[Fot] Loss of power over 4000 RPMs?

[Duncan Charlton]

Bud,

I think the important clue is probably the increase in fuel demand at speed
which suggests fuel restriction instead of loss of spark.  Is the #1 EGT
reading still zero when revving over 4000 when the engine is NOT under load?

As a quick check you might try putting an induction timing light on each of
the front two plug wires and observe the strobe to see whether the signal
stops when the engine is revving over 4000.  That would suggest that something
is mechanically affecting the ignition but this seems unlikely given that this
happens on a straight where there are no constant G-loads felt by the engine.
If the timing light method doesn't tell you anything you might try removing a
spark plug, laying it on the engine to ground it and then running the engine
up over 4000, observing the spark, or lack thereof.

You didn't say whether the engine is using two carburetors, but if that is the
case, I would remove the float needle valve in the front carb and flush
everything in the vicinity thoroughly.  There might be a tiny piece of crud
stuck in the carb inlet which lies in wait until vibration and increased flow
moves it downstream enough to restrict flow.  Do you happen to know whether
the orifices leading to the front carb's float chamber are any different from
the ones leading to the rear carb's?

Duncan


On Jul 31, 2013, at 2:37 PM, Bud R wrote:

> Mark Eden was good enough to let me drive his #42 Spitfire at High
> Plains Raceway this last weekend but we just couldn't get it to run
> correctly once the  RPM got over 4000 when under load on the track. It
> was fine in the paddock and on the grid reving easily to over 4000 RPM
> but once I got on the track and under load it lost power and the dual
> EGT gage on the #1 cylinder exhaust would drop to zero while the EGT
> gage was reading 1100-1200 degrees on the #4 cylinder exhaust.
> Basically the #1, and maybe the #2 also, cylinder was not firing at
> all. It certainly felt like only 2 out of 4 were firing but since only
> two exhaust pipes are monitored with the EGT system I couldn't verify
> that.
>
> We checked fuel pressure, fuel lines, spark plugs, changed rotor,
> wires, distributor cap, relashed valves, checked timing, and few other
> things I probably forgot but no joy. It dropped power going straight,
> on curves, uphill and downhill, but once I got to the hot pits and
> paddock under 4000 RPM it would come to life and the EGT gage read the
> same on both sides.
>
> So what else could it be? Advance mechanism in the distributor bad?
> Valve float (we visually inspected them to see if any were broke)? And
> why under load would something like this suddenly manifest itself when
> there's no sign of a problem when not under load? And only on 1 or 2
> cylinders?
>
> Thanks for any suggestions.
> Bud Rolofson