Stumbling Race Engine at higher rpm's

[The Watson]

Hello -

I have a '65 C race car. Well prepared several years ago by a former owner but it has traded hands a few times since with no documented history. I have taken it out on a couple of DE's, most recently yesterday. Problem: engine is very weak/stumbling from around 4,500 to 5,500 rpm, then it comes alive again. It will run past 7,000 rpm. The electronic tach jumps around a bit also. Here are a few data points:

- 1720 cc, scat crank, high compression around 11:1, Norris 356S Cam with 304 deg intake and exhaust duration.
- fully charged yellow optima (no alternator system)
- Solex P40-II with 160 main jets and 160 air correctors, floats adjusted, accel pump minimized to a dribble as for racing, Skirmants velocity stacks
- installed new coil (yesterday, with little effect)
- has an electronic points insert (I have to check make/model today) - seems like a likely suspect
- none of the problems occur unless under-load. Revs to redline nicely in neutral. This seems like a key point, but not sure what it tells me??
- have experimented with fuel psi between 3 and 4.0 without noticeable change.
- running race gas of 100 or 110 octane

I (and my local independent P-car mechanic/racer) have been working on this off and on for several months. Could it be anything other than the electronic points module? Help!

Chuck

[ganun]

have you considered a harmonic in the intake, how difficult would it be to vary the length of the vel stacks.

[wayne robson]

Distributor vacuum advance,check it, or has it got vacuum advance,if not
require it

[hobbycars]

This might sound stupid, but when I tuned up my 356B a couple of years ago it missed and stumbled at higher rpm. When I did the tune up I had a choice of rotors. One of the choices was a rotor with a rev limiter built in, which apparently was in my car. When I changed out the rotor it ran great and wound up like it should.
Ted