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Stromberg carb manifold gap  Rating:  Rating
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 Posted: 06-01-2013 12:37 am
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DDrake
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Has anyone determined what the gap should be between the carb flange and the intake manifold ?
The WSM  indicated 0.070 in one section and 0.040 in Fig,RM5

In the past I have defaulted to the 0.040 distance for the simply reason that 0.070 looked pretty loose !

D.Drake
73 JH

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 Posted: 06-01-2013 01:05 am
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Jensen Healey
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Just snug them up until they feel about right. Then use a feeler gage to make them even from front to back.

I have run them at .040 and also completely clamped down. There was no noticeable difference in performance.

Kurt

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 Posted: 06-01-2013 11:01 am
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DDrake
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Thank Kurt
I wonder why there was concern with that gap at all?
It's not as if there w as a calculated air leak that was supposed to happen there.
DDrake

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 Posted: 06-02-2013 08:50 pm
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Frank Schwartz
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I think the gap was to allow for vibration....probably not a real problem...but that is what the "book" says  .040....I doubt it is critical...

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 Posted: 06-02-2013 09:31 pm
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Esprit2
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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.

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 Posted: 06-04-2013 04:44 am
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Jensen Healey
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Apparently the o-rings reduce heat transfer from the manifold as well as dampening the vibration from the engine. Obviously they need to be air tight.
This is not a critical tuning parameter, it's adjusting the slop in the system. 
.070 or .040  or anything close will work just fine. If you have older O-rings, make sure they seal and have an even gap.
I doubt the various "anti popping spacers" and o-rings are manufactured to any specific spec. If we knew the thickness and durometer rating of the original o-rings the gap could be set to .070 with confidence.
Use common sense and snug them up 'till they feel right.
 
 

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 Posted: 06-05-2013 02:00 am
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DDrake
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What are the symptoms caused by frothy fuel?
I would suspect it to manifest by fuel starvation ,which I do experience in hard left hand turns .is there any other ways I might notice it ?

I will try your setup technique this weekend an se if it makes a difference
Thanks
DDrake
73JH

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 Posted: 06-05-2013 04:51 pm
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Esprit2
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Jensen Healey wrote:
Apparently the o-rings reduce heat transfer from the manifold as well as dampening the vibration from the engine. Obviously they need to be air tight.The O-ring's primary function is vibration damping, and any thermal barrier benefit is incidental. Of course, it's important to use the right O-ring. But given that, the tighter it's screwed down (the more narrow the gap), the less effective the damping is. If it's too loose, there can be air leak issues, and too tight they're no longer effective as dampers. The goal should be looser rather than tighter, and the specified 0.070" gap was the loose-optimum without tempting air leaks.

Jensen Healey wrote:
This is not a critical tuning parameter, it's adjusting the slop in the system. 
.070 or .040  or anything close will work just fine. If you have older O-rings, make sure they seal and have an even gap.
I doubt the various "anti popping spacers" and o-rings are manufactured to any specific spec. If we knew the thickness and durometer rating of the original o-rings the gap could be set to .070 with confidence.
Use common sense and snug them up 'till they feel right.
There's more to it than you give it credit for.

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 Posted: 06-05-2013 06:13 pm
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Jensen Healey
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Tim, I appreciate your attention to detail and agree that care must be taken when tightening the mounts. There is a fine line between vacuum leaks and foaming fuel.

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