Single carb (again)

trident sam · Apr 27, 2020

Matchless said:
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.

Thanks Martyn, you've told me that in the past, but you also said the power delivery was flat if I remember correctly ?
What I'm trying to achieve is a bike that goes really well throughout the rev range, but not having to balance two carbs.

MexicoMike · Apr 27, 2020

A friend and I routinely "test" his various single carb configurations by doing side/by side WOT runs. SO far, in every case, my 850 with twin amals pulls away from his 850 at around 5000 RPM. Below that, the bikes are pretty much even - to the point where on any given pull, either bike might be a length or so in front of the other. But the twin Amals always pull away when the RPM gets north of 5k.

cliffa · Apr 27, 2020

Sam, have you thought about Don’s (madass140) single cable gantry? it offers all the benefits of a single carb with the performance of dual Amal’s. Oh, and a lighter throttle.

Cheers,

cliffa.

lcrken · Apr 27, 2020

EstuaryBoy said:
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.

Thanks, Brett. I should have that issue. I'll drag it down from the rafters, and if I can find the article, I'lls can and post it.

Personally, I've only ever run a single Mikuni on a Commando once, on a bike that came to me with it fitted. It ran fine, but didn't seem to have the top end power it should. I also much preferred the twin carbs from an appearance standpoint, so replaced it with a pair of 36 mm Amal Mk2 carbs. Nothing wrong with a good single carb setup, and with a large enough one, you will still have reasonable top end. FWIW you would probably need at least a 45 mm carb to get the same air flow at higher rpm as with a pair of 32s.

Ken

trident sam · Apr 28, 2020

cliffa said:
Sam, have you thought about Don’s (madass140) single cable gantry? it offers all the benefits of a single carb with the performance of dual Amal’s. Oh, and a lighter throttle.

Cheers,

cliffa.

Yes, I contacted Don about a month ago and he said there would be some in the future but wasn't sure when'

EstuaryBoy · Apr 28, 2020

Unless you're going racing I still champion the SU, though of course it's no longer available 'off the shelf' And it's where Norton were headed before the plug was pulled. Was in correspondence with one guy who'd taken on board a host of BMC 'A' series tuning ideas and had knife edged the butterfly and shaved the throttle spindle, whispers of 90mpg and no loss of performance anywhere..

Matchless · Apr 28, 2020

trident sam said:
Thanks Martyn, you've told me that in the past, but you also said the power delivery was flat if I remember correctly ?
What I'm trying to achieve is a bike that goes really well throughout the rev range, but not having to balance two carbs.

Sorry Sam but that was meant as a humorous comment. There is no way a 2" SU would fit. The 1.75" you are thinking of does go flat over 90mph though.

concours · Apr 28, 2020

MexicoMike said:
A friend and I routinely "test" his various single carb configurations by doing side/by side WOT runs. SO far, in every case, my 850 with twin amals pulls away from his 850 at around 5000 RPM. Below that, the bikes are pretty much even - to the point where on any given pull, either bike might be a length or so in front of the other. But the twin Amals always pull away when the RPM gets north of 5k.

Like Dad taught me at a young age:
“If it’s got a motor & there’s more than one of them you can RACE it!”

Matchless · Apr 28, 2020

EstuaryBoy said:
Unless you're going racing I still champion the SU, though of course it's no longer available 'off the shelf' And it's where Norton were headed before the plug was pulled. Was in correspondence with one guy who'd taken on board a host of BMC 'A' series tuning ideas and had knife edged the butterfly and shaved the throttle spindle, whispers of 90mpg and no loss of performance anywhere..

I have one that I fitted to my 850 a long time ago, & whilst it ran really well up to 90 the biggest problem I could see was the inlet manifold which was a poor compromise to enable the rather large carb to fit in the small space available. I have no doubt that given more space & a better manifold the SU would work really well at higher speeds.

