1974 125 Project

[Bill Campbell]

I am just about the start creating a 1974 125. I have a Sachs 6B motor that has the original intake for a 26mm carby. As I have no small Bing carbys I will be putting a Mikuni on it. I thought I would put a 28 or 30mm one on. How do I modify the inlet to accommodate one of these carbys and what porting would have to done to take advantage of the larger carb. I don't want to go to a full "D" specification but I thought I would modify it a little. Hope someone out there can help. Cheers Bill

[Doug Knight]

Find or make a plastic sleeve and epoxy it to the outside of the intake spigot. B motor should be able to handle the 28-30 Mic. Connect with a hose boot and hose clamps. I had a "D" motor on a '74 back in the day. Very different 125 from the early Pentons. Especially with the frame change  and using 35mm Ceriani that were used on the Harescrambler.

[Bill Campbell]

Thanks for the reply Doug. I also want to bore out the inside of the inlet tract to match the bore of the carby. I need to know how much I can bore it out before I have to completely alter the inlet tract. I f i have to do that how do I do it?  Cheers Bill

[Mike Lenz]

28mm is all you can go. And be careful, its paper thin at 28mm and will distort if the drill bit catches. 27mm is safer. Im running the 100 with the intake opened up to 28mm with a 27mm 53 Bing.  On the 125 I cut off the old intake, opened it up to 33mm, and had a 33mm ID pipe piece welded on (OD same as the 84 carb spigot). I am running a 32mm 84 Bing. If you can its better to have the intake a mm larger than the carb. However, unless you have the extra boost ports as I do you may be better off with a 30mm carb. The intake can be opened up to handle a 32mm carb but the transfers are pretty small, and without the boost ports a 32 carb may run worse than a 28 or 30. And by the way two different carb reports Ive seen say you need to run a 1 to 2mm larger Mikuni, than Bing to get the same flow. I say Bing for MX. Mikuni for woods.

[Bill Campbell]

Thanks Mike for that sage info. I also got a little from the Facebook group. Getting a bit too old for MX Cheers
Bill