Pilot jetting advice for my XJ650 sought

Hi there,
I'm new to the forum and I have a question regarding pilot jetting.
I’m sure its be asked many times, but maybe not for this reason.


This is the issue:


Some years ago I swapped out the engine in my 4K0 XJ650 (UK spec, Non YICS) for a USA 5N8 engine with YICS.

Otherwise the bike is ‘stock’ with original exhaust and airbox (and recent filter!).
The valve clearances are spot on, the carbs spotless inside and out and the diaphragms are not damaged.


The engine runs very well except for a slight glitch or ‘bogging’ around 2000 rpm – especialy under load such as exiting a junction or up slight hill.
These symptoms don’t seem to fit with leaking throttle rod seals as I am able to balance the carbs (using the blanking tool)
After much carb cleaning I finaly realised the problem was not dirt but with the ‘do-not-adjust’ pilot mixture screws and I wonder if the carbs no longer ‘match’ this new engine


After many adjustments and road tests I have all but eliminated the ‘bog’ however these pilot screws are now at 3.25 turns up from the base (the datum is 2.5 turns up)
This has put the screws right at the top of their adjustment with very little spring pressure to hold them firmly in place.

So my idea, to bring the pilot air screw adjustment back into a sensible range, is to DOWNSIZE the pilot jets (since I cannot get any more air into the pilot mix)
They are currently 45’s (which is odd as the UK spec for the original 4K0 engine is 43!)


I am thinking of changing them to 40’s


Would this be too much of a jump?
Or am I going in completely the wrong direction!!


Your comments / opinion / direction to other threads would be appreciated.


Cheers, Steve P

 

A 40 pilot is the perfect jump. That is exactly what Yamaha put in the carbs for the YICS engines (except in the U.K.).

You'll probably find that you will need to change the main jets as well. The YICS engines call for a 110 main, and a Y-10 needle instead of the Y-11. The air jets are also sized differently.

I'd recommend starting with changing the pilots and going from there. particularly since you found nonstandard (for any market) pilot jets.

 

Isn't it the other way?

Moving out from soft seat is richening the mixture. You will need to increase the size of the pilot jet to reduce the number from 3.25 to approximately 2.5.

 

i think there's more going on here than just jet size. already two sizes over stock, with the screw out that far it should be really rich. i wouldn't go any bigger on the jets until the carbs are cleaned again. then go back to stock jets and start over.

 

Perhaps not as Polock suggested. I think just about all of us have thought this way, and later discovered it wasn't quite so as some of those passages particularly on the pilot circuit can be very stubborn to clear.

http://www.xjbikes.com/forums/threads/in-the-church-of-clean.14692/

You could also look at fuel level and wet setting to be sure the fuel level is set correctly. Too low will cause a lean condition and you might be compensating by backing the mixture screws out.

http://www.xj4ever.com/setting fuel levels.pdf

Has the plug color or color tune been done?

 

try not using the tool once.
3.5 turns out isn't really that far off, could be float levels as Rooster said.
maybe the o-ring on the mixture screws isn't doing it's job. so as the screw comes out to richen it some air leaks past. try a little piece of tape over the holes and see if something changes