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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.
Contents |
Overview
NOTE: For in-depth info on carb tuning and performance, buy or borrow a book the type of carb you have. Try your local library or http://www.amazon.com.
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 changes to the induction system are made such as a high-overlap camshaft vs stock profile, or a cold air intake vs stock heated air intake
- major changes 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.
Indications
If after proper adjustment, you encounter any of these symptoms, the carbs may need tuning or proper installation fitting.
* poor fuel economy * unacceptable low speed bog (hesitation) * hesitation when accelerating suddenly * overly loud induction noise * carbs getting hot enough to boil over fuel * White spark plugs * Black (dry) spark plugs * Hiccups/coughing * Backfire/popping * Black smoke
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
First Step - Venturi Sizing
For best operation of a street vehicle, choose an appropriate carburetor size. The critical factor is the venturi size.
1200cc engine - Max RPM 6000 * Hitachi DCG/DCH (stock A-series carburetor) * Weber/Dellorto/Mikuni 40 or 45
For a racing engine, or street use where you don't care about smoothness of operation:
* Hitachi DCH340 * Weber DGV (32/36) * Weber DGAS (38/38)
See main article: Carburetor Sizing
With sidedraughts, you can use either 40s or 44s/45s if the correct size venturis (chokes) are fitted.
Baseline Jetting
See:
* Carburetor Specifications for Hitachi jetting * [[Weber_DCOE_carburetor#Jetting|Weber DCOE carburetor jetting] * Weber DGV Tuning
2nd Step - Emulsion Tubes
3rd Step - Main Jets & Bleeds
See Jets for selection of part numbers
4th Step - Power Valve
Full throttle high speed/high load jetting
5th Step - Acellerator Circuit
Dyno Tuning
A dyno operator can assist with tuning and see immediate results via the power output.
Home Tuning
You can do it!
Wide-band O2 sensor tuning
Air/Fuel ratio meters use a Lambda sensor (oxygen sensor) to tell you if the carb if running rich or lean.
Zcar Magazine two excellent articles on using Lambda sensor Jason's Weber page