SU carb on Commando?

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Nov 12, 2006
Anybody have any experience with an SU carb on a Commando?
I have a chance to pick up an SU, and I wonder if it's worth experimenting with.I know some Harley guys that swear by them,and another guy that restores Triumph(cars),MGs and Austin Healeys, says they are the best carbs ever made,once properly set up. Any opinions on the subject?
At N-V, we considered it, but never got as far as testing the idea. Consensus was that the metering piston would be too badly affected by the vibration of the engine.

I would think that, if you're into experimentation and have the machining skills to make the adapters, it would be worth a try. An SU off a 1.8 MGB, or maybe a Keihin (Japanese SU copy) off a Datun 240Z would be a good start. I wouldn't bother with the smaller twin-carb packages, like the MG 1100.
SU carb on Commando

Regarding SU carb application on Commandos:
Check out click technical, click Commando, click carburettors-alternative, scroll down to SU carbs.
There is a picture and settings chart, and a few postings regarding installation and performance.

I have used one in the past.
It was a kit from Phoenix motorcycles UK for 750 & 850's.
The carb was 1 3/4 " SU H1F6 similar to a Jaguar car carb but was developed by Phoenix.
It had great economy with 79mpg average in tests and a great even tickover with only a small loss in top end speed.
The only downsides is they are not available anymore and if you use it in the stock commando frame the dashpot can be difficult to top up.
I eventually changed to single 34mm MK2 amal which is also excellent and only needs one cable, SU needed two choke and throttle.
Regards Hursty :lol:
Bu it's cunning design the SU carb succeeds in providing the right mixture most of the time, unlike most other carbs. Consequently it achieves high efficiency and often very good mpg. However, it needs patient setting up with different needle profiles and, in my experience, does not give the same immediate response to sudden throttle openings. Whereas a Weber (on a car) or an Amal ( on a bike) can deliver rapid blast of fuel on sudden throttles and hence a pleasing and responsive kick of power, the SU is more studious. It ponders the new demands of throttle and carefully controls the mixture. It still gives good performance but in a more controlled and mild-mannered way.
I think the route which hursty has taken, using a single 34 Amal, is going to be a better bet. You can even go to 36mm if maximum power is important, although the 34 will probably give better mpg and low/mid range torque. It also gives you a much lighter throttle action than twins.
Guys - Those of you that have achieved good results with a single Amal MK2: any issues with carb flooding? I tried for three weeks to get a MK2 to work reliably but the thing just kept flooding. I changed floats, needles, adjusted float height, tried different gaskets - all to no avail. Seems the float needle was not reseating properly somehow. I came to the conclusion my engine setup / crank balance or whatever meant it was being bounced around too much on the rubber manifold to carb mount. A single MK1 has worked flawlessly for 5000 miles, but I'd like to fit something with better build quality!
I did some tests at N-V on a single-carb version of the Norton 650SS, which was my commute-to-work ride. We'd had some customer inquiries about the possible improvement to low-end torque with such a configuration.

Tests showed that indeed an improvement on low-rpm torque did result, but that top end power was down about 12 percent. I rode the 650SS with the single carb installation as a commuter most of the time I worked for Norton-Villiers. Even with a daily commuter run of about 60 miles, I was hard pressed to tell the difference from the twin-carb set-up.

I beleive N-V published the results of our tests, which were primarily of interest to people who used the Domiator and the Atlas as a sidecar hauler.
I had a similar problem years ago, it turned out to be the little lugs on the gasket that lay over the float hinge pin. They would catch the float and prevent it working. Don't cut them off completely, just trim them a little.
Best of luck.

In respose to hal20308 I have experienced flooding on the MK2 in the past.
It can occur if you don't use the bike frequently.
I have found a delicate tap with a wooden instrument will usually do the trick.
Occasionally you may have to change float needle as I think the modern fuels & additives damage them.In an extreme case I have had a leaking float.
You can check for flooding by removing the float bowl and feeding with fuel to see if it overflows (it is just like a household toilet cistern).
As regards balance factors I currently run two 750's one on standard B/F of 52% & one on B/F 77% & it makes no difference.
It is a great set up & RGM do a complete kit.
Regards Hursty.
Nick, what exactly do you mean by flooding?Does it run rich all the time,or drip fuel,or spill over into the intake,when you stop the engine? :?:

Thanks, everyone for your feedback.I think I will put the SU on my Sportster.
Everything is available for a bolt on install. It may be the cowards way out, but........ :wink:

Guys - Thanks to all who answered on the "hijacked" subject of the MK2! To answer - the bike is a daily rider but I never really got it out of the garage with the MK2.... The bike would fire up OK, run and then usually foul the plugs after a couple of minutes as the float bowl overflowed and fuel started going down the intake. Once, the engine had stopped, fuel would usually start to pour out of the two "overflow" rubber tubes or out of the air filter (K&N pod-type) indicating the float needle was not doing it's job somehow. I tried all manner of tricks, including trimming the gaskets/replacing float & needle/adjusting float height - all to no avail. I did notice that the carb vibrated significantly more than a solidly mounted MK1. Eventually I gave up and just went to a single MK1. Thanks for all the input from those that are running one successfully, guess I'll have to put my experience down to a secondhand MK2 with some sort of problem..... :c) CHeers - Nick
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