SU carb.

[Ken Morton]

I have fitted a one inch SU carb to my Chummy engine, What is the recommended needle and spring?

[Bob Culver]

SU parts ca nbe interchanged and influence the needle. There are various weights of piston assembly and some have dashpot, lackingif original.vital the jet orifice is not worn. Apart from the carb exit the innards of 1 1/8 seem the same so presumably the same needles apply.

[Hugh Barnes]

First of all, what Carb is it? If it is a 1" OM series, for example, it doesn't have a spring.

[Jeff Taylor]

It could also be an 1" UBA (sometimes described as a VOM) off an early Morris 8 (vertical stud manifold flange whereas OM series has horizontal stud flange). Like the OM the UBA has a weighted suction piston and does not use a spring nor does it include a damper. The UBA carb usually has the number 1930 cast into the right hand vertical side of the carb body near the idle screw brass fork fitting on the butterfly spindle. A photo of the carb would help identification.

[Ken Morton]

The carb has "617 Q "on the side and F 32526 2 stamped on the inlet flange. I believe it came originally from an early MG. It does not have a spring at the moment.
Thanks for the comments.

[Jeff Taylor]

It would appear to be a 1" OM carb, the body number 617 being fitted as a pair of carbs on MG's from the 1932 12hp 6 cyl, 1933 850cc J2 4 cyl, 1934 to 1936 P & P.B. 8hp 4 cyl and as a triple carb set on the 1933 K.A. & K.B. 12hp 6cyl. On none of these applications was a piston spring ever fitted. Some had a weighted brass piston and some a weighted zinc piston. Recommended needles varied widely between the four MG models, so best to wait for someone on here who's using an OM carb on their A7 to recommend a needle.

[Hugh Barnes]

I have run my OM with both AN or GG needles, AN currently. Both seem to give reasonable performance and economy..

hth...

[Bob Culver]

If you get involved in comparing profiles note that with the rduced lift of smaller carbs the part of needle utilised in normal running is reduced. A feature of the SU is that by tryibg the effct of slight start enrichment ("choke" substitute) the effect odf a richer needle can be assessed at all throttle openings.