Only things I can think of are:
Valve clearances, or a weak intake valve spring. If the valve is not closed
enough before ignition. As could be possible with a weak spring not able to
close the valve in time at higher revs. Thus it ignites the mixture in the
intake.
ATF fluid is not the correct thing to use in the dash pots. You should use
the same oil as recommended for the engine as for the temperature range you
are using the car in. This is so that the damping level will be the same if
you use it in 30c below to 30c above. If the damping is to little this can
lead to the piston hunting around a level the mixture will thus be incorrect
going from too lean to too rich.
The only other possibility is that some how you cam timing has slipped
around. For a spitfire engine.
Inlet opens 18 BTDC closes 58 ATDC
Outlet opens 58 BTDC closes 18 ATDC
Adjust tappets 7 and 8 to 0.080 in (2.032 mm)
Turn the crank to tdc on the compression stroke of cyl 1. Remember to turn
it clockwise because there is backlash to take up in the cam.
Check that valves 1 and 2 are close and 7 and 8 are "on the rock". Use two
feeler gages to check for the same clearance.
If try moving the crank until this condition is met, remembering to allow
for back lash by rotating it clockwise until the crank starts to move.
If it has slipped around one tooth they will be on the rock at @6 deg btdc
etc.
Consult your factory workshop (not Hayns) manual to adjust. I couldn't find
the procedures in the Hayns Spitfire manual.
|