Carburetor tuning

 

To do this the motor need to be run at speed and in a straight road with no traffic and if possible run it uphill or against the wind to ensure that the engine is puling at the greatest load. The engine must be in perfect condition and at its normal working temperature.

1. 1.       With the throttle wide open, the mixture is controlled only by the main jet. Test the motor at top speed on full throttle then cut the engine quickly and check the spark plug. A medium brown color around the insulator is ideal. A darker color means a rich mixture. A lighter color indicates a weak mixture. A slightly rich mixture is better than a weak one. A weak mixture could cause overheating and may damage the piston.

1. 2.       Throttle openings from one fourth to one fourth are governed mainly by the needle and needle jet.

1. 3.       Beyond the one fourth open position; the airslide cutaway has the most effect. It provides for a smooth pick-up with no ‘flat spots’ when the machine is driven away from a standstill.

1. 4.       When the throttle is closed, the pilot jet supplies the fuel. A second pilot outlet increases the fuel supply for a clean, smooth engine pick-up.

1. 5.       The carburetor should be tested in these positions. Get the engine running smooth and clean then check the spark plug.

1. 6.       To richen the mixture, fit a jet with a larger number. Go one stage at a time and in any case, opt for a slightly rich jet.

1. 7.       Adjust the tickover to get it reasonably slow and even.

1. 8.       Test the motor in the one fourth to one fourth open positions. It should run evenly at a steady throttle opening and pick up quickly and smoothly when accelerated. Check the spark plug.

1. 9.       If the mixture is seriously amiss, a larger or smaller jet will be needed. A larger means richer on all jets. It might be possible to fit a needle with a different taper. It the motor runs good on half throttle but misbehaves below this, change the airslide. Here a larger number means a weaker mixture.

1. 10.   Test the motor at low speeds and for a clean pick-up when gently accelerating. This is affected by the airslide and the needle. When OK, check for full-throttle acceleration.

1. 11.   Go over the slow-running adjustments carefully to get the best tickover. These tests often have to be repeated several times to get the best results.