SU Oddity?

[Steve kay]

References to speedboat carbs got me thinking about carbs on Meadows engines. In car use almost always SUs, but I cannot recall whether or not the Meadows in double diagonal speed and RAF boats had SUs or another make. There used to be a couple of such  on Loch Lomond and the Scottish Maritime Museum certainly had at least one ex-boat Meadows in the stores, maybe one of our distinguished Scottish members might investigate. There are certainly none on the Mon & Brec, otherwise I'd go out into the rain and peer into the engine bay.

[Mike Costigan]

Many years ago I bought a brand new Meadows 4EC marine engine, which was fitted with an upraft Solex carb; this illustration is of a mid-1930s 6-cylinder Meadows marine engine, and it looks like that also has an updraft Solex:

   

[Charles Levien]

OK, will one of you clever chaps explain this to me:

Moderate tune Chummy. 1 1/4" SU. Semi downdraught. Pumped feed.
Tuning on the road with an AFR meter on a Lamda on the downpipe.
Steady, fairly steep hill. Holding 5000rpm flat out in 2nd gear. AFR steady at 12.3, happy! Holding same revs around  left hand corner and AFR drops to 12.0. This was repeated a couple of times. 
What's going on to make it go richer on corners?
What's the cure?

This paints an exciting picture — might be helpful to know if you were going up or down the hill. If the latter I would suggest changing into top. According to Bill Williams tyre constants that will give you 68 mph on 19 inch wheels. 
Can’t  think of any useful suggestions concerning cornering but in view of cars performance I should try to avoid them.

[bobthompson]

What a wonderful batch of diverse replies!
The test in question was going uphill, so under max load. 
5.67 axle ratio, so not doing 68 mph in second on a 3 speed box!
Float chamber is vertical.
Blue spring fitted.
Damper oil is 3 in 1, so pretty thin, but thicker oils always seem to give so much resistance in winter temperatures - any thoughts on this being contributory?

[Alan]

Being pragmatic about this - is there any other symptom apart from a momentary drop in AFR?

[bobthompson]

No other symptom - just curious to find out what's going on!
I knew one of the more experience chaps would have some answers........

[Charles Levien]

What a wonderful batch of diverse replies!
The test in question was going uphill, so under max load. 
5.67 axle ratio, so not doing 68 mph in second on a 3 speed box!
Float chamber is vertical.
Blue spring fitted.
Damper oil is 3 in 1, so pretty thin, but thicker oils always seem to give so much resistance in winter temperatures - any thoughts on this being contributory?

Hello Bob,

68 mph was assuming a change to top gear!

More sensibly can I ask what needle you have in your 1.25 SU. I have the same carb on my special with a blue spring and no 6 needle. A little rich but goes extremely well.

[Davemayle]

Hi Bob, if when cornering it throws the piston upwards I think it would momentary weaken the mixture which is even more odd!

Dave

[bobthompson]

I'm not sure you are right there Dave - if the piston goes up, it takes the needle up which increases the gap around it and the jet, hence letting more fuel through.
I am using a no. 6 needle which I have modded a little in search of perfection - which of course I never find!

[Davemayle]

Yes Bob, it seems daft but as the venturi opens less suction hence less fuel even though the needle should allow more fuel (at a given RPM & butterfly opening).

Dave