SOHC.co.uk Forums > CB750
Carb rubbers
royhall:
Thanks for all your interest guys, was beginning to get demotivated with this.
Am off work today so going to take the carbs off yet again and have a really close (magnifying glass) look at this. Am wondering if the rubbers were just too hard before they were softened up. Have got a used set on the way from DK for £26 so I have a comparison. I did notice before fitting the rubbers that the engine side is about 50% thicker than the carb side, and was a fair bit harder. Maybe due to the extra thickness they haven't softened as much on that side. Will know better this afternoon after I pull them again. Will report back.
On another note. My other F2 is fitted with replica rubbers from the USA that are also leaking now. Looking online a great many people are having problems with these, most with air leaks they cannot get rid of. They look okay, but they are very soft and have a shiny finish. Don't think they are made from the correct material. There are some reports that at about a thousand miles these rubbers go rock hard, and one guy reports having a carb actually detach (snapped off rubber) whilst riding. Got these from Vintage CB750, so if anyones tempted to order these DONT. £136 wasted me thinks.
Further thrilling instalments to come. Watch this space. ;D
AshimotoK0:
Know what you mean Roy about getting despondent but we need to get all of our facts. I can easily get the polymer type of anything done at Hull Uni. All I would need is a sliver of the polymer from the aftermarket ones, from a non critical area and he could tell us if the material is fit for purpose or not. Then we can join forces with the USA guys and complain as a united force. If DS cannot tell me that the pattern ones he is selling are correct with data from the supplier to back it up , I am going to suggest he donates one for me to test out. He should be agreeable as he has a degree in Chemistry, I think, so he should appreciate our concerns.
I have another friend who runs a rubber moulding company in Hull (and a keen biker) who could mould parts in the correct material (i.e. nitrile Rubber) but, as always, it's the tooling costs that kills it. He worked with myself (and the Chemist) at Fenner PLC in Hull yonks ago and they made parts for Jaguar /Rover/Delphi and Mitsubishi/Volvo plus others, so he knows what he is talking about.
I am no expert on Rubber but as I remember the Key properties are:- 1) Correct Polymer type (including any fillers , 2) IRHD Harness, 3) Solvent / chemical resistance 4) Compression Set, 5) Resistance to ozone cracking 9hence black colour) , 6) Temperature resistance.
The most highly specced polymer parts I have found in Honda's are the little D shaped things that cushion the bottom of the cam chain tensioner on the CB250/350 K's. They are a dark blue colour which probably means they are made of a fluorocarbon material (like Viton). They are quite soft but never seen one that's hardened up.
Ash
royhall:
Should be able to scalpel off a tiny slither of rubber for you Ash if that's all you need, will have a go tomorrow.
Progress with the F2, it appears its leaking at the head side of the rubbers only. Carb side seems good. Have re-tightened the clamps as much as they would go then ran the engine until it was all as hot as possible. And Bingo, got another turn on the screws. Repeated this a couple of times during the afternoon and have now got it 99% or so leak free. When I test it with WD40 the engine does not react, but I get a slight reaction with carb cleaner as its more volatile. Bike is running okay though and ticks over okay but at 1200 rpm not 950.
Going to leave it at that now and commission it for the road and use it for a while. Never know I may get a bit more on the clamps yet that may get it leak free.
Will post some pictures when its all back together and the small teething troubles are ironed out.
Thanks all.
UK Pete:
I have a Nos Honda set of rubbers which i was going to sell but reading through this thread i think i should keep them as i have 3 bikes with f2 engines
pete
royhall:
If you change your mind Pete, can I have first dibs?
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