SOHC.co.uk Forums > CB750

Carb rubbers

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royhall:
Well I never, so he does. When I rang him just before Christmas they said they didn't stock them. The little tinkers. ;D ;D

AshimotoK0:
Got this back from my mate who analyzed a sliver of the rubber from Roy's Aftermarket F2 carb 'rubbers' sourced in the US. I have asked him if he can explain now why the aftermarket parts leaked in service (possibly inferior compression set properties). The CB750K0 OEM inlet 'rubber' he tested for me in 2010 was also found to be  nitrile rubber.

Roy has the rubbers fitted to his bike now that I softener for him. We have both come to the conclusion that you need to soften in the fluid and THEN  post cure them in an oven afterwards.

"Ashley, your small  black rubber sample is a good quality NBR containing a process oil, nitrile rubber and carbon black with  no extending inorganic fillers. 

I don’t have micro IRHD (micro-hardness meter) but looking at its overall composition, in particular that it has about 150pphR of carbon black, it will be around 80 IRHD. It still has about 40 pphR of plasticizer in it so wouldn’t have thought it had lost much.

So it is a good quality hard NBR   (Nitrile) "

royhall:
Ash. Is Nitrile rubber good for these parts. Aren't they supposed to be Viton? The ones you softened for me are working pretty good, but cant seam to get rid of the last tiny leak on No.2. Cheers.

AshimotoK0:

--- Quote from: royhall on May 22, 2015, 02:33:29 PM ---Ash. Is Nitrile rubber good for these parts. Aren't they supposed to be Viton? The ones you softened for me are working pretty good, but cant seam to get rid of the last tiny leak on No.2. Cheers.

--- End quote ---

Got this back from the guru  Roy. I don't think Viton was ever used...too expensive for such a large part:

"It depends how hot, this bit was NBR, the original may have been HNBR which has higher temperature capability. Also with standard NBR for hot oil a high acrylonitrile polymer is likely to have been chosen, however on balance if this part failed and the OEM was OK then I would guess the OEM was HNBR as oppose the standard NBR"

I replied  ....It would probably see about 150 degrees at the clamping point interface  on its outlet side  and has fuel/air  vapour passing though it , no significant contact with oil..

REPLY:-
150 is far too hot service temp for NBR

IS MY ESTIMATE OF 150 deg C realistic ? ? ? ? ?

royhall:
What's the outcome then Ash? Are these expensive new rubbers any good. Sounds like they may be made from the wrong rubber. If they are this NBR type, what's the likely outcome from prolonged use. Cheers Roy.

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