Calling all Mikuni guru's...

regarding pilot jet set ups:

On a warm engine the idle will begin to dip when I turn the screw inwards from about 1.5 turns in.

Peak idle appears to be past 3 turns out and it's not until the screw is almost un-threaded that the idle begins to dip again.

Would this indicate that I need to go up in PJ size, or down?

 

My initial guess would be it's too lean. I would think that if its to rich as you backed the screw out it wouldn't take much adjustment to notice a change in idle. With it being lean you can't richen it enough by backing out the adjustment screw. Just a thought, could be totally off.

 

Forget about lean/rich. It is very simple.... If your screw is all the way out, the jet your controlling is too small. Get one size up.

 

Yeah, it sounds like one step up on the pilot jet or one step down on the air idle jet. What pilot jet are you running now? Is your air idle jet the standard 1.0? If you notice in the tuning manual for this carb, Mikuni leans a little more toward changing the air idle jet over the pilot jet. Both will affect the fuel mixture at idle...along with the idle screw...but the air idle jet seems the better route for both idle and the first transition from idle when you first crack the throttle to accelerate from idle.

On my KLX300 setup...actually 292cc with the same compression as stock 250 at 11:1, I run a 17.5 pilot jet and went down to the .9 idle air jet. Remember, though it initially sounds counterintuitive, the smaller idle air jet richens the mixture because it allows higher vacuum to the fuel pilot jet. I liked the way my engine responded to an idle air jet change versus the pilot jet...perhaps because the vacuum signal to the carb in the smaller engine with a larger carb responds better to increasing the vacuum signal via the idle air jet versus just adding more fuel via the pilot jet.

 

Thanks TNC.

The PJ that came in the carb is supposed to be a 12.5 according to the vendor.

specs are listed here:
Mikuni 36mm Accelerator Pump Flat Slide Performance Carburetor TM36 68 | eBay

https://i.imgur.com/puv1WAa.png

it seems to idle pretty nicely, and it will accelerate fine if you're gentle with the throttle. Quick turns will yeild a large hesitation before accelerating again.

 

If I understand the pilot system correctly - on the TM36 - that is an air screw controlling the air flowing into the pilot jet. (not a fuel screw as on the CVK)

Doesn't that mean that IN would be richer - cutting off air to the PJ, and OUT would be leaner - allowing max air to the PJ....? Exactly opposite the net effect of the CVK fuel screw..?

 

From the Manual:
this would seem to support TNC's observations.

 

Well,,, So it is a "fuel screw" then - like the CVKs' - and controls the amount of A/F coming off the pilot jet into the carbs' air stream..

 

That seems to be correct. A friend said that air screws are typically on the airbox side of the carb while fuel screws are on the engine side.

Google gives somewhat contradictory advice when looking up Mikuni Tuning... Given TNC's advice, the manual I've got and My friend's advice, I'm pretty sure it's a fuel screw and not an air screw.

I'll buy a smaller Pilot Air jet and go bigger with the Pilot jet (not likely both at the same time mind you).

 

Ya, and when your riding if it doesn't pick up off idle very well, just raise the idle a little.