View single post by Esprit2
 Posted: 06-02-2013 09:31 pm
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Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 320
The correct soft mount gap is 0.070". That's the dimension quoted in a variety of Britcar/ Stromberg manuals dating back into the early 1970's (I've got at least five).

The Jensen-Healey illustration RM5 is the only place I've seen 0.040" quoted. Lotus, in it's own 907 manuals, specified 0.070". If you think about it, it would have been pretty easy for some non-technically minded copywriter to mis-read a hand written 0.070 as 0.040. Personally, I've always discounted the J-H RM5 as a typo.

Many of the engines of the day were relatively rough running little shakers, and not very refined. With the carbs mounted to the side on inline engines, out on the ends of rather long manifold runners, engine shake and vibration can cause the fuel in the float bowls to froth, and that plays havoc with fuel metering. The soft mount breaks the rigid coupling with the engine, and provides some vibration isolation & dampening.

The soft mount must be installed loose enough to allow the carb to move relatively independent of the engine, but tight enough that no air leaks are produced. Rather than being hard-mounted to the engine, the carb flies separately, but in close formation.

If the mounting nuts are cranked down too tightly, the carb is subjected to more engine vibration, and the fuel frothing becomes progressively worse... metal to metal contact being the worst case.

If you're using the Thackeray washers (coil springs) under the mounting nuts, they're known to fatigue over time, even fracture. But with fresh Thackerays, the gap between the coils will be about 0.050" when the soft mount gap is 0.070". As either gap becomes smaller and smaller, the fuel frothing problem increases.

I lightly smear the O-rings with Hylomar. Hylomar never hardens so the O-ring remains free to squirm as required to maintain a seal; but the tacky Hylomar provides a more air-tight seal.

Then I make a wire spacer loop of about .070", place it in the joint during assembly, near the outer perimeter where it can be easily pulled out later. Use Vice Grips or clamps to draw the joint up tightly against the spacer loop all around. Then install the Thackeray washers and tighten the Nyloc nuts until the gap in between the spring's coils is 0.050". Pull out the temporary spacer loops, and the gaps should remain pretty much as set.

It seems that a lot of modern owners pooh-pooh the need for the soft mounts and crank the nuts down tight. If that's what you believe, do what you wish and I won't argue. I'll just say it once going in, that's a mistake.