Mikuni Carb Rubbers

Golgother

Golgother

Hero member
Hey All

I have Mikuni RS 36mm carbs mated to a RGS head via alloy inlet manifolds.

I have used rubbers to connect the carbs to the inlet manifolds, supplied by a well known supplier that have served well but now have cracked beyond repair.

My solution to date has been 42mm ID radiator hose cut to length 40mm, so far so good, we'll have to see if the tube will withstand fuel vapour longterm.

I have so far resisted the temptation to butt the carb directly to the inlet manifold in an attempt to prevent any damage due to vibration, reduce heat transfer to carb and reduce potential for fuel frothing. Currently I have approx 5mm of gap between the carb and manifold that I assume would not be optimal for gas flow.

For optimal gas flow should the 5mm gap be zero?
For optimal gas flow and vibration protection should the 5mm gap be taken up with a rubber insert of the same diameter as the carb/manifold ?

Any thoughts or advice ..........

                              Cheers
                                      Matt
 
The bloke who fitted the Kehan cv carbs to my bike used,his discription,Turbo hose,its a sort of brownish red with approx 3mm wall thickness,its been on for maybe 5 years and is still ok.It fits over his stubs on the head and over the ends of the carbs,buts up without a step.I also thought people were looking at snowmobile mounts,dont know how that went.
 
Who knows Matt , it might even help! I can understand your concerns and I don't think I'd like a gap there, experts might have other opinions. Most OE carb rubbers have a moulded profile for the carb to butt up against and I wonder if it might be possible to replicate this by bonding in a hoop of smaller O/D rubber tube, or maybe just a hoop of nylon?

I think I'd be looking for the correct Mikuni mounting rubbers and machining up some inlets which will fit to it.


 
Matt
Dump the rad hose, I used it, it doesn't work.  Proper rubbers have a  step which abuts both carb and stub which helps to smooth the transition although conversely my Genesis used to have a promounced ramp between both. (following shit advice in the 80's) I carved it back to a step to match the ID of both and the performance went down.

Proper rubbers will be available from the usual suspects, I use PHM mounting rubbers pt number 130929C .

Also the thin jubilie clips that you buy, poss 9mm wide works but you'll notice OEM dont use them but a bespoke clamp that doesnt have an 11mm wide screw slide. That wide part distorts the rubbers so if anyone comes up with an OEM part that fits these rubbers, I'd be interested in these.

Clearly spent too much time looking at these parts :)


CHeers Rob
 
Hey all

Thanks for the input.

The radiator hose was fitted during this years TT to get me home after terminal failure of the tailored parts.

I have also noticed that jubilee clips can deform the rubbers and have never sat happy with the tailored rubbers ability to support the carbs especially once hot.

I'm in the process of designing some new support brackets that hold the carbs at the correct spacing that will be more accurate in the hole spacing and stronger than my current alloy ones, one of which has fractured contributing to the problem. Que, Keith, I got those in stock .......... :D

My carbs actually blew off after I shut down from a loaded, prolonged WOT run, in a  perverse way I almost hoped I had holed a piston or two so that I could justify bigger pistons.......  :LOL:  

Incidently, on turboed/supercharged bikes you will often see that the carb/plemum chambers are very solidly fastened to the bike to stop them blowing off once the throttle shut.

Keith, Do you have supercharger kits in stock ?

                            Cheers
                                    Matt

 
Vince said:
thought people were looking at snowmobile mounts,dont know how that went.
Click to expand...

Vince, I use Wolfgang's snowmobile rubbers because they're curved and I wanted to get my Mikunis sitting a bit lower (so they didn't hit the bottom of the tank) and a bit more upright so the float bowls would hold a bit more petrol. I tried Hinckley Triumph ones but they didn't have enough curve. The ones Wolfgang sells are stepped on the inside so there isn't a big turbulence-inducing discontinuity, and they do a nice job of dropping the carbs down enough to clear the tank.
But they really don't fit all that well, they don't have an inside lip to fit the groove on the carburetors. When they're new they're very hard and difficult to fit, but once they're a few months old they start to get a lot softer and that's when they start to fall off on overrun. I have a bracket from the frame to the threaded hole on the side of the middle carb that holds them up, works well, although definitely very DPOish.
And, one of mine split wide open at some point, and that carb started running just a little lean :o. So I bought 2 new sets, one to keep as spares.
I think though that I might just get some of Red's angled stubs and use the Mikuni rubbers. I'm currently using the CNC ones the guy on UK eBay sells, nice and strong and don't crack, although I did have to have them turned to a smaller OD and shortened before they were much use. Oh, and flared internally 32/36mm.

Ken
 
Hey Ken

I know what you are talking about ........... ;)

                      Cheers
                              Matt
 
Sounds like you have a plan now.

My carbs dropped off only when I used Radiator hose and for that I made a support bracket. Once I used the rubber (BOLOGNA) moulded into the sides of them, I took the bracket off, didnt ever need it.

The carbs do move and the rubber stubs actually pulse on idle (at least on mine), so they do take a bit of stick..must be the Vacuum with the slide shut.  Seem to recall cutting a groove into the stubs (ELEGOG Mfr)  to match the rubbers and the carb side matched up ok with the Internal lip inside the rubbers. I now have the Redax angle ones tapered 38-36.
 
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