Single carb (again)

trident sam · Apr 27, 2020

Guys and Gals

What is the biggest single carb that will work well out of the box on an 850. I've had Mikuni vm34 in the past and it's ok but does run out of breath above @5000. I don't want replies on why I should stick to Amals thanks, just owners with experience of big single carbs- Keihin or Mikuni.
Thanks
sam

Fast Eddie · Apr 27, 2020

Didn’t you have my 36mm Sam ?

trident sam · Apr 27, 2020

Fast Eddie said:
Didn’t you have my 36mm Sam ?

No mate, I let it go to another owner a long while ago .

Fast Eddie · Apr 27, 2020

Well, even the 36mm ran out of puff at around 5k Sam. But as a Comnoz has pointed out in the past, the manifold on such kits is a huge restriction.

A bigger carb on a less torturous manifold does work according to some, but the frame gusset requires cutting away to facilitate.

baz · Apr 27, 2020

Unfortunately yep you have to cut a notch out of the frame gusset to fit a TM 40
When I had mine fitted to my 750 it would rev past 7000 rpm easily

baz · Apr 27, 2020

I couldn't tell any difference between a VM 34 and a 36 when I tried them

trident sam · Apr 27, 2020

baz said:
Unfortunately yep you have to cut a notch out of the frame gusset to fit a TM 40
When I had mine fitted to my 750 it would rev past 7000 rpm easily

OK ta Baz.
How big is the notch ? is it at the top or bottom of the gusset, can you still fit an air filter on , sorry for all the questions but a TM 40 does appeal as long as I can get it working without too many probs.

MexicoMike · Apr 27, 2020

I have a friend who has tried various single carbs on one of his 850 Commandos, including some not-very-common carb options, he has not found one that won't run out of breath around 5k RPM compared to the factory setup. As noted, it might be the single manifold setup as opposed to the carb itself...

Roger Middlebrook · Apr 27, 2020

I ran a single PHF 36 Dellorto pumper on wifie's 850's, only other engine mod was 4S cam, went well always started and never fluttered when coming to a stop at lights. Seemed to go ok but never really put it through its paces.

baz · Apr 27, 2020

trident sam said:
OK ta Baz.
How big is the notch ? is it at the top or bottom of the gusset, can you still fit an air filter on , sorry for all the questions but a TM 40 does appeal as long as I can get it working without too many probs.

My frame came to me with the gusset completely removed
The frame was also chrome plated
If you do a search you will find details of the notch because I remember seeing it on here
I had to change the main jet and needle jet to get it to rev out properly
I had one of the standard mikuni kit manifolds first of all and I thought that was the reason it had a flat spot at 6500rpm
So I made a new manifold
That made no difference, so I marked the actual throttle positions on the twist grip
And realized it was the needle jet
Then it would rev as hard as I wanted to
Rgm were going to make a TM 34 kit I believe that would bolt right on
Don't know what happened to that one
Cheers

Craig · Apr 27, 2020

Baz , do you remember which size needle jet you got the good rpm from ? Thanks

baz · Apr 27, 2020

3, 2013
comnozVIP MEMBER
It is no surprise that the 40 mm carb makes good power.

The most power I ever got on my 880 street bike [before fuel injection] was with a 44 mm Keihin carb from a "big twin" on a thoroughly reworked and ported manifold. It was not a CV carb but it had a very nice wide powerband. Jim

May 3, 2013
pete.v
May 3, 2013
jaydee75VIP MEMBER
PV, what kind of manifold do you use with that 40? Where do you get it? How much did you have to cut the gusset? Pic?
Jaydee

May 3, 2013
pete.v

I went up an 1" and across 3 1/2" under the grommet for the wile loom. My real regret is that I didn't do a neater job, but I will clean it up maybe next wu=inter fro a frame re-paint. This also require re drilling the side cover bracket and relocating the hole for the Dzus connecter.

