Jetting Calculator

Veso: “I’m more from karting and of course there also are some usual jetting practices. Also there are always superfast guys. The engine pulls like a train and nobody knows from where. Here I’m talking sealed equal engines without tuning. So where is the difference? Carb is the only possibility, as all other factors regarding engine are standard. So I bought the calculator from Michael and entered the data. What was a mainstream jetting practice in the calculator was nowhere near good. So we made the changes, my kid went on track and good progress was made instantly. He is 8 years old and came out after 3 laps just to tell me the engine is better than before. So in summary the calculator helped me, and all this for only 15$. it is nothing compared to buying 300$ of atomizers, needles, jets…”

“We are using Michael’s jetting calc, and as long as you get your base jetting right in terms of main and idle jets, it’s very helpful to give you guidelines about your needle setting, which is it’s primary intention. No software in the world will spit out jetting setting for you because every engine has so much variables, so it’s nearly impossible for a software to tell right main jet size for your particular engine setup. But for tuning around needle, it does it’s job more than well.”

Bjorn: “We are using the spreadsheet when fine tuning the needle and it is way better than the normal guess work.”

What engine was it out of curiosity? I see VHSB as the carb and I can’t recall a common engine in karting that uses the VHSB other than Rotax Max which has spec jets, needles and nozzles.

Since this is basically an ad I’m moving it to classifieds for now.

I don’t remember which carb Veso had. I just put that screenshot of the Dellorto sheet here just to give an idea of what the program looks like. It has separate sheets for the Mikuni VM, Mikuni TMX/TMS, Keihin, and Dellorto. You can review its features at Needle Selection

Oh wow, this is an interesting exchange. Evan was just being funny, he is a great contributor to this forum and I’m sure that you also have good intents. If this were my software I’d be proud of it too.

I ran Rotax (Gen1) and other float carbureted motors for almost a decade (unfortunately, everything I do is now measured in decades) and I got pretty decent at tuning but I’m sure that I had advantage over some because I could tune the carb.

A jetting calculator will get the carb in the ball park by selecting the main jet, that is if you’re way off beyond that I’ve found that the jetting calculator needs to be tuned for the engine and carb over a period of time. I always tried to use the same carb on different motors to keep the # of variables down. The rest of the carb tuning has to either be done by driver feedback or maybe through the use of data, driver feedback and the jetting calculator.

The other thing that the jetting calculator does is, it gets you thinking about how each element could affect the engine performance, so that you might ask the right questions to your 8 YO, asking an inexperienced 8 YO; “How’s the engine running” will not generally yield useable answers.


I found when designing my calculator that there are too many variables to be able to calculate the needed main jet so the user has to jet for power (not plug color) and then use the program to get these right:

  1. needle to jet clearance. 2. cutaway height. 3. needle shroud height. 4. needle height. 5. needle taper
    And it does have a “whatever” carb section if it’s not a Mikuni, Keihin, or Dellorto.

Let’s try to keep our arguments reasonably productive. Evan made a good point but I can see how the 8-yr old bit rubbed you wrong.

Play nice everyone! It’s really easy for internet speak to get misinterpreted. The web is tone deaf.


This topic was automatically closed after 29 days. New replies are no longer allowed.