Carbs For 125 SD's

[tofriedel]

I have been reading all of the information on this subject that has been previously posted and I am a little confused about the differences in carb openings.  Maybe I am a little dense, my wife has suggested this to me a number of times.

My question is, if you mount your larger carb (Bing or Mikuni) directly onto the cylinder spout, which has a 27.5mm ID, doesn't the ID of the spout control the air-fuel mixture more directly than the diameter of the throat of the carb?  It seems to me that the smallest diameter of the flow is the control and not the larger carb ID.

I understand the need to change some of the jetting & needles.  What am I missing other than getting the larger carb to fit in the available space between the cylinder, pipe & airbox?

Thanks,

Tony

[Ron]

Hi Tony,
Your right. Any smaller diameter in the intake track becomes the intake diameter.
Going to a larger carb would necessitate boring the cyclinder intake to the same ID, to take full advantage.
That's not to say people don't adapt larger carbs without opening the cyclinder tract they do, but there's generally no increase in performance.
I have a 30mm Mikuni on my 6A/125 that has an aluminum flange welded around the spigot and then bored to 30mm.
I'm having a heck of a time with the air boot though. Right now I'm thinking I'll work the boot inside the airbox instead of the usual clamp on the outside and use an edge trim to seal the two inside the airbox. Hopefully this will have the effect of shortening the boot and make it fit. If I install the boot the way it was intended it smashes against the back of the carb and will restrict airflow.
RonW

[bentrims]

Tony,

As Ron said in agreement with you, these are intake limiters that can be opened up some.
 
I have collected a good amount of circa 72-4 Sachs 125 articles on porting. The one I am looking at shows and tells of opening the intake spigot in the 29mm neighborhood while maintaining a 2mm wall thickness. I have a 32 Mik on my bike. Without a dyno or flow bench all you have is seat of the pants (Larry Perkins quote)testing but it is dramatic over the stock Bing I had.

It also allows 1-2 kick starting and actually finishes races. Jets are pretty easy to find on race day if needed. I would gladly send you the articles, jetting of the 32mm (Donny Smith's) and anything needed.

[email protected]
Tom B
PS- Another article by EC Birt says those ports should be polished like a mirror...man my fingers are getting sore from hand polishing![]

[john durrill]

Ron.
 Here is a short cut to a message from 2002. Its one way to get a Mikuni to fit on a six day. I was able to ride 2 days in the mountains on the Ark. Oklahoma boarder with this member and his son. The bike worked well with the mikuni on it.
John D.
http://pentonusa.org/forum/topic.asp?TOPIC_ID=1330