S.U carb question

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I must admit the thought of a SU carb fitted to a Commando intrigues me quite a bit..... I have more than a little experience with SU's from messing with British, and Japanese, cars for almost 40 years.

Something occurs to me...... My Bugeye Sprite has a 948cc engine that carries two, petite SU's...... hmmm.

Rod R
 
Rod R said:
I must admit the thought of a SU carb fitted to a Commando intrigues me quite a bit..... I have more than a little experience with SU's from messing with British, and Japanese, cars for almost 40 years.

Something occurs to me...... My Bugeye Sprite has a 948cc engine that carries two, petite SU's...... hmmm.

Rod R

I think the difference is that the Sprite carbs are feeding 4 cylinders and the Norton is more like 1 1/2.
 
I was looking at it this way, on the Sprite one SU feeds (948/2 = ) 474cc's..... On the Commando, one Sprite SU would feed (850/2 = ) 425cc's.... (Compression, valve timing, and a thousand other things I don't fully understand notwithstanding)

If you were using an off-the-shelf MGB SU carb, for example, a single SU feeds (1798/2 = ) 899cc's. Granted that's a little closer match than the Sprite setup, but I like two carbs on a Norton, despite the fact my 850 Interstate has a single Mikuni.

Like another post said, "two Amals look better than one Mikuni"..... two SUs would have to look cooler than one, especially hanging off the side of the engine like those cool Dell'orto setups I used to see.....
 
Rod R said:
I was looking at it this way, on the Sprite one SU feeds (948/2 = ) 474cc's..... On the Commando, one Sprite SU would feed (850/2 = ) 425cc's.... (Compression, valve timing, and a thousand other things I don't fully understand notwithstanding)

If you were using an off-the-shelf MGB SU carb, for example, a single SU feeds (1798/2 = ) 899cc's. Granted that's a little closer match than the Sprite setup, but I like two carbs on a Norton, despite the fact my 850 Interstate has a single Mikuni.

Like another post said, "two Amals look better than one Mikuni"..... two SUs would have to look cooler than one, especially hanging off the side of the engine like those cool Dell'orto setups I used to see.....

Actually I wasn't using simple math, rather taking into account narrow distance between the cylinders, which is why you can get away with a single Mukuni so easily. The dual carbs on the 4 cylinder were because of the wide spread between the front and rear cylinders. The long lengths of the intake manifold would have led to an uneven distribution of the fuel/air mixture.

Not saying don't do it, but you may need to do some fettling to get the mixture right. And hope there's enough vacuum to make the SUs operate correctly.
 
Yes, I understand..... Highly unlikely I would attempt it, given the expense and uncertainty.... And, too many projects sitting around here already to start that experiment..... I would be happy if I could just get a nice set of Amals back on my Interstate. The PO did the Mikuni conversion, and somehow the Amals got away.

Happy Holidays one and all !
 
.... A Weber 2bbl will look better than anything else ! I saw somewhere a picture of the manifold and the list of jets etc for it....
Philippe
 
If you're talking a DCOE Weber, I happen to have a few of those laying around...! Still, sounds abitious if starting from scratch.

Back from the in-laws Christmas thing..... No Norton parts under their tree....

Rod
 
I ran a DCOE on my fuelie bike for many years before I went to EFI. Still have the setup but I wouldn't recommend it to anyone. Jim
 
Scooter62 said:
The big advantage an SU has is that its a constant velocity carburetor that allows over carbureattion but still prevents the drop off the cliff power when you whack the throttle open.It also could give better fuel mileage since it only allows the slide to move up in direct relationship to the air flow. This was an excellent engineering approach at least until fuel injection was perfected. Many of the pre FI Japanese superbikes used CV Keihins to allow massive oversized carbs on them to maintain rideability and some semblance of fuel economy and air quality.

I have want to try a single Bing CV carb from a BMW airhead on my 750 Commando. I think they are 38mm (can't remember from my old BMW days.) I know that people have been using single Mikuni carbs on Commandos for years without ill effect but I have had two of them lock open on me and now I am a little gun shy. The CV carbs have nothing to stick and the cable pull is on the side. It is my understand that a lot of the Mikuni sticking problems came from cable routing and the lack of sufficient space between the carb top and the frame backbone. A carb with a side pull would eliminate this problem. These old Bing carbs are readily available with a good supply of jets and parts. I have no experience with SU carburetors but I have nightmares remembering the Zenith Stromberg carbs on my TR7. (I liked my TR7 but it ate head gaskets and the transmission was not its most robust feature.)
 
