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Recently, an air leak introduced itself in my Triumph TR7. After some playing with soap and propane, I found out the leak was from the rubber carburetor (SU HS6) mounting plate. As with many things on this particular English car, it was incomprehensible that they couldn't think of a better design (i.e., one less prone to failure). This is the failing part, it consists of a metal plate with a rubber gasket vulcanized to it. part
The bolt ends are also vulcanised to the rubber, but they're not fixed to the metal plate. You can wiggle them. Consequently, the carbs are mounted to the manifold only with those vulcanised connections to the rubber. They're doomed to come loose because of the vibrations of the engine, which is exactly what is happening.

I could buy a new set but i don't think they'll last that long, especially the lower quality aftermarket ones. So, I was thinking of prying the rubber loose, and welding those bolt ends to the mounting plate. Then, the rubber gasket would be replaced with a nylon one. But I'm worrying about defeating the purpose of the original rubber gaskets. They're meant to protect the carbs from vibrations. Welding those bolt ends to the mounting plate and using harder nylon gaskets may let through more vibrations to the carbs. On the other side, the MGA for instance, with the older H4 SU's do use bolts directly mounted to the manifold, and harder rubbers.

I hope some of you have experience with this kind of problem. Do you think I can remake this carb mounting as explained, or will that damage the carbs over time?

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Some thoughts on this:

  • Are those the original mounts? If so, they are 30 to 40 years old, unless the TR7 isn't the car I think it is. Obviously they could be the 2nd or 3rd set, but, on the chance that they are original maybe we shouldn't be too mean to their designer.

  • The original design may not be as bad as you suggest – it could be that the bolts aren't studs embedded in the rubber. They could be something like elevator bolts (below) with a "head" that provides both for a large sealing area and that helps to resist being pulled through the rubber.

stainless steel elevator bolt

  • Since the TR7 is, relatively speaking, a much newer car than the MG A. It seems very likely to me that the move to the "softer" mount was motivated by a desire to improve the isolation of the carb from the engine vibrations and/or heat. This seems particularly likely since I would expect the HS6 (TR7) design to more expensive to produce than the H4 (MG A) design.

All of this leads me to think that it wouldn't such a bad idea to go with replacements if you can find them, or if you can't to try to reproduce something like the original that provides some isolation for the carbs from the engine heat and/or vibration. Perhaps with the improvement of embedding some kind of head or mounting plate for the studs in the gasket material.

Even if you did that we'd still have the English electrical system to make jokes about…

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  • Yes, but the actual mounts are dried out and cracked. Judging from the softness of the rubber on the inside, i think it's been replaced once. It's highly probable that the bolt ends are something like elevator bolts, or else they would've been ripped out of the rubber within hours. But the vulcanised connection to the metal plate keeps being the 'weakest link in the chain'. On top of the poor design, i think the rubber will degrade much quicker with modern alcohol/additive containing fuels, hence my idea to 'redesign' those mountings. - continued
    – Bart
    Commented Dec 21, 2016 at 18:31
  • Wonder if you could do something similar to the originals using the mounting holes on the bottom plate to grab a top plate that holds the gasket portion which holds the mounting bolts for the carbs? I'm just inclined to think that there is probably a reason for the contortions of that design.
    – dlu
    Commented Dec 21, 2016 at 18:35
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    Like you said, the MGA's way of isolating the carbs will probably offer less protection. I can't really think of another way to create a stronger connection, without letting through too much vibration. To simply replace them will only postpone the problem for a year tops, i fear. This guy even sells solid aluminium 'gaskets': classicregulator.com/carb-mounts/solid-carburetor-mount-kit If that doesn't shake the carbs to pieces, i'll eat my shoes. And about the english electrical system, if there's one way to learn how you shouldn't design a wire loom, look at an old english car...
    – Bart
    Commented Dec 21, 2016 at 18:41
  • Yes i'm too, but if you google su hs6, there's much other cars that just bolt it to the manifold with metal to metal contact, or even without any vibration protection. i'm thinking of something that still doesn't have metal to metal contact, but which is not as weak as rubber vulcanised to metal. A kind of bracket that keeps the rubber to the metal plate, like you proposed, would be a solution. Then the metal plate has the rubber embedded in it, so that the rubber is crammed inside the metal plate. But it's kinda hard to make, since there's not much room. The carbs cover the entire rubber.
    – Bart
    Commented Dec 22, 2016 at 7:59

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