Intermittent miss fire

[Mike Wattam <101714.1343@domain.elided>]

Hi Nick

I would look at the following:

FUEL PUMP
This is known to stop working in extreme heat, or deliver low fuel (which
in turn can cause overheating due to lean fuel mix).

VAPOUR LOCK
Normally the fuel will keep pumping, which prevents vapour lock as
relatively cool fuel is coming through and keeping temepratures down.
However, in the extreme heat and particularly if your fuel pump is
delivering low fuel so things are not being colled properly, this symptom
just could occur.

One thing very often overlooked in these circumstances is the float chamber
breather pipes.  If these are  blocked, the extra fuel evaporation at high
termperatures will push vapour into the engine (causing weak mix and
overheating) and in extreme cases make the fuel pump stall.  Easily checked
by blowing up the lines (exit is down front of timing chain cover).

LUMENITION
It is known that the amplifier module (this is probably on the bulkhead
above and behind the engine in the hottest possible area) tends to break
down under extreme heat.  I reckon this is your problem, especially as you
describe high/rising temperatures and engine speed related conditions. 
This fault is so common you would not believe, so much so I have not even
bothered to ask if you actually have Lumenition (or other electornics such
as Piranha) fitted.  If you can temporarily convert back to 'points', this
will prove it one way or the other.

THE ROOT PROBLEM
One thing I must mention.  You seem to have a basic overheating problem at
idle/low speed.  If you haven't heard about the radiator fan cowls which
are being re-made by Glenn Merrell (the Superstar), I suggest you very
seriously consider buying, as these will encourage a much more satisfactory
air-flow through the radiator at idling.  

Let us know the results please.

Mike Wattam
Triumph Stag Register