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[Datsun 1200 encyclopedia]

Carburetor Tuning

From Datsun 1200 Club

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Category: Fuel System

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.

For adjustment of the carburetor, see Tune-up



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

Since carburetors 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.


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


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 40s (single or dual) 
** with small venturis
* Weber DGV (32/36)

For a racing engine, or street use where you don't care about smoothness of operation:

* Hitachi DCH340
* Weber DGAS (38/38)
* Weber/Dellorto/Mikuni 44s/45s (single or dual) 
See Main Article: Carburetor_Sizing

Baseline Jetting

To start, evaluate the jetting in your carburetor.

If your carburetor runs well, but only needs a slight change, identify which parts are fitted and compare to a baseline.

If it runs really poorly, pull out the parts and notes which ones you have. Then get the baseline parts. This is your starting point for Tuning.


* Carburetor Specifications for Hitachi jetting
* Weber DCOE carburetor jetting
* Weber DGV Tuning

Once the engine is running you can proceed to evaluate it. Tune it up, adjust the carburetor to specification (especially the float level) and see what performance you get.

2nd Step - Emulsion Tubes

Emulsion tubes are responsible for the transition points -- when the low-speed circuit give over to the high speed circuit -- and so it's important to get these right before changing the jets. Proper E-tube selection can often get rid of "flat spots".

3rd - Idle Circuit

Idle jet primarily controls curb idle. Be sure to set the idle at the recommended speed (around 700 rpm for most stock Hitachis)

  • If the mixture screw needs to be turned out too far for proper mixture, your idle jet is too small, go up one size
  • If the mixture screw needs to be turned in too far (less than one turn from seating), your idle jet is too large, go down one size

Secondarily the idle jet has an effect on the off-idle performance (low throttle openings). For this reason DO NOT raise the idle speed to make the carburetor idle "right" -- this will expose too many transition holes in the throttle body and ruin the off-idle performance.

EXCEPTION: If you have a large duration cam, 
it may require a high idle speed. Use your 
cam vendor's recommendation.

4th - Main Jets & Bleeds

If -- after choosing the correct idle jets -- you have a flat spot or hesitation when you first take off then the primary main jet is too small.

See Jets for selection of part numbers

Drive your engine for a few minutes above 2000-3000 RPM then shut the engine off without letting it idle then check the spark plugs. Look deep down inside the insulator. Black? Too rich. White? Too lean. Tan? Right on.

Next, if the engine transitions under load fine from idle to 3000 RPM, but hesitates or falls on it face when the secondary is opened you need to work on the Secondary main jet.

5th Step - Power Valve

Full throttle high speed/high load jetting

6th Step - Accelerator 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 in a particular scenario. Proper venturi sizing and emulsion tube selection is still a must as it determines the dynamic characterstics.

An 02 gauge won't tell you if the carburetor basics are right, only if under certain circumstances it is rich or lean.
Zcar Magazine two excellent articles on using Lambda sensor
Jason's Weber page

Using the meter, you can easily tell if the crusing A/F mixture is correct. Say at 50mph/80kmh in fourth gear.

See Main Article: Oxygen_Sensor#Air Fuel Ratio Gauge
LOWTECH: I just got a portable Air/Fuel ratio meter from I used it to jet my Weber 32/36 last week! It uses a wide band O2 sensor to measure your mixture in real time. You just need to weld in a sensor bung in your exhaust just after the collector(the meter comes with a bung and a plug) This is one of the few affordable meters that's supposed to be really accurate. 

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