Dellorto Carburetor Tips

Dellorto Carburetor Tips
Dellorto Carburetor Tips
CLEANING CARBURETOR
Because some of the jet passages in the carburetor are very small it is possible for very small particles of
impurities in the petrol to block or impede these passages. This can cause misfiring in the engine.
If you experience misfiring, the carburetor should be carefully dismantled and all jets and passages blown
out with compressed air. If you are unfamiliar with the carburetor consult the dealer from which you
purchased your engine.
TO CLEAN THE CARBURETOR
1. Undo the 3 bowl retaining screws and remove bowl, the fuel floats should stay located in the
bowl.
2. To clean the jets, an aerosol carburetor cleaner or compressed air is the most suitable method to
clean with. The jets can either be cleaned in place or preferably removed one at a time and
cleaned and refitted. The important jets to check are the Pilot jet, the Emulsion jet and the Main
jet.
3. To clean the needle and seat, firstly remove the fuel float pin and the fuel float lever arm. This will
then allow you to remove the needle & seat to clean. Refit and check float lever height and adjust
if required.
4. Assemble the carburetor carefully making sure everything is replaced correctly.
5. One of the important jobs to remember when cleaning the carburetor is to clean the fuel filter
located on the side of the carburetor.
This tiny filter should be cleaned regularly and then placed back into the carburetor.
TRUST THE MANUAL!
When tuning your Rotax the best way to get the maximum performance is to read your jetting chart first.
Study the chart to find what the recommended jet is and use that as a starting point, from then on you will
only be required to make minor changes.
BASELINE CARB SETUP
Follow these steps to assist in dialing in your Dellorto VHSB34 carburetor.
FLOAT LEVELS
1. IT IS VERY IMPORTANT THAT THE FLOAT LEVELS, WHICH OPERATE THE NEEDLE &
SEAT ARE, SET CORRECTLY.
2. REMOVE THE BOTTOM FLOAT BOWL COVER OFF THE CARB. TAKING NOTE OF THE WAY
THE FLOATS FIT IN.
3. WITH THE COVER AND THE FLOATS REMOVED HOLD THE CARB UPSIDE DOWN AND LET
THE FLOAT LEVERS SIT DOWN. THESE LEVERS MUST BE PARALLEL WITH THE BASE OF
THE CARB.
4. IF THEY ARE NOT PARALLEL DON’T BEND THE LEVERS, INSTEAD YOU SHOULD BEND
THE SMALL TAG WHICH SITS ON TOP OF THE NEEDLE AND SEAT.
5. IT IS VERY IMPORTANT THAT THESE LEVERS ARE PARALLEL WITH THE BASE OF THE
CARB. NOT SLIGHTLY UP AND NOT SLIGHTLY DOWN.
6. WE SUGGEST THAT THIS JOB IS BEST PERFORMED BY YOUR ROTAX DEALER IF YOU
ARE UNSURE.
JETTING
1. FIT THE CORRECT MAIN JET AS PER THE CHART IN THE OWNERS MANUAL.
2. THE NEEDLE ON THE THROTTLE SLIDE OF THE CARB SHOULD BE SET ON THE RICHEST
SETTING WHICH IS THE LOWEST CIRCLIP GROOVE.
3. RE FIT THE CARB MAKING SURE THAT IT IS PERFECTLY VERTICAL AND NOT TILTED
EITHER WAY.
4. FIT A REAR SPROCKET WHICH IS EQUAL OR UP TO 3 TEETH LESS THAN A NORMAL HPV
or Yamaha Pipe GEARING FOR THE TRACK.
5. TEST THE KART. IF THE ENGINE IS RUNNING A LITTLE ROUGH AT LOW RPM OF 6 TO
8000 ADJUST THE CIRCLIP ON THE NEEDLE UPWARDS 1 GROOVE. IF IT IS RUNNING
QUITE ROUGH YOU CAN ADJUST 2 GROOVES.
6. IF THE ENGINE DOES NOT WANT TO REV PAST 12,400 FIT ONE SIZE SMALLER MAIN JET.
IF THIS IS AN IMPROVEMENT BUT NOT QUITE RIGHT CHECK THE PLUG AND PROVIDING
THE SETTING LOOKS RICH TRY THE NEXT SMALLER JET.
7. IF THE FIRST MAIN JET CHANGE TO A LEANER JET MADE THE ENGINE RUN WORSE
THEN TRY A JET ONE SIZE LARGER THAN THE ORIGINAL ONE YOU STARTED WITH.
IF FITTING A SMALLER (LEANER) MAIN JET HAS IMPROVED THE ENGINE IN THE TOP
END RANGE IT IS SOMETIMES NECESSARY TO RESET THE SLIDE NEEDLE TO A RICHER
(LOWER) GROOVE TO COMPENSATE. THIS MAY IMPROVE BOTTOM END POWER.
BE CAREFUL! NOT TO FIT A TOO SMALLER JET OR YOU COULD CAUSE A SEIZURE. THE MAIN
JET CHART IN THE MANUAL IS VERY SAFE. IF YOU PLAN TO TRY LEANER (SMALLER) AGAIN BE
CAREFUL AND CHECK SPARK PLUG READING FIRST.
IF THE MAIN JET IS SET TOO LEAN THE DRIVER WILL NOTICE POOR POWER IN THE 10,000 TO
12,000 RANGE AND SHOULD RETURN TO THE PITS IMMEDIATELY.
PRIMING THE FUEL PUMP
Prior to starting the engine, it is highly important to make sure the fuel in the lines is all the way up to the
fuel pump. There are three ways of achieving this, either by removing the spark plug and then turning the
engine over briefly, removing the fuel line from the over flow bottle and blowing into the fuel tank which
will pressurize the tank forcing fuel up to the fuel pump or by blocking the tubes on the airbox when
starting the engine.
It is also very important not to hold the start button on for too long, as this will cause the starter motor to
over heat causing internal damage.
8.
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