FZRSupport: the FZROnline Archive

Ever wonder why DynoJet made up their own numbers? Here is the response I was given:

We are often asked for a "cross reference" sheet that compares our main jets to Mikuni or Keihin. The fact is you cannot directly interchange the jets for a given size. Many things affect fuel flow through a jet of the same orifice size. The entry and exit tapers of our main jets differ from those of other companies. Due to this, the fuel flow changes as a function of velocity through the carburetor venturi. This means that two jets of equal orifice size will have a different fuel flow curve relative to intake air speed. At a certain speed the two may flow equally, but a change in velocity changes the flow characteristics.

The Dynojet main jet hole size is measured in millimeters. For example, a DJ142 has a 1.42mm hole.

Michael Cory
Research & Development
Phone: 800-992-4993

I was under the impression that Mikuni 's jet #s are also the hole diameter.

???

its my humble opinion that DJ is full of hooey and does it to make you buy jets from them

haunter wrote:its my humble opinion that DJ is full of hooey and does it to make you buy jets from them

I'm almost certain of that. At least, unlike Factory Pro, you can actually find stuff on DynoJet's website!

I can find stuff on their website.

And Marc is extremely helpful when you email with questions!

ragedigital wrote:Ever wonder why DynoJet made up their own numbers? Here is the response I was given:

We are often asked for a "cross reference" sheet that compares our main jets to Mikuni or Keihin. The fact is you cannot directly interchange the jets for a given size. Many things affect fuel flow through a jet of the same orifice size. The entry and exit tapers of our main jets differ from those of other companies. Due to this, the fuel flow changes as a function of velocity through the carburetor venturi. This means that two jets of equal orifice size will have a different fuel flow curve relative to intake air speed. At a certain speed the two may flow equally, but a change in velocity changes the flow characteristics.

The Dynojet main jet hole size is measured in millimeters. For example, a DJ142 has a 1.42mm hole.

I stumbled on this page which is just as mentioned, a "cross reference sheet" I don't read the text, but the chart is very useful
Linkhttp://r-246-web.hp.infoseek.co.jp/tca_j.htm

Good call... here is a screenshot for those interested. Maybe someone can get this into the WIKI...?

duckling wrote: I stumbled on this page which is just as mentioned, a "cross reference sheet" I don't read the text, but the chart is very useful
Linkhttp://r-246-web.hp.infoseek.co.jp/tca_j.htm

Hi I prefer the english version :-}
http://tidyville.roadstarclinic.com/jet ... nchart.htm

Forgetting which company bumps out what smoke & mirrors suggest you read:-



http://www.saltmine.org.uk/martin/jet_sizing.html

ragedigital wrote:Ever wonder why DynoJet made up their own numbers? Here is the response I was given:

The entry and exit tapers of our main jets differ from those of other companies.

I think they're REALLY splitting hairs here. So basically he's talking about the taper that surrounds the bore right at the base of the screwdriver slot, correct? By extension, I'm guessing he's referring to a similar taper on the opposite side... (yeah, I'm a genius, right? LOL!). I tend to agree with Haunter in that this just sounds like a bit of marketing "our jets is better than their jets because (insert excuse, reason, BS here)". I had a similar experience with DJ back when I was looking into getting some jets for the 400. I pulled out the conversion chart that I often have guys reference (see link below) and threw out a couple numbers and the guy basically said the chart was worthless. I WILL say however that the numbers didn't seem to jive for the 400 so I think a little more investigation might be in order (when I have time!). I can't imagine that it would be too difficult for there to be a straight bore to bore number conversion of some kind (which is what the link probably refers to). From there, one would just need to understand how the DJs perform (with the different taper) compared to Mikuni / Factory Pro (IF there's really any difference at all). I have DJs in my 600 and it runs great. Personally, I think there would be more of a performance difference between needles. Even then, I think most people would be hard-pressed to really notice any difference between the two. I pretty certain I wouldn't be able to tell...

Sorry for the ramble..

Brandon

http://www.jetsrus.com/main_page.htm

try this link instead:-

http://www.jetsrus.com/FAQ_mikuni_vs_dy ... _sizes.htm



As to all this BS about jet sizes and tapers within the jet read the link I posted last and you will see what manufacturers use for numbering tolerances !!!!!!

yamaha_george wrote:try this link instead:-

http://www.jetsrus.com/FAQ_mikuni_vs_dy ... _sizes.htm



As to all this BS about jet sizes and tapers within the jet read the link I posted last and you will see what manufacturers use for numbering tolerances !!!!!!

Thanks Geo! That's actually the DIRECT link I was looking for (time to bookmark!). As for the tolerances, I agree, once you start dealing with really small numbers (such as those used for jet sizing) and how they relate to difficult to measure variables such as air flow (velocity & density), quantity (not to mention quality) of gas, engine speed, and whatever else is changing on a moment by moment basis - It ends up being inconsequential which brand of jet you use (tapered or not-so-much). I think that as long as they're within a size or two of the the range they should be in (regardless of brand) - you're good to go.

Off topic, but related to tolerances...

This reminds me of trying to measure valve shims. As we all know the shims have a number stamped, printed one side so you know what size the shim is. However, go ahead and measure 5 shims that say "170" on them. More than likely, none of them will actually be 1.70mm if measured with a caliper. In fact, you'll probably end up with 3, maybe 4, different measurements. If anyone needed to be ultra-picky about tolerance - THIS is where I would spend more of my time. The "within spec" range is 0.09mm (0.11 - 0.20 & 0.21 - 0.30). Most of the shims I actually measured were 0.03mm different than the number on the shim (can't remember off-hand which direction they're off)! Talk about loose tolerances! So... those who don't know any better may want to break out the caliper next time you go to check valve lash because throwing in that "170" (which actually measures 1.67mm or 1.73mm - as I said, can't remember which way they're off) may actually throw your clearance OUT of spec rather than making it any better.

Of course, you're gauges and caliper have tolerances also which compounds the tediousness of the process even more...

Brandon

FYI :


The MIKUNI hexagon main jets ( up to 500 ) are flow rated in cc per minute.

* Jets from size 50 --> 195 are available in steps of 5
* Jets from size 200 --> 500 are availble in steps of 10

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The MIKUNI round main jets are aperture sized.
The largest jet size available is a 250
Which has an aperture of 2.50 mm




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