[Fot] Intake manifold vacuum and no high RPM

[Mike Harmuth]

I'm tracking down a problem with the Spit race car (1296 Small Journal,
12;1 compression, APT TR74 cam 295 duration and 0.324 lob lift, head ported
as per Kas's book). Rebuilt European SU HS4 carb setup with AAA needles
(manifold and carbs checked for vacuum leaks), Advanced Distributor for
spark (34 degrees at full, mechanical only, advance @ 3.5K RPM)

The problem I have has followed me on two different short blocks, I can
easily pull 7500RPM in 1st-3rd ( Close ratio TR7 gearing in a Spitfire
case, 4th gear is 1;1,  4:11 locked diff ) gears. In 4th I'm struggling to
get over 6K, no matter how long the straight. Just before the last race of
the season, I installed an air fuel ratio gauge on the header (stahl) and
found out I'm running (progressively)  very lean (19:1) in 4th over 5K. At
2K I adjusted it rich, to 12;1 and it keeps in the 13.5- 14.5 range until I
get over 5K when it really leans out. If I pull the choke to 1/2  full over
5K, I get a good ratio again, 13.5- 14%.

To me that seemed like the carb pistons aren't pulling all the way up so I
drilled and tapped the manifold to get a vacuum reading. It was steady but
low at 6K, ~20 Inches but would jump to ~28 briefly as the revs dropped if
I blipped the throttle. Under 6K, it was in the 15 inch range. Normal air
filters are K&N with stub stacks but I get the same results with open carbs.

Am I correct in assuming that the vacuum is too low or is it that due to
overlap with a performance cam? Any ideas on where to look to get the ratio
"normal" at higher revs? Fuel pump is Holley Red, adjusted to 3 PSI at the
engine compartment and steady through all revs.

I have dyno time scheduled in a few weeks but I'd like to use that time to
tune not debug things.

thanks
mike h
63 spitfire
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