Had a rough weekend, running the Spitfire at the Blackhawk Classic. I
had a terrible miss which we chased all weekend. Substituting a new
distributor, coil, plugs, wires, cap, rotors changed nothing. Changing
needles had no effect (the EGTs showed one carb running well (1400
degrees) and one not well (1100 degrees), and the air-fuel meter showed
VERY rich when the engine was running poorly.
Ultimately, it started running on 3 cylinders and we discovered a
pushrod popped loose (on #7, intake). Lots of aluminum in the upper
end, done for the weekend, time for another rebuild. This is a 1296
small-crank motor, with an A-10-A cam, larger (Ford) lifters, roller
rockers, APT's "full race" valve springs, etc. The A-10-A is a 0.517
lift, 265 duration.
I spent the 3-hour drive home developing theories about what went wrong
here. One theory is that the 100 pounds of seat pressure wasn't enough
to keep the valves from floating and I ultimately popped the #6 pushrod
loose.
My notes from the head assembly show that we used 0.060" valve shims to
get to 105-ish pounds of seat pressure. Full open pressure, according
to the chart APT sent me, is 278 pounds.
Another theory, floated by a more knowledgeable source than I, is that
it was a sticking valve that caused both the miss and the popped
pushrod. I've had a stuck valve with this engine before, leading to
some pretty comprehensive damage. Jack always recommended a lot of
valve guide clearance. The machinist I use (roundy-round V8 builder)
really doesn't like to use a lot of clearance. I can't locate the spec
he used on this head, but my recollection says it was something in the
neighborhood of 0.0025". That could be wrong.
The sticky valve theory might well be corroborated by the fact that one
EGT was consistently low while the other was fine.
So the questions: (1) should 100 pounds of seat pressure be sufficient?
And (2) what's the right valve clearance to avoid sticking valves?
Scott B.
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