SU Puzzlements never end

[Richard McDonel]

Since getting my 240Z restoration safety checked, licensed and on the road, last fall I've put on about 200 miles.  At times it has run well, but there have been too many other times.

Back in August, it quickly, without warning started running rough.  I pulled the plugs and they were all showing a lot of soot - running too rich.  I cleaned them up, re-installed, and nothing changed.  I've now put in a new set of plugs,  started the engine with minimum choke, shut the choke off quickly, and ran the engine for 10 minutes.  It idled OK; just a bit rough.  Open the throttle and it really stumbled until it cleared 2,000 rpm.  Beyond that, it sounded fine.  Pulled the plugs, and got really confused.  Number 1 looked like new; 2 had some black soot; 3 and 4 were badly sooted; 5 had some soot, and 6 was almost as clean as 1.  

I read that as too rich at the centre of the engine, but leaning out as we move towards the ends.  I could lean down the carbs (set at 2.5 turns), but what would that do to cylinders 1 and 6?

Any advice or wisdom would be appreciated.

Richard McDonel

ps    Back in the '60s I bought a used Triumph TR-3, with its twin SUs, and ran it for a year.  I endured a number of issues with the car typical of English sports cars of the day, but nothing ever went wrong with respect to the carbs.  But now...

[240260280]

Sounds like air leaks in the intake manifold somewhere and maybe floats or fuel needle valves sticking.