Topic: Raising the Needle Jet

[gwallis]

My 1973 CB750 is hesitating at the 3,000 to 3,500 RPM range.  It seems as though just when it is going from the pilot circuit to the main jet.  After the slight hesitation the bike runs strong through the rest of the power range.

If I raise the needle will it smooth out this transition?

The needle is already on the third notch down from the top.

Dlekevic headers with an air box from Cognito Moto.... Air Screws one half turn out from seated.

'Thanks'

[flybox1]

Re-test this, and note what throttle position you are in when the hesitation occurs, as this will show you where to make your adjustment.
(rpm doesnt dictate what jets are being used.  ie.  3500rpm in 1st vs 3500rpm in 5th.)

[gwallis]

The hesitation occurs at the 1/4 position on the throttle.

So.... do I move the clip on the needle jet down one more notch?

[HondaMan]

To fix this, turn the air screws out 1 full turn from the closed position. The airbox is disturbing the airflow into the emulsifier air holes at the back of the carbs, so this flat spot will ALWAYS exist with that airbox. It will be less with the air screws set nearer 1 full turn, but that's all you can get without the velocity stacks supplied inside the OEM airbox, which are designed to create the still-air zone at the bell of those carbs, in order to feed the emlusifiers. Without that, the engine has to try to suck up the fuel from the jets, instead of it being pushed UP those jets by these emulsifiers, so you will suffer flat spots in the RPM range.

[gwallis]

Hi Hondaman:

Makes sense... I knew about the Velocity Stacks inside the original airbox and its function of returning unused gas back into the carbs.  But I did not realize their effect on jet emulsifiers.

However, when I turn the airscrew out more than one half from seated the bike is running in a very lean condition.  I have played with several main jet sizes (110,120,132 & currently 140) just to get it to run half way decent.  I still have the 40 pilot jet installed because the bike idols fine... until I goto 1/4 throttle.

I know that you are going to tell me to go back to the original airbox.... right?

[evanphi]

Hi Hondaman:

Makes sense... I knew about the Velocity Stacks inside the original airbox and its function of returning unused gas back into the carbs.  But I did not realize their effect on jet emulsifiers.

However, when I turn the airscrew out more than one half from seated the bike is running in a very lean condition.  I have played with several main jet sizes (110,120,132 & currently 140) just to get it to run half way decent.  I still have the 40 pilot jet installed because the bike idols fine... until I goto 1/4 throttle.

I know that you are going to tell me to go back to the original airbox.... right?

You need to enrichen your idle circuit. Bump up your pilot jets.

I have to do the same thing

All you're doing by increasing your main jets is increasing fuel flow at 3/4-full throttle.

[flybox1]

  I still have the 40 pilot jet installed because the bike idols fine... until I goto 1/4 throttle.

A bike with an incorrect idle mixture can still idle.
How about a idle plug chop to get an idea of how close your mixture is/isnt.

[gwallis]

My idol/pilot jet is currently at a #40.  How much higher should I go.... bracket?

[HondaMan]

Instead of increasing the idle jet, try instead raising the float levels. Typically, today's fuel metering needles are MUCH too stiff (if they were replaced since 2002 or so), which results in low fuel level in the float bowls. I run them at 24-25mm (one float at 24, one at 25) if I am forced to use the new float valves, just to get the low-speed jetting correct. This won't have a lot of effect above 5500 RPM, but if it does then you can modify the needle clip position for that.

The idle circuits in these carbs are not terribly efficient, being such a tiny air passage and jet, and their full adjust range from non-working to working (and back) is 3/4 to 1-1/4 turn: going beyond that either way closes off the dynamics of the air flow (in) or fuel (out), so you're then trying to work in a range where it doesn't. You may also want to either thoroughly clean the little emulsifier holes in those pilot jets or, if they are the ones we (again) get in the modern kits, add another pair of holes. I have also lately been flaring those holes (there should be 8, BTW) with a sharp countersink on both the pilot and main emulsifiers, because it seems to help aerate the ethanol a bit more.

The ethanol is increasing this phenomenon at the very range where you are working: for a test, see if you might be able to find some real gas somewhere for a tank, for comparison. You might be pleasantly surprised!

[flybox1]

My idol/pilot jet is currently at a #40.  How much higher should I go.... bracket?

How about a idle plug chop to get an idea of how close your mixture is/isnt, and then dial it in with your IMS