Twin Carbs versus Single Mikuni 36mm (2012)

Hi All,
quite a few years ago I put a single 36mm Mikuni on my 750 interstate.
Now i,m thinking of putting back new twim Amals on it.
I remember the power seemed a bit down when I fitted the single carb on it but it does start 1st kick if the battery is charged.
Anyone else gone back to twin carbs ?
Is it better or worse ?
Is acceleration better ?
Any feedback welcome.

Just seen a great sticker,
Sell my Norton !
I,d rather stick wasps up my arse.

Sorry about the rude words.
Cheers Don

The twin Amals on my '74 have 14,000 miles total, start first kick and carburet cleanly from bottom to top. Plugs are clean and soot free. Fuel economy is great. Power is as expected. I have 40 years experience tuning Miks, less with Amals, but for the life of me can't see where all the twin carb drama comes from. Inspect, make sure they're not worn, set-up as book says, sync, enjoy. YMMV :lol:

Hi Don,

Yes, I ran a single Mikuni for 15 years, until two years ago when I bought a pair of JSmotorsport's 32mm flat slides.

With both the single and now the double set ups, my bike starts first kick and idles like a rock.

The difference is in the performance over 4000rpm when i open the throttle the twin carbs feel much stronger,
and this has been dyno tested by many, notable our own Jim Commuz here who if memory serves posted that the same bike and motor showed that the twin carbs produced a solid five or so horsepower over the single.

This makes sense, as the single's manifold flowing into two chambers is a disturbed pattern as compared to the straight flows of the twin carbs

Certainly, racers ought to know best, they are all about both acceleration and top speed, and I have never heard of a competitive Norton, Triumph, or for that matter any twin pot racer using a single carb, they all choose twins.

Who the hell knows til you try it on your particular bike set up. General consensus is duals better for top end and singles better for low end response and economy. In my Peels case I found dual 32's more snappy down low to 4000-ish but 34 Miki significantly better after that and not much less snap down low either. No way to play with eager sports bikes on dual Amals but sure could with the Miki. Surprised me as I started with the Miki for run in only thinking to eliminate one variable on break in mildness. Then there's Peters 38 mm flat slide with accelerator pump he's about besides himself on the extra power off idle to who knows how fast yet. Split manifolds always come up as main issue to over come in single carbs performance. Mine had accidental vortex generators at carb flange and again at head face lip so don't know it that's what helped me enjoy 34 single over the 32 duals.

i have just fitted the 36VM after trying to hook up a TM36-68 pumper without success ( i was advised by the mikuni dealer and the dipshit mechanic that it would bolt straight up....no probs mate!!!) so i have not enough miles on "Marea" (73 850 so named after my mother....long story but they both cause me grief) to really value add yet. I did immediately notice that the same response or snappyness was not present so i will continue to tune away in the hope that i will get some punch. Run-In comes first though so the sedate response will do me for the time being.
The JS carbs were definately quite snappy and on reflection, theyy have been the "best so far" option. i purchased them a good 2 1/2 years ago and this was probably quite early in the development. I had some issues that in the end, i got rid of them as i had lost confidence emotionally and probably, when stuck on the side of the road, started blaming them for everything that was wrong ...you know what it is like when those relationships with something you used to like goes to shit and there aint no saving it. Much like my first wife but then again she was just a F^$^&% Head. I will say this though, Jim was an absolute pleasure to deal with and a very helpful gentleman at that. If i lived in the states, i would probably have kept them but each time an issue arose, it was an email event and in the end it just shat me. i also know that he was not responsible as some of the parts were incorrect from supplier and incorrectly made/stamped.
i ditched em and went back to my flogged out amals which other than throwing fuel all over my cases upon start up, were probably my fav option so far.
i also looked at a pair of smoothbore Mk 2 amals and will continue to look at a pair of these as an option, just need to sort out if i can have an idle screw tapped into em.

