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Re: Advice on MGA 3rd gear noise mystery

To: "SCC" <>, <>
Subject: Re: Advice on MGA 3rd gear noise mystery
From: Barney Gaylord <>
Date: Tue, 05 Jun 2001 01:33:22 -0500
At 11:26 AM 6/4/2001 -0700, SCC wrote:
>* Frequency of sound is proportional to engine speed while under throttle.
 Throttle off, sound diminishes greatly.
>* No trace of sound in any other gear (including reverse)
>* Sounds like fanblade (metal) contacting rubber or plastic, but it isn't
(fanblade is clear)
>* Could also be described as a vibration or warble as if a shaft is
>* Similar to a high frequency version of thumping one would hear with warn
suspension bushings, for example.
>* Sound is difficult to locate, but is definitely not from the rear of the
car.  Driver or forward.

Rapid ticking noise in 1st 2nd or 3rd gear could be a broken gear tooth.  A
little gear whine in 1st gear is normal for the MGA gearbox, as it has
straight gear teeth for 1st gear.  Gear whine in 2nd and/or 3rd gears is
usually caused by a worn lay shaft, which causes the gear teeth to be
misaligned a bit, putting the gear tooth contact point too far out towards
the end of the teeth.  For heavy load conditions this can also lead to
broken gear teeth, as it overstresses the teeth.  It is normal to be very
quiet in 4th gear, as this is a direct drive coupling of the input shaft to
the output shaft with no load going through the gear teeth.

>.... someone .... said it could just be a worn universal joint.  I would
have thought that would affect more than just 3rd gear. ....

Worn u-joint usually makes noise/vibration relative to road speed, usually
above 30 MPH (unless it's really bad), generally not affected by which gear
it's in.

>The only other issue .... is an occasional backfire when cruising at 60+

Check the valve lash (rocker arm clearance).  If you have a slight case of
valve recession the valve clearance can disappear, which will cause "funny"
things when the engine is hot and under load, usually a mild backfire
through the exhaust, but possibly through the intake.

Also check the integrity of the electrical connections on the primary
ignition circuit.  A loose wire connector can cause intermittent ignition,
which in turn can cause a backfire.  These connections include both of the
small wires on the coil, the wire on the side of the distributor, and
grounding of the distributor on the engine block.

There is also a small multi-stranded wire inside the dizzy, underneath the
moveable contact breaker plate, that grounds the moving plate to the body
of the dizzy.  If this wire is broken inside of the insulation it can cause
an intermitent ground, which can cause the ignition to cut in and out,
especially in reaction to the operation of the vacuum advance unit.

>and a tendency to stall occasionally when decelerating from cruising speed
under engine braking until she's really warmed up.

Cold carbs run lean.  A warmer thermostat can help.  Or maybe the fuel
mixture is a little too lean.  Otherwise a litle manual choking is in order
until the engine is up to running temperature.

Barney Gaylord
1958 MGA with an attitude

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