I would like some diagnostic advise before I go ripping into areas I'd
really rather avoid, like removing the carb, inlet and exhaust manifolds (an
all day task).
Here's the situation:
1) Engine has been running great except for loud single backfire when
shifting above 3500 RPM.
2) A crack (was welded) has opened up in exhaust manifold directly under the
heat shield (and carb).
3) Engine starts and idles OK from cold.
4) Fuel can be seen in the filter located at the carb.
5) Timing is spot-on at 10 BTDC
6) Mixture is blue-white using ColourTune.
After driving about 5 miles or so I have the following symptoms:
1) The idle has gradually increased and now is around 3300 RPM.
2) Fuel filter located at the carb is empty
3) Removing the fuel pump line going to the filter results in pressurized
fuel squirting out all over the place (definitely not a smart thing to do on
a hot engine!).
4) The fuel tank is venting well (no vacuum)
5) After the car has rested and cooled off the fuel filter is again full of
fuel, and when started, the engine idles OK.
My preliminary diagnosis is that the additional heat coming from the crack
in the exhaust manifold is heating up the carb creating pressure in the
float bowl which in turn is working against the flow of fuel from the pump,
resulting in the mixture REALLY leaning out.
I do have a spare cast iron manifold plus I also have stainless steel
headers on order.
Any advice or suggestions will be welcomed.
Thanks.
Reid
'79 Spitfire (original owner)
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