…cause I am fairly sure one of mine is.
Performance is a lot less than expected, fuel economy not so great! Could this carb diaphragm make a lot of difference?
Yes.
If the diaphragm is split the needle won’t lift nearly as well as it should.
Cool, I was looking for a reason my bike seemed to be restricted (fine on normal roads, but won’t pull in 5th or 6th at motorway speeds to save it’s life!)
is your idle spped high too? that is sometimes a sign, especially if it wont adjust lower.
Nope nothing other than lacking the punch at speed. Copied from another forum:
Have had the bike to bits, no problems with airbox, oil filters or exhaust, the only thing we didn’t do was the carbs and one of them was suspect.
I take it you’re struggling with a straight four, an old Zephyr.
If so, then yes a hole or split in one (or more likely more than one) diaphragm will cause problems with both mid-range and top end. It will probably start fine, as on idle the fuel is metered with the needle at the bottom of its travel.
It is only when you twist the throttle that problems will arise: if any air is passing through holes in the diaphragm the the vacuum above (in the top of the carb) will be reduced, so the needle won’t rise up as it should, and so less vapourised fuel will enter the cylinder. The engine will therefore run lean on that cylinder, which can cause overheating and a holed piston. You’ll notice a dramatic flat spot in the middle of the rev-range, and a less obvious lack of power at the top end, but it will pull sluggishly and erratically through the flat spot in the power-band. A properly set-up Zephyr has a very smooth continuous power curve, so when the diaphragms leak it’s pretty obvious that something’s wrong.
If the rubber is split then that cylinder probably isn’t firing at all except maybe on idle. More commonly you can see tiny holes when you hold the rubber up to the light.
Don’t try to fix them (with a spot of superglue): I got away with it on an XS1100 for a year or so, but that was in desperation as I couldn’t find a service kit back then. I ran the risk of a holed piston, and it still didn’t feel just right until I replaced the diaphragms. A complete kit costs about £50, more if you have to buy it with a slide and/or needle.
listen to the man above… 
A classic zephyr, yes! 
Hoping to get a whole carb set and just swap them over (once I’ve checked the new set!) - why would I be risking a holed piston?
Sounds like it’s a classic problem, at least I know now - I was beginning to think it was restricted somehow!
Holed piston?
I’ve forgotten the whole physics, but very lean burning engines burn either very hot or very hot in places inside the combustion volume.
These hot spots can, bit by bit, sear their way through the piston crowns.
Some pistons will put up with hot spots, some won’t. It’s a pain in the wallet if you have pistons that don’t.
If you decide to replce the whole back of carbs, and that’s often a very sensible plan, make sure that the carbs come from an identical bike that you have actually seen running right. A smashed low-mileage bike bought as a favour from your mate while he’s recovering in the ICU would be ideal. He may under those circumstances offer you a good deal. Concussion can do that.
Otherwsie you may well end up a month from now with a pile of equally knackered carbs, none of which were worth the £200 you paid on eBay.
Replacing diaphragms is easy, and if the rest of the carbs are fine - not leaking etc. - why risk disappointment?
BTW, w/r the “identical” bit, you may be surprised to learn how often the carb specs on Japanese models were changed. Some even have different jets for the middle two carbs to accommodate heat/cooling discrepencies. So to be safe, if you swap a bank of carbs, you will need to find an original properly-running bike of the same model and year.
Hmm, you sound like you know what you’re talking about!
How about, I get the new (used) carbs, dismantle and check the diaphragms in both, then swap the damaged one in my current carbs for one of the new (used) ones?
Fine, so long as you are sure that the donor carbs are exactly the same as your Zephyr’s knackered ones. You may well find that one or more of the donor carbs have intact diaphragms.However, there are two immediate problems with this plan:
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you’ll be lucky to find a decent and identical bank of carbs for less that 200 quid, especially on eBay. Cheap ones will be worn out, exactly as yours are: holed diaphragms, gummed up jets, leaky floats - ie all the stuff that wears out both with use and standing around idle. Junk that will pile up in a stinking heap.
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even if you find apparently intact diaphragms, they are still old, and the rubber will still be showing its age, so they may work now, but next season you’ll be no further ahead.
So my advice would be to bite the bullet now and, assuming that there’s nothing else fundamentally wrong with the original carbs, just replace the diaphragms now and be confident that you’ve fixed the devil you know.
And it really isn’t a big job - just remove the tops of the carbs, figure out how to disconnect the slide/diaphragm, replace, reassemble, ride. No need to remove the bank of arbs at all (ie leave the air filter(s), don’t wrestle with hardened intake rubbers etc.) and there’s no need to replace jets, floats, seats etc.