"It runs badly" doesn't get you far when looking for a fault. The more useful question is at which throttle position it happens. In a Dell'Orto several circuits work one after the other, each responsible for a different range. Once you know the ranges, you know which part to look at.
The ranges at a glance
| Throttle | Responsible | Typical complaints |
|---|---|---|
| idle to about 1/4 | idle jet and mixture screw | needs the choke all the time, won't hold idle, pops on the overrun |
| about 1/8 to 3/4 | needle (notch) and needle jet | snatching at steady speed, stumbling when opening gently |
| about 1/2 to full | main jet | weak at the top, runs hot, or burbling at full throttle |
| snapping the throttle open | accelerator pump (PHF and PHM only) | a brief flat spot or stumble when opening suddenly |
| cold start | starter jet (choke) | hard to start from cold |
Idle to 1/4: the idle circuit
With the slide almost closed, so little air flows that the needle and main jet hardly deliver anything. The mixture then comes from the idle circuit. The idle jet meters the fuel, and the mixture screw sets how much reaches the inlet. Before you change jets here, clean the idle passages. They are very fine and are the first to clog after a lay-up.
1/8 to 3/4: needle and needle jet
The further the slide opens, the more area the tapered needle uncovers in the needle jet. This pair controls the range you ride in most of the time on country roads. The notch shifts the curve: clip lower means needle higher and therefore richer. The size of the needle jet sets the basic level. Needle and needle jet rub against each other and wear. A worn needle jet slowly makes an old carburettor richer without anyone touching a jet.
From 1/2 to full: the main jet
With the slide wide open the needle sits so high that the main jet is almost the only thing limiting fuel flow. Full-throttle problems are therefore nearly always a main jet issue. And a plug reading after full throttle tells you nothing about the idle. The throttle slide is part of the factory set-up as well. Its cutaway shapes the transition from idle, but it is not normally changed on a standard engine.
Accelerator pump and choke
PHF and PHM carburettors have an accelerator pump. When you snap the throttle open it squirts a little fuel through the pump jet and covers the brief lean moment. If the engine stumbles exactly then, check the pump as well. The choke has its own starter jet and only richens the mixture for cold starts.
Using this when looking for a fault
- Match the problem to a throttle position. Ride that range on purpose if you need to.
- Change only the part responsible, one step per test ride.
- Work from the top down, as described in Tuning a Dell'Orto.
- Use the factory jetting as your starting point. It is in the jetting database for every model.
Which needle jet or needle fits?
Needles, needle jets and jets for your carburettor are listed on your motorcycle's page, with delivery time. For anything that isn't there, just ask.