I continue the search for for my ideal option as i get the feeling that the VM36 is good, but not "that good"

Hi Guys,
thanks for the replies so far.
I replaced my original Amals as they were worn out & would not idle very well & I could not seem to balance them.
I did notice immediatley that the acceleration was not as good with the single carb as it was with the twins but thought I just need to get the correct jetting.
I am not to bothered about top end speed but I would like acceleration increased. My engine has all standard parts internally.
At the moment i'm polishing it up for the NOC international rally in Skipton, UK. Lets hope the rain keeps at bay for that weekend.
I had not thought about smooth bores, has anyone tried those ?.
I dont want to buy somthing & find its no good. I do live in Yorkshire & we have a reputation to keep up here about not spending money without need.
Cheers Don

I nearly bought a pair of 34's (smoothbore amals) but being so far away from a supplier, i thought i will get what i can service locally hence the Mikuni. The issue i discovered with the smoothbores was that there was no idle screw and as such, idle would need to be set via cable adjustment which is do-able, but probably not relaiable and as acurate as say an idle screw adjustment. I will however look at whether they can have an idle screw tapped into the body of the Carb and then go from there.
Being in England, you would be in a much better position than i to contact Amal directly and talk about a custom job http://www.amalcarb.co.uk/buildCarb_SBore.aspx

I am also keenly interested in the Gardner Flatslides of the 60's/70's but i beleive that parts would probably be a nightmare to get.
I also seem to remember that both options are banned from AHRMA for reasons i am not quite aware of. Maybe because they are that good that they give a clear advantage to the bike they are fitted to (haha).

The other option is perhaps a pair of Mikuni TM33 pumper carbs with ACK manifolds but i am pretty sure that the cable mount needs to be modified to get it clear of the frame for the LHS carbie.

Amal Mk2 Smoothbores are my next adventure.

DKT

i'm going to try the Lake with and w/o boost involved. Here's some reading on this to entertain and tease.
The trick is setting up for best idle, then go though tedious road tests and measures to file the needle for upper power joys. External float bowls are available to adapt to these things.

http://www.jockeyjournal.com/forum/show ... hp?t=16479
I've done a 67 Triumph with a set of Lake's on it, after I figered out how to tune them they worked great, just don't forget to turn the gas off. the secret was needle to slide height for running mixture and then the indexing of the "D" shaped needle for idle mixture. A little adjustment goes a long way with those carbs and the label "fuel injectors" was a joke, they are just bowl-less carbs.

I've always just used the cables for idle adjustment and never had any problems. If the cables are adjusted properly all the idle screw does is create slop in the cable.

Hi sorry to hi-jack the thread but reniek, are you running smoothbores?
Do you have any jetting info and slide info? Also is this setup for an 850 or 750?
Also, the lake setup looks a hell of a lot like gardners without remote bowls.
Are they worth the trouble? I read some of the blog. Sounds like drama to me
Cheers
Dkt

My carbs are 932 concentrics. I just plugged the throttle slide stop screw hole with silicon. I think a lot of wandering idle problems are caused because there is too much stuff (stock wiring harness) jammed between the frame and the gas tank tunnel. This can put pressure on your carb cables or extra bends or turns. If you make sure your throttle cable has a clear unobstructed path through there the adjustments stay in place. Adjustment on top the carb and on the cable below the junction syncs the carbs and single top cable and grip adjustment sets idle speed. Unnecessary slack between the inner and outer cables must also be taken out with these adjusters of coarse. I've run my combat 750's and my mk2a 850 like this. (I also removed the choke assembly). FWIW I had 5 Nortons from the early 70's to the mid 80's and this is how I ended up setting them up eventually. Although I've been totally out of the bike scene for 25 years I now have a few in the stable however none are currently on the road so I can't help with jet/slide sizes and needle settings etc.

I have no experience with the lake injectors or other flat slide carbs.

I've always just used the cables for idle adjustment and never had any problems. If the cables are adjusted properly all the idle screw does is create slop in the cable.

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For you, that may work, but many bikes have cables which move around as you steer and the tighten the cable.

The Lake and Gardners are kissing cousins with a few others of that era. They can be made to work a treat but it takes many road tests and markings then measuring flows then scratching the flat of the needle and measuring flows, at each throttle setting to dial in optimal from idle to WOT. External bowl can be added so not to flood when not running. Other wise these simple gravity feed flat slides can only be set to start and idle well or set for top end WOT and lose the down low behavior.

Dual carbs are a bit more effort to work on long runs if that matters. I like to be able to diddle idle for conditions so prefer thumb screws myself but do have to route cables with proper tension so steering and bottle necks dont add and subtract slide motion.

After my own experiences and now Peter's with over size 38mm single with an accel squirter I vote single can best cover full range of performance but not unless the manifold resistance and slow initial lean mixture compensated for somehow.

After my own experiences and now Peter's with over size 38mm single with an accel squirter I vote single can best cover full range of performance but not unless the manifold resistance and slow initial lean mixture compensated for somehow.

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Steve, please explain why a single carb has a "slow initial lean mixture"?

