English French German Italian Spanish

SU carb on Dommy 99

Submitted by niall_topping on Sat, 15/12/2012
Forums
Heavy Twins

Hi to all,

A few years ago I made a manifold adaptor to fit a 1 1/4" SU HS2 Carburettor on to a Dommy 99, somebody gave me the carb which they had had as a brand new spare for a Morris Traveller, I was just playing and never tried it out. Now I'm building up another 99 and would need to find a carb, I thought I might have a go with it. I've seen this done a few times on chops etc, has anyone any information on needles etc, as a starting point? I talked to one guy I met at Worden Park with one on a Triumph 6T and he said it just worked, but he didn't even know what car it had come off, or what settings he had.

I've set them up in the past on Minis etc, but all the info is readily available for that.

Thanks

Submitted by gordon_johnston on Thu, 20/12/2012

Permalink

I have known Commandos to be fitted with an SU carb. A bit of a guddle I believe. Might be worth asking the rubber-mounted riders for any info. Gordon.

Submitted by Anonymous (not verified) on Thu, 20/12/2012

Permalink

The SU carburettor conversion for Commandos is well known. The idea was derived from the Norton 76 project and Bernard Hooper, trading as Phoenix Motorcycles, later sold the conversion kits. The SU used was a 1?" HIF type and as I understand it, the needle profile was developed specifically for the Commando.Notable features of the conversion were:-

- A very tight fit indeed under the Commando frame tube- Almost impossible to top up the dashpot oil with the carb. in place- Difficult to start as the SU does not prime easily, especially on a low tank- Popping on the overrun - mostly solvable by fitting a valver in the butterfly

+ Lean running and excellent economy. 70 m.p.g. easily and more. I got 99 m.p.g. once+ No choke required most of the time+ Good balance between the cylinders as with any single carb. setup+ Spares easily available from Burlen

The SUs that preceded the HIF were considerably less sophisticated and did not include all the emissions and temperature compensation gubbins. I am sure somebody must have tried using them on motorcycles but I have never come across anything written about the needles. I believe the needle profile is determined by looking at it as 10 consecutive measurements as it tapers from thick to thin. If anyone has a Commando SU conversion kit around and can measure the needle in that way, it might be a starting point. I might have one tucked away somewhere and if I can find it, I'll measure it up.

Submitted by bob_fensome on Thu, 20/12/2012

Permalink

Further to what has been stated, an "SU TYPE MC2" was fitted to Triumph Thunderbirds (6T), in the early fifties up to the mid fifties.The needles quoted for this carb were :___

M9 for solo use.

M7 for sidecar use.

In my copy of" Triumph Instruction Manual No.17" there are eight pages of info about this carb which I will EMail to you, if your'e prepared to leave your EMail address on the members only side of this site.

Submitted by niall_topping on Thu, 20/12/2012

Permalink

Thanks for the replies, but the carburettor on the Thunderbird wasn't an MC2, of the type fitted by Triumph to the early 6T (and Police Model Speed Twins as well, incidentally), it was an HS2 of the type fitted to BMC 'A' series engines (see pics), and I've seen one on a Dommy 99 Chop as well, and many, many on H-Ds. The MC2 was a smaller thing specifically designed for bikes. Incidentally, the clearance problem for the dashpot wouldn't be an issue with a FB frame.

http://i207.photobucket.com/albums/bb72/niall4473/SU005_zpsa23e7b16.jpg

http://i207.photobucket.com/albums/bb72/niall4473/SU004_zps0d825b78.jpg

http://i207.photobucket.com/albums/bb72/niall4473/SU002_zps27c2dc74.jpg

http://i207.photobucket.com/albums/bb72/niall4473/SU011HS4Cables_zps63acaa20.jpg

Chris' comment regarding the needles is correct, they do not have a linear taper like Amals, but are individually profiled to suit different applications, and selecting one would make my brain hurt, because the part numbers would appear to be (but probably aren't of course) completely random because of this. Back in the old days my local SU carb specialist (and former TT rider) Bert Houlding would just say,'needs to be a bit richer on the third stage' and just pick one out of about 600 different little drawers, and it would be right, but he'd been doing that for 50 years.......

Submitted by louis_carey on Mon, 24/12/2012

Permalink

I fitted a second hand phoenix conversion to my Commando :

- never had a problem starting , even from a 3 year layup - 3 priming kicks and brrrm

- needs the choke from cold - it is a proper choke valve circuit

- dashpot can be topped up by removing seat and using long spout oil can

- runs well , mine is an "850" model fitted to a 72 combat 750 - idle is great with the single carb

- lean running very low fuel consumption - handy when you have the small roadster tank

- Air filter is probably biggest problem with 750, came with a sock foam filter on spring to fit the 850 airbox I think, I modified a piper SU foam cone filter by shortening it and turning it upside down. Certainly the various options sold by Mini specialists will give you some options.

I would just fit it and try it , there were some details of the needles in the NOC technical section I think, you can tune the mixture a fair amount with the older SU , the HIF type used in the Phoenix conversion is even easier to tune - screw on side aises and lowers the main jet.

Submitted by niall_topping on Sun, 30/12/2012

Permalink

Thanks Louis,

I'll give it a go when I get to that point, as you say there is a wide range of adjustment on the early SUs, and at least I know what needle I'm starting with, in any case, I suspect that the 99 won't create enough depression to open the 1 1/4" SU up fully anyway, so the jet size and half the needle won't matter, the guy L spoke to with the 650 Trump said he just played with it until it worked, so I'll just try the same.

Surprised no-one else here seems to have tried it though, will post report.

 


Norton Owners Club Website by 2Toucans