Jackpiner Mikuni Jetting

[Mike Leon]

Hoping to get some help.

I have tried doing the searches on Mikuni Jetting but haven't found a clear answer.

I have a 1974 175 and will be running a 32mm mikuni but need to get a good starting point on the jetting.

Your help is appreciated.

Mike Leon

[firstturn]

Mike,
  Try this...


Posted - 01/31/2007 :  9:47:29 PM      
I know lots of you are running Mikuni carbs on Jackpiners. I read all the past postings on the topic and understand the mounting issues because I converted a Six Day to a Mikuni a year ago. I can work through the mounting problems but I really need suggestions on jetting, that's what drove me crazy on my Six Day. As supplied by Pro-Flow the Mikuni's come with pilot jets way to large, it took me months to figure it all out. I have a 75' 175 all set for the conversion. Since this is my trail riding bike I will be going with a 32mm. Does anyone have a good baseline starting point on needle, main jet, pilot jets?

Thanks
Chris
AndyL
Advanced Member

USA
152 Posts

 Posted - 02/01/2007 :  2:18:41 PM      
Chris,
I have a 1975 Jackpiner with a Mikuni on it. It does run well, I use it as a back-up MX bike and run Vintage Harescrambles with it as well.

I don't have any of the carb details here at work but will get them tonight and post.

Andy
#14u MOVMX and AHRMA
1975 Jackpiner
1970 Steeltanker
83 YZ 100 and 250

AndyL
Advanced Member

USA
152 Posts

 Posted - 02/01/2007 :  8:44:17 PM      
Chris,
Here is what I am running in my 32mm Mikuni on my 75 Jackpiner.

It seems pretty close, could be a bit fatter down low and a smidge leaner in the mid but it starts easy, doesnt foul plugs and pulls like a tractor down low.
140 main
25 pilot
2.0 slide
6DH2 needle
Clip in the leanest slot.

Andy
#14u MOVMX and AHRMA
1975 Jackpiner
1970 Steeltanker
83 YZ 100 and 250

Chakka
Advanced Member

USA
394 Posts

 Posted - 02/01/2007 :  10:44:06 PM      
Thanks Andy! That's outstanding, you saved this rookie tuner a long time messing around. I have learned all the steps of restoration but the one thing that eludes me is the ability to fine tune an engine. I think it's a lot art and a little science.

Chris

chicagojerry
Advanced Member

USA
617 Posts

 Posted - 10/15/2008 :  2:46:17 PM      
hey andy, while researching jetting for a 175 mikuni i stumbled on this posting,any idea what your needle jet is? you have everything but that listed. thanks for any help chi jer

brian kirby
Advanced Member

963 Posts

 Posted - 10/15/2008 :  2:56:10 PM      
Another thing to keep in mind is old ('70s model) Mikuni carbs are different internally than new ones so the jetting will not correspond, they might not even be close for identical size carbs. Having said that, I am somewhat alarmed at the 140 main jet, I run a 34mm Mikuni on my Can-Am 175 and its got a 230 main. I know its a different size carb and a totally different engine, but a 90 jet size difference is huge. Not that I am trying to cast any doubt on Andy's specs, but I would be careful.

Brian

'73 Berkshire Team d-Con
//www.d-conproducts.com America's #1 Rodent Control Brand

Merlin
Advanced Member

216 Posts

 Posted - 10/15/2008 :  10:48:25 PM      
A 140 reverse jet in a long hex Amal type would be around 230, more or less the same amount of fuel. The 140 is an orfice diameter and the 230 is a cc per minute flow rate.

Quote: Thomas Jefferson, We are all born ignorant, some work to remain that way.
Quote:Merlin, "it is impossible to teach those that wish not to be taught".

brian kirby
Advanced Member

963 Posts

 Posted - 10/15/2008 :  11:07:27 PM      
Ah! I didnt know that, so older Mikuni jets are orifice size and new ones are flow rated? If so how does one tell the difference?

Brian

'73 Berkshire Team d-Con
//www.d-conproducts.com America's #1 Rodent Control Brand

Merlin
Advanced Member

216 Posts

 Posted - 10/16/2008 :  06:06:03 AM      
Reverse type jets are round and have slot not a hex, they also come in small (some power jets use them) and large, large is most common for main jet use. The name "reverse" come from the fact the jet meters accurately in either direction, hex type jets flow accurately in one direction only (from the small hole to the thread end). The increment of adjustment is much finer with hex jets and are what you will find in most two strokes although some two strokes have used round (Kawasaki comes to mind)in spite of the radical changes caused by thier design exacution.




Quote: Thomas Jefferson, We are all born ignorant, some work to remain that way.
Quote:Merlin, "it is impossible to teach those that wish not to be taught".
Edited by - Merlin on 10/16/2008 06:09:12 AM


Ron Carbaugh

[Mike Leon]

Thanks for your help it is much appreciated.

I did see these posts when I did my search.

I was confused with the old vs. new mikuni specs.

As mentioned in the posts the old 140 = new 230 but I wasn't sure if there was a conversion needed on the other needles/jets.  This gives me a starting point.

Thanks again