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 runnig 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 facetory 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. This page then is dedicated to the trials, tribulations, and final solutions to the Twin Cam carburetor vibration problem.

Tuning with high-compression ratio 10.0:1 and larger carburetors
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Fit 1 3/4 in. (44.45mm) dia. SU carburetors ....
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To prevent vibration of the carburetors it is adviseable to use a synthetic rubber gasket (Part No. AHH5791) between the carburetors and the manifold and a 1/8 in (3.18 mm.) thick double-coil spring washer under the carburetor fixing nuts, so that the carburetors may be left not quite tightened solid. Wire the nuts in pairs to prevent them becoming slack.

At 10:36 AM 5/12/05 -0500, John Barber wrote:
>"I have a set of them and initially put them on. The problem is that the spacers move the carbs to close to the inner fender for the air cleaners (new original type from Todd Clarke). The front carb air cleaner rubs on the inner fender. .... I am watching for signs of the vibration causing a problem in the float bowl and leaning out the mixture and debating whether to reinstall them and go to some other type of air cleaner."

At 02:11 PM 5/12/05 -0400, Bill Spohn4@aol.com wrote:
>"Guys - no need to reinvent the wheel - just buy something like this: www.racetep.com/avibe.html
>The bolt spacing can easily be adjusted to match diagonal bolts on the H6 carbs.
>
>If you want to get fancier and have a machine shop handy, just groove the manifold for O rings and use double wound washers (and possibly longer studs if necessary) - this may add less to the air cleaner position and minimise clearance problems.

At 07:10 PM 5/12/05 +0000, A. Counts wrote:
"Rick Botting originally produced the mounts and he may still have some or could maybe make some more. His E-mail address is: rick_botting[at]hotmail.com ...."

At 08:49 PM 5/12/05 -0400, Mark Harris wrote:
>"Warning, I abandoned the Weber flex mount double O ring set up on my 40 DCOE's several years ago because of repeated spring fractures and crimped O rings. Perhaps this will not happen when flex mount kits are adapted for SU's but I have encountered many people who loathe the double O rings for Webers. My solution was to buy Mukini (44mm I think) flex mounts from Malvern Racing, do some minor alternations and fit them to the intakes--they have worked fine for thousands of miles. Webers are extremely sensitive to vibration and mine practically float out there in twin cam heaven far away from the evil mechanical humors of our beloved engine.
>
>PS I have never checked to see whether Mukini mounts will fit the H6 and original air cleaners."

The fancy double wound washers are called [Thackery] washers. If you tighten until you leave a small gap between the coils you get the required amount of flexibility. If you tighten too much you are essentially going back to the rigid mount that causes the problem in the first place. You can be reasonably tight in terms of a non-leaking seal and still be able to grab the carbs and move them up and down a fraction of an inch. For the Thackery washers the recommended gap between coils is about .040 [inch]. Use Nyloc nuts so the setting doesn't change, of course.

You can find another nice O ringed mount here www.rdent.com/pages/carblink.html. They carry lots of other stuff including the washers and alternative rubber washers - and this interesting advice:

Installation Tip: Use the rubber cushion mounts on the top carb studs (this is where the thackery washers tend to fatigue and break), and use the thackery washers on the bottom carb studs where the inevitable fuel leakage from the carbs will cause the rubber mounts to soften and fail."