SOHC.co.uk Forums > CB750
Carb rubbers
AshimotoK0:
--- Quote from: mickharrow on February 28, 2015, 08:12:05 PM ---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.
--- End quote ---
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
royhall:
Hi All. The carb rubbers shown being softened in Ashley's witches brew pot are off my 750F2. When I sent them to him they were as hard as Bakelite and no way would the carbs go on. As I said earlier in the thread, I actually lifted the bike nearly off its stand trying to get the carbs on without success (didn't do much for my back either).
When they arrived back I gave them a good look over. They are now way softer than they were, and no visible cracks, splits, or surface bubbles nor any other form of reaction damage. They fitted easily one at a time onto the head, then with an almighty heave the carbs popped on (not forgetting to tape the front brakes on first). Not started the engine yet as I'm still waiting and waiting for Lancaster Bronze to send back my rechromed hose clips, but it all looks promising.
Anyway thanks for that Professor Ash saved me a fortune (if there's no air leaks when the engine starts of course). By the way the replacement F2/K7 carb rubbers I bought for my other F2 came from the States but were not genuine Honda just reproductions. And with the cost, P&P, import duty and the Fedex charge they cost £136 and you don't even get new clamp bands with them. Next Honda will be a K series as F2's are just too expensive to restore (if you can get the bits at all).
Will report back when the engine starts. Cheers.
royhall:
Sorry for the delay in reporting back. Had to wait 7 weeks for Lancaster Bronze to replate my carb clamps (Jeez). Anyway, there back now and the carbs went onto the softened rubbers a treat. Engine started and no air leaks. Mr Ashley you are a weird and wonderful genius. Cheers all.
AshimotoK0:
Don't try softening the K0 airbox to carb 'rubbers' . Tried to soften 4 from a '69 K0 and they came out rock hard after a full night's immersion at >70 deg C. Think they are made of Polyurethane not rubber. Good news is that the funny grommet cover thing on 750 Oil pressure switch softens OK. I remember having chemical analysis of the polymer types on K0 carb "rubbers" done in 2010 and was told the K0 airbox 'rubbers' were PU then. Seems that the softener makes them harder !! K0 carb to head 'rubbers' are nitrile rubber, as are later K7/F2 versions, so they are fine.
Ash
royhall:
Sorry Ash its all gone pear shaped.
Put the carbs on Tuesday night and started it. Sprayed some carb cleaner around the rubbers and all was well. Not touched it again until today. Tried to balance the carbs with the vacuum gauges today and they were all over the place, couldn't get it to steady out. Tried the carb cleaner again and now got leaks on 1, 3, & 4. Have even tried clear silicone on the rubbers to stop the air leaks but barely helped at all.
Any chance the rubbers have gone porous.
Bit stuck for where to go with this now. May be the final push I need to bin the PD's and convert to F1 carbs.
Anybody any bright ideas to help out as I would like to use the bike sometime this summer ;D
Cheers all.
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