S.U carb question

Thanks all for your input , I have some experience with SU carbs on classic british cars, and I was aware that coversion kits for Commando's were offered in the past by some British suppliers.
Again thanks all for educating me on this subject. This site is a great source of information indeed !

Peter.

Hi All
The trick is in the needle and getting the correct profile is very time consuming in dyno and road test time. Needless to say, the profile is entirely unique to the application and cannot be purchased from normal SU sources. However, if you contact Pete Hooper at Bernard Hooper Engineering, it may still be possible to purchase a conversion kit from him. If you google you should come up with Bridgnorth and Shropshire, which will lead you to an e-mail, phone and address. I was privileged to work for Bernard thirty years ago and bought a kit for my 750 Commando. It's been on my bike ever since and provides very smooth economical running and probably improved longevity due to less bore wetting with petrol. I would be highly suspicious of any kit other than one of Bernard/Pete's because I cannot believe anyone else would have invested the hundreds of hours of dyno/road hours required to get the mixture right at all the various engine loads and speeds, cold start mixture etc... Failure to to this will lead to holed pistons, burnt or dropped valves or seizure. And that's just what happens to your bike engine at the start of the incident.
All the best

A mate of mine has had a pheonix SU kit on his 850 for years and loves it, starts , runs and idles very nicely .
I bid for one on ebay about a year ago , but it went for more than I was prepared to pay .
Unfortunately they are no longer availiable new .
From what I have seen, they work well .

Hello, my 1st post! Can I first say how much I enjoy this site? Learned a lot too! Thanks.
I`ve covered 30,000 miles with a home made SU conversion with no problems. Yes it runs lean (or just efficiently?) but I cruised at 80mph for 3 hours Paris - LeHavre with no overheating damage. I can`t report on top speed, because I`m never flat out, but performance seems fine up to 90.
I run an HS4 1 1/2" with standard spring and needle on an 850mk3 . My first SU was 2nd hand from a scrapyard Austin1300. SU s don`t really wear out, but a NOS HS4 can be found on UK Ebay for around 30GBP. I use the AUD red jet series. 70+ mpg uk gallons. I would avoid the modern SU as more difficult to set up
RGM list a single manifold for 55GBP, and you will need to make up an adapter block because the SU mounting bolts are at different spacing to an Amal. A spacer block also gives more clearance to the top frame tube. Many SU s have a lug to ensure vertical float chambers but this is easily filed away to match the Commando engine angle.
A single Amal is worthwhile, giving around 60mpg. The original twin Amals were constantly going out of tune, and synchronising them far more of a chore than topping up the SU dash pot. Twin Amals gave 45-50mpg. A bonus with the SU eliminates the awful cable drag and gives an easy throttle action.
A DIY SU conversion is well worth considering
Rick

There was quite a lot of discussion when I was at N-V about switching from Amal to SU. Two things stopped it being adopted. The first was that motorcycle dealers and mechanics weren't familiar with the SU. The second was that the very high vibration levels on the engine transmission "pod" when it bouncing around on the ISO's would mess up the damping of the SU piston.

Because of the first objection, the idea was killed off by management and the experimental department never got to test for the second.

I had one fitted to my 750 Fastback and it ran very well with excellent pick up and solid tickover. Economy was good too but the only reason it had to go was the poor starting. IMHO the problem stems from a lack of suction in an engine initially being turned over by a kickstart. A fast cranking electric start would have probably sorted it but we all know there is no such thing in Commando land. JS Flatslides get my vote if you can afford the import duty. Best option is a holiday in the states and bring them back scattered around you and the wifes luggage. (Not that I would try and avoid the taxman) My Rotary has two SUsand they are almost maintenance free, but that is a thirsty beast and another story.

Thanks Frank, it is always fascinating and valuable to get a factory insiders view. I don`t doubt your 2 points given against the SU, but it does seem odd, as at the time the majority of British cars ran SU, so nearly everyone would be familiar with them. Apart from topping up the dash pot they are virtually maintenance free.
I haven`t found any evidence that the engine vibration does affect the dashpot. I wonder if that was just an excuse for resisting change? Or possibly my running the smaller 1 1/2" helps by raising the gas speed.
When I fitted the scrapyard SU, I was expecting a lot of experiment would be needed. But it ran after about 3 kicks, and the standard jet/needle seems fine for road use.
I suppose a more serious objection might have been that any loss of performance would have impacted sales. The standard 850 was already slow enough ! Were there ever any performance figures released for the Norton 76?
As you can see, I`m sold on SUs, but with buyers fixated on top speed the Norton Management might even have been right to stick with Amals
Rick
edit. Gripper`s post overlapped. I haven`t found any starting trouble but again maybe the smaller 11/2" has better gas speed. Also the earlier red jet SUs have a crude but effective choke. The jet is just lowered. Later models have more complex arrangements

gripper said:

My Rotary has two SUsand they are almost maintenance free, but that is a thirsty beast and another story.

Click to expand...

First of all, Jealous! I have always fancied a rotary, but in the states they are even more rare and when they do go up for sale they cost more than my truck.


Second of all, I would have experimented with a wrecking yard SU carb, if we had any over here.
From what I have read and seen on TV, the UK is lousy with them. Plus you have really cool small displacement V8's.

