wants a vintage motorcycle of any kind in their life.

IF you really are struggling with the stock carbs, maybe there is a real issue that you should sort. Maybe the slides are bad an you need to replace the carbs?
From what I have seen you can buy two new Amals for the price of the single Mikuni conversion, so what is the point?

But I also don't see why points should be replaced on a stock bike with electronic ignition either.
Maybe I am odd.

Jag. I don`t think you odd .... maybe a little bit judgmental .... I say to each his own .... if some one wants a single carb .... then I am willing to offer what I know .... which most times is not much really ... as far as points .... my bike came with boyer and when lead wire to pickup cracked I upgraded to the Mark 4 version .... can`t tell from looking and so far so good .... no different then big bore kit , brake upgrades and such .... who can leave anything alone with an engine in it .... the popular craze right now is cafe bikes .... not my cuppa .... sure do like looking at them though and marveling at the skill it took to create .... why try for controversy when some free advice offered makes us all feel like good guys ... :wink:
Craig

Craig said:

Jag. I don`t think you odd .... maybe a little bit judgmental ...

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I have been called worse....lol

Me, on the other hand, is sort of wondering "why" on both types of carburation; I mean why 2 carbs if 1 is enough, and why 1 if 2 gives (possibly) more power? I´m totally with Craig on the "each to his own", no doubt. There´s so many ways to do this, and so many different setups that work, so there can not be a wrong or right way, IMHO. And then you have the people who absolutely accept nothing else than the original setup, and the ones who would never wanted to be seen driving an original bike..... everybody is doing the right thing, their thing!
I´m thinking of buying a TM or possibly a HSR, I´m pretty sure it´ll be a one-carb setup. The snowmobile versions of Mic VM that was on my bike are not going back on. Is there any experience of a big (-ish) HSR, 40 or 42mm? From what you can read about them, they seem to be almost a "giant leap" in carburation development, would be interesting to compare a big one with a standard dual 32mm Amals setup on the same engine. Speaking of size :wink: , could you go "too big"? I mean where is the upper limit for a single carb? By the way, I have standard (at least as far as I know ) 850cc, a RH4 head, I think the engine is a 1973.
Tommy

My first conversion was on a Mk3 850 and was simple:
The original LH 32mm Amal carb fitted to a Stewart Engineering manifold.
Worked fine, didn't cost much, maybe lost a wee bit of power high-up, did not worry me.
Later I used a 32mm Mk2 Amal with a new chromed brass slide.
So sweet, perfect idle all the time, shutting the throttle without having to worry about stalling gives you peace.
Have no experience with Mikuni.

Anyone ever do some dyno runs with different set ups?

fiatfan said:

Is there any experience of a big (-ish) HSR, 40 or 42mm?
Tommy

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Ok, I'll chime in. I run a TM40 (aka HS40). I got a nice cam from JS and did some mild head work. For some reason the cam needed 10 degree advance to function, and I could not find a carb or ignition system to work well including old Amals and new premiers.

Cruising the ebay I noticed many Easy kits up for auction. The Mikuni Easy kit was a salution for Harleys and the TM/HS 40 was offered for the 883. It seems to me that these proved to be a bit small for the 883 and they started offering the 42 and 45 and 48 in the Easy Kit form. So on a hunch I found a supplier and jump into the pool. To say the least, after much work and testing, I have not had any need what so ever to look back. For me it has been a great success and have been loving it since the start of the 2012 riding season.

That being said, I do not recommend this to anyone looking for a plug and play solution, not to mention slight modifications required due to space constraints. Again, mine is all about the cam, head, Mag, and big bore exhaust all on a 750.

I suggest finding a TM34 or get the proven kit from Rocky Point. If you do not get his kit, you can get everything else from him particularly the 36 to 40mm manifold. This is what I use with my TM40 and will fit a 36mm nicely. No matter what you get, be prepared to do some jet work. The main jets are pretty easy but the combinations of needles and needle jet are many. Many will offer starting point but it seems no two are exactly alike when tuning is completed.

Here's some history. Enjoy! 40mm-flatside-pumper-t13023.html#p153742

I bought a flat slide Kehin single from JS Motorsport. Jim Schmidt has some nice products but most are a bit spendie. The kit came with the manifold, carburetor, air filter, throttle cable and twist grip and it was less that a Mikuni set up. I was moving to eastern Arizona and he included extra jets with the deal. One of the things that I like about the set up is that the cable has a guide tube so your cable doesn't have to do an immediate bend right out of the carburetor.
Spoke to Jim about a year ago and he is only doing the dual carb set up now which is sad as they were a nice set up.

John Ebert
Texas

I bought a single Mikuni conversion kit from Rocky Point Cycle several years ago. They asked me the elevation where I do most of my riding and set the jetting accordingly, before shipping it to me.

I bolted it on (7 or 8 years , and around 12K to 14K miles ago).

Have never given it another thought since. That's the way it should be, as far as I am concerned.

Yesterday I did a very brisk (speeds generally in the 70mph to 90mph range) ride of about 300 miles. The bike performed flawlessly. For those who like to point out a drop in top-end accelleration/power with a single carb, please meet me at the junction of Highway 78 and the Imperial Highway (in the Anza Borrega desert in Southern California) next Saturday morning at 10 AM, and I'll be glad to give you the opportunity to prove your point.

Ok, at least a few people have responded, I have a feeling there must be more of you out there !
Pete: Do you have an opinion about a 40mm TM on a standard 850? I´m not really that interested in hi revs power, more the low revs grunt of the 850. But I won´t say no to it either.... :twisted:
Keith: What type and size is your Mic? Your experience with this setup sounds really interesting to me, considering you have a 850....
Tommy