carb needles question

I have a 1980 KZ1000LTD with the stock mikuni carbs. I recently cleaned and rebuilt them. However on all the diagrams I've seen the jet needles have three grooves to change the setting from lean to rich. The needles in my carbs have only one groove and no apparent way to richen or lean the fuel mixture, except by the air mixture screws located on the sides of each of the carbs. Is there a way to adjust the air/fuel mixture, or am I missing something. I believe the carbs are running lean by the color of the spark plugs and I dont want to burn my valves. Any help or advice would be appreciated.

dodak1967 wrote:

I have a 1980 KZ1000LTD with the stock mikuni carbs. I recently cleaned and rebuilt them. However on all the diagrams I've seen the jet needles have three grooves to change the setting from lean to rich. The needles in my carbs have only one groove and no apparent way to richen or lean the fuel mixture, except by the air mixture screws located on the sides of each of the carbs. Is there a way to adjust the air/fuel mixture, or am I missing something. I believe the carbs are running lean by the color of the spark plugs and I dont want to burn my valves. Any help or advice would be appreciated.

Sometimes a tiny washer may be used to shim the jet needle so it sits slightly higher in the throttle slide and thereby enrich the fuel mixture during mid-range.

But remember that the mainjet (not jet needle position) governs the mixture at wide open throttle.

The pilot circuit (mixture regulated by the side located pilot air screws) governs mixture from idle to about 1/4 throttle, where the jet needle governs mixture from 1/4 to 3/4 throttle, where the main jet governs. All these circuits overlap -- for example, the pilot circuit at idle fully controls the mixture, but as rpm increases, the needle jet and jet needle in association with the throttle slide position assume more and more control over the fuel mixture as the pilot circuit loses influence.

At wide open throttle, the main jet controls the mixture (the throttle slides are up out of the bore and the jet needle is up out of the needle jet atop the bleed pipe.

The best way imo to determine correct mainjet size is plug reading after a high speed throttle chop (with simultaneous ignition kill), which gives a true picture of what's happening inside the combustion chamber at wot.

The bike might indeed be lean in mid-range with the oem single clip jet needle. But would first diagnose the mainjet situation (where bleed pipes and needle jets and main jets are all clean and clear) before fooling around with the jet needle position. Just my 2 cents.

Good Luck!

The obvious question here is to ask if you have modified the stock airbox or exhaust system. If not, the stock jetting certainly won't hurt your bike nor run excessively lean. Do your carburetors still retain the stock jetting? To verify, are they VM28SS?

The airbox is stock as well as the carb jets. There is however a 4 into 1 header in the bike. At this time I believe the carbs are VM28SS.

dodak1967 wrote:

The airbox is stock as well as the carb jets. There is however a 4 into 1 header in the bike. At this time I believe the carbs are VM28SS.

Is an oem paper air filter being used, or perhaps a K&N?

dodak1967 wrote:

The airbox is stock as well as the carb jets. There is however a 4 into 1 header in the bike. At this time I believe the carbs are VM28SS.

If the header has better flow characteristics than stock (and the good headers do), that's why it's running lean. You could try going up one size on the mains and see if it runs better.

Changing needle position generally cures the problem if the engine has a "weak" stumble going from very light throttle to maybe 1/4 throttle. As mentioned, at wider throttle openings the main jet size does all the talking.

There is no need to change the jet needle position for this setup. I would leave it alone. I would use some larger main jets perhaps... #107.5 and ensure the accelerator pump assembly is functioning as it should but even if you do nothing, it won't be all that lean or burn the engine up for sure.