Introduction by G of K.G.B of T & T

A couple of months back I was hanging out on the 2 Stroke list @ Dorje dot com when Jim Lyons mentioned a Tuner I had not heard of before. This chap, Martin Kieltsch, lives in Germany and I know a fair few of the German Smoker crowd and this guy's name had never cropped up. So I went to the link (see below the Article) that Jim gave us and had a good old read.

Having had my interest tweaked, I decided I would buy Martin's tuning book on Tuning the Water Cooled variety of RD/RZ350, which is very modestly priced I may add. My only wish is he offered one on the aircooled versions :-{   because what I got, was, a wealth of coverage on mild to wild tuning on YPVS models.

Having said that a lot of the knowledge he imparts could be applied to any 2 Stroke! I might add that he has also written a book on the RD / RZ 500 models which I have not read but based on what I have seen of his work is very probably a good read as well.

Now I have been with the RD scene since it started ( almost ) and I do not impress that easily, but this tuner had found something, that I have never heard mentioned by any tuner I know and yet I had the same "hunch" he had, way back in the past. Now being German and thorough he jumped in and investigated. Below is what he wrote:-

The problem of Jet Sizes

By

Martin Kieltsch

A small problem in finding a set-up for my RD500 following a reed conversion led me to a fundamental carburettor 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 rich in mid-range and top-end and I was wondering why, because the conversion would have required a slightly richer jetting, then I changed main jets to #170 (Mikuni) and it made no difference at all

Just before going nuts I had the idea to measure my jets in my jet store – with some amazing results     As you will see here:-

The Mikuni jet number should indicate the fuel flow which is not the same scale for all measured jets. Mikuni claims to deliver production lots of a jet size with a maximum variation of ± 10 This means if you buy a main jet labelled #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 represents 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 Ventura) leads to the jet number. Obviously the method changed about a decade ago (I’d guess 1985).
I found that some of my jets had a serious mismatch between label and fuel flow of upto #30 numbers and more.!!!!!

The stock RD500 #195 jets all had a fuel flow of about #165 (present Mikuni labelling). The #180jets 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 behaviour.

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 using a small kitchen measuring jug 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.

• Clean the jet very carefully and pour in a full bottle charge of water before starting.
• Make about 5 to 10 measures for one single jet. (Typical values would be: 129s, 125s, 122s, 122s, 121s, 123s, 120s)
• Clean the values from obviously too high/low times (here 129 and 125), calculate the average time and standard deviation (mean-square error). See your math teacher or use a scientific calculator to perform that point. (Average = 121,6 ; standard deviation = 1,14 => The right time is between 120,459 s and 122,74 s with a security of 68,3%)
• The flow rate in ccm per minute is calculated by: 125 [ccm] x 60 / flow time [s] = flow rate [ ccm / s] (121,6 s lead to 61,67 ccm / s). Make the same calculation for the min/max values of the standard deviation to judge the accuracy of this single jet measurement.
• Make a diagram (scale paper or PC/EXCEL) where the x-axis is the jet size (labelled number) and the y-axis represents the flow rate.
• Connect the average points with a straight. This is your reference straight for future measurements with your special equipment (Here: Y=0.397*X).
• To calibrate your individual test rig to the statement flow rate = jet label you have to introduce a proportional factor k to convert the straight to Y=k*0.397*X. For our "good" jets (which are not too far away from our reference straight) this is calculated: k = Jet No. / Flow rate [ ccm / s] (For the jet #170 with 110,4 s and 67,93 ccm / s the value for k is 2,5024). Calculate the k value for all jets and use the average k value (here 2,5188) for your converted reference straight.
• Now you can directly use the measured times to compare to the reference straight by calculating the corrected flow rate: y = 2.5188 x 125 x 60 / flow time [s].

©  Geo of K.G.B. of T & T.