It would surely be a fun experiment to run SUs, mostly for the freak-out factor for anybody remotely familiar with British hardware. They are great carbs, overall. I am sure they could be made to work pretty well in a dual setup, although a single would be most sensible. There were some short-top types that reduced the height of the dashpot. But I recall Jim Comstock saying he put a pair of Honda CV carbs on a Norton and it was a great way to make the bike into a dog...

P.S.: The one in the photo earlier looks to be an HIF with a concentric float bowl. Be your best bet on a motorbike, as we know from Monoblock to Concentric Amals.

A reason for SU's reduced (removed?) presence from the car business is that their transient characteristics make it very difficult to meet emission regulations, particularly in the US. Back in '68, emissions weren't an issue (except for noise) and I believe are still less stringent for bikes compared to cars.

The introduction of digital electronic fuel injection in mainstream cars has virtually eliminated carburators - and a good thing too, IMO. A whil;e back, I saw a definition of the word --"Carburator is French irregular verb that means "Leave well enough alone"".

gripper said:

I had one fitted to my 750 Fastback and it ran very well with excellent pick up and solid tickover. Economy was good too but the only reason it had to go was the poor starting. IMHO the problem stems from a lack of suction in an engine initially being turned over by a kickstart. A fast cranking electric start would have probably sorted it but we all know there is no such thing in Commando land. JS Flatslides get my vote if you can afford the import duty. Best option is a holiday in the states and bring them back scattered around you and the wifes luggage. (Not that I would try and avoid the taxman) My Rotary has two SUsand they are almost maintenance free, but that is a thirsty beast and another story.

Click to expand...

The poor starting could be from a number of things.
I have been looking for the downsides to fitting a single SU and this appears to be the only issue.
When you fitted I assume that your engine was in perfect tune with no starting problems, then, the only way to go from there is downhill.
I would if using a SU on a worn engine that couldn’t get the suction on startup, fit a oilcan containing petrol/gas and via a small bore tube to the carb inlet squirt gas into the inlet before kicking over to overcome this problem if it demanded it.
Below is from a Triumph Thunderbird SU information website;

http://www.britishonly.com/tech/tips/su.asp

Gas speed is important on SUs, and I`m going to suggest that the HIF6 1/34" used in the Pheonix is too large. In addition this beast had temperature compensation, and an emission friendly choke which are harder to set up. The earlier HS series had a simple choke where the jet tube just dropped, effectively raising the fuel level, and much easier for starting. There was also a piston extension which you could raise to check tune. An integral float (as HIF) isn`t necessarily an improvement , because a remote float allows you to compensate for an angled carb. Angling the carb can gain you precious dashpot clearance.
Twin SUs wouldn`t be a good idea because of the low pulsating gas speed.
The Lightning Spares website had exhaustive experimental info on Rocket 3 with an SU. I believe they found the11/2" better than the 1 3/4", but the pages seem to have been taken down.
My advice is to buy a used HS4 (11/2") from UK Ebay. Dirt cheap, even with US shipping.
I doubt you will have problems beyond making the manifold adaptor. I `m sorry to repeat myself, but recent posts seem to raise difficulties I`ve not found with the HS4.
Rick

There are HIF6 and HS4 available here if you really want one?

http://www.sucarb.co.uk/Restoration.aspx?id=35

Rick:

Back then (mid-1960s) car and motorcycle people rarely mixed, so mechanics in one specialy wouldn't know much about other's. All motorcycles ran Amals, and the Japanese invasion was only in its infancy, so familiarity with Keihin and other carbs was minimal.

After emigrating to the US, I've often wondered how a Commando would run with a big 4-barrel Holley! My 1970 Mercury station wagon had one. Must've held a half-pint of petrol in the float chamber. The engine was a 429 cubic inch (7.1 litre) V-8 - just a tad bit bigger than the Norton! I could lay rubber if I floored the pedal at about 75 mph, but I had to also lay rubber to slow WAY down for corners! When the fuel crisis hit in the early 1970s, a big, heavy (over 4,500 pounds, IIRC) car that got 9 mpg on a good day was a liability. I sold it to someone who drove demolition derby races. When we relocated from coastal Virginia back to Seattle in 1972, I towed my Cortina GT behind the Mercury on a rigid tow frame (no driver in the Cortina). The Cortina went much faster on the tow than it ever did under its own power and I couldn't sense that it was even there.

dillinghamp said:

Hi All
The trick is in the needle and getting the correct profile is very time consuming in dyno and road test time. Needless to say, the profile is entirely unique to the application and cannot be purchased from normal SU sources. However, if you contact Pete Hooper at Bernard Hooper Engineering, it may still be possible to purchase a conversion kit from him. If you google you should come up with Bridgnorth and Shropshire, which will lead you to an e-mail, phone and address. I was privileged to work for Bernard thirty years ago and bought a kit for my 750 Commando. It's been on my bike ever since and provides very smooth economical running and probably improved longevity due to less bore wetting with petrol. I would be highly suspicious of any kit other than one of Bernard/Pete's because I cannot believe anyone else would have invested the hundreds of hours of dyno/road hours required to get the mixture right at all the various engine loads and speeds, cold start mixture etc... Failure to to this will lead to holed pistons, burnt or dropped valves or seizure. And that's just what happens to your bike engine at the start of the incident.
All the best

Click to expand...

You'll need to start with an SU HIF6, Pete may be able to provide the needle profile - SU used to have a little lathe that machined the needle to profile, then you'd just need a manifold, which wouldn't be difficult in these days of CAD, steriolithography and lost wax...