Rotax Jetting Questions

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  • #42770
    Scott Mowrey
    Participant

    Hello all,

    Well, got the kart on the track Saturday at Centennial. That was the good news. The bad news was that I could only get about 9 laps in before the engine became undriveable. It would bog down and stall at lower RPMs, and would only run well between about 6000 (which too a lot of finesse to reach) and 11500 RPMs, which I think are the speeds that I am on the pipe (according to the manual). It’s a Rotax engine running in the Master TAG class. It was set up with 12/77 gearing.

    I suspect that the jetting was not correct. I made the mistake of not picking up some jets at Shockwave when I picked the kart up. Anyhow, I would assume that a 152 jet would be way too rich for a hot day….Any ideas? Also, I was thinking about picking up the Jet Max software- any opinions one way or the other about it?

    Thanks for any feedback!

    Scott
    #40

    #61620
    cgordon
    Participant

    Scott,

    I was at The Track on Saturday and in those conditions a 152 jet would be OK, though you might have been able to go to a 150.

    For TAG Master, 12/77 is a bit tall. Try 12/82 for a starting point. You’ll get more revs everywhere and the engine will pull much better out of the slow corners.

    Charles

    #61621
    Scott Mowrey
    Participant

    Thanks for the info. I don’t have any weight on the kart but I weigh about 225 with all my gear….

    I thought that gearing may have been the issue as well. I’ll pick up a couple from Doug this week, an 82 and 81. Do you run a Rotax Charles? What gearing would you recommend for IMI?

    #61622
    cgordon
    Participant

    I race a Spec Shifter now, but I raced Rotax for several seasons. For IMI, 13/78 is a good starting point.

    Charles

    #61623
    Jeff Welch
    Participant

    Charles is right on for gearing. His jetting advice is very sound too, for MOST Rotax engines. We’ve found over the years that a few Rotax engines like to run different jets than is usually considered normal. We had one engine that was an absolute dog until we put a 140 in it, and we were running 148-150 in all our other engines!
    The MaxJet is useful as a baseline tool, but you will still need to experiment to find the optimal jetting for your kart.
    In this case, gearing was most likely your biggest problem. If you haven’t thoroughly cleaned your carb and power valve in awhile, I’d do that too just to make sure everything’s working the way it’s supposed to.

    #61624
    Scott Mowrey
    Participant

    Thanks Jeff. I think I talked to you last Thursday or Friday about this subject. I think I will pick up a couple of gears and try that first….along with a set of jets. Do you set the kart gearing to max out the RPMs on the straight or to have more power out of the corners?

    The carb and power valve were sparkling; it was the first time out since Doug put the new head on it, so everything was clean and adjusted. One thing I did not mention was that the engine would not hold an idle. I understand that this is how some Rotaxians set their engines up, but I am not sure why. My son’s Mini-Max will hold an idle and he can scoot!

    Scott

    #61625
    Jeff Welch
    Participant

    We set up our engines (Rotax and otherwise) so that they will not idle. The reason is, when I’m not on the throttle and under braking, I do NOT want the engine pushing me forward, not even a little bit. It’s a very minor difference, but it does help just a tiny bit under braking. I don’t mind holding the throttle open slightly when idling in the pits.

    Also, if you allow the engine to idle when you’re coming off the track, that tends to load the engine up with fuel, which makes it difficult to get an accurate spark plug reading to determine jetting. Better is to “hot chop” the engine. Basically, this entails running the engine at full racing speed (high RPM) then completely releasing the throttle and coasting into the pits.

    Gearing… that’s kinda a judgment call. Both the straight and the slowest corner are important considerations. Generally the slow corner will be most important in a TaG, particularly if you running against a variety of engines (TaG as opposed to Rotax only). It’s often very worthwhile to work specifically on driving around the slow corner faster, which allows you to run a lower gear which gives you a little more top end.
    A couple general rules:
    -If you’re not revving out on the straight but are ok in the slow corner, you may want to add more gear
    -If you’re in the powerband out of the slow corner but are topping out halfway down the straight, that’s probably fine.
    -If you’re not in the powerband out of the slow corner, you should do everything possible to get there – unless you like getting passed 😉

    #61626
    Rick Schmidt
    Participant

    Scott,

    I don’t know how long you have been running the Rotax, so don’t be offended if you already know this.

    All good advise you’ve received on this thread. One of the biggest things new Rotax drivers need to adjust to is, not letting the slide close all the way. You don’t have to power brake the hell out of it, just keep the slide lifted enough for the brakes to be effective. If you do let the slide slam shut, the carb goes back to the idle circuit and takes time to create venturi vacuum causing a delay or as some call it “a bog”. Also, don’t just jamb it coming out of a corner, just give it what it can take and ease into it.

