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Re: High Idle & Plugs

To: "Neil Cotty" <>
Subject: Re: High Idle & Plugs
From: Barney Gaylord <>
Date: Mon, 03 May 1999 11:52:53
At 03:40 PM 5/3/99 +1000, Neil Cotty wrote:
>>.... I have my MGA running nicely now, .... (Sorry Barney but she won't
take 20 degrees advance - not sure why..!).

Mine runs regularly an cheap gas with 20 advance at idle, no pinging, and
doesn't even run on at shut down.  Then again it's usually a little low on
compression, so not much use for high test gas.  Yesterday it seemed just a
little off it's normal pizzaz above 5000 rpm, so for the duration of the
last half of an autocross I pushed the timing up some more, a few degrees
at the dizzy being maybe another 6-8 degrees at the crankshaft.  That made
it a little hard to crank over the first piston to start, but it idled okay
and ran great otherewise.  And more pizzaz for sure, picked up another 1st
place trophy for the effort.  I did set it back to its normal 20 degrees
advance before the trip home.  I guess it may be time to push the top end
advance limit on the Mallory up a a little higher.

>.... I can't seem to get the idle down to sub 1000rpm before the idle
adjusters bottom out. ....

Too much air getting in there somewhere.  With the idle screws bottomed out
it should flat out die, actually closing the carbs completely off.  You
might have an air leak somewhere between the carbs and the head, but then
when you shut the carbs down low it would go very lean and probably idle
rough.  If you then adjust the carbs to idle right you end up with it
running rich at road speed.

Most likely the carb butterflies are not closing all the way.  First check
that the fast idle screw is also backed out so it isn't sitting on the cam
while you're trying to get the idle down.  Then loosen one clamp on the
throttle shaft between the carbs so you can adjust each carb separately.
Maybe one carb is closing completely while the other is still being held
open because the shafts are out of synch.

Next best bet is that the throttle shafts are worn so the butterfly valve
may be hitting the side of the throat and stopping before it closes all the
way.  Wiggle the shafts up and down to check the wobble.  They should move
very little, hardly any perceptible amount at all.  If you get anything
like 0.010" motion there you need to renew the throttle shafts, and maybe
consider drilling or rebushing the throttle bodies while you're at it.

Good luck,

Barney Gaylord
1958 MGA with an attitude

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