My idea to improve idle and reduce overrun 'surging'

[keleto]

This might be obvious to a lot of people but it occurred to me that in a carburettor of the BVF type (where the throttle remains closed at idle and all the air is drawn through the bypass system) any leakage between the throttle slide and the carb bore will upset the idle mixture, much more so than with the more typical "throttle stop" design where the throttle remains slightly open at idle. And if your BVF carbs are like mine with significant wear and scoring on the throttle slide this might be a significant factor in the idle and overrun problems which seem to beset BVF carbed machines. I have therefore carefully sealed the joint between the throttle cable dust cover and the throttle cable sheath, and put a cable tie on the skirt of the dust cover so there will be no air drawn in through that route. (Of course there will still be some tendency for air to be drawn through the throttle cable itself but hopefully that will be insignificant, or could be cured with some heavy grease in the cable). I don't know if I am fooling myself with a placebo effect but this really does seem to have made a difference to my bike; better idle, better overrun. I would be interested to see what results (if any) other riders may notice with this idea. Cheers !

[vicar]

Find a mikuni. 30 or 28 mm carb or a bing 85 30mm they are both conventional carbs. The bing is std fitment on some mz 250s. I have a 251 that runs without issue on a bing.

[keleto]

That would be optimal, but having a budget tending towards zero..........

[Blurredman]

As far as I can see, the the 30n3 is the only MZ carburettor (for the 250#s) that has the air bypass screw. It is a little tougher to set up but once you understand it it makes sense. The slides with wear equally regardless of whether it has an idle stop or a natural fully closed position. Infact to have an air bypass screw is more typical of a car carburettor and I actually prefer it to be honest. For a start, there is actually more wear that will become of the throttle slide the extra variable being the stop screw itself wearing the inner ledge of the slide.

From my experience the 30n3 is much smoother in terms of idling too.

[keleto]

Thanks. I guess I am just more used to the typical throttle stop design. It is the only sort I have worked with before. As per your other post I will try getting the idle mixture right first THEN adjusting the tickover speed with the bypass (if I have understood that correctly).....