SOHC.co.uk Forums > CB750
Carb rubbers
Sparrow1:
I'm relieved we are looking more closely at air leaks.😳 although I could be wrong of course.
Bear in mind that the hose clips supplied by Silver and others (from a realiable source) may a little too big and one shouldn't really be down to the last turn on those clips. You should have a least further 5/6 turns left. How do you know it's at the right tension when fully screwed in? I have learn't from this - air leaks are the scourge of a smooth running engine.
I really hope you are now heading in the right direction. Good luck.
Nige. Ps, as I said earlier, the Dave Silver rubbers (and airfbox ones) are working perfectly. If I have any rough running or incresing tick over or reluctant to return to tick over I immeadiatly focus my attention to those rubbers! I re tentision/tighten those clips backed up with carb cleaner leak testing, providing that doesn't damage the engine paint! Thread Locking those screws is not a bad idea..
AshimotoK0:
Emailed this to DS:-
Hello David,
In 2010 I came up with a method of re-softening carb. inlet rubbers for the CB750K0 (no pattern parts were around then). This is a chemical used in the rubber processing industry and not the 'essential oil 'wintergreeen' treatment used by members of the USA SOHC forum.
To this day my sample retains its softness and compliance but has never been fitted to an engine. There was no significant swelling due to the treatment.
However, I softened a set of K7/F2 750 carb inlet ones for an SOHC.co.uk forum member and he is reporting inlet leaks so I am not sure on the 'Compression Set' properties on the treated rubber (the CB750K7/F2 ones are really expensive and as far as I know not reproduced as aftermarket parts).
However a few forum members have purchased aftermarket inlet rubbers and report similar leaks shortly after fitting, which worries us a little about fitting aftermarket parts. As I understand these aftermarket parts were sourced from the USA. I know you now sell aftermarket inlet rubbers for some models (including the CB750K0). Do you have any specification details of the rubber used in these please from your supplier ? I had a set of original '69 inlet rubbers analysed in 2010 and they were made of nitrile rubber. The airbox 'rubbers' were a form of polyurethane, borne out by the fact that, in the softening solution they actually hardened more ! If you don't have the technical spec. of the rubber used on the aftermarket parts, I could have these analysed FOC but I would need samples to try, which may be a destructive test, if I need to get a compression set value measured.
I am not a rubber expert but, as I remember,the Key properties physical & chemical properties are :-
1) Correct Polymer type (including any filler)
2) IRHD Harness
3) Solvent / chemical resistance
4) Compression Set
5) Resistance to ozone cracking (hence black colour)
6) Temperature resistance.
7) Swelling after immersion in petrol.
If a manufacturer cannot supply or understand these properties then I would question their integrity on the reproducing of these critical parts.
I am also interested in the type of rubber used on repro. cam chain tensioner parts. As far as I am aware, nobody reproduces primary chain tensioner polymer components for Honda's.
One member has fitted your pattern inlet rubbers ( I think to a 750K0), so that looks promising. What do you fit to your 400/4 resto's?
Hope to see you at Stafford.
Best regards,
Ash.
Lobo:
Ash... speechless at your tech knowledge. School told me rubber came from trees in Malaysia & signed the topic off there... they woefully let me down!
A big thanks to this thread... my K2 was lately popping at idle... one of those jobs I kept forgetting. Anyways, took a mini socket to all 8 inlet clamps yesterday... every bloody one took a turn or more. Job done... fantastic.
Slight topic drift Ash... but you're the man... storage... better to spray WD40 over the metal parts... or silicon? My bro-in-law telling me WD40 overspray can degrade plastics / rubber, whereas silicone won't. Comments?
Ta,
Simon
AshimotoK0:
--- Quote from: Lobo on April 07, 2015, 06:09:48 AM ---Ash... speechless at your tech knowledge. School told me rubber came from trees in Malaysia & signed the topic off there... they woefully let me down!
A big thanks to this thread... my K2 was lately popping at idle... one of those jobs I kept forgetting. Anyways, took a mini socket to all 8 inlet clamps yesterday... every bloody one took a turn or more. Job done... fantastic.
Slight topic drift Ash... but you're the man... storage... better to spray WD40 over the metal parts... or silicon? My bro-in-law telling me WD40 overspray can degrade plastics / rubber, whereas silicone won't. Comments?
Ta,
Simon
--- End quote ---
I worked for a large company for yonks before I went self-employed about 18 years ago. After many rounds of re-organisation we were wittled down but we had ended up with a really a good technical team of engineers / technologists. One was a chemist, myself electronics/physics, a rubber technologist, a mellalugist, an engineering plastics expert and lots of mech. eng. experts. We have all gone our separate ways now but we still draw on each others specific areas of expertise. Sadly the metallugist is very poorly.
I will ask the chemist guy about affect of WD40 overspray on polymer parts.
Ash
archmill:
Just done my carb rubbers with Wintergreen oil in the slow cooker, 1:50 mix oil and water and cooked them for a couple of hours ( hope she who must be obeyed is not reading this ). Worked a treat.
Archmill
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