|
 |
| Features |
| Tech Talk: Bob Hogg's - Understanding the Dell’Orto Carb–Part 3 |
| |
By Bob Hogg
Editor’s note: Canadian Bob Hogg provides us with part three in his Dell’Orto carb series. In the first segment, he reviewed the history of the carb explaining other models and how things developed. In part two he provided readers with a pictorial of the carb broken down and explained how he takes apart, cleans and cares for his carb. This segment, Hogg begins to get more technical explaining two types of carbs, the enriching circuit with a small valve and jet and one with an idle and transition circuit which is more popular. The entire series can be found by clicking on the "Tech" section at the top of the EKN navigation bar.
- RMB
Part Three in Dell'Orto Series
Do you ever wonder why the Rotax carb doesn’t spew fuel out the vent tubes? The reason is, it comes with a small 1.5 inlet valve. You have the overflow problem with moto engines as they usually have a 3.3 or larger valve designed to work with the gravity fuel feed on a motorcycle or ATV.
You can break the Dell’Orto down into three segments of operation. Below we will describe roughly how each section works and blends into the next. The three main segments are; the enriching circuit commonly called the choke, plus the idle circuit and main circuit.
Carburetor Number 1
The enriching circuit has its own little valve (plunger) and jet. Not many fool with this so we will go on. As a point of interest, recent model factory bikes are having special Mikuni’s built with that part completely left out. They are trying the save weight and get the carb smaller.
Carburetor Number 2
 | | Figure A |
The second little carburetor is the idle and transition circuit. We could fill the page with this, but I’ll try and keep it brief. This circuit is the one that gets the engine going and provides the transition from the slide sitting on the stops to just off the stop. The circuit is more important on moto cross and snocross as the attitude of the vehicle in the air, off jumps, is controlled by throttle and brake. Give her gas and the nose rises, pull in the brake and it drops. For the pros – yes it is also controlled in the beginning on how and when you let off the gas as you leave the jump. An idle circuit too rich allows the engine to run on thus lifting the nose for a tail landing. A good working idle circuit helps in engine braking as well. And – yes – too lean and it will seize the engine when the throttle is slammed shut at high speed.
I believe the Dell’Orto has the best idle circuit design except the airscrew could use a bit longer thinner taper for finer tuning. It uses two pilots, however it seems only the outer is used by most karters. The inner emulsifying jet is almost impossible to find at most dealers.
With the slide flat on startup, (Fig. A) air velocity is not fast enough to suck fuel up the emulsifying tube or needle jet to get it going. The idle circuit has a hole downstream of the slide and a hole just under the slide. Air enters the hole under the slide and gets the engine side idle tube to send out a small stream of fuel into the engine.
 | | Figure B |
As the slide lifts, ever so slightly, the Dell’Orto enters the transition stage moving to the main circuit. Air is now traveling faster under the slide and over the idle jet holes. Fig. B. The hole under the slide takes on the new job of helping to introduce fuel into the throat instead of sucking in air. This is the stage you either get the buuu– buuu or the whaa whaa.
It is a common misconception to think of ICC’s not having torque when in fact it is probably poor carburetion. The transition from the idle circuit to the main should be seamless. The low speed air needle is working as well and we will go over that the next issue. In Fig. C you will see that the transition is passed, the idle jet is helping and air is beginning to rush under the slide now picking up fuel from emulsifying tube.
 | | Figure C |
Fig. D is simply an enlargement of Fig. A to C with some detail. Again you will notice the Dell’Orto has two pilot jets. Very few run the inner emulsifying jet and they seem to be hard to find at dealers. The Mikuni and Keihen both have emulsifying pilots only. The key to carburetion is emulsifying the fuel correctly instead of letting it flow in globs. The low speed air needle controls the air mixing with the upper pilot.
 | | Figure D |
So – why two pilot jets? The upper jet is the emulsifying jet the lower is the pickup. The next great thing that it is designed to form another chamber between the two jets to help maintain a steady flow of fuel in the pilot circuit. Will it run with one or the other? Yes, the Dell’Orto will work using either one of the pilots. The factory recommends running both the same size. I usually run a bigger outer pilot and use the inner for tuning.
Carburetor Number 3
 | | Figure E |
Finally we get to everybody’s favorite, the main circuit. See Fig. E. OK – so we have lifted the slide off the stops. The slide is used just off the transition and to hold the needle. Much research is being done with the Mikuni and Keihen carbs these days in that area. The narrow design (flat slide) is used to improve the pillow and speed of air in and around the slide. The Dell’Orto still uses the flat face design while the Asian manufactures have advanced to buffeting with a “D” shape. Again see Fig. F with the Dell’Orto on the left Mikuni on the right.
The correct front cutaway is the key to instant low-end response. Typically, the ICC engines use slides ranging from a 40 to 60, the latter being the leanest. The higher the number, the higher the cutaway the more air it let’s in making it leaner. We always check the slide movement before starting an engine out of habit.
 | | Figure F |
Once the slide lifts off the base to about ¼ throttle it begins to work on the needle and emulsifying tube. Refer back to Fig. E. This is very hard to get right, as you will find as you delve deeper into the jetting phenomenon. From this point up, it is a balance of many things. The good news is you will never get it perfect so keep working. Sure – you can have it very close and will probably win if everything thing else is in order but it is an elusive goal. For us, we can run for example, a 150 main jet and change the balance slightly and have a 140 main with the same EGT reading on the same day! We don’t have room for a book so we will go on.
The emulsifying tube or needle jet comes in many sizes as well, and is removable. Mikuni used to allow you to fool with this in the old VM and TM models. The late model Mikuni TMX, TMXx TMS and Keihen have deleted this part and use a fixed Needle Jet. It works perfect but the needles are designed around the characteristics of the engine they come on. Our YZ125 liked a European needle that was more suitable to the high revving pipe we used. Having a fixed emulsion tube also solved the problem of customers using too small of needle jet, jet needle combination that can virtually take the main jet out of the equation. We will look at the main jet-sizing problem later.
Typically the ICC Dell’Orto uses a DP or DQ tube in the low 260’s to the high 260’s. What is the difference? The DQ is leaner than a DP of the same number sort of. Basically, switching to a DQ from a DP it is like lowering the needle about two notches. That means if you were running a DP266 then the next step leaner would be the DQ266. There is one more catch in the equation however. The fact that the DQ is closer to the throat may have some affect on atomization. If you had a DP266 you could simply lower the needle two notches (raise the clip) and end up with a DQ266 affect, except for the slight affect of the tube being closer to the throat.
The needles or jet needle selection is huge as well from Dell’Orto. Obviously, the fatter the needle the leaner things are. The key is getting the taper correct for your particular engine requirements.
You will notice an air hole entering from the air box side at the bottom of the throat of the Dell’Orto into the emulsifying area. The air enters into what Dell’Orto calls their emulsifier and I think appropriately called. This is the brass piece you see sticking into the throat of the carb. Some would like to get rid of the huge obstruction but it is key in mixing the fuel and air as it enters the engine. This fixture is incorporated into the body of Mikuni and Keihen carbs. Dell’Orto has several to choose from.
In the next sesion, we will move on to actual jetting.
|
|
|
Go Top
|
|
|
|
|
