Engine running despite the state of the SU

[Jeff Taylor]

When I was putting the special together I put a H1 (1 1/8") SU on the engine that I found on my shelves with the intention of sorting it later.  I have now travelled about 500Km and it ran very strongly but for other reasons the engine is out again and as I am waiting for some parts, I thought it a good time to have a look at the SU.  First off it has a damper with a weight but also has a spring also it has a 3/16" hole on the top between the Damper and the butterfly I assume for a vacuum line ...but it was running with OK apart from the plugs being very black and lastly the needle is a GG also I have now blocked the hole.

The engine is standard other than a 37 head that somewhere in the past has had about 1/16" shaved off it and a set of extractors 4 into 2 into 1

Lastly I had a 18mm bung put in the exhaust so I can put my air fuel gauge in to get it right but I do need a starting point with needle and spring or no spring recommendations.

Hi Mark,

As a long term owner of a 1936 Morris 8 Tourer (Australian's know them as the 8/40 Roadster), it's more than likely that the SU H1 (1.1/8") carb you have was originally fitted to either a 1939 to 1948 - 918cc Morris 8 Series E or a 1948 to 1952 - Morris Series MM Minor - i.e. the early Minor with the 918cc 8hp side-valve engine. There should be a cast number on the side of the carb body - if from either of the above vehicles it would be 3457 or 3457/1. 

These carbs used a weighted suction chamber piston and therefore were not fitted with a piston spring. They did however have a brass (oiled) damper acting within the hollow piston shaft. The 3/16" hole you mention was not part of the original carbs specification, so probably a much later addition perhaps to fit a vacuum gauge ?

The jet size is 0.090" and when new and fitted to a Morris 8hp side-valve engine the recommended needles were:
Rich: M9
Standard: EK
Weak: MOW

Obviously with the mix of today's modern petrol and the fact the carb's fitted to your Austin 7 these needle specs may no longer be applicable to your needs. 

Here's a link to Burlen's Parts Lists for the Morris 8 Series E and Series MM Minor. Bear in mind that the exploded colour illustration is essentially generic and of a later SU carb - i.e. it shows a piston spring - however I believe the Full Parts List is of the correct 1930's specification and doesn't include a piston spring within the parts listing.

http://sucarb.co.uk/carbspec/carburettor...s/id/1848/

http://sucarb.co.uk/carbspec/carburettor...s/id/1849/

http://sucarb.co.uk/carbspec/carburettor...s/id/1896/

SU also produced a 1.1/8" downdraught carb fitted to some Wolseley cars in the 1930's / 40's - for obvious reasons (the piston being horizontal) these are fitted with a piston spring. 

Just to add to what Gary was saying about his SU 1" OM-type carb - these carbs were originally fitted to pre 1934 Morris Minor's and early MG's - the needle specs for the 1933 Minor were Rich: M7, Standard: M9 and Weak: MO - the needle he's currently using.