May 3, 2013

1up3down · Apr 27, 2020

Years ago Cycle World Magazine did a series of road tests between a Commando first with a set of well adjusted Amals and then
with a single 34 Mikuni. They found that the Mikuni bike was "stronger" between 50 and up to 85mph than the twin Amal bike.
From my own experience I feel that my Mkuni equipped Commandos started easier and idled much better.
Cycle World's quarter mile testing showed the twin carb bike flowed more air above 5000rpm and so was a couple of tenths
of a second quicker.

I have tried both the 34 and 36 single carbs and could not feel, tell, any difference.

concours · Apr 27, 2020

Is there a scan of that old Cycle World article?

“stronger” is quite subjective, and, could have easily been documented with a measured mile roll on test using a stopwatch.

let’s learn more of that....

RoadScholar · Apr 27, 2020

For me the point of having a single carb is the simplicity and the stronger low and mid range power delivery; a 34mm VM serves that purpose very well and does not require the purchaser to cut anything but expenses. And, yes, the ton may not be in reach anymore.

The Commando I am building is aimed at the 40 to 80 MPH range; the engine and suspension components were selected to operate with maximum magic in that range; this includes a 34mm VM single carb setup.

trident sam said:
I don't want replies on why I should stick to Amals thanks

I won't, but I feel that you may have missed the point of selecting a single carb setup; I'd suggest the Mikuni 34mm VM kit. Or see if the Mifuni-Kronus twin cam, supercharged V-12 is still offered; when originally offered it came with a pit pass to the Pearly Gates paddock....

Matchless · Apr 27, 2020

Sam,
I've got a 2" SU off a Jag. stashed away somewhere. Might have to chop a bit of frame away to fit it though !

Martyn.

lcrken · Apr 27, 2020

1up3down said:
Years ago Cycle World Magazine did a series of road tests between a Commando first with a set of well adjusted Amals and then
with a single 34 Mikuni. They found that the Mikuni bike was "stronger" between 50 and up to 85mph than the twin Amal bike.
From my own experience I feel that my Mkuni equipped Commandos started easier and idled much better.
Cycle World's quarter mile testing showed the twin carb bike flowed more air above 5000rpm and so was a couple of tenths
of a second quicker.

I have tried both the 34 and 36 single carbs and could not feel, tell, any difference.

Can you give us a date for the CW article? If you can, I'll scan it and post it here.

I thought I'd scanned all the Cycle World articles (at least all the ones not already included in the printed collections "Cycle World On Norton 1962-71" and "Norton Commando Gold Portfolio 1968-1977") about Commandos years ago, and I don't have anything comparing twin Amals with a single Mikuni. There is one that gives flow data comparing stock 32 mm Amals, a factory prepped head with twin 36 mm Amals, and a Jerry Branch tuned head with twin 34 mm Mikunis.

Ken

baz · Apr 27, 2020

Craig said:
Baz , do you remember which size needle jet you got the good rpm from ? Thanks

Sorry can't remember
I'll strip it and check when I get time cheers

EstuaryBoy · Apr 27, 2020

Although it's obsolete now (I would think) British Bike magazine Dec '94 featured a 920 'Flat track' style Commando sporting a single 38mm Dellorto, which had an accelerator pump! Manifold by APS?

Brooklands Books 'Ultimate Portfolio' reprints an article from 'Cycle' March '75 featuring the Mikuni Vs Amal conversion.

Craig · Apr 27, 2020

I tried twin 32 VM’s , was interesting project , think manifolds were hard to find , then had to make up my own cables , find air filters that fit etc. , the VMs had come supposedly pre-jetted , not great , I tore down my 34VM and just copied it’s jetting .... yeah it worked pretty good .... I don’t recall that it was astoundingly better than the single though , I found the position of carbs splayed outward was a distraction while riding and kinda gave up on them and reverted back to the 34 , in hind site if I had, had more patience I may have been able to dial them in much better , also my riding style with the Norton was then and is now not very aggressive had already slowed down and didn’t push the bike very hard ever, just didn’t see the point as I had much faster better handling bikes in shed waiting for me , never why I got the Norton , it was just my memory of Commando from back in day when I could not have one ...

Single carb (again)

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