When is a 220 main, not a 220 main?

[lecroy]

I got this used Titan micrometer thing. After getting it all working, I was playing around with it and measured some stock Mikuni jets for fun. A 160 main for example should be 1.60mm diameter.

I looked at a 220 and a 230. The 220 measured 0.09230" and the 230 measured 0.09380". If anything that 220 should have been marked a 230. No way they could be this far off, so I measured some standards, nope, they are that far off. I figured they were precision made.

I guess when you jet your in-line 4, buy all the jets from one lot.

 

[49er]

I got this used Titan micrometer thing. After getting it all working, I was playing around with it and measured some stock Mikuni jets for fun. A 160 main for example should be 1.60mm diameter.

I looked at a 220 and a 230. The 220 measured 0.09230" and the 230 measured 0.09380". If anything that 220 should have been marked a 230. No way they could be this far off, so I measured some standards, nope, they are that far off. I figured they were precision made.

I guess when you jet your in-line 4, buy all the jets from one lot.

Mikuni jets are rated on what volume they flow per minute. Dyna, Dellorto and Weber jet numbers are equal to the oriface diameter in millimeters, ie a 180 main will measure 1.80mm.
I have a jet drill set and do all my own re-jetting in mm. If you have the right formula it saves being hampered by the inaccuracy of incorrectly sized jets. With a comprhensive dill set you can check that all your jets are sized correctly. Never take for granted that a jets size is as marked.

 

[lecroy]

I agree, drilling is a good way to go if you have some extra cash laying around to buy the set. In my case I don't have a choice but to drill.

I have this old book, I need to double check but I thought it stated that Mikuni used both standards (flow and diameter) depending on the jet type. I thought the hex shaped parts were in flow, while the round ones were in diameter.

Strange that the round ones I measured were within one jet size to the stamped diameter.

Could you tell me how they rate the flow? I did a quick search and a lot of chatter about them using flow, but no details, just flow. Thanks.

 

[49er]

I agree, drilling is a good way to go if you have some extra cash laying around to buy the set. In my case I don't have a choice but to drill.

I have this old book, I need to double check but I thought it stated that Mikuni used both standards (flow and diameter) depending on the jet type. I thought the hex shaped parts were in flow, while the round ones were in diameter.

Strange that the round ones I measured were within one jet size to the stamped diameter.

Could you tell me how they rate the flow? I did a quick search and a lot of chatter about them using flow, but no details, just flow. Thanks.

I believe the following explantion is correct. The example pertains to two stoke mikuni tuning but is relevant to 4 stokes as well. This was taken from the web.

"A small problem in finding a setup for my RD500 following a reed conversion led me to a fundamental carburetor problem.
After rejetting from stock #195 main jets (stock YAMAHA with Mikuni marking) to #180 (G?tz) with #22,5 power jets (using RD/RZ 350 idle jets with 4 mm thread) the engine was running very poorly. The mixture was far too fat in mid-range and top-end and I was wondering why, because the conversion would have required a slightly fatter jetting. Then I changed main jets to #170 (Mikuni) and it made no difference at all.
Just before getting nuts I had the idea to measure my jet store ? with some amazing results - .
The Mikuni jet number should indicate the fuel flow which is not the same scale for all measured jets. Mikuni claims to deliver lots with a maximum variation of ? #10. This means if you buy a main jet labeled #180 it?s just sized in the range from #170 to #190.
The jet number is linear dependent from the fuel flow which means a #120 main jet has 20% more fuel flow compared to a #100 jet. Do not mistake fuel flow with jet bore diameter! This dependency is not linear!
In an older jet chart Mikuni also claimed the jet number to represent the fuel flow in ccm per minute. This is only true for a special test combination of fuel, pressure and jet-type. As far as I know nowadays the jets are measured with air and the difference in pressure (before and after the jet venturi) leads to the jet number. Obviously the method changed about one decade ago (I?d guess 1985).
I found that some of my jets had a serious mismatch between label and fuel flow of #30 numbers and more.
The stock RD500 #195 jets all had a fuel flow of about #165 (present Mikuni labeling). The #180 jets from G?tz had almost the same flow rate as the #170 Mikuni (#173 and #171). What made it even worse was that the #22,5 idle jets had a bore which compared to Mikuni #60 power jets (Mikuni number N100606). With that knowledge I dared reduce the jet size to stock #195 (= front #163, rear #168) combined with #30 power jets which lead to a usable engine behavior.
For this reason I?d advise anyone who is going to change jetting to make his own jet measurements".

Makes you want to do all your own jet sizing, don't it? Knowledge really is power.

 

[lecroy]

Agree, if what your told is always right, that knowledge is a great thing. The RD is very old and I believe what they are saying about these jets being flow rated. However, it does not mean that all Mikuni jets are rated this way.

This following was taken from the book I spoke of:

"Some carburetor jets are classified with reguard to fluid flow; the number imprinted upoin it signifies the number of cubic centimeters of fuel passing through its orifice at a given pressure and specified time. Other jets are classified according to orifice diameter."

"Flow-rated Mikuni main jets are identifiable as hex-headed jets. There are both large-hex and small-hex jets but they are both flow-rated...."

"Mikuni's round-headed jets, on the other hand, are classified by aperture size."

It goes on to calculate the flows for the round headed jets. All of my Mikuni jets are the round headed types. My guess is that the book is right and that they are +/- one jet size with these. This would match with what I see with the Titan. It would be interesing to see how Mikuni does their flow tests.

I think your ok to buy jets predrilled for the in-line fours as long as they all came from the same lot. I have done this with several bikes, no problems. But it sure makes you stop and think about it.

 

[49er]

Mikuni mystique

Mikuni mystique

Thanks for the clarification. This Mikuni anomaly has obviously caused many problems for tuners in the past. You're not able to compare apples with apples without the knowledge that Mikuni are using both these rating systems.
All the jets I have used in my VM's are all round headed and were close to the oriface size that they should have been. I thought it was a coincidence that the flow value was nearly identical to the oriface diamenters.