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Re: high idle

To: "MonteMorris" <mmorris@nemr.net>, "MG list" <mgs@autox.team.net>
Subject: Re: high idle
From: Barney Gaylord <barneymg@MGAguru.com>
Date: Mon, 21 Apr 2003 00:40:53 -0500
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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