SU carb conversion

I have the chance of obtaining a virtually unused complete SU conversion kit, so
what are the pros and cons.
It will be taking the place of a single 34mm Mikuni which I've been happy with.
I'm particularly interested in replies from owners who have (or had ) these devices fitted.
performance --- acceleration, not bothered about top end speed as long as it'll do a ton
economy
ease of use and fitting
any special requirements
Bike is mk 111. 850 stock motor

thanks
sam

trident sam said:

I have the chance of obtaining a virtually unused complete SU conversion kit, so
what are the pros and cons.
It will be taking the place of a single 34mm Mikuni which I've been happy with.
I'm particularly interested in replies from owners who have (or had ) these devices fitted.
performance --- acceleration, not bothered about top end speed as long as it'll do a ton
economy
ease of use and fitting
any special requirements
Bike is mk 111. 850 stock motor

thanks
sam

Click to expand...

Would you be kind enough to get some good, clear close up images of all the manifold, hardware, bracketry, as well as the carb and post them? :mrgreen:

There was a post about a week or so ago on this. Shouldn't be too far down the list.

Hi Sam,
I had the SU conversion on my 850 for a few years & I liked it. The economy was a lot better. Figures of up to 80 mpg were suggested by some people. Maybe possible if you ride like an old lady but I could get up to 60 mpg. The dashpot needs topping up from time to time which is not difficult but a bit of a faff. Idle was good, it had a very light throttle action & acceleration was also good. The reason I dumped it in favour of the original 32mm Concentrics was lack of go at 85 plus. I'm glad I tried it to satisfy my curiosity. Would I put it back on? Only if I started despatch riding on a Commando which is very unlikely. If you ride at legal speeds all the time you will like it, but as I doubt that, keep the Amals. They really are rather good.
Martyn.

A mate of mine has one on his commando , It goes well all through the range . If I could get the whole kit for a fair price I would not hesitate .

Hello Sam
I,m the chap you sent the old Sprag,sprocket and gearwheel to, thank you, the mod i did to them (al, Ken Merrick mod, see noc website) works a treat. i.e. no risk of the sprag flipping.
Back to the SU, don't hesitate, buy it. When i bought my MK3 back in January from a chap in Birkenhead it came fitted with a mikuni. Performance wise it went really well, easily in the 55 miles per imperial gallon range and it would do the ton as well, much to the dissatisfaction of my son who followed me home when i bought it! As i mentioned in my previous posts I had an SU fitted to the MK3 I had 30 years ago and the memory is still fresh of the revelation that the SU was over the original Amals. I had the opinion that the amals were the closest to uncontrolled fuel injection that you could get! Needless to say I obtained an SU and fitted it as soon as I could. Performance wise the SU is a match for the Mikuni but with the added bonus of even better economy and it will still do the ton.
Fitting wise, you might have to make up some ferrules to fit the cables properly into the manifold (throttle cable) and carb body (choke)
One thing i wasn't happy about is the bending that the throttle cable has to cope with exiting the manifold casting before it connects to the throttle lever. My solution is photo,ed on my previous posts. The air filter fitted to the mikuni will fit the SU,s ram pipe. Use 20/50 in the dashpot, thinner oil will work just as well but will disappear quicker.
It is generally acknowledged that a single carb is a couple or so of horsepower down compared to a twin carb setup at the TOP END.
My question now is when did you ever have your bike flat out and I mean flat out wide open throttle, i doubt many people have ever done it, or if they have it will have only been once. All this makes arguing about how much 'horsepower' a bit of a moot point, it is really how the bike rides that is far more important and unless your bike has got a fairing how long can you hang on at a 100 miles per hour? Not long I,ll bet!

Another advantage of the SU over the Mikuni is that you have a proper choke and can use the original choke lever. The Mikuni has a cold start device operated by a lever on the carb body which is a bit fiddly to get to when you have gloves on. Attention all Mikuni users, you should be able to get the parts to convert the Mikuni to a cable choke so you can have a handle bar cold start lever. Back in the day (late 1980,s) I built my own F2 sidecar using a TZ350G engine and i fitted the 38mm reverse slide mikunis with the cable choke bits so we had a thumb operated fuel enrichening device that we used for initial engine tuning (Accelerating WOT and dab the thumb lever to see if the engine power surged or faltered) not really clearcut but it gave you good idea of where to go with jetting and the like. I still have the Mikuni book that we used when racing and it lists the part numbers for the cable type starter systems. I will photo the relevant page for anyone who is interested.
regards
Peter Shand

1975 850 MK3

Many thanks for the replies chaps, some great info.

Peter , I'm glad that stuff was of some use to you !
I'm sure I will fit the SU , in the winter, as I'm going to France on the bike next month, I haven't got the SU carb here so it's a bit difficult to understand your throttle mods at the moment.
I agree about the flat out bit aswell, it's rare that I do it these days, and anyway, the bike I'm converting is my long distance tourer so economy is of more importance than speed ( I have another Mk. 111 that is fairly stock (except brake/ignition mods) and I will keep it that way)
Concourse, when I fit the carb I'll get my mate to do some pics and post them, I still haven't joined the 21st century fully yet, and don't own a digital camera etc.
Martyn, my buddy lives up by you, rides a red/black T160, and a black/white T150T, maybe you've seen the bikes.
cheers
sam

trident sam said:

Many thanks for the replies chaps, some great info.

Peter , I'm glad that stuff was of some use to you !
I'm sure I will fit the SU , in the winter, as I'm going to France on the bike next month, I haven't got the SU carb here so it's a bit difficult to understand your throttle mods at the moment.
I agree about the flat out bit aswell, it's rare that I do it these days, and anyway, the bike I'm converting is my long distance tourer so economy is of more importance than speed ( I have another Mk. 111 that is fairly stock (except brake/ignition mods) and I will keep it that way)
Concourse, when I fit the carb I'll get my mate to do some pics and post them, I still haven't joined the 21st century fully yet, and don't own a digital camera etc.
Martyn, my buddy lives up by you, rides a red/black T160, and a black/white T150T, maybe you've seen the bikes.
cheers
sam

Click to expand...

Great! Also pics BEFORE fitting, of the parts themselves, manifold, etc. Thanks

I had an SU fitted to my 1970 Fastback and it was ok with good pickup and solid tick over. The reason I took it off was the difficulty in starting. Whilst there is a choke. the carb lends itself to the engine being cranked over to create a depression to make it work. That's difficult with a kickstart. There is a primer pump available and used on some Harley SU conversions. I would investigate this device before considering fitting one now. My RH knee just couldn't take the damage. A good head of fuel makes starting easier.

Thats typical with S.U.s on cars ,
Grind over to create vacume to activate carb . They dont fire on the first cylinder .
Ah Well - he's got a lelectric start . :twisted: