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)?

I ran my commando on a 32mm CV for a while
It was from a bank of carbs for an fj1200
I shimmed the needle and fitted a larger main jet
Bottom end and mid were good but you couldn't maintain 90 + mph it'd drain the float bowl
And top end felt flat
This is a clip of it with the single 32cv

dobba99 said:

Does it have the manifold to fasten to the engine? If not, you are in make your own manifold territory, The Phoenix modified SU's had the dashpot height shortened to fit under the frame tube.

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I will make my own intake, no problem.

cliffa said:

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)?

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I actually forgot to ask, just assumed it is a HIF. Have to check that. What do you mean with "choke the engine"?

baz said:

I ran my commando on a 32mm CV for a while
It was from a bank of carbs for an fj1200
I shimmed the needle and fitted a larger main jet
Bottom end and mid were good but you couldn't maintain 90 + mph it'd drain the float bowl
And top end felt flat
This is a clip of it with the single 32cv

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Very interresting, I forgot to mention that I´m not looking for horsepower and top speed, and what you describe that you had good bottom end and mid range power is good enough for me! Good info!

Others will know better than me but the SU kit is not known for good starting if the tank is low on petrol
That's why some fit a lawnmower primer
The dashpot isn't that easy to top up
But most say how good and smooth the bike runs with some incredible MPG reported

fiatfan said:

I actually forgot to ask, just assumed it is a HIF. Have to check that. What do you mean with "choke the engine"?

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The HIF, has an integrated float bowl. The HS series have a separate float bowl (think of it like the difference between Amal concentric and an Amal Monoblock). The Phoenix kit uses a HIF44, and needs the dashpot damper casting machined, and also I'm pretty sure the damper tube in the piston reduced in height to match, and possibly also need a shorter damper piston.

If it's an HS series, it may be better, as the body can be set in a "downdraft" position, and the float bowl rotated to vertical. This may avoid the need to machine the dashpot etc. but it would be very much trial and error.

Have a look here, and you will see what I mean. https://sucarb.co.uk/technical

By "choke the engine" I meant too small for it and not allow it to run properly. Again, it would be trial and error.

As @baz mentioned the bike can be hard to start with the kicker, as it won't draw enough fuel up, so I and many other folks have fitted a primer bulb to the overflow, which act a bit like a tickler. ( this isn't a problem if you have an electric start).

I purchased a Phoenix kit years ago and really liked it. My 850 would pull smoothly from low revs and I had no problem starting it. I'd probably still be running it but I parked it when I got a new Triumph Sprint and later when I went back to it ethanol gas had gummed it up. I cleaned it up but it never ran right so I went back to a single Mikuni. It's sitting in a box and "one day" I'll get around to setting it up again. My Mk III is running Premiers and they work great but it won't do the ton. before Spring I'll check the main jets as I'm running original Dunstalls and suspect it's set up for black caps.

triumph2 said:

I purchased a Phoenix kit years ago and really liked it. My 850 would pull smoothly from low revs and I had no problem starting it. I'd probably still be running it but I parked it when I got a new Triumph Sprint and later when I went back to it ethanol gas had gummed it up. I cleaned it up but it never ran right so I went back to a single Mikuni. It's sitting in a box and "one day" I'll get around to setting it up again. My Mk III is running Premiers and they work great but it won't do the ton. before Spring I'll check the main jets as I'm running original Dunstalls and suspect it's set up for black caps.

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When you do get around to it, check the o-ring (or rings on later carbs) on the rotary enricher device. My bike ran fine one day, and then the next really, really rich. The o-ring had become hard, so passed neat fuel all the time. I replaced it with a Viton one, and all good again.

cliffa said:

When you do get around to it, check the o-ring (or rings on later carbs) on the rotary enricher device. My bike ran fine one day, and then the next really, really rich. The o-ring had become hard, so passed neat fuel all the time. I replaced it with a Viton one, and all good again.

