In search of Mikuni Z8 (341-14941-88-00) needle jets

[bosco659]

I’m in the process of rebuilding a set of BS38 carbs. The needle jets in the carbs were in pretty bad shape so I was looking to buy new ones. I have since posted a request to buy a set in the classified section.
If I’m not successful getting a “new” set, I may try to reuse the old ones. Before I do this I want to make sure that the ones I have are actually Z8 needle jets.

I measured the hole diameters in the sides of the jet and the larger holes are .030” and the smaller are .024”. Was wondering if anyone had a set that they could measure for me, to confirm I have the correct jet. Unfortunately there are no markings on my jets. To measure the diameter of the holes I used a set of number drills, using the shank of the drill as a measuring tool (didn’t want to use the business end of the drill bit for fear of enlarging the hole).

On another note, does anyone have a go to method for removing a petrified oring in the needle jet groove. It’s as hard as the brass jet and removal with a sharp dental tool won’t touch it. Though maybe I could soak it in some chemical to soften it up.

Any help would be appreciated. Picture for attention.

 

[DogBunny]

A genuine jet should be stamped in the area shown, to the left of the "Z" there should be a Mikuni "square-within-a-square" symbol.
Your .030 and .024 measurements are correct, however, they may be more to it than that, such as I.D. and length.
I can sell you a used pair, but I am south of the border. Hopefully someone in Canada will help you.

 

[bosco659]

Thx DB! Looked really closely under my illuminated magnifying glass and found the markings so I’m good to go. I’ll give the a good bath in the ultrasonic cleaner too. Hopefully that will help get the old orings off of them. Thx again for your help, that puts me one step closer to getting these carbs back in service. Just ordered new mains, pilots, shaft seals and screws too.

 

[5twins]

The holes down the sides of the needle jets are air bleed holes and while they may be important, the actual fuel metering orifice is probably the more important one. That's located at the top of the jet about where the o-ring goes, and down in the main bore through the jet. If you look into the top of the jet, you'll see the bore wall steps out slightly about 1/8" or 3/16" down into the bore. That's the metering orifice, quite large compared to the holes in main and pilot jets. Sometimes the needle passing through it wears one side of the orifice, making the hole through it oval. You can usually spot this because the step will be gone, worn away, on one side.

 

[bosco659]

Had a look. The step is there and

The holes down the sides of the needle jets are air bleed holes and while they may be important, the actual fuel metering orifice is probably the more important one. That's located at the top of the jet about where the o-ring goes, and down in the main bore through the jet. If you look into the top of the jet, you'll see the bore wall steps out slightly about 1/8" or 3/16" down into the bore. That's the metering orifice, quite large compared to the holes in main and pilot jets. Sometimes the needle passing through it wears one side of the orifice, making the hole through it oval. You can usually spot this because the step will be gone, worn away, on one side.

My jets do have a step but not much of one. Can someone roughly measure the bore of that hole? Perhaps my jets do require replacement. My jets are in the ultrasonic now but I will report the approx bore once I pull them out. Thanks.

 

[Jim]

MikesXS has the Z8 jet. Prolly a Chinese knockoff though.

 

[bosco659]

MikesXS has the Z8 jet. Prolly a Chinese knockoff though.

Thx Jim, checked that out but shipping for the two little pieces was quite high. After shipping and exchange, they would cost me about $43 Cdn each. I think the old ones will be ok now that I’ve confirmed they are the proper size.

 

[bosco659]

The holes down the sides of the needle jets are air bleed holes and while they may be important, the actual fuel metering orifice is probably the more important one. That's located at the top of the jet about where the o-ring goes, and down in the main bore through the jet. If you look into the top of the jet, you'll see the bore wall steps out slightly about 1/8" or 3/16" down into the bore. That's the metering orifice, quite large compared to the holes in main and pilot jets. Sometimes the needle passing through it wears one side of the orifice, making the hole through it oval. You can usually spot this because the step will be gone, worn away, on one side.

Hi 5T, had a look at my jets and a 1/8” drill bit passes through with no drag. Hopefully they are still usable.

 

[5twins]

I've never measured any because I couldn't really figure out an accurate way to do so but yes, the hole is quite large. The needle passes and works through it. The numbered drill bit idea may work but I think the difference between sizes is quite small, just like on main and pilot jets. Big enough to make a difference in fuel flow yes, but probably so small that measuring would be difficult.

 

[5twins]

I think all these replacements are knock-offs. I don't think you can get originals from Mikuni or Yamaha anymore. Some other models and even some from other manufacturers used similar carbs and needle jets but I've yet to run across any in the sizes we need. All I've seen so far are smaller.

 

[grizld1]

Bosco, trying to gauge the NJ orifice with drill bits won't tell you much of anything. The difference in orifice diameter for Mikuni NJs is .0002" per number step--and no, there's not an extra zero in there, that's two tenths of a thousandth of an inch. My advice to you is not to borrow trouble. When there's wear in the NJ it's not usually even around the diameter, but localized, so that you'll see part of the step that's thinner than the rest, and wear will usually be visible on the needle as well. If you don't see those defects, your NJs are probably fine, and I'd trust them over any Chiwanese knockoffs. The most obvious "tell" for worn NJ's is elevated fuel consumption, since the NJ is the primary metering component in the cruising range. If you get 45 mpg or better with the motor in a decent state of tune, leave well alone.

 

[bosco659]

Bosco, trying to gauge the NJ orifice with drill bits won't tell you much of anything. The difference in orifice diameter for Mikuni NJs is .0002" per number step--and no, there's not an extra zero in there, that's two tenths of a thousandth of an inch. My advice to you is not to borrow trouble. When there's wear in the NJ it's not usually even around the diameter, but localized, so that you'll see part of the step that's thinner than the rest, and wear will usually be visible on the needle as well. If you don't see those defects, your NJs are probably fine, and I'd trust them over any Chiwanese knockoffs. The most obvious "tell" for worn NJ's is elevated fuel consumption, since the NJ is the primary metering component in the cruising range. If you get 45 mpg or better with the motor in a decent state of tune, leave well alone.

Ok thx Grizld1. I’ll try out the ones that I have with fingers crossed. The needles themselves had no signs of wear.

 

[5twins]

Yes, I had a bike with worn needle jets years ago. It seemed to run OK but the plugs were black and it barely got 30 mpg.