Can somebody help me analyse a little problem I’m having dialing in
my new SU carbs? We’ve got them about 98% there. She runs just
great, strong and smooth, up to 2,800 rpm at which point she burps
and stumbles and then smooths out again. It presents this problem
shifting from third into fourth more so than second to third.
I’m sure that under hard acceleration it would do this in high gear
too - but because the engine is very fresh with only 25 or so miles
on a total rebuild I’m not ready to take her much past 3,000 rpm.
It’s a 4.2 motor with brand new SU carb conversion. Also new plugs
have been installed. It’s running with a new Pertronix electronic
distributor. We’ve already dropped higher flow needles in and that
helped a lot but there’s just a bit more fine tuning to go. Maybe
an even higher flow needle? Anyone else have this problem solved?
Thanks
Randy–
Randall Harris
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In reply to a message from Randall Harris sent Sat 27 Mar 2004:
Randy,
If you haven’t modified the engine (high compression, large
valves, high-po cams) you should not need to deviate from the
stock UM needles. Even with modified valves/cams/etc., you should
not need to go beyond UO needles. So, I would suspect your problem
lies elsewhere, and would recommend going back to the UM needles,
and trying to sort out the real problem.
Since the problem is transient, rather than persistent, that
suggests one of two likely sources: sticky/incorrect timing advance
or sticky carb pistons.
First, disconnect and plug the vaccum advance line for now, to
eliminate that as a potential problem source, as it will make no
difference in performance. Double-check the ignition timing. Are
you certain your coil, wires, distributor cap and rotor are in good
condition? You’re not using the little black Champion bow-tie
spark plug caps, are you? This could be caused by arcing on the
asprk plug wires.
What kind of distributor are you using? Have you verified the
mechanical advance is working properly, providing proper advance,
and moving smoothly? At 3000 RPM you should have at least 30-35
degrees total advance, and as RPMs rise from idle, advance should
be monotonically, and smoothly, increasing. You can use a timing
light with advance wheel to verify the advance curve.
Do all three carb pistons move all the way up with no
binding? When you ‘‘blip’’ the throttle at idle, do they all rise in
unison? Try a run with the damper pistons removed, and see if
there’s any difference in response.
How are you setting mixture?
Also, are you absolutely certain your cam timing is correct?–
Ray Livingston - '64 OTS Santa Cruz, CA
Santa Cruz, CA, United States
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