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 (Götz) 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 Götz 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. The values will be different but the intention is to compare "unknown" with "known" jets and to judge if the jet label is in a valuable range.
I used a rinsing bottle for battery acid (diameter about 70 mm, about 180 mm height) with an 80 mm hose with 5 mm inner diameter (to screw in the jets). Pour in an exact amount of water (small bucket with scale or letter scales) and measure the time in which the whole amount has flowed out. To avoid deviation in measuring you should follow exactly the following instructions.

My test setup had the disadvantage that you have to measure quite exactly. If you use a higher test volume (250 ccm) the measurement will be more accurate but it will double the time you need!
As an example I’ve listed my measurements below.
 

Table 1.: Jetsize measurement for the in-range-jets
 

Table 2.: Jetsize measurement II (Out-of-range jets)

Figure 1.: Reference straight for the corrected flow rate
The conclusion out of the whole thing is: Do not trust any jetting specifications without comparing it to own experience or at least two other independent origins. If you want to find a carb setup use a complete set of new jets purchased completely from one source. Do not work with old jets unless you’ve measured them thoroughly.
Wolfenbüttel/Germany May 98
Martin Kieltsch
If you have any questions concerning this stuff or RD500 RD/RZ350 tuning specs: Martin Kieltsch
(You can mail in English and what is even more appreciated in German)

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