[Spits] 79 Spitfire with SU carbs

[Jim Muller]

On 26 Jun 2007 at 17:43, David Woerpel wrote:

> The carbs are set correctly.

Your puzzle still puzzles me.  How do you mean the carbs are set 
correctly?  I assume that means you've gotten it to idle well and the 
problem is with actual running, not idling.

An air leak would normally get less significant as you opened the 
throttle, so if it idles well that probably isn't the cause.  
However...

A PO installed SUs on my GT6 but it had running problems.  At part 
throttle it ran lean, and full throttle rich.  I finally figured out 
that the piston springs were too soft, being for an MGB's 1800 4-cyl 
instead of the bigger-breathing 2000 6-cyl.  Projecting this problem 
on to you (probably not valid, I realize), if the springs are too 
soft, the mixture gets too lean quickly as you open the throttle.  
You didn't say where these SUs came from, and it's hard to imagine 
they came from a shallower-breathing engine than the Spitfire, but 
it's possible.  On the other hand, supposing one or more was broken 
or missing.  You might not notice it at idle because you aren't 
measuring just how much the pistons rise with a bit of throttle, but 
under running conditions it could be significant.  Have you checked 
them?  You being an MGA guy you probably did, I imagine.


-- 
Jim Muller
jimmuller at rcn.com
'80 Spitfire, '70 GT6+



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