I have been using a HIF6 SU carb. supplies by Phoenix in 1988 on a Mk III. A good bit of kit.
Since then I have done about 18000 miles. It provides a reliable tick over which I could never get with the Amals and a really smooth pick up. Over the 1300 miles I've done this year I've got 65m/imperial gallon.
As Gino reports the bike runs lean and takes ages to warm up. However in recent years this has got better as though something is wearing to provide a richer mixture. Even so shutting down for even a few minutes will require choke again. So far the engine has not suffered from the lean burn though I do treat it fairly gently. Starting has never been a problem though I always use the electric foot which may be a factor.
In the early days I struggled unsuccessfully to stop the exhaust popping on over run. This was a minor problem with the original balanced exhausts with black caps but much more of a PITA with black caps that I now run. I now just live with it. I did consult Peter Cooper son of Bernard when he was working at his engine development company at Halfpenny Airport near Stourbridge. A helpful man who spoke about further developing the kit for Triumphs. Presumably that did not happen.
The fit under the frame is tight but I have never found topping up the dash pot a problem: just use an oil can to dribble oil in with the cap raised against the frame which gives about 1/2inch clearance.
Remembering the hours I spent messing with Amals the SU just does the business with low maintenance.
An letter to the UK NOC mag many years ago gave the correct neddle as BFD SUNZX8052, the spring as red SUAUC4387. I can verify the accuracy of that.
Seasonal greetings everyone.
Gordon
 
Hi Gordon,

With later SUs, or for that matter earlier SUs with the jet not correctly centred , the needle rubs on the jet and wears both. Jet wear isn't as big a problem as the needle. It's the wide section of the needle that wears most ie the tickover area, and to correct the rich tickover the jet has to be raised weakening the rest of the needle.

I think your SU is a little weak and off memory it might have a temperature sensitive jet and that makes it a real pain to set up. There might be a pin hidden under the rim of the dash pot. With the motor at running temp lift the pin slowly. Increase in rpm = rich, drop in rpm or stall = weak, a rise then fall back = correct. Later carbs don't have this pin but you can lift the dash pot wih a screwdriver from the carb mouth with the filter off. Don't forget this is only tickover the fixed needle profile controls the rest.

SU had a needle catalogue that covered the huge range of profles making setup a lot easier. However, for a single scarb you could polish a spare needle in specific areas to get the results you need. I used an electric drill some fine wet and dry and a micrometer. Be carefull cos a couple of thou is a lot.

Have fun,

Cash
 
I had a SU on my Commando Mk3 in the 80s. It was a HIF6 and I think it was the Phoenix conversion. The upside was much better economy and ease of setting up but the downside was slightly less performance and a constant need to put oil in the dashpot. As the dashpot top was right up against the frame, this wasn't easy. All in all though, it was a positive experience.
 
A letter to the UK NOC mag many years ago gave the correct neddle as BFD SUNZX8052, the spring as red SUAUC4387. I can verify the accuracy of that
.
Should have written that I cannot verify that information.

Gordon
 
That's interesting, a red spring, if the standard blue spring is left in the mixture will indeed run lean above tickover, and to me would suggest a needle compromise. (I'm sure red is rich and yellow weak)

Cash
 
In the 70s I managed a small motorcycle shop in Hampshire UK, We had a customer who had an SU carb on his 750 Triumph.
It ran very well with no loss of performance and the fuel consumpsion was superb.
 
Does anyone know how Riviera do to have no more oil in the dashpot in their SU Harley kit?
"NEVER use any oil inside the dampner. Oil will slow the rise of the piston causing an overly rich mixture.
Every 30 days unscrew the dome cap and lightly spray around the piston shaft with WD-40. The piston must float
freely at all times to accomplish the constant velocity principle of allowing the engine to determine its position."

http://www.riveraprimoinc.com/SU_Instru ... 0-2009.pdf


Philippe
 
prmurat said:
Does anyone know how Riviera do to have no more oil in the dashpot in their SU Harley kit?
"NEVER use any oil inside the dampner. Oil will slow the rise of the piston causing an overly rich mixture.
Every 30 days unscrew the dome cap and lightly spray around the piston shaft with WD-40. The piston must float
freely at all times to accomplish the constant velocity principle of allowing the engine to determine its position."

http://www.riveraprimoinc.com/SU_Instru ... 0-2009.pdf


Philippe

The oil in the dashpot helps damp out induction pulses and to give a rich burst when the throttle is snapped open to give better responce much the same as a power valve/pump thingy fitted to most car fixed jet carbs. It's the spring that determines the the mixture control over the whole needle profile. SU recommended engine oil in the dashpot, however some did get good results by using thinner oil. I never did. Why they recommend no oil I don't know but if it works what the heck. :roll:

Cash
 
Hi Ebsbury,
Boy! that brought back some memories. I would spend ages looking for scrap MG Magnettes and Rileys as they were fitted with HD4s. The only SU that was fitted with spindle bushes etc, had the long choke, quick lift dashpot, remote jet adjusters and a butterfly bypass for easy reliable tickover setting, perfect for a well tuned Mini. IMHO the best SU ever.

I hope you guys don't mind me rabbitin on, I'm house bound and going stir crazy, I've had a knee replacement and it's not healing too well. :(

Cash
 
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