[Healeys] Austin Healey 100 Misfire Diagnosis.

m.g.sharp at sympatico.ca m.g.sharp at sympatico.ca
Wed Sep 28 18:48:20 MDT 2022

Mike,  my first thought was coil, as I have heard they can fail gradually.  For the small amount of work it is, I’d swap it out and see if it makes a difference if for no other reason than to eliminate that possibility.


Best, Mirek


From: Healeys <healeys-bounces at autox.team.net> On Behalf Of Michael Salter
Sent: September 28, 2022 6:25 PM
To: healeys at autox.team.net
Subject: [Healeys] Austin Healey 100 Misfire Diagnosis.


This problem has plagued me for over 1000 miles.

Today I had to drive some 200 miles and this is how it went:

Starting from cold I experienced a persistent and irregular misfire. 

Pulling the choke out made no perceivable difference.

Power was not much affected and the car cruised along well in top overdrive with just regular periods of misfiring.

Climbing slopes really made no difference, even when full throttle was applied for 10 - 15 seconds.

Misfire continued for some 90 miles with no change when the engine was fully heated up.

I could really do nothing to influence the rate of the misfire although it did seem to be slightly less evident at low throttle settings.

After 90 miles I stopped briefly to get fuel and upon starting off it was quite evident that the misfire had disappeared almost completely and remained that way for at least 80 miles after which it started to misfire just a little as I came to the end of the trip.


1.      Plugs … not like a bad plug. The misfire seems to be more than 1 cylinder and never came close to being the regular beat that 1 plug misfiring produces. All the plugs have a healthy brown/ grey appearance.

2.      Coil …  In my experience coils are very reliable but when they do fail they tend to fail as they warm up and the problem gets worse as the coil warms up

3.      Points … a possibility but the dwell is set at exactly 60 degrees and does not waver at all when the engine is running in the shop.

4.      Fuel pump … if the engine was starving for fuel because of a fuel supply issue caused by a blockage or failing pump the problem would get worse when the demand for fuel increased, as in climbing a hill of full throttle. This was absolutely not the case.


Any ideas folks?


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