    A 152 jet does not sound that bad. The Rotax seems to like fuel anyway. There is inconsistency between the different engines and carbs. Our first Rotax would require a 148 jet when everyone else seemed to run a 155. Last years motor never saw a jet leaner than 155 in competition. (qualifying is a little different) If you lean it to where it gives you an occasional (one a lap or so) pop, you’ll probably be ok. We found we could go lean and be fast for the first handful of laps then it would start to lose some power. It’s a trade odd for sure. The Rotax has a pretty small radiator and a pretty big carb. Possibly check your float level if you haven’t done so yet. And keep the carb clean. We also had problems last year using the Rotax legal fuel filter. Our problems sound somewhat consistent to what you are experiencing on top. The filter element media seems to be breaking down and getting inside the carb.

    Roman ran a 12-80 gear at the Rotax Grands, (clockwise) and had a trap speed of 79 mph at the braking zone going into one. (mid 56 second laps) Gearing is always some kind of compromise. At The Track in a Rotax it seems the hairpin is the corner to concentrate on, if you can’t get off of the hairpin, it’s going to be tough to generate good lap times.

    Also, as has been mentioned numerous times, battery charge has an impact on the motor being able to rev as well as the exhaust packing baffle. Stick a flashlight in the end of the pipe and make sure there are no breaks in the perforated tube you wrap the packing around.

    Be religious about cleaning your power valve and carb is the best practice.

    Rick

    #61627
    Scott Mowrey
    Participant

    Rick and Jeff,

    Thanks for taking the time to divest your knowledge. Saturday was my first time out there with the kart and I am sure that I need to learn the “Rotax Finesse” with the throttle. I’m guilty- I was letting off the throttle completely going into the turns and mashing it out of the corner, so I am sure that did not help, just used to the rental karts I guess. I was really bogged down exiting the hairpin (and the pits for that matter), and I am sure gearing and throttle played a big part in that…With the 12/77 setup I was clocked at 83 going into turn 1, but doing 1.09 to 1.11 laps, and I could not get power out of the turns, so it seems the gearing was a little high, and my finesse a little low!

    I will check the floats this week, change the gearing, and make sure the fuel filter in the carb and the air filter are clean. I checked the exhaust baffle and everything looked good there. What’s a good range of jets to have on hand? And, well, this is a stupid question, but I forgot which jets are leaner, higher or lower?

    Scott

    #61619
    Rick Schmidt
    Participant

    146 – 158 would more than likely do it.

    The smaller the number the leaner the jet.

    #61628
    Garrick Mitchell
    Participant

    Late to the party, but oh well. All I’m going to add as a fellow n00b who doesn’t have a MaxJet is…

      If there are other Rotax runners at the track, ask how they’re jetted that day. Over time, you may find what works for others works for your engine too, or you may need to consistently go one way or the other.

      Keep notes of what jet you ran, the temperature at the time (I look up the hourly observation at Centennial Airport (KAPA) on my phone), and what RPM you were able to reach on the front straight.

      If you’re not sure you’re jetted correctly, go up or down by one jet and see the difference.

      Learn symptoms of running lean or rich. I’m still working on that myself… There’s the infamous “lean pop” at high RPM, but sometimes you can pop when rich too, I’m told. Look at your spark plug for excessive oil (rich) or a whitish, “cooked” appearance (lean).

      Personally, I release the throttle fully under braking, but I don’t slam it open or shut. Works for me, anyway.

    I have just enough sessions under my belt to know that a little jet adjustment on the Rotax can make the difference between a sweet ride and a lead sled. That, and you were running an enormous gear! 🙂

    #61629
    Scott Mowrey
    Participant

    Wow, thanks for all of the info!

    I went ahead and ordered some gears and jets from Shockwave. I plan on heading out to The Track on Friday afternoon and do some experimenting. Before then I will check the floats and filters…

    A little off-topic- can I run pump gas through the engine instead of paying 10 bucks a gallon at the track? I never thought that 4 bucks a gallon would seem like such a bargain!

    Scott

    #61630
    Garrick Mitchell
    Participant

    Premium pump gas “will work” in a Rotax. The first year I had my kart, that’s all I put in it (well, a dash of oil too :rotate: ).
    The oxygenates in pump gas may affect your jetting, in turn requiring a bit of experimentation on your part, and I’ve heard racing fuel is just way, way more consistent. However, if it gets you seat time, I say go for it.

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