......Thanks. That's what I thought, but wasn't sure if some sort of adhesive was implied.

Okey dokey, that is good news because it implies that as the bike get used the heat generated over time will in fact make the carb rubber to reed valve box seal better and better.

Here's the thing. . .

These bikes are 40-50 years old.  NOS gaskets may have been sitting on the shelf for as long.  Things tend to warp, degrade and change over many years.  Furthermore, Yamaha has been known to change materials and specs over time.  Just saying that some sort of adhesive *may* be necessary to get a good seal, even with an OEM gasket.  Yamabond 5 is a good one.  Personally I use Hylomar Blue on *difficult* intake gaskets.  If you end up using a sealant do so sparingly.  If it's noticeably oozing out after tightening you've used waaay to much (IMHO).  

Good luck, hopefully the new gasket will work fine.

They are talking about the boot "welded" to an aluminum plate...  basically the rubber is formed over the plate.  They are NOT talking about the boot plate being "welded" to the reed cage.  They are separate parts and always have been.

Ahhh yeah.  Re-reading the post showing the pic of the Yama manual I get it now.  Thanks Mark.

Yep, the Penny's dropped for me too. Thanks for the explanation Mark. 

Well the NOS carb gasket arrived and although from the on-line pictures it looked thicker than my existing gasket there wasn't a lot in it.  Checked thickness and both were a shade under 0.7mm.  Anyway I fitted it in place and repeated the leak down pressure test from 5psi(g) and no appreciable change in test result.  It still leaks from the gasket joint (5 to 2.5psi(g) over 10mins).  I didn't really want to go down the next route because it is messy and hassle if I need to take the reed cage/carb boot unit off the bike again, but I have smeared silicone sealant over both sides of the gasket (used sparingly as RT360Fan advised) and sandwiched it all back up with the bolts lightly done up.  Therefore now another 24Hr curing period, so that's all for the moment.  Andy. 

I'm still going to warn you that I'm 99% sure that sealer will fail eventually.  Silicone sealants like Pro Seal are at best "oil resistant" and when exposed to modern gasoline will soften and disintegrate with use over time. 

Bad news.......then finally good !!  .   Apologies for the extended delay in submitting this post.  The delay was caused by my decision to heed the advice I'd received regarding the silicone sealant I'd used on the gasket between the reed cage and cylinder (jug) inlet metal face.  The bad news then is the additional work it gave me. I discarded the gasket with silicone sealant on it, cleaned all the faces free of silicone and instead lightly smeared my original gasket with Hylomar Blue which is definitely compatible with gasoline (it says) and tightened everything back up to 8Nm. The good news is that using my brake bleeder as a pressure pump (see pic 1) I pumped everything up to the test pressure of 5psi(g) see pic 2 and it passed easily, maintaining pressure for over 15mins after which I ‘relaxed’ it back down to 0 psi(g). More good news was that I flipped the brake bleeder pump over to suction (pic 3) and vacuumed the crank plenum down to -5 psi(g) as shown in pic 4 and again it held for well over 15mins till I relaxed it back to 0 psi(g).  That concludes this leak down test and I can now continue getting the rest of the build on track.  Thanks for all the help forum members have offered and it's been a great learning curve for me as a leak down test 'newbie'.  Andy.

  

Congratulations!  Nice job! 

(And yes, Hylomar blue should work great)