SU carb - too small?

Matchless said:

That depends on where you live

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True

Matchless said:

It might well do 100 or a bit more, but wouldn't get there anywhere near as quick as twin Amals or Keihins, at least not on my bike.
I do think that with a little more space for a decent manifold it would perform as well as twin carbs. I seem to remember someone on this site running a 40 or 42mm Mikuni, with the frame gusset plate chopped to make room.

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That is Baz with a TM Mikuni

I’ve told this before, but here goes…

Some years ago Dave Degens (RIP) built a special Dresda with the idea of creating a classic cafe racer that would do over 100mph AND over 100 mpg.

It was a 650 Triumph twin in a Dresda chassis. It had a lightweight crank, special cams, super lightweight valves and springs, a squish head set up and… a single SU carb !

I thought the SU was a stupid idea and took the piss. It was mounted on a fabricated long Y shaped manifold.

Dave asked me to put some laps on it at a Mallory practice session. I asked him what to rev it to and he said “whatever it feels comfortable at…

Well, that bike revved like a Banshee, sounded like something properly exotic, and freakin’ flew !

I got back to the pits and Dave said “that sounds good, what were you revving it to”

“9,000” says I.

Dave did a bit of a Basil Faulty at that point, but my defence was “that’s what it felt comfortable at” !!

Anyways… point of the story being… I stopped taking the piss out of SUs that day, when set up right, they freakin work !

trident sam said:

That is Baz with a TM Mikuni

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Yep I run a 40 mm TM
The frame gusset has been completely removed on mine but you can fit a TM 40 with a couple of notches out of the gusset

fiatfan said:

I have been offered a SU carb that is restored and set up for a Triumph 650, it´s a 32mm which would make it a 1 1/4" if my math is correct? It has never been used. I have a standard 850 -73 engine. The most common SU used on Nortons seems to be the HIF6 which is 1 3/4" or 44.5mm. That´s a big difference. Is there knowledge out there about this, could it work?

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As others have already said it will probably choke the engine. I run an SU, and as you said it’s a 44mm. Do you know if it’s an HIF series, or the earlier HS ( separate float chamber)?