Repairing diaphragms ??

[Guest]

Just a whine per an otherwise fine drawing- the drawing doesn't show that the slides are a tight smooth fit in their bores- which is important, I think to stop air being drawn into the engine and to keep the venturi pressure away from the "atmospheric area" (though the effect of that would be small?)... anyways, I recall some CV carbs actually scribe rings into the slider. My guess is that they would do this to ensure a more airtight fit without increasing the friction.
oh no! the compressibility of air versus orifices! I was hoping it wouldn't come down to fart jokes.

I was going to point that out too, but as it shows the reference air coming in from the rear, I figured it didn't matter. But as drawn, you wouldn't get past idle.

I think the rings you mentioned were on the old SU [Skinner Union?] carbs that used to adorn British cars back when. I think a few bikes even had them, but they were pretty costly compared to a basic 'controlled leak' Amal.

The first SUs had a leather bellows, but the version I recall had just a big piston and no spring, plus an oil filled damper pot on top that served as a bearing and bounce restrictor. The can leaked, but the orifice was big enough to flow even faster, so the slide rose anyway. The jet block was adjustable up and down with a knob of the bottom, rather than having to raise or lower the needle. The biggest problem they had was a tendency to wear out at the butterfly shafts, which led to symptoms similar to bad intake boot o-rings. Someone in Design forgot to consider the return springs side loading the throttle shafts and/or that they didn't need to lift the driver's foot as well; that could be done otherwise.

Quite what the rings scribed into the piston did seemed a mystery to me as well. Maybe they just thought a piston would look silly without some sort of ring theme on it.

[Gorminrider]

just a loose end per discussion...

I think the rings you mentioned were on the old SU [Skinner Union?] carbs that used to adorn British cars back when

-No, more modern. Here it is- a ringed piston in a cv carb from a Honda. and NO diaphragm -that's the raison d'etre maybe...

non-diaphragm-ringed piston.jpg <-click me larger

[MatBirch]

Just a quick update-
Thanks for the tip on JBM Industries! I picked up a pair of new diaphragms which fit perfectly. I love their no nonsense "if you can't follow directions, don't buy from us" attitude!

I highly recommend the company and the product!

[60ratrod]

Glad they worked as well for you as they did for me! I think the hardest part for me with the ones i bought was getting the plastic rings cut off of the old boots

[Grimly]

just a loose end per discussion... -No, more modern. Here it is- a ringed piston in a cv carb from a Honda. and NO diaphragm -that's the raison d'etre maybe...

[ATTACH=CONFIG]43199[/ATTACH] <-click me larger

Possibly the Hitachi copy of the SU, made under licence. They were factory fits on the Datsun 240Z also.
The rings were on the originals too, and did indeed provide controlled rates of air friction without requiring very close metal/metal tolerances, which would bring their own costs and problems in a cooking engine. Even so, it's easy to ruin an SU bell assembly by over cleaning it on the inside - once the diy bodger takes some grit paper to the piston or bell, it's toast.

[520eek]

just a thought...I remember reading somewhere for a quick emergency repair on a diaphragm one could use those "skin" patches that are used for bicycle tubes now days. Any one ever try this or know if it even works?

[Grimly]

just a thought...I remember reading somewhere for a quick emergency repair on a diaphragm one could use those "skin" patches that are used for bicycle tubes now days. Any one ever try this or know if it even works?

It would be difficult for the diaphragm to work properly, but it might get you home.
Tha link to JBM Industries is useful - I think I might need diaphragms for the XJ at some point, and $20ea is better than 40 or 50Euros.

[jdvorchak]

I have repaired them using a patch made from a 5 mil rubber glove, like the cheap blue ones you get at harbor freight and high temp RTV. Use the RTV sparingly and cut the patch so it is about 1/4 inch larger all around than the tear. Apply the patch on the top side of the diaphragm so it never comes in contact with the carb slider or throat. Let it sit at least overnight before assembly.
Good luck.

[JJ]

I have repaired them using a patch made from a 5 mil rubber glove, like the cheap blue ones you get at harbor freight and high temp RTV. Use the RTV sparingly and cut the patch so it is about 1/4 inch larger all around than the tear. Apply the patch on the top side of the diaphragm so it never comes in contact with the carb slider or throat. Let it sit at least overnight before assembly.
Good luck.

How long did the repairs last?

Until you sold the bike? LOL

[jdvorchak]

I fixed it for a friend of mine two years ago. It's still holding.

[Guest]

I have repaired them using a patch made from a 5 mil rubber glove, like the cheap blue ones you get at harbor freight and high temp RTV. Use the RTV sparingly and cut the patch so it is about 1/4 inch larger all around than the tear. Apply the patch on the top side of the diaphragm so it never comes in contact with the carb slider or throat. Let it sit at least overnight before assembly.
Good luck.

The blue ones I see are nitrile rubber which is theoretically 'fuel proof'. Latex would be a disaster; the coating on the diaphragms is probably nitrile [??] and silicone is pretty inert, so this system looks like a viable one. There's also vinyl gloves, but my guess would be they they'd harden over time, if they didn't dissolve. Nitrile rubber is the 'right' rubber for oil and gas. They're also pretty tough and handy for working on dirty stuff and solvents, etc. Cheap by the box at the industrial supply.

[Gorminrider]

nitrile, eh? gets me thinking about that other troublesome diaphragm "needing emergency repair"- the petcock.

[TxGSrider]

You may want to us "The Right Stuff" instead of RTV as its resistant to oil and gas, where RTV doesn't seem to care for exposure to gasoline.

[Guest]

You may want to us "The Right Stuff" instead of RTV as its resistant to oil and gas, where RTV doesn't seem to care for exposure to gasoline.

In the carb diaphragm the edge of the adhesive is exposed to gasoline vapour, but otherwise sealed in. The underside is dry and vented, but everyone seems to want to repair it on the top, despite the pressure holding the patch on being on the underside. In theory, you would patch an inner tube on the inside, but?..

On a petcock diaphragm it's wet on one [piston] side; until it fails.

What is in The Right Stuff anyway? Chemically, I mean?

[Guest]

A better gasket making material from Permatex. You can use it to make valve cover gaskets etc. Meets OE specs for Honda Nissan and Mazda. Trigger can application. No clue how well it would work on a diaphragm. http://www.permatex.com/products-2/p...t-maker-detail