KeiHin Carburetor Tuning - The basics of how and at what range the jets and adjustments take effect.

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       Clip    Clip position.                             

The clip position effects the most common riding conditions, from about 1/4 to 3/4 throttle as shown above in blue.  By default, the jet needle clip is placed in the center groove, but this may or may not be suitable for a particular bike...wasn't on mine.  As these bikes get older, or if you buy or find one no telling where, it could have any number of modifications, quirks or a poor mechanic somewhere in its history.  If when, you inspect the plugs, or notice blackish smoke in the mid-range, feel the bike is "bogging down" or is less responsive than it should be in normal or average rpm ranges, the clip position may need adjustment.  As an example, if the bike is running too rich in the midrange the jet needle** clip would need to be raised a notch at a time dropping the needle further into the needle jet** leaning out the mixture some.  On the other hand, if the bike is popping in the mid-range, and you've done the timing and valve adjustment, than there's a good chance the clip needs to be lowered, raising the needle to get a richer mixture. 

**Note the difference - "jet needle" is the actual needle that goes up and down with the slider into "needle jet" when the throttle is used.

Ma   Main Jet.

The "Main Jet" should be should be check for scaring, wear and corrosion, as all the parts....and needs to be sized as the factory suggests unless modifications in the intake (air filters), valves, cam or exhaust.  Some of the later model CBs will convert from the air-box to single pod filters without changing jets, but almost always it IS necessary to get bigger main jets when going from 4x4 exhaust to a 4x1 header.

Ma    Air Screw/ Slow Jet

In the early sohcs the KieHin carburetors (thru '76), the default setting for the air screw is one turn out, and unscrewing or turning the air screw counter-clockwise increases the amount of air allowed thru the main jet.  On later carbs ('77 and '78), the opposite is true.  The honda manual states, after setting at the default 1 turn out, fine tune the each air screw by adjusting one at a time and find where the bike idles highest.  Of course this is done after the basic idle is set first.  Many people make the mistake of setting the idle before the bike is fully hot - then find after riding the idle is too high.  The idle must be set after the engine is at full operating temperature.  I found it impossible to hear or note any changes in idle speed when performing the air screw adjustment, but accomplished it by (after idling only) checking the individual plugs and adjusting from there.  My bike has a high performance cam and it's just a little too radical to make some adjustments as the manual prescribes, but with a little commonsense and trial and error, or experience there is always a way to get things fine tuned.

The slow jets should also be set as prescribed by the factory.  Even with many modifications, it is unusual for any need to change its size.


This info is straight from the Honda Service Manual.

Year/Model Main Jet Idle Jet
'70-'75/K1-K5 120 40
'76/K6 105* 40
'76/F1 105 40
'77/K7 115 35
'77/F2 105 35
'78/K8 110 35
'78/F3 105 35


'76/K6 105* - I believe my Honda manual is in error about this number.  The manual goes directly into the "F" model.  I think the K6 with 4 into 4 pipes requires the #120 jets. 40




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