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I found this out the hard way years ago when I was running a Reliant Scimitar. As standard these cars ran a Ford Essex 3L v6, but it wasn't all that powerful, & was so heavy it would have made a good boat anchor. I built a 3.5l Rover v8 & fitted a Boxer manifold using four 1 3/4" HIFs. The o rings I was supplied with by an SU agent turned out to be the wrong ones & caused me no end of agro. as removing the carbs from the manifold would have taxed a gynecologist. Later on I built a 4.6 Rover & fitted Lucas fuel injection.

dobba99 said:

I fitted the phoenix SU kit to my first Mk3 back in the early eighties and when i got back into bikes and commandos in 2014 i wanted an SU on Mitzi.
She will still do 100MPH plus, but when ridden like a granny i've acheived 90mpg, Usually its in the region of 70 mpg.
Granted the manifold isn't the best (Phoenix had to squeeze the manifold into the available space so it isn't optimum) but it is worth bearing in mind if production had carried on into 1976 and beyond the bikes would have been fitted with SU's. so maybe the frame and manifold would have been modified to a better shape. Bernard Hooper disliked the Amals

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Mitzi is a motorcycle?

Lineslinger said:

Mitzi is a motorcycle?

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My second Mk3

When you do get around to it, check the o-ring (or rings on later carbs) on the rotary enricher device. My bike ran fine one day, and then the next really, really rich. The o-ring had become hard, so passed neat fuel all the time. I replaced it with a Viton one, and all good again.

I took me 1 whole riding season to figure this out. I had just rebuilt the engine on my 73 ironhead, and while I suspected carb, I suspected more something in the top end. I cleaned carb multiple times, changed needles, changed spring. Couldn't go 20 miles without really fouling plugs. Didn't even consider that O ring, I didn't realize that that o ring was air flow not gas flow... baffled a few of us. And Im not new to SUs.

trappnman said:

When you do get around to it, check the o-ring (or rings on later carbs) on the rotary enricher device. My bike ran fine one day, and then the next really, really rich. The o-ring had become hard, so passed neat fuel all the time. I replaced it with a Viton one, and all good again.

I took me 1 whole riding season to figure this out. I had just rebuilt the engine on my 73 ironhead, and while I suspected carb, I suspected more something in the top end. I cleaned carb multiple times, changed needles, changed spring. Couldn't go 20 miles without really fouling plugs. Didn't even consider that O ring, I didn't realize that that o ring was air flow not gas flow... baffled a few of us. And Im not new to SUs.

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It took me a while to find the issue too.

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?

Click to expand...

My first race bike was a Triton 500 which my mate had built and would never let me ride. I bought it after he sold it to somebody else who blew it up. I think my mate had built on advice from Baldo Meli. Meli used to work for the Triumph factory, and in 1949 got a 12th in the Senior TT on the IOM on a 500cc Triumph. I was about 18 and knew everything. I thought he was a total Irish idiot. He said two things to me - 'when you start racing, force yourself to go slow' - and 'SU carburetors are good'. There was an SU carburetor on my 650 Triumph, I replaced with a type 6 Amal.
There is a lot of bullshit about carburetors. Big carburetors always look better, but carburetion is about vacuum and mixture control - not flow. You guys get peeved when I talk about methanol fuel - but its major benefit is it hides-up the tuning errors. With an SU carburetor, you have automatic compensation for differences in vacuum, so throttle response does not rely on restraint by the rider. With a really well-tuned motor, it is usually necessary to wind the throttle on in a controlled fashion to get the best performance. The slightest bit too rich, and the motor will always be slower. With petrol as fuel, tuning errors have twice the effect when compared with methanol, and with methanol the tuning errors are critical. An SU carb probably gives more hope of getting the tuning right.
The normal thing to do when chasing speed, is to know where the power band is in the rev range and keep the revs within those limits. If you get on a bike and whack the throttle open, you will usually be slower than if you wind the throttle on slower - an SU carb would allow you to be more heavy-handed. In a normal carburetor, the taper on the needles is significant. Slower taper needles give more speed but require more restraint from the rider.