Topic: K8 Jet Needles

[Wisconsin Jeff]

First off, 1978 CB750K8.

Stock air box, 4-1 CB750F header with Kerker can.  Just installed brandy new air filter and spark plugs.

It seems as though I am running rich at mid-throttle.  I see that the K8 jet needles are not adjustable.  I have read that you can shim them up, but that would make a for a richer condition at half throttle. 

Is there another solution, like K7 adjustable needles?  Do I have to replace the needle jets as well? 

Is there anything else that would cause a mid-throttle rich condition?

Any input is appreciated.

[Wisconsin Jeff]

Bump

[Coyote13]

Not 100% on this, but mid throttle should be your pilot jet.  On the PD42's these are press-in, and as far as I know shouldn't need to be changed with the modifications you describe.  Have you done plug chops to confirm the mid-throttle rich condition you're describing?

[Wisconsin Jeff]

I didn't do plug chops per se, but the plugs were awful sooty when I took 'em out to check.

Pilot jet/air screw is only going to be effective up to 1/4 throttle.

[Coyote13]

I think you're confusing the Idle Screw (which does control up to about 1/4 throttle) with your pilot jet, which controls 1/4 to I think about 1/2 to 3/4 throttle, where your main jets take over.  I would say do plug chops at each interval so you can say without a doubt what condition your bike is running in on each cylinder at each interval.  It will take some time but you can be sure it will tell you more than I can about your motor's state of tune.  Good luck!

[Wisconsin Jeff]

Thanks for the advice Coyote, I'll try that.

[Wisconsin Jeff]

This is the chart I was referencing when refering to the jet needle/needle jet at mid-throttle.

[Coyote13]

There is a chart like that one that is specific for the K8 (no needle clip).  I know I've seen it floating around the forum before, I'll see if I can find it for you.  Same idea, just better for your specific carbs.

[Skunk Stripe]

The fuel screw has a wide range of control, same with the idle jet.
Raising the needle will not affect the low rpm range and starts affecting the mid rpm range some, a bit sooner.
What air filter did you put in? I hope you didn't cheap out on those emgo filters. They apparently are too restrictive compared to the stock honda filter.

[dougandarthur]

Here is another Keihin Chart that is a little different. I wonder which one is closer to correct for 054A Carbs? I am looking for leaner Jet Needles too.

[Wisconsin Jeff]

I went ahead and cleaned the slow jets again.  They seemed to all be clear.  I also did the clear plastic tube test trick to check my float level.  It looks like the bowls are only about half full on cylinders 1 and 3 and almost all the way full on 2 and 4 (about 3mm below the flange).  I am going to attempt the correction tonight by bending the tangs as well as properly setting the fuel screws.  I'll keep you guys updated on the results.