Having run single Mikunis on various bikes, I am not aware of why they need to be set up presumably slow jet"lean" as you say, how does their manufacture make them different from say twin carbs in needing this leaness?

thanks

Ugh, sorry, was only referring to a single big enough that could out power smaller dual carbs, so its bigger throat would slow intake velocity and the fuel it picks up on off idle throttle opening. If same or smaller size single fitted than than duals then one would expect better off idle and down low response. Again accelerator pumps cancel a hell of a lot of big throat initial bog. I don't know what or if rules limit carb size in racer classes but they all seem to get by better on duals. So my confused initial comment, who the hell knows until tried in a particular bike and usage. I had no concept a Miki carb Combat could scare me accelerating and turn me into a local demon to seek out, but by golly its addicting but Peel's spunk went to poop when I put on brand new 32 Amals. Then thought it was the small head so switched to 6 mm stem CHO and lost even more spunk to point Peel lost drags with normal Cdo's at big rally and no danger w/o white knuckled reflex death grip 1st.

AH Mam I still remember my involuntary forced Grimice turning to big Grin when Ms Peel 1st shot out from under me on 1st bravery to nail her down low.

Presumeably a single carb at low rpm's has better atomiseation through double the twin set ups airflow .So wouldnt need to be rich to let the fuel come out .

Typically a single will stifle the thing at ( or before max. revs.Where with twin , its ' other things ' .

A big Bad 38 mm Mk II smooth bore Amal would be ok for a single thingo , if theyre on the Lancastrian scrap heaps . Ex 2 stroke will need a 4 stroke float .

Mk IIs flow better than Mk 1s , but whatevers on it , tip top condition rateher than all scoured out is what enables accurate carburation.A 40 DCOE Weber
would do to , if itll fit .Or DHLA Dellorotto .They say they hit the frame gusset, You have to do the Mk II manifold so they clear that too .

The manifold is the real issue once size is adequate. Norton even firing twin is the best configuration for single carb use as each jug sucks equal intervals on the carb. Basic rule for power building is what ever cfm WOT top rpm requires, then every carb, single or multiple should be able to supply that. What might please us most on natural inhaled engine would be dual 34 mm carb each with an accelerator pump. Here's some shapes to image-in.

To get the full advantage of the ram effect / pulse scavangeing / induction enhanceing , one uses a choke on Ea Cylinder .



Olde L.J.K. Setright had a term for infernal combustion engines describeing the dynamic atributes of the efficency. Blowed if I can recall it.
But along the ines of ' Its the holes in there that provide the capeabilities . There shapes the important bit . The metal just CONTAINS those forces.

Hopefully . :lol: :shock: :idea:

1up3down said:

Hi Don,

Yes, I ran a single Mikuni for 15 years, until two years ago when I bought a pair of JSmotorsport's 32mm flat slides.

With both the single and now the double set ups, my bike starts first kick and idles like a rock.

The difference is in the performance over 4000rpm when i open the throttle the twin carbs feel much stronger,
and this has been dyno tested by many, notable our own Jim Commuz here who if memory serves posted that the same bike and motor showed that the twin carbs produced a solid five or so horsepower over the single.

This makes sense, as the single's manifold flowing into two chambers is a disturbed pattern as compared to the straight flows of the twin carbs

Certainly, racers ought to know best, they are all about both acceleration and top speed, and I have never heard of a competitive Norton, Triumph, or for that matter any twin pot racer using a single carb, they all choose twins.

Click to expand...

re; Certainly, racers ought to know best, they are all about both acceleration and top speed, and I have never heard of a competitive Norton, Triumph, or for that matter any twin pot racer using a single carb, they all choose twins.

Not so :!:
I guess the 2 stroke twin cylinder Scott Flying Squirrel that got 3rd place in the 1929
Isle of Man TT doesn’t count , does it :?:

Dave Dregens did this on his Manx featherbed Triumph twin in the Barcelona winning bike back in the 1960s when he beat the works Honda 750s-there was a story of it somewhere ……..

Dave Dregens did this on his Manx featherbed Triumph twin in the Barcelona winning bike back in the 1960s when he beat the works Honda 750sthere was a story of it somewhere ……..

Click to expand...

Ok, so one guy some 45 years ago was competitive with a single carb set up. I stand corrected.

And no doubt someone will point out another example

But seriously, from dyno testing to decades of race tests, it is clear that twin carbs flow more air at higher rpm and as such create more horsepower and top speed than all things being equal single carbs.

However, wishful thinking and desire to be factually contrary for the sake of being different can argue otherwise.