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 Mikuni VM28 on the Berkshire
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johnbowman
Advanced Member

USA
137 Posts

Posted - 09/07/2021 :  4:23:50 PM  Show Profile  Reply with Quote
While out riding this weekend to get ready for Diamond Donís, the Berkie started the all too familiar loading up routine when hot. Iíve had this same issue more than once with a Bing. From past research and talking with some of the gurus on here, the consensus was to replace the slide (if smiley faced) and/or needle and needle jet. I have gone down that road a few times over the past year. Each time, it did solve the issue and the bike ran great again. Unfortunately, my Bings seem to like a steady diet of slides, needles, and needle jets. And yes, I replace my air filter with a clean one after every race weekend. If itís really dusty, after every race. I also run an in-line fuel filter and check for leaks in the intake system. After my last invoice from Bing Agency and a long delay in getting parts, I decided that it was time to go to the dark side and installed a Mikuni VM28.

Getting the airbox boot on was the hardest part. I tried for over an hour to get a new boot that I had purchased from Al to work. It was so pliable that it kept making contact with the pipe. I could just see that boot having a hole burned in it by half a lap. Finally, I remembered that I had an old boot from a bike that had a Mikuni on it. I cleaned that one and it went straight on! I guess years of sitting there attached to a Mikuni "preformed" it correctly. Itís still soft, but not as flexible as the new ones that Al sells.

I saw some old posts by Ernie about the jetting. I decided to go with 160 main, 45 pilot, and a 2.5 slide. I'm not sure what needle and needle jet are in it. It seems to run really well. Starts easily. Idles well once warm. Comes off idle crisply. Pulls through all the gears. I've checked the plug twice now. Once after extended idle. Once after ten minutes of running it through the gears. Plug has the light brown paper sack look.

I just hope that it runs okay at Diamond Don's. I have already had a DNF a couple of years ago due to carb issues. I really donít want to repeat that performance. Fingers crossed, the little Berkie will pull through for me again. It sure has been fun to ride.

Richard Colahan
Advanced Member

USA
293 Posts

Posted - 09/21/2021 :  9:23:03 PM  Show Profile  Reply with Quote
John, there are many many folks who are dedicated to their beloved Bings. I'm not one of them...unless it is for a correct restoration of course.
For a Vintage XC bike...get the parts that work and are reliable.
When it comes to carbs, imo that means a Mikuni!
Best wishes for a great race at D/D's.

Richard Colahan
1969 V1225
Upper Black Eddy PA
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johnbowman
Advanced Member

USA
137 Posts

Posted - 09/22/2021 :  12:09:06 PM  Show Profile  Reply with Quote
An update on the Mikuni

The bike ran great at Diamond Don's. It seemed smoother than with the Bing, but I think that the Bing has more airflow and is snappier. The difference is kind of like the difference between a Yamaha and a Honda, if that makes sense. They both have about the same amount of power, it is just delivered in a different way.

The Mikuni makes the bike a lot easier to start. Put on the choke (enricher), give it a kick or two, and the bike fires right up. It idles smoother with the Mikuni. I always had a problem with the Bing after transporting the bike to a race. Sometimes, I had to kick that sucker forever before it would start. The bike started on the second kick with the Mikuni at Diamond Don's.

I took the plug out after the race and it looked great. Nice light brown paper sack look.

Overall, I'm happy with the change to a Mikuni. If it works well for New Blaine, then I'm going to stick with it.

Now, for my garage queen Penton, I'm going to keep a Bing on it.
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Mickey Sergeant
Advanced Member

USA
340 Posts

Posted - 09/24/2021 :  11:41:12 AM  Show Profile  Reply with Quote
RULES WERE CHANGED A COUPLE YEARS AGO TO RUN A 30MM ON THE 100CC. I HAVE A 30MM MIKUNI ON MINE. 28MM CARBS ARE HARD TO COME BY. HEAT AIR BOOT UP WITH A HEAT GUN IT WILL MAKE INTALLING EASIER
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