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| Features |
| Tech Talk: Bob Hogg's Tuning the Dell’Orto |
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Editor’s Note: We hope you enjoy this new tech series focused around Dell’Orto Carbs from Canadian tuner, Bob Hogg. This is the first part of an ongoing series from Hogg who has an extensive background in professional motorcycle and snowmobile racing. Over the past two years, he has moved into karting, tuning for his son Ian. As with any tech articles, they are based on the thoughts and personal opinions of its writer. As we have come to know Bob, we feel he has a lot to offer as you will learn in some of the coming articles as well as in this beginning piece.
Bob’s son, Ian, began his shifter kart activities with a YZ125 (placing second in his series) and now competes with an ICC where he won his series championship as an independent team. Both father and son live in St. Davids in the town of Niagara-on-the-Lake, near Niagara Falls with Bob's wife and Ian's mom.
In the late seventies, Bob’s full time occupation was with the Yamaha Motor Canada who also sponsored his snowmobile racing career where he claimed wins at every major track on the professional circuit in Canada for both money and fame. From there, he moved to the fun side of the sport in the early 80’s competing with a Yamaha prototype quad and then with a Honda Canada sponsored ATV. He moved from there into a custom parallelogram suspended flat track bike. In 1999 and 2000 both father and son were sponsored by Yamaha Canada and USI Skis to race snowcross.
He currently earns a living designing and building liquid level controls and feels very lucky to have raced with the very best a factory-racing department could produce. He feels this experience helped them understand that there are no magic engine parts. In karts, handling and well setup motors run like a fine tuned instrument under talented teams.
A Clear Vision of Basics and Carb History
by Bob Hogg
No, this is not another copied carburetor tuning article loaded with math designed to glaze our eyes over. The procedure and ideas are based on one person’s view, correlated from actual winning experiences and information gleaned from the support of factory personal. The information will also be in segments to prevent boredom for both the writer and readers.
To be honest, we have been out of the carburetor loop for three-generations of models. Only in 2002 did we get involved again, which was probably a good thing. It was fun to look at development and direction engineers are taking. Later, we will describe very briefly, our experience with the wicked fast circle m pulse fuel injectors designed by the famous road racer Kenny Roberts / Yamaha and the Mikuni experimental 55 mm float bowl butterfly carb.
 | | Two Slides |
To see the advantage and disadvantage of the Dell’Orto, we need to look at a few other brands first. I don’t think the recent TMS is around so the latest USA Mikuni is the TMX. The first flat slide, Mikuni TM, was actually developed thanks to Suzuki looking to fit a narrower carb on the bike. A good selection of needles and removable needle jets were also made available for the TM. You will see the TM used extensively on the 80 cc shifters. Brass needle jets are the removable brass tubes also found in the Dell’Orto. Further slide carb development then moved on the TMX to offer superior throttle response and fewer parts.
 | | The TMXx |
Was that the end of development, did the TMS finally indicate we were at maximum? Nope, the Asian bike manufacturers wanted more power and throttle response off the turns. The distance across the slide and how the pillow of air is formed is crucial to a stronger negative signal that the needle reacts to. So – they built the TMXx with an amazing new heavy “D” flat slide. Yamaha’s design soon became the standard to build to.
A problem with using that carb in a kart moto engine is the needles are designed for the specific power curve of its OEM motorcycle engine and pipe. When you run out of needle, say on the lean side, you would simply go to the next needle available on a bike. The Keihen is similar to the TMXx but may have something more to offer at high end. For 2003, Yamaha used the TMXx 39’s on the YZ125 dirt bike and the 39 mm Keihen on the road race high RPM TZ250 (Twin 125cc cylinders.)
 | | Fig. 3 - The carb and bowl - see caption notes in text |
Our personal carburetor learning curve has done a complete 180 on the use of Dell’Orto carbs. In my opinion, it would be the ultimate carb for moto engines as well as ICC. To begin with Dell’Orto has several inlet needle valves as described above. Many karters opt for pump around or simple tee to overcome the major problem of too much fuel entering the float bowl. The reason is the OEM inlet needles are designed for gravity feed and pumping in all that excess fuel simply blows the valve off the seat. The bottom end will be way too rich and not tunable. We solved the problem on a TMXx 39 mm by finding a 1.5 inlet assembly from a TM and re-machining it to replace the stock 3.5 TMXx valve. Only a highly qualified carburetor technician should perform this procedure. Stick with the pump around or tee.
Figure 3 Notes: You will notice a strange tube exiting from the bowl of the TMXx. We installed a level tube so we could easily check the fuel level at rest or on the stand. Float levels are critical and with the carburetor at such an angle on the moto, it is sometimes difficult to judge. With our visual level tube, what you see is what you get.
 | | Inlet Valves |
The Dell’Orto has a full line of rubber tipped inlet needle sizes keeping everybody happy. Be careful as too small of an inlet needle will not allow enough fuel to maintain the proper fuel level and can result in a seized engine. Caution should be used when replacing the valve, as you don’t want brass fillings falling in the passageway
The Dell’Orto just keeps looking better in this area. Not only do they offer many inlet valves but the pump is designed with a pressure relief valve that senses slight backpressure and can actually compensate to help prevent flooding. Mount the pump above the engine intake pulse line to avoid oil and fuel from flowing back into the pump diaphragm cavity. A pump can work basically from anywhere, but being a parts mounting rebel in the sport, we mounted ours beside the gas tank on the floor. We had no problems with our 2002 moto; however, the pulse line on the ICC is more conducive to fuel flowing back due to the location.
 | | 15 Needles |
That’s it for today; we only wanted to present a base line of information that can be referred to as we get into the very tunable Dell’Orto. Stay “tuned” for more in this Dell’Orto series of tech articles exclusively on Ekartingnews.
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