Mike---An interesting set of circumstances surrounding your lurch/clunk. Not
all that unusual, tho. I think the clunk is the result of accumulated
tolerances in the drive train, like the kind you'll see when rotating an
off-the-ground tire forward and backward.
The valve overlap with the S-2, along with the lightened flywheel could amplify
this surging in the engine. I had a time with this myself not long ago, but it
went away before I could truly identify the source. Mine occured when running
off throttle, when the rpm would slow and hit the 1500 mark and down to 1000. A
slight increase in throttle, would stop this.
Some SU carbs have a 1/8" hole in the throttle plate. It's about 1/4 inch up
from the bottom center. Not entirely sure as to why this hole is there, but
suspect it helps to hold the air valve up a little, when the throttle is
otherwise closed. Some SU's have an idle circuit, so fuel gets around the
throttle plate even when closed. Don't know how adding a hole to the ZS would
act. You could try it as sort of a last resort, and braze it up if you don't
like it.
I would check to make sure your distributor weights returned to rest. If they
stay flung out, the timing could stay too far advanced, at least momentarily.
Squirt some WD-40 down there, and wiggle the rotor back and forth.
Also check any changes made to the float levels or mixture, prior to when all
this bucking started. I'd be interested in what you find, should this return
to my car!
Dick
-----Original Message-----
From: michael lunsford
Sent: Sunday, September 20, 2009 2:28 PM
To: 6pack@autox.team.net
Subject: [6pack] Lurching/cluinking
I know this is a strange sounding thread but.....
I have been trying to run
down an annoying tendency my car has to lurch when I get off the throttle or
under light constant throttle. I thought the lurching was related to the
clunk but I have been unable to isolate the problem. I've checked all the U
joints, replaced the differential with an LSD diff from a Q 45 and the
lurch/clunk is still there. I have checked the trans mount bolts and the axle
bolts on the trailing arm and they look fine.
I am beginning to suspect
that the problem may be related to the S-2 cam and lightened flywheel I
installed during my engine rebuild. My son said lurching of this sort was a
problem on Mustangs with high lift cams and that the fix was to drill a small
hole in the butterfly of the carb. Any thoughts on the butterfly hole idea or
any other suggestions as to the source of the problem?
Mike Lunsford, 1970 TR
6
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