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Re: MGA Fast Idle

To: "Mike Razor" <>, "MG LIST" <mgs@Autox.Team.Net>
Subject: Re: MGA Fast Idle
From: Barney Gaylord <>
Date: Mon, 18 Oct 2004 18:41:03 -0500
At 04:16 PM 10/18/04 -0400, Mike Razor wrote:
>What would cause my MGA to stick at around 1200-1500 RPMs.  I can bump the
>gas pedal and sometimes it will ease back down, sometimes it won't. ....

One (or both) throttle plates are sticking before going completely 
closed.  Ah, such a common problem.  I think I need to post this one on my 
web site.

If you're real lucky, your throttle return springs may be "soft", and might 
be adjusted to fix the problem.  Each carburetor should have a torsion 
spring on the shaft next to the throttle body.  There should also be a 
third spring on a bracket at the end of the throttle cable to aid in cable 
return when the pedal is released.  This should make the throttle snap 
closed smartly when released.  If the springs on the throttle shafts seem 
weak, they can be snugged up by loosening the spring retainer on the shaft 
outboard of the spring and winding it up to properly pre-tension the 
torsion spring.

If that doesn't cure the problem, then this is commonly caused by a worn 
throttle shaft allowing the butterfly plate to be misaligned in the 
throat.  This is common with the MGA, having brass shafts, which are 
usually well worn by 100,000 miles of use.  The only reasonable cure for 
that is to replace the throttle shafts.

The bores in the throttle body do not wear nearly as much as the brass 
shafts.  The first time you may get away with only replacing the standard 
size shaft.  The second time around you might drill the throttle body 
0.010" oversize, and install an oversize shaft.  The third time you may 
replace the oversize shaft.  This is my current situation, with my MGA now 
having accumulated over 340,000 miles, and about due for new shafts 
again.  The fourth time around it may be time to have the throttle body 
bored out and bushed, and return to a standard size shaft.  If all goes 
well you may not need to do that until close to half a million miles.  If 
you install powder metal bushings they may never again wear enough to need 
replacement (but the brass shafts do still wear).

Occasionally this problem may be caused by loose screws securing the 
throttle plate to the shaft, also allowing the throttle plate to be 
misaligned in the throat.  Here's hoping you didn't already lose a loose 
screw down the throat, which can cause catastrophic internal engine 
damage.  Cure for the loose screw syndrome (before anything breaks) is to 
back off the idle screw to allow the butterfly to close completely, realign 
the throttle plate to seat all the way around the throat (zero air gap), 
and tighten the screws.  Be sure to expand the original type split end 
screws to prevent them from dropping out, or use a drop of thread lock 
adhesive on each screw.

If you're on a crisis tight budget and need a temporary Grapes of Wrath 
repair for a worn throttle shaft, you could intentionally loosen the 
throttle plate retaining screws to realign the plate (as previously 
described), and then tighten and reset the screws so they won't work 
loose.  This can allow the engine to idle again, but be aware that air 
leakage around the throttle shaft would give it a lean fuel mixture.  When 
you readjust it slightly richer to have the correct mixture at idle, it 
will then run a bit rich at all higher speeds (which can carbon up things 
inside the engine).  And you should still plan on replacing the worn shafts 
at earliest convenience.

Barney Gaylord
1958 MGA with an attitude

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