EstuaryBoy · Apr 28, 2020

Matchless said:
I have one that I fitted to my 850 a long time ago, & whilst it ran really well up to 90 the biggest problem I could see was the inlet manifold which was a poor compromise to enable the rather large carb to fit in the small space available. I have no dought that given more space & a better manifold the SU would work really well at higher speeds.

Burlen 'borrowed' mine when floating the idea of releasing a new kit, but sadly nothing came of it.
If you read the only contemporary road test of the Norton '76, the rider was more than impressed with the SU over Amals, but top speed comparisons were scuppered by the incorrectly set up braking....

trident sam · Apr 28, 2020

Matchless said:
Sorry Sam but that was meant as a humorous comment. There is no way a 2" SU would fit. The 1.75" you are thinking of does go flat over 90mph though.

Oops Martyn, I would never have known, got no experience of SU carbs on any bike

Fast Eddie · Apr 28, 2020

I’ve said this before, but worth repeating here me thinks...

I rode a 650 Degens built Dresda that had a single SU on it, albeit on a fabricated ‘Y’ shaped manifold, much longer and straighter than the Commando ‘kit’ manifolds. That bike went like stink.

I ran it in for him at a Mallory Park test day. I asked him what to rev it to and he just said ‘whatever it feels comfortable at’ (kinda daft thing to say to a young ish racer type).

After a lot of laps I came in and he says: ‘that sounds sweet, what were you reving it to?’

“9,000” says I innocently.

“NINE ?!?” He yells before giving me a right bollocking.

“Well... that’s what it felt comfortable at” says I in my defence.

But that was when I woz younger and stupider of course...

Anyway, mildly amusing story not withstanding, my point is, on a decent manifold an SU carb WORKS !

acotrel · Apr 28, 2020

When you fit twin carbs to a Commando engine, you usually use supposedly identical needle jets and needles and set the needles at the same position. You rely on the manufacturers' tolerances. With petrol, the difference between good and bad is bugger-all. If you have only one carb, you are more likely to get the jetting spot-on.

lcrken · Apr 28, 2020

Found the March '75 Cycle article in the extensive Left Coast Racing archives

shown here

This is the full text of the article:

Ken

alan hodge · Apr 28, 2020

ken an excellent article

Craig · Apr 28, 2020

Have not read yet but going to now , thanks Ken for digging it out and posting it up !

Craig · Apr 28, 2020

I remember that now ! , one of opinions I had looked at when deciding to keep the Mikuni , there used to be ,a myth maybe ,that one certain brand manifold out performed all others , I can’t recall the brand name ..... really too bad no one ever actually built the “best manifold possible” as mentioned in article that would possibly outperform the twin Amals in all situations , instead of just out performing Amals in real world rides ..... After all these years I sure do appreciate not having to fiddle with my carb , ever .... thanks again Ken , took me back a ways

worntorn · Apr 28, 2020

"Your ( Mikuni equipped) Norton will idle, theirs still won't."

Just not so!

45 year old original Amals 26,000 miles. Not sleeved but has had needle jets replaced.

Glen

Craig · Apr 28, 2020

there are always exceptions , eh .... you sure they original, just askin' .... also, most interesting take away for me was the real world performance testing , I always thought my Mik equipped bike was performing as well or better than the twin Amal equipped bikes was with at times , but it was always same old story the original equipped twin Amal bikes are faster and quicker, from most even on this forum , the minority with singles sang an unheard song , nice to see some real world testing, vindication after personally feeling and seeing the improvement over the years ... I rarely hit 6500 rpm so as I have said for years , right set up for my type riding ... looks like from this piece I was correct .... and yup it nice to have that rock steady idle , that is mentioned too

jbruney · Apr 28, 2020

My old Amals are still ticking much the same.... I'll never complain of them again after wrestling 4 Keihin CV heartbreakers off the wing.
Amals can be done roadside in a pinch.

Single carb (again)

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