A slide carburettor has several parts that each control the mixture over a part of the throttle range: the pilot (idle) jet and air screw, the slide cutaway, the jet needle and the main jet. The ranges overlap, so the order in which you tune them matters. The Amal tuning guide linked from the course gives a clear order that suits most slide carburettors: main jet first, then pilot, then cutaway, then needle, then idle again.
Do this only after the basics are right. Many "jetting" faults are really a dirty carburettor, an air leak or a blocked filter.
Safety
- Test on a quiet, flat road at a speed you can control. Never read a plug at full throttle in traffic.
- The exhaust and plug are hot. Let them cool before you touch them.
- Petrol spills when you remove the float bowl or main jet. Work outdoors, away from flames, with the tap off.
Before you start
- Clean carburettor and clear jets: Clean the carburettor.
- Correct float height: Set the float height.
- No air leaks at the manifold or rubber: Find an intake air leak.
- Clean air filter fitted, exhaust in good order, ignition timing and plug correct.
- Throttle cable with a little free play, and the slide closing fully.
Steps
- Mark the throttle grip in quarters so you know where you are: Mark the throttle grip.
- Main jet (three quarters to full throttle). Ride with the throttle fully open. If the engine runs heavy and lumpy at full throttle, the main jet is too big. If it pulls better with the throttle closed slightly, or with the choke partly on, the main jet is too small. Fit one size at a time: Change the main jet.
- Read the plug after a short full throttle run and a clean cut: Read a spark plug and choose the jet by throttle position.
- Pilot (closed to about one eighth). With the engine warm, set the idle with the throttle stop screw, then turn the pilot air or mixture screw slowly until the idle is fastest and smoothest. Lower the idle with the stop screw and repeat: Set the idle screws.
- Cutaway (about one eighth to one quarter). Open the throttle gently from idle. Spitting back through the carburettor here means weak; heavy, lumpy running means rich. First try a small turn of the pilot screw. If that does not cure it, the slide needs a different cutaway: a larger cutaway leans, a smaller one richens.
- Needle (about one quarter to three quarters). Ride at part throttle. If the engine pinks or feels weak when you open the throttle, raise the needle one groove (clip lower). If it runs heavy or sooty, lower the needle one groove: Set the jet needle clip.
- Idle again. Go back to the pilot screw and idle speed for a final setting, because the other changes affect it.
- Write down every part number and clip position you end with.
The choke test for rich or weak
At the throttle position of the fault, close the choke or air valve a little. If the engine runs better, the mixture there is weak. If it gets worse, it is rich. Open the choke fully again before you go on.
Signs to read
| Rich | Weak |
|---|---|
| Black smoke, sooty plug | Spitting back through the carburettor |
| Heavy, lumpy running, "eight stroking" on a four-stroke | Engine runs better with the choke partly on |
| Fuel dripping from the carburettor intake | Overheating, poor acceleration, pinking |
Check your work
- The engine picks up cleanly from idle to full throttle with no spit back or bog.
- The plug reads a light brown after each test.
- Idle is steady when warm and the engine returns to idle when you close the throttle.
If it did not fix the problem
- A rich mixture at half throttle is not cured by a smaller main jet. Lower the needle instead.
- Wear changes jetting: a worn needle or needle jet makes the mixture rich in the middle range. Replace both: Remove and refit a needle jet.
- Twin carburettors must also be set equal: Synchronise twin carburettors.