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Justin,
Presuming that you’re running a Senior and using a K98 needle, a very general guide is that you’ll be in the 152-162 range; probably a 152 or 155 most summer days. But that won’t be precise enough to give you maximum performance, and you also risk going too lean and sticking the motor. If you’re interested in software, there are two primary solutions:
MaxJet: http://www.g-rpm.com/ (Canadian, but sold and serviced locally by Dave Galegor)
Jet-Tech: http://www.rotaxjetting.com/(UK-based, but you can purchase and download).
I’ve used both, the MaxJet for our Rotaxes and recently got Jet-Tech for our stock moto. For the novice, who basically wants to know what jet will get him close, MaxJet is the way to go. (IMHO, of course.) That said, Jet-Tech seems to be increasingly the more popular option, and allegedly has more advanced tuning capacities. If I could figure out how to get mine to work, I could tell you more. (Seriously, the Jet-Tech requires that you calibrate the device with lots of testing to create a baseline. Not so easy for someone who’s just trying to learn the package and isn’t quite sure how to know if the jetting is correct.)
In either case, you’ll also need a weather station for temperature, humidity and barometric readings (unless you have a smartphone and want to get the data from a local airport, which is particularly easy at The Track or GJMS, but not so easy for IMI, Bandi, or PPIR). Both companies sell weather stations, the one that comes with the MaxJet integrates directly with the software, which is nice, but it’s not self-standing.
Trial and error was my method when I raced my Rotax. Maybe I was too cheap to buy a jetting program, but I just tried out different jet combinations to see what worked for me. Provided you don’t go too lean (I had a 150 in my tool box, but rarely used it. Like Joe said, 152 is about as low as you’ll go in CO) it’s tough to hurt a Rotax. The beauty of Rmax rules (provided that’s what you’re running) is that the Jetting is pretty restrictive, so there aren’t that many combinations to try. When i was first starting out, I would just ask other people what they were running for jets/needle and tune from there…gave me a decent baseline.
My advice would be to play with it. When you’re out test & tuning, change something different on the carb each session and try to watch what it does. Every engine is a little different, so not everyone’s setup will work on your engine (my second Rotax liked being about a jet size leaner than my first). Listen for high pitched ‘popping’ when you’re WOT on the straights; if you hear that, you are (likely) too lean…bump the main up a size.
Your mileage may vary, but that’s what worked for me.
Thanks for the replies. Having at least a starting point is helpful for me. What seems to happening at The Track (running downhill) is I enter turn 1 at 3/4 throttle (yeah, I know it is a flat-out turn) and then by the time a get to the entry of turn two the engine seems to be loaded up and it isn’t until turn 4 that I can slowly roll back on the throttle. Does that sound like I am too rich? Obviously key details are missing to help describe, but I am struggling which direction to go.
Justin,
When you had that problem what jetting were you running? Needle and clip, main, pilots/floats, air screw, float height.
Rotaxes really dont like being off-throttle. If you lifted for turn 1 and it had hesitation when getting back into the throttle that may just be a Rotax thing. Try not letting fully off the gas, or giving the engine small burts of throttle when your on the brake, or just dont lift for turn 1 😀
Another thing that could be going on is float height, if it is set too rich then it could sputter a bit as it has too much fuel in the bowl. Also if the float height is too lean then you will run the carb out of gas going down the straight, then when you go into the turn your remaining fuel will lsosh to one side of the bowl leaving no fuel for the main as you get back into the gas.
I dont think you really have to worry about lean sticking it, we have run as lean as 135 in our motors and have never had any problems. I wouldnt really ever go leaner than a 150 jet though, as the leaner you go the more power you use. You may want to try jetting the main at 155 and setting a k98 to clip 1. It will run rich enough to get good pull out of the corners, and having the needle at clip 1 will still let it rev out on the straights.
One more thing to think about is engine temp. The temp that you run the engine at will effect the tuning of the carb as the intake air/fuel will change temp with the motor. Make sure that if you try different jetting in the carb you are running the engine at the same temp every time.
Also if you call us at the shop the day after you were at the track and fill us in on what temp it was and such we can probably help you out more.
Greg,
I will look tonight how my carb is set up. I was out last Friday and it was about 92 deg out and I was running a 158 main jet and about 2 turns out for the air screw. I assume that I am running a K27 needle and probably the 5.2g float but I am not certain – are there stamps on the float and needle indicating what type they are? I will check out all of the other settings on the carb this evening. I am planning on spending the day at the track tomorrow with a forcast of 72 to 80 over the course of the day. I figured I would start at a 155?
I understand that the Rotaxes don’t like being off throttle, but I after the engine bogs at the exit of 1, I don’t get any throttle response at all until the ledges at the exit of 4.
Thanks for the help
Justin,
The needle is stamped–and it makes a big difference (about 2 jet sizes vs the K98, as I recall). We’ve run the K98 for years, so I don’t remember typical K27 jetting.
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