Leaking Bing Carbs R80G/S
 

I have currently serviced and refurbished a few sets of Bing Constant Depresion Carburettors.
These carbs were from an 82 R80 G/S and 80 R100T.
Floats, float needles and gaskets were replaced as well as new fuel hoses ordered from BMW.
On both sets, the left carb would leak but it seems to do so only when the motor is not running.
If there is someone that can shed some light it would be most welcome.

Have you correctly set the float levels? If they are not adjusted properly, they will alow the bowl to overflow. When the bike is running, it takes enough fuel to eliminate the overflow.... It is the float that turns the fuel flow on and off.

If I remember this proceedure correctly, it should be adjusted so that at dead level it is just pushing on the needle to close it. The needles can also wear, and alow leaks, but it sounds like you have replaced them anyway.

I have set the float levels often and have also set them to even close prematurly but no luck, I suspect that the needle is not seating properly.
The seat is fixed to the main body of the carb, so the only solution is to replace the whole carb as BMW does not sell just the body of the carb. This is not really cost effective especially after already replacing all the parts in the existing carbs.

On setting the float level the right method to use is to run the bike warm for app. 10 min or so. On your return swich off the bike while stil rolling and close the taps right away.
You then take the float bowl of and measure the distance of the fuel from the edge of the bowl.
I do not know "yet" what the distance is but maybe someone will be able to share that info.
If I find it through another source I will also post it here.

...although i haven't tried it yet, i've always been skeptical of this meathod. After you turn off the fuel, you still have a full line from the tap to the carb. When you un-clip the bowl and lower it, the float drops, spilling a small amount of fuel into the carb before you can get away (I use this meathod for starting stubborn camp-fires. We call it liquid indian http://www.horizonsunlimited.com/ubb/smile.gif ).How can this be an acurate way of measuring the fuel level??

I was told by a bmw mechanic that you can "lap" (like valves) the seat, by using an old needle and a small amount of grinding compound. put the compound on the needle and spin it lightly in the seat... I always assumed it was a last ditch attempt to save a carb, before buying a new one.
Cheers

I agree with Ron - that's not an accurate method. Correct is to turn the carb upside down and VERY carefully set the float "level".

In other words, the float tang that lifts the needle (in normal orientation) should just touch the needle when the float is carefully lowered to the level position. Bend tang as needed.

It's not ultra-critical so no need to be anal, just get it as close as you can by eye.

As for lapping the seat, the seat isn't supposed to be lapped in - in fact it's supposed to be a square edge (the seat) contacting a tapered needle - the contact is therefore only a narrow line, not a flat surface. All that matters is that they are both ROUND, and the seat edge is SQUARE. (if that makes sense http://www.horizonsunlimited.com/ubb/smile.gif) The needle should be replaced when there is ANY sign of a groove worn around it.

You MIGHT be able to salvage the seat by lapping the surface with a square ended, hardened steel, round shaft of appropriate diameter to get the top surface flat and square - could be worth a try. Use a drill to power the shaft! http://www.horizonsunlimited.com/ubb/smile.gif

Otherwise, look for a good used carb - they're not too hard to find at a wreckers.

hope that helps, and best of luck,

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Grant Johnson

Seek, and ye shall find.
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One world, Two wheels.
www.HorizonsUnlimited.com