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For adjustment of the carburetor, see Tune-up
The carburetor mixes air with gasoline in the proper ratio for all conditions. 14.7 to 1 by weight is the chemically correct ratio, but engines can work with 8:1 (very rich) to 22:1 (lean burn). For best power, it's about 12:1, but for best economy is is about 18:1. So you can see the job of fuel regulation is very important: you want the best power when you need it, and the best fuel economy when you don't. And smooth operation at all times. For technical reasons, this is very difficult to do, so carburetors are by necessity very complex.
Overview
Since carburtors works on airflow, not engine size, most carbs jetted for automotive use with light vehicles work acceptably on a variety of engines -- without changing the jets. Each carburetor works within a range of airflow, so if the carburetor sizing is appropriately chosen, good results are found on a range of engines sizes. As the airflow changes, the carburetor meters the fuel change accordingly. But "work acceptably" is not the same as "optimum". Changing the jets is necessary for fine tuning.
Correct jetting is important for:
- For maximum fuel economy
- For maximum power
- For maximum smoothness of operation (flexibility)
Generally, stock engines of varying sizes and manufacturers have similar jetting requirements. But if engine modifications are very different, then jet tuning is a must. When buying a used carburetor, this is often necessary because you never know what state of tune the carburetor was previously used for.
Jet tuning is a must for any of these reasons:
- Properly adjusted carburetor creates black smoke out the tailpipe
- Properly adjusted carburetor returns poor fuel economy
- major modifications to the induction system are made such as a high-overlap camshaft or a cold air intake
- major modifications to the exhaust system are made such as headers/extractors
When in good condition, and not worn out, the stock Hitachi carburetor does an excellent job of providing superb economy but providing a rich power on demand. The Hitachi keeps in within the appropriate range for all throttle openings, temperature and loads. No dyno tuning is necessary when using the carburetor designed for your stock-condition engine. If it doesn't run correctly it is either worn out or something else is wrong. It needs troubleshooting, but not re-jetting.
Jetting
The term "jetting" means tuning of the carburetor for your particular engine and use. It is accomplished by changing variety of parts not just the jets.
- Venturi sizes - most important factor
- Float Level - critical to proper tuning
- Emulsion tubes - for transition phases
- Main jets & bleeds - for cruising power
- Accelerator Pump - for changes of throttle
- Idle jets & bleeds - for low throttle openings
- Power Valve - for maximum power at full throttle