Hi all.
I have completed a couple of modifications:
- Removed airbox and installed a K&N cone filter.
- Change stock exhaust over to a long shot pipe welded onto the outlet of the header pipe and inserted a 13 inch baffle wrapped in exhaust packing.

This is a 2016 Canadian model bike and has a 145 main jet and a 45 pilot jet along with the standard 0.110 inch white spacer.
Currently with the stock jetting setup, I am getting a lot of popping on deceleration along with a bit of not extremely loud backfire. It seems a bit sluggish in the midrange but improves slightly if the choke enrichment is pulled out to the first notch. Currently I have the idle mixture set at 3 turns out for the highest idle speed point. It doesn't seem to change much (maybe 50 rpm) from 2 to 4 turns out.

My question to all of you who have some jetting and spacer washer experience is what jet sizes and spacer size if any should I begin with?
I am in Calgary, Canada which is at an elevation of 3300 feet and the climate is very dry. Riding temperatures range from 10 to 26 degrees Celsius which is 50 to 80 Fahrenheit.
I really appreciate all input as I have no experience with carb jetting.
This will at least get me in the ballpark as a place to begin.
Thanks so much folks.

Thanks Modsquad!

One more thing.....don't jet for what happens when you are riding and close the throttle.  The popping and banging from the exhaust doesn't mean anything about your jetting.  You should be jetting for how the bike idles and runs when you are "ON" the throttle - not off.

When you allow the throttle to close completely when shifting gears or when decelerating - you have basically shut off most of the air flow and nearly all of the fuel flow.  The high engine vacuum pulls air past the throttle plate - and the pilot circuit flows a little bit of fuel.  The fuel/air mixture is very lean and the spark plug cannot ignite the mixture....and the mixture goes into the hