pilot fuel screw adjustment question

Hi all,

The choke on my XJ550 is always engaged, otherwise the bike will stall (obviously the engine temperature is accounted for).

I know it's not dirty carbs because I cleaned them zestfully and rebuilt them partially (including swapping out all pilot jets).

I know it's not an air leaks because I replaced intake manifolds/air boots and did liquid checks to check for leaks and there aren't any.

That leaves the last option--an improperly set mix screw. It seems that the pilot fuel screw is not letting enough fuel through as such, partially open choke compensates to create the appropriate air/fuel mixture to keep the bike running.

I looked in the manual and unfortunately didn't find any material/info on how to regulate the mix screw for the Mikuni carbs. Wondering if anyone has any pointers/information maybe one of those helpful PDF's written by saints on this forum hah

Currently wondering:

1. What is the ideal RPM to have on a XJ550 in line 4 at idle? With the pilots set at 2.5 turns, idle RPM is currently at 1500. Is that too high? Too low?

2. Is turning the pilot screw IN going to enrich or lean out the mixture?

3. Is there a way to dial in a proper setting without having to constantly unrack the carbs? I noticed carb 2 has a bracket that secured the throttle cable above the carb head, effectively blocking access to the pilot screw...

Many thanks
~Dre

 

2.5 turns for your mixture screws is perfect, as a starting point. RPM should be at 1200, ideally. If you turn the screw in, it's going to lean out the mixture. Turning it out enriches it. If you're curious, this shows your carbs all cut up so you can see the circuits and visualize it easier:

Something New, Something Naked | XJBikes - Yamaha XJ Motorcycle Forum

I found the stubbiest slotted screwdriver I could in the garage, then tested to see if it would reach with the tank off.

As for tuning, I'm using a Colortune Plug (Bought it from user @chacal) as the consensus I've heard is that they work the best. you'll need to do some swapping of spark plugs, but at least they're easier to get to? I'd recommend looking into one. I haven't tuned my engine yet, so I haven't used it, but from what I've read from the gurus on the forum, really fine adjustments (one or 2 degrees) are needed to really hit the sweet spot.

Good luck, and if you have any questions just ask!

 

Also, I just realized these articles may be helpful:

The Information Overload Hour | XJBikes - Yamaha XJ Motorcycle Forum

THE SECRET LIFE OF CARBURETORS | XJBikes - Yamaha XJ Motorcycle Forum

 

If you are running with the choke lever on, after warmed up, you still have carb issues

 

I agree with hogfiddles. After cleaning the carbs again try boiling the carb bodies for about 15 minutes. This will melt and remove any qum missed in the cleaning. After boiling, blow them out with compressed air and carb cleaner.