At 06:34 PM 4/20/03 -0500, MonteMorris wrote:
>I've just gotten my newly rebuilt HS4's synchronized and tuned but I can't
>get the idle down below about 2500 RPM. The butterfly levers seem to be
>fully closed and I've backed off the idle adjustment screws until they
>aren't even touching anything. The same with the fast idle screws-they are
>not touching the cam.
The fact that the engine is running at all is proof positive that at least
one of the butterfly valves is not closing completely. If the butterflys
were fully closed, a vacuun leak would introduce only air with no fuel, and
it wouldn't run. With no vacuum leak and the butterflies fully closed, the
engine will not idle, as the air passage through the butterfies is for
practical purposes zero. Either one (or both) of the butterfly plates is
not properly aligned, or some lever or clamp on the throttle shaft (or
binding bushings) is preventing one of the carbs from closing all the way.
The throttle plate is oval (eliptical). When fully closed it sits at a
small angle to the throat of the carburetor, and appears to be a perfect
circle when viewed in line with the axis of the carburetor throat. The
edges of the plate are beveled to be parallel to the inside surface of the
throat, so it makes a near perfect seal when fully closed. If you install
the plate backwards the bevel on the edge will be angled the wrong way, not
parallel to the throat wall, and the butterfly will not close all the way,
leaving a large gap and allowing for lots of air flow.
The next problem is centering the plate. During assembly the plate
retaining screws should be left loose so the plate will float in the
slotted shaft. With the idle screws backed off, and the arms touching
nothing, push the throttle plate fully closed with your fingers and nudge
it around a little as required to get it to seat against the throat all the
way arond for a complete seal with no visible air gap. Then tighten the
plate retaining screws while holding it in that position. Once secured,
peen the screws so they cannot come undone, or otherwise use thread locking
adhesive to secure them.
If new shaft bushings were installed in the throttle body during rebuild,
it's possible that the shaft may be binding in a bushing, or that the
throttle plate may be binding against the inner end of the bushing if it
protrudes into the throat. You can hone the journals with a piece of drill
rod and lapping compound (or stem of an old intake or exhaust valve if
still standard bore size). For a bushing intruding into the throat, a high
speed hand grinder can remove the excess material (when the shaft is
removed). But do be careful not to grind away any part of the throat of
the throttle body in that area as that would introduce a leak point that
would prevent complete clusure of the throttle plate.
Assuming new throttle shafts were installed as part of the rebuild, you
should check to be sure none of the idle control arms are touching any part
of the throttle body when the butterflies are fully closed. It may be
possible that one of the cross pin holes was drill in the wrong orientation
so the arm hits the mechanical stop on the body before the butterfly is
fully closed. Unfortunately the cure for that problem is usually
replacement of the throttle shaft (again) with another new part so the
hole(s) can be drilled in the proper orientation. If the fast idle control
arm is hitting on the cam plate, it may be possible to reposition the fast
idle cam to allow some clearance for the arm.
Another possible problem may be that a throttle shaft clamp on the
intermediate shaft between the twin carburetors is secured in an improper
position such that on of the butterflies is held partly open when the other
is fully closed. That could be cured with standard set-up adjustment of
the linkage.
With carbs in good mechanical condition, when the butterflies are fully
closed the engine should be as dead as if you had placed rubber stoppers in
the carburetor inlets. With misadjusted, misaligned or badly worn
carburetors, all bets are off.
Barney Gaylord
1958 MGA with an attitude
http://MGAguru.com
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