Home Forums Tech Talk Tuning the idle jet with EGT Reply To: Tuning the idle jet with EGT


Rob Kozakowski

Some good info from Al, however I think his lack of knowledge of the Dellorto and similar carbs is a bit off when he says he doubts the needle clip position has much impact on the low speed.

Ray, do a google search for Rotax carb tuning, and go to rotaxjetting dot com and do some reading – John Savage provides some good info on the Dellorto’s – it’s obvious that the needle clip position is very important for low to mid range performance.  Also, Tom Barth has some good info on his site, greenflagkarting dot com

While a “carb is a carb”, there are some practical differences in how most approach the tuning of a Walbro on a KT vs. a Dellorto on a Rotax.

The beauty of the Walbro is that it can be adjusted on the fly so you can reallly fine-tune your carb to ever-changing conditions.  While I doubt that anyone would ever dispute the value of the EGT for tuning these carbs, there are LOTS of drivers who can tune these carbs just as “effectively”, with experience, by the seat of the pants.  The benefit to this is you focus more on your driving and less on your gauges.  The downside is if you don’t know what you’re doing, you can easily go “too far” or the wrong way and stick the engine.  An air density gauge in this case will help you learn and predict where you might want to set things for different conditions, but you can get by just fine without.

The beauty of the Dellorto and similar carbs is that they can’t be tuned on the fly, so who really cares what your gauges are telling you in real time while you’re driving – especially now with capabilities of the Mychrons, etc to store other data for analysis after you’re off the track.  The downside is they are very complicated to fine-tune to extract maximum performance from them – especially idle to mid-range, which is progressively affected in different ranges by various pilot jets, air screws, selection of needle, needle clip position, emulsion tube, etc.  Fortunately, top end is much more simple because you’re really just talking main jet.   There is almost no disputing that an air density gauge or weather station is critical to getting maximum performance from these carbs.  The trouble is conditions can occasionally change faster than you can adjust for, so while you might be able to tune to a set of conditions before you hit the track, if those conditions change while on the warm up lap, you’re suddenly not getting maximum performance.  The counter argument to this is that it becomes so hard to tune for “maximum” performance, that most people will be happy enough to accept something “close enough”.  One of the articles on Tom Barth’s site summarizes these carbs best when he says something to the effect that these carbs really are meant for serious tuners, otherwise, just run the thing, read the plug and change the main jet.

Which goes back to John Savage’s advice of setting the idle speed for a smooth idle and focus on mainjet and needle clip position in response to changes in air density.