Morris Minor Owners Club

I had a very pleasant ride on my motorcycle this morning and was accompanied by my neighbour was was riding a 1926 BSA 250cc 4 stroke motorcycle. He informed me that the engine had recently been rebuilt and that the piston used was a 948cc Minor low compression one with a few thou skimmed off the top.
These Minors get everywhere

good old british bikes

Quite a few bikes use SU carbs Phil

Yes I have seen SU carbs on motorcycles but are not widely used like the Amal.
I once did a conversion on a 1 1/2" H4 SU card so that it had a detached float chamber like the racing Amal with its twin detached float chambers. Fuel was fed from the float chamber to the bottom of the jet by a flexible pipe. Because the vibrations from the engine did not affect the float level it would rev like hell.

Ahh - but did you convert the SU so the throttle lifted the piston - thus eliminating the power robbing butterfly? This is one I have wondered about over the years - but never had the time (or facilities) to do the job....

bmc
As regards mechanically opening or raising the piston no did not do that as I could see no benefit. The whole idea of the SU is to make use of the variable depression acting on the piston. I did all the usual tricks to increase the air flow through the throttle body and make the throttle response better which when I had finished with the carb was much better overall.
Of course the Fish/Minnow carb went some way to overcoming the deficiencies with the SU carb but have to admit I have never used either but they were/are supposed to give a large improvement over the standard SU.
Overall the SU carb is a fine instrument and can be improved considerably if you spend the time and the effort.
The later SU carbs are more about pollution control than performance.

Indeed - but eliminating the butterfly will allow more air to flow.. And the Amal works ok by lifting the 'piston' so it has always seemed to me that it would be worth trying... BUT - this is only worth while if competing in a restricted inlet bore class... I used to do Sprints with 'single carb 1.5" class' - and it would have been perhaps worth it then... but since I won the class anyway..... Otherwise - just fit a bigger carb !

Might work for racing but you'd lose responsiveness for street use.

The idea behind having the vaccuum lift the piston rather than doing it directly with the cable is that with the latter setup getting on the gas too quickly will cause the engine to stutter which makes it more difficult for use in the street.

Yes food for thought; but quite a bit of development work required me thinks.
I note that many Japanese motorcycle engines have given up the slide throttle (aka Amal) and gone over to a type/variation of the SU carb.
I do not know how much % HP/air flow is lost to the butterfly and spindle but doing the carb mods as set out in the C&CC tuning books must get a good portion of those losses back but I would agree not all.
I have never seen or found reference to a SU with mechanical lift to the piston and they may never have been made because of the easier option of fitting the Amal (founded 1927 and which has been around nearly (but not quite) as long as the SU founded 1905) as a direct replacement for one or more SU carbs and to that end a variation of the manifold fitted to the Arden eight port head allowed 4 Amal carbs fitted. Other makes of performance head had the option of fitting Amal carbs.
I have seen a 'A' series twin carb conversion with Amals fitted as a direct replacement for the twin SUs which resulted in less work and development time.
Some technical reports indicate that the slide throttle carb is very wasteful of fuel if not at a constant speed (that waste (pollution) believed to be the main reason for change on Japanese bike engines) and of course the air flow through the SU allows only the fuel needed by the engine but of course the needle shape/taper has to be carefully determined.
It is interesting to note that Amal and SU are now under the ownership of Burlen Fuels and located at Salisbury.

The idea behind having the vacuum lift the piston rather than doing it directly with the cable is that with the latter setup getting on the gas too quickly will cause the engine to stutter which makes it more difficult for use in the street.

IIRC, with the SU carb the mixture gets richer as soon as you open the throttle & vacuum advance helps too (bigger pressure drop over the throttle as it rises also means that more fuel gets sucked during the rise time + vac advance keeps the spark early enough to make use of it) - which for responsiveness (on normal & aggressive road use) is a real benefit that would be reduced with a direct-actuated throttle slide. You would get the reduced restriction, but fuelling & spark timing could reduce the difference..
<not actually sure if the bike has a vac advance, but Minors do>

On modern engines, despite the 100% different hardware, this is an area where you suffer if the MAF sensor is dirty - when the "hot wire" is clogged up the ECU doesn't see the air inrush spike on throttle opening, so you miss out the extra fuel = poor response. I only mention that as I finally replaced my 13yr old MAF on my modern (well, modern-ish) and got a massively improved throttle response! It's like a different car now..

A lot of people now are converting carb models to throttle bodies ie Jenvy and also using motorcycle carbs

Everything goes round.
Just look at the current diesel controversy.
We will be using fixed choke Zenith/Solex carbs next
What ever happened to running on water (using the H of H2O). Was hailed as the new saviour of the planet.

I Had a racing scooter which had a Wal Philips fuel injection system terrible to set up .Do you remember the fish system?

Fish and Minnow (both operate on similar principles) both mentioned above and the Fish was highly recommended by Mr Vizard.
Also there is the Dellorto, a direct replacement for the 1 1/2" H4 SU carb.

Fetching high prices nowadays

Still got my Wal Philips injector! Was fitted to a 225 Lambretta, wish I still had the Lambretta.

Discovered today that my carpet fitter is a big scooter fan and has a LI 150 which he restored in the sitting room to the chagrin of his wife; so he HAD to listen and view The Animal Jack Band CD/ you tube 'I can't strip my Lambretta down in the kitchen (SKA) blues'.
ALL scooter enthusiasts should own a copy
He has taken the CD which Animal Jack gave me to circulate. He went away a very happy man.
And yes the quotation was good too so he had my order

They are quite good heard them on youtube

I do not know about being "quite good" they are brilliant!!!!

Never heard of them or the song before - so had to go and look/listen on YouTube. Good fun, I enjoyed it (and liked the LCV sitting outside the garage).

I suspect your carpet fitter will look forward to returning to do your job!