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Deciphering Mikuni Needle Codes


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Looking for some help here! The needle currently in my 1988 YZ250 is bent so I need a new one. The one currently in it is a "6f62."

 

Thanks to reading Graham Bell's book "Two-Stroke Performance Tuning" I understand that the first "6" means the 6 series Mikuni carburetor. The "f" means the taper is 1*30* degrees. The second "6" followed by the "2" are what have me stumped. Would anyone happen to have the knowledge of what that means? I have learned a lot from reading the book and reading stuff online, but I am having a hard time understanding those last two numbers. Any help would be greatly appreciated!! 

 

Of course I could go measure the needle with my micrometer and find something equivalent to it, but I think it would truly be better to have more of an in depth understanding of this. Being 20 years old and in my situation I don't have a bunch of money to spend... even on carburetor needles. So I have to make every purchase count. 

 

**Not directly related to this post but for those of you curious~~Thank you for those of you who helped me a while back. I posted a question regarding the crank seal and spacer. Because this is my first dirt bike and I got it in a box of parts and didn't know what anything looked like before or even knowing if everything was all there, the countershaft spacer seemed like it went there but didn't fit right. That was prior to me knowing that it was the countershaft spacer haha. Turns out that the crank spacer thing wasn't with everything else and I had to find one on eBay. Also, I did in fact have the crank seal flipped the wrong way too. Besides all that, I got it all together and it ran. It freaking ran!!!!!!!! Super proud of myself for that and I learned a LOT by doing all of it myself. Everything is currently all apart again and frame is getting powdercoated. I tore the motor apart again and polished the shift shafts and forks along with grinding down the shift star. It shifted a million times better on the bench than it did before so I am super excited to get this thing running again. I do need to make a power-valve for it though. Fortunately, I have the non-actuating half I can make a copy of and just add a stem to it to connect to the linkage. Looks like I'll be using my drill press, micrometers, and my eyeballs to get that part done because I don't have a lathe. This should be interesting... Riding it without the power-valve was a little scary. From a stop it accelerated quickly and then suddenly it felt like a rocket motor lit off once the rpm's hit the port timing it liked!**

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You have a Yamaha specific needle.  You will likely only find that needle at the junk yard or Yamaha. My guess is a 6F6 set on the second from top needle clip. If your needle is bent replace your needle jet too. It will be worn. 6F9 and 6F8 used to be common, you should be able to substitute one of them with a leaner needle jet.

 

Paul

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You have a Yamaha specific needle.  You will likely only find that needle at the junk yard or Yamaha. My guess is a 6F6 set on the second from top needle clip. If your needle is bent replace your needle jet too. It will be worn. 6F9 and 6F8 used to be common, you should be able to substitute one of them with a leaner needle jet.

 

Paul

 

Thank you! I think that answers my question as to why I didn't understand it. I ended up getting a 6DH7 needle. It was VERY similar to my stock needle as to how far down the taper starts, thickness, and end taper thickness. I believe (could be wrong here but it looks like-) it is slightly leaner than 6F9 until the very end judging by the measurements I am finding online. I'll look into getting a new needle jet too as per your suggestion. Thanks!

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