Having read this post I was dreading trying to put on the carb rubbers today on my rebuild. The rubbers were supplied by David silver and went on with a hard push and wiggle so a very happy chappy now.
You won't have any problem with new ones or possibly a bike unused for yonks with low miles. It's the one's like my 750K0 that did 40k miles with all of that heat and fuel exposure that harden up ( cause of many a broken alloy carb. link plate
). Evidently, Vic World the sandcast resto guru in California checks the hardness of the airbox-to-carb 'rubbers' on any sandcast/K0 bike he looks at to buy, in order to gauge if the mileage is authentic.
One thing I am experimenting with as we speak in the CB250k ( and 400/4 as I remember) softening, as they have rubber to metal bonds which don't want to be weakened by the softening process. I have painted the interface of metal/rubber with industrial epoxy so that it stops ingress of the softener into the bonding, during the long softening process.
There are a lot of doubters on the other bike forums about my method but all I can say is I softened a rock hard '69K0 inlet rubber 5 years ago and it is still soft but never run yet on the bike. I have a video of before and after will have to put on Youtube. I am also trying to soften the little cover on the CB750 that covers the oil pressure switch. On other forums they bang on about using essential oils and glycerine, which may work too for all I know. However, I tackled the problem scientifically and asked someone who is a qualified industrial chemist and worked in the rubber processing industry for 20+ years and now operates from a university giving advice to industrialists, which he has done for about 18 years ( one of his main areas of work is looking at the crabby materials used in the caravan industry.. anyone who owns a caravan will know the havoc ingress of water into the structure has). We tried several methods whilst I was at his lab doing some other work in 2010 and we came up with the Refos stuff as the best method of softening nitrile rubber.
Having said all of this since 2010 you can now get brand new pattern rubber parts for some bikes at reasonable cost but on the F2/K7 750's you are still talking a lot of money for Honda replacements. Plus some parts like the oil pressure switch on the 750 are rare or unobtanium.
I wouldn't try the softener out on cam chain/primary chain rollers or other internal parts though ( e.g. primary drive cush rubbers).
I have also got some other stuff I want to try for treating the finish on other rubber components ( grommets, fork ear isolators etc) which are still soft but have had ozone exposure and develop a hard surface layer. I have some stuff I resurrected a colour laser printer with by removing the surface layer on it's paper feed rollers with 'rubber roller rejuvenator', so this is what I will try out.
Ash
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, to please daughter) and all is fine now. Cheers for all of your advice/comments. These electrical random faults are bu**ers to find sometimes.
