I have a 1622 MGA, with a pair of 1.75" HS6 SU (originally from a Volvo
I believe). Perhaps this problem will strike a chord with someones
experience.
The problem is that when the engine is switched off, if the float
chambers have not been first dried out by idling with fuel pump off, it
will not immediately restart unless a carburettor slide is first
manually lifted. If left (wet) for 20-30 minutes it restarts easily.
This pattern is valid at any engine temp from dead cold to screaming
hot.
Either drying out carbs or lifting slide is inconvenient. The need to
occasionally manually lift the slide prevents me from fitting air
filters which I really want to do.
On an earlier strip down, (before I learned the drying out trick), at
the point in each aluminum manifold where the downdraft becomes
horizontal, I was amazed to find a small pool of viscous dark purple
liquid. I believe this was "distilled" pink die from racing fuel,
suggesting that fuel is collecting at this point in the manifold, then
slowly drying out.
Various float levels (including the correct one) have been tried with no
effect. Fuel pressure has been fiddled with endlessley, with no
apparent effect. I am currently running a 2.5psi Facet fuel pump with no
improvement. This style of pump allows the line pressure to bleed back
past the piston back into the tank when the pumpmis switched off, though
I dont know how quickly.
Since the problem occurs at any engine temp, including dead cold, fuel
vapor lock is not an issue. These HS6 have a compliant mount of float
chamber to carb body however I wonder whther there may be some
"thrashing" taking place at low RPM, suffienet for the float to seal
poorly and cause temporary, momentary flooding. (I fitted ball style
float valve without effect)
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