The MGA With An Attitude

Apparently the single most important lesson to learn about the MGA Twin Cam engine was not figured out by the factory until after this model had ceased production. The engine had been plagued by burned pistons, quite mysterious at the time. The factory first retarded ignition timing, then lowered the compression ratio (both reducing power output). They finally gave up on production when the engine could not be made to run reliably.

Sometime later avid driving enthusiasts were to discover the cause of the problem to be a resonant vibration of the SU carburetors at certain select engine speeds. Such vibration would cause foaming of the fuel in the float chamber, which in turn causes a lean running condition, which in turn leads to low power output and burned pistons. The problem may be overtly obvious if the car is run on a rolling road with power monitored as the engine speed increases, and there may be a sudden unexpected dip in the power output right in middle of the sweet spot on the power curve. Using some modern technology, an O2 sensor in the exhaust may also disclose the lean running condition.

The solution then is to install vibration isolating flex mounts between the intake manifold and the carburetors. Once this was resolved, Twin Cam enthusiasts have enjoyed may years (even decades) of high speed motoring enjoyment with a generally reliable engine. If this problem could have been known by the factory it might have been fixed during production, and the MGA Twin Cam model may have survived for a much longer production life. The Twin Cam engine might also have followed on to subsequent MG models. Can you say MGB Twin Cam without a flutter? There was at one time a Twin Cam engine MGB prototype. These pages then are dedicated to the trials, tribulations, and final solutions to the Twin Cam carburetor vibration problem.

Addendum: Here is another version of the root cause.

At 06:59 PM 8/18/2009 -0400, Frank Graham wrote:
"In 1972 or so I had the chance to meet both Cecil Cousins and Alec Hounslow. Cousins was the very first employee of the MG Car Company and later became the first head of the MG Competition Department. Hounslow worked in comps in the 30's and was Tazio Nuvolari's riding mechanic when he won the Irish TT in a K3. Hounslow was still around in the late 50's working on record breakers and Twin Cam development. In the early 70's I was one of a very few people interested in Twin Cams. The first I acquired had a hole in the top of #4 piston. The second engine had a hole burned in the top of #3.

Over a few whiskeys Alec explained that when reports began to surface about numerous engine failures on Twin Cams it was assumed that owners were either over revving them, running on too low an octane fuel for the 9.9:1 compression or had their ignition timing too advanced. However none of the fixes such as centrifugal advance units, low compression pistons etc. seemed to work. Alec was told to investigate this by John Thornley and find a fix. So they ran engines on the test bed advancing the timing way beyond spec, revving them until the bottom ends blew out but they were not experiencing holed pistons. Dick Jacobs had never holed pistons on his racing twin cams either but they were running Webers.

What they did note was that the balance of the production engines was well below the levels of precision that engineering had recommended and that the engine had two natural vibration periods at about 3200 and 5600 rpm. With stock gearing the 3200 rpm period coincided with typical highway cruising speeds. So they ran engines right at the rpm where they vibrated and found that the float on the rear carb would hang on its spindle and not drop which caused the rear carb to go full lean and hole either #3 or #4. Virtually every twin cam I have ever heard of with a holed piston had it on either #3 or #4. So flex mounting the carbs along with better balance was the solution but it came too late to save twin cam production".

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