I have a riddle for all of the armchair mechanics out there, but it is a
longish saga so anyone not interested hit 'delete' now.
I installed a very racey 302 into my Mk I last winter and have spent all
summer trying to sort it out. I am on my third mechanic (four counting me, and
I did used to be one) with no answer in sight yet. The symptom is an annoying
'surge' or 'buck' at lower RPM in steady state cruise, most noticeable at
about 3000 RPM but present at 2000 RPM. This is distinct from the lumpy idle
that the cam produces, it just seems to erratically fluctuate making it hard
to hold a steady speed. Full acceleration is crisp, and in fact startling at
3000+ RPM.
The engine is a 69 302 with a .512 lift cam, ported Dart heads (2.02 intake),
balanced blueprinted and Dyno'd at 345 Hp by a reputable engine builder. The
Carb is an Edelbrock 600 cfm, but I have tried two different Holley carbs to
try to isolate the problem: same symptoms existed. Ignition is an MSD 6AL,
including the distributor. Ignition is set at 7 degrees initial and 34 degrees
total advance. We have also tried three different coils, mostly due to the
mechanics' unfamiliarity with CD ignition patterns, in my somewhat skeptical
opinion. I have checked the fuel pressure and it seems fine, I put a pressure
regulator on it to ensure I wasn't over pressuring the float bowl.
Can anyone think of something that will cause my engine to accelerate well yet
not hold a steady RPM well. I would expect a over rich or lean carb to be
somewhat consistant, and a faulty ignition to get worse at high RPM or WOT.
Once off of idle the vacuum is quite steady and it is not too bad even at
idle, so I doubt a vacuum leak is the answer either.
Help! I am tired of paying for mechanics to make the same guesses that I made
six months ago, but I still think that even a high performance engine should
be able to hold a steady RPM once it is in its powerband.
Thank you in advance for your input
Michael McBeth
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