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| Features |
| Tech Talk: Tuning the Dell’Orto Carb – Part Two of Series by Bob Hogg |
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Editor's note: This is part two in a series of Dell'Orto tech articles written for Ekartingnews by Canadian tuner and former professional racer Bob Hogg. This piece focuses on identifying, taking apart and cleaning the carb and utilizes photos in figure form of the carb used for the Rotax Max FR125 engine. Just click on any photo to enlarge. Special thanks go out to Bob’s friend, John Short, who supplied the Rotax Max carb for our use. John’s web site can be found at http://www.shortracing.ca/accparts.shtml
In upcoming segments on tuning, an ICC carb with pictures will be utilized.
by Bob Hogg
I usually spray my carb with brake cleaner to remove dirt before ripping into it. If it’s really dirty give it an external bath to remove dust and sand. Draining your Dell’Orto after every race weekend is good practice as well. This helps prevent the little passages from getting clogged.
 | | Figure 1 |
Figure 1 Figure 1 shows the 3 bolt (5 mm hex) float bowl removed - that was easy. Get the main jet out along with the slosh shield next. The location is center sitting out the furthest. The MJ sits very low in the float bowl, actually inside the bottom bolt. The shield helps maintain fuel in the jet area as the vehicle bounces around.
 | | Figure 2 |
Figure 2 Figure 2 has the above-pictured jets removed. The next set of jets, going towards the engine side of the carb is the main pilot jet. Under the main pilot jet is the emulsifying pilot jet. What a trick idea. Read about, “why did Dell’Orto do that” later. The last jet at the bottom of the picture is the enriching jet or choke tube – not a very popular item. NOTE – be careful removing the emulsifying pilot jet. Use the correct size flat blade screwdriver to avoid breaking the top of the jet. Too wide a screwdriver could burr the threads and then you’re screwed.
 | | Figure 3 |
Figure 3 Figure 3 is a picture of the emulsion tube or needle jet removed. The ICC carb uses a primary type tube while the Rotax is a bleed type, sort of. The design with the holes near the bottom tends to lean the mixture at high revs. Larger holes at the top would richen. It has to do with the fluctuating fuel level in the bowl. A true form of bleed type has a variety of holes top and bottom. The wall distance between the O.D. of the emulsion tube and the sleeve is also a tuning factor. True bleed type tubes are usually used in four strokes today.
Another Form of Tuning
Another form of tuning, I won’t go too far, is a washer or ring mid way down the bleed type tube on the O.D. of the tube and internal of the outer tube. This builds another emulsifying area. We had some trick Mikuni carbs to test on our 440 cc snowmobile, years ago, using that principle. They were monster 54 mm butterflies with a float bowl. They had to use butterflies, as I couldn’t lift that size slide with my thumb plus it would take too long. The secondary internal circuit was used to compensate for the oversize hole in the carb. Anyway, we only had that package for one weekend, as too many other things were wrong. I can still remember the tremendous acceleration it had.
 | | Figure 4 |
Figure 4 Figure 4 illustrates the slide removed as well as the jet needle. Be careful re-installing the slide and needle. We usually give it a good visual and movement test before putting the air box on. An easy way to remove the clip is to lay it flat (open clip side down) on a hard smooth surface. Simply push down on the needle with one finger and place a finger over the clip so it doesn’t fly. You can re-install the same way. What about that Rotax spring – wild! Put a clown face on the end and it would make a great toy, or did somebody already invent that?
 | | Figure 5 |
Figure 5 Figure 5 is of the low speed airscrew being removed. Before you take it out – screw it in until it bottoms counting the turns as you go. That way you have an idea where to put it back to when the time comes.
Look back at Figure 3 and you will see the inlet valve under the two prongs. The valve has a rubber tip and can be destroyed by some carb cleaners. Carefully pull or lever the pin out using needle nose pliers from the serrated end of the pin. Re-install the same way. Drop the valve out and check for dirt or wear on the tip. You will notice Rotax has used the 1.5 inlet valve size to prevent fuel from overflowing out the vent tubes. Most motorcycle carbs have a 3.3 inlet valve size suitable for gravity feed.
Remove the balance of the gaskets as well. I think the best way to do a good cleaning is to soak it in a small container of carburetor cleaner obtained at most auto stores. It comes with a basket to lift it out. Follow instructions, as it’s pretty strong stuff. Then wash it in warm water and soap and rinse followed by blowing out all passages. On a week-to-week schedule we usually spray all the passages and jets with Honda brake cleaner and then blow with an air gun. Follow air gun rules as well – very dangerous. We also use an ultrasonic cleaner once and a while but that’s a bit extreme. Oh – we use the Honda stuff, as it is the only spray I have found that doesn’t contain oil.
Nest session, we will try to squeeze into one page, what each jet does during the transition. I can assure you, in the field of jetting, there is no end. Hopefully, by the end, you will realize why every jet is important for ultimate speed, not just the main.
To review part one in this series about the basics and history of carb designs, visit http://www.ekartingnews.com/news_info.php?n=2235
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