Carb overflow tube issues and replacement

So this issue is not exclusive to the CBX but also impacts many other models and makes, not just Kehein. I have analysed the issue in detail. The issue is two-fold, in a limited number of cases the overflow tubes corrode due to a galvanic reaction caused by ethanol oxidising into acetic acid and the decay process is exacerbated by the acid and galvanic reaction. The second and more common issue is the tube micro fracturing due to the residual stresses from the cold rolling process during tube manufacture. The tube is pressed into the bowl using an interference fit which reduces the pressed in diameter by approximately 0.05mm or two "thou". This causes further stress and over the years "season cracking" micro-fractures permeate up and down the base. In some cases, these fractures are so small even with an eyeglass they cannot be seen. Soldering, JB weld (epoxy polymer) etc will not work long-term (>10 years) whatever anyone tells you when ethanol-based fuels come into the equation, I have seen many failed attempts in this regard (and tried them all myself, yes I could have taken the easy route and bonded them). The crack will permeate through to the pressed-in part of the tube where the solder will not adhere and it will leak. Ethanol softens the polymers in epoxy-based adhesives and shortly again it will fail especially with the heating and cooling effect as the bike cycles from a cold carb caused by the venturi effect to a hot carb when the bike stops. We have therefore provided a durable pressed-in lead seal that overcomes all of these risks.

It is essential that the overflow tube has an internal diameter >2mm and the hole at the top is restricted to <1.1mm. Keihen and other manufacturers do this for a reason, to stop it from getting blocked by residual deposits. In addition just putting a straight tube in allows for fuel to "splash over" down the tube and out the overflow tubes while in motion hence the reason the 1.1mm hole at the top. This hole is only a few thou deep is to ensure it does not get blocked, again all of these are design features that we meticulously follow in our manufacture of these tubes (can you tell the difference!).

Testing

To test firstly make sure the drain screw is tight and the overflow passage is clear using a 1.9mm drill bit. Fill the bowl with water, using compressed air at NO more than 40 PSI put your finger over the top of the tube and gently release air into the overflow outlet at the base, you will soon see bubbles coming to the surface if it is fractured. The replacement tube has been supplied with a soft lead seal to avoid the stresses previously mentioned and should never fail in the future.

You can either send the bowl for repair to us at The Super Sport Shed or alternatively fit them yourself, this is a relatively simple process but requires a sturdy drill press to undertake the job (the small cheap ones are unlikely to be man enough).

You can buy the individual part here for the CBX & DOHC:- https://thesupersportshed.com/product/carburettor-bowl-overflow-tube-qty6/

https://thesupersportshed.com/product/dohc-750-900-1100-carb-over-flow-tube-qty-1/?currency=GBP

Removal

Tighten the old overflow tube in the drill chuck and gently twist back and forth, either the tube will release or snap at the base, if it comes out great, don't worry if it snaps off. If it snaps use a 3mm (second preferred) tap in a pin vice (or similar) and tap down through, the broken bit will release keep twisting in the same direction and pull it out, this has never failed me yet and I have done many dozens to date. Use a pair of pliers to remove the remaining tube from the tap.

Firmly grip the old tube, twist the float bowl back and forth whilst holding the chuck firmly
In this case, it broke off, you can clearly see in this photo the fracture to the base
If the tube breaks off at the base use a 3mm second tap in a pin vice or similar keep tapping/tightening and the broken piece will break free
The broken piece has come free, use a pair of pliers to grip it and remove it from the tap

Insertion

To insert the new tube put the replacement overflow tube in the drill chuck, tighten it firmly but not overtight which will crush the tube! Pack the bowl underneath to ensure its level and press the tube in a "one-shot" action, if you stop it will be very difficult to press it in further so I recommend removing it (by twisting back and forth) and trying again. If you overshoot a little don't panic, again very lightly twist back and forth and pull down gently to get to the desired height (level with the top of the float bowl), you will have a limited opportunity to get this right otherwise it is likely to leak if you miss more than once. It has never failed me yet!

Firmly tighten the new tube in the drill chuck - do not overtighten
Pack the bowl underneath so its level, press the tube in with a "one-shot" action
The tube needs to be set level with the base, I allow 1 mm tolerance, either way, the carb on the right is protruding "proud" on the right-hand bowl (in tolerance)
All good now!


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