The factory jetting is meant for standard condition, with the original air filter and exhaust. Open up the intake or the exhaust and more air flows through the same jets. The mixture gets leaner, and in most cases the jetting has to go richer.
Why a modification changes the mixture
The jets are sized for a certain amount of air. Open bellmouths instead of an airbox reduce intake resistance. A freer exhaust changes the back pressure. Either way, the engine draws more air. Without the airbox the flow in front of the carburettor changes too, and the engine responds differently. If it pulls weakly at the top after the change, runs hot or shows pale plugs, it is lean.
What usually needs adjusting
| Part | How often affected | Direction |
|---|---|---|
| main jet | almost always | bigger, in steps from the factory size |
| needle position, then needle jet | often | richer, otherwise everyday riding snatches |
| idle jet and mixture screw | depends on the change | often one size bigger if it pops on the overrun |
| float level | check first | set to the factory figure |
How much richer depends on the individual bike: bellmouth length, with or without mesh, exhaust, camshaft and engine condition. That is why we don't give blanket sizes here.
Why nobody can give you the right jet
Forums are full of combinations that worked for someone else. As experience they are valuable, as a shopping list they are not. Two modifications that look the same often run differently. The practice reports on our carburettor pages show this. On the PHF 36 a Le Mans III ran far too hot after a conversion to open bellmouths; the jetting was evidently too lean. On the PHF 30 a T3 bored out to 995 cc ran rich with every combination that was tried. The advice in the forum in the end: measure lambda instead of guessing.
How to go about it
- Finish the modification and check the basics: no leaks, ignition, valve clearance, float level.
- Fit your model's factory jetting, or note down what is fitted. The figures are in the jetting database.
- Tune in the right order: main jet, then needle and needle jet, then idle. One change per test ride. More in Tuning a Dell'Orto.
- Check by reading the plugs, or better with a lambda reading or on a dyno after bigger changes.
- Write down the result with the complete jetting. It helps you next time, and the next owner too.
Road legality
Changes to intake and exhaust usually affect type approval and noise. That has nothing to do with jetting, but it is part of every modification.
Need jets in several sizes?
With the main jet set you put together the sizes around your factory value yourself. It is on your motorcycle's page in the jetting database. Questions welcome.
