Fuel Mixture

Turn in the mix screw all the way until it stops. Now turn it out again for about 1,5 turns. This is what's called the base line setting.

As a general rule this base line setting show be working ok. However, in order to get the best out of the carb and to adjust the settings 100% correctly one should drive now for some kilometers and afterwards have a closer look at the spark plug condition. The condition of the spark plug will exactely tell you how well setup your carb actually is.

There is basically 3 types of the elektrode's condition:

... grayish into bright white: the fuel mixture is too lean. Too much air but too little fuel. Result: The engine is not getting enough petrol with the consequence of low power and reduced lubrication.

--> Slightly turn out the fuel mix needle to adjust the mixture accordingly.

... dark black and oily: the fuel mix is too rich. Too much petrol and too little air. Result: spark plug, manifold and silencer soot. Too much gas is consumed whilst the engine is not providing enough power. Consequently the spark plug wouldn't last long either.

--> Slightly turn in the fuel mix needle to adjust the mixture accordingly.

OPTIMAL Setup: Brown to Grayish: Electrode showing a clean and brown to grayish colored surface. The mix is optimally adjusted and the engine is getting just enough petrol in relation to air. The optimal power and performance is provided and the needed lubrication is secured.

After all corrections have been made, insert the spark plug back again and conduct another test drive. Finally confirm the correct settings by repeating the previous steps.

Fuel Mixture

Idle Setting

Principle:
The idle screw sets the idle RPM of your engine. The screw pushes on the arm that will lift or lower the throttle slide. Tightening/screwing in the screw will increase the idle rate (it will push on the arm which will open the throttle slide of the carb). Loosening/turning out the screw will decrease the idle rate and slow down the engine (it will release the pressure from the arm which makes the throttle slide close the carb inlet). A too high idle speed results in a higher fuel consumption when stopping at traffic lights etc. A stalling engine may be due to a too low idle speed, hence you should turn in the idle screw until it maintains a reliable idle speed. Please note that such settings should always be made with a warmed up engine.

The optimal idle speed is the lowest RPM possible without the engine stalling and dying out.

Before setting the idle speed, make sure you have: correctly adjusted ignition, proper spark plug, clean jets of correct size (please find more information on how to determine the correct jet size in another chapter of our page), clean and proper working throttle cable and last but not least no false air, neither from the engine nor the carb. (Hint: many folks seem to not pay any attention to the little felt or rubber ring that seals the carb manifold against false air! This sealing is of paramount importance for a proper idle speed and engine performance!).

LOOK-OUT: Before you kickstart your engine, make sure the idle screw is rather turned out for a lower idle speed. By that you will avoid a wild rev up of the engine but you may incrementally increase the idle speed. In the event you shouldn' be succesful with the basic idel speed setup, please check what has been listed above: Fuel mixture, throttle cable etc. Make sure none of the single strands are lose and prevent the throttle cable from moving back and forth. Eventually you have a small cable barrel attached to the end of the throttle cable which may got stuck. Rarely it could also be due to a badly moving throttle tube or defect mechanics of the handle bar.

If all of the previous didn't help, please carefully check for any false air intrusion. The best way to check for false air is to just keep the engine up turning on a very low speed while a second person sprays starter spray against all critical areas such as carb housing, carb manifold and engine halves connections. A spontanious acceleration of the engine may indicate some leakage at the spot the spray was pointed at.

A major prerequisite is of course to have all simmerrings and gaskets in proper condition! A defect or damaged simmering will either add oil (gears side) or air (iginition side) to the fuel mixture and thus negatively influence its ignition quality. The best carb setting won't fix this.

Idle Setting

Main Jet

The main jet regulates the amount of fuel mix made available to the engine at high RPM (3/4 to full throttle). Thus a too small diameter of the main jet may result in low performance. This goes specifically for all type of tuned engines, even if same cubic size. Please check the manufacturer specs for the genuine jet size. Most often there is two different sizes to chose from.

Tuned or modified versions make the whole setup much more complex. Additional boost-channels and modified pistons often require big sized main jets in order to achieve the optimal perfomance. Very important though is the combination of main jet and pilot jet. Unfortunately there is no real common setup to be used. Only trial and error will provide the best result. Especially the pilot jet can make the difference. Please note: the pilot jet can easily be replaced without disassembling the carb. It sits on the outside of the carb.

Main Jet

Pilot Jet

Like explained before, the choice of which pilot jet should be used depends mainly on trial and error. But there is still some initial guidelines which one could start with. First of all, always use new jets rather than dirty and choked ones being found in some corner of the workshop. The 16/16 SHB was basically equipped with two different sizes: 38 and 42.

The rule should always be to start with a rich mix and changed the jets towards a lean mix. As described before a lean mix has a higher potential to cause damage (less lubrication, missing cooling). Thus start with the 42 jet. In case of the famous DR50 you may even begin with the 50 jet.

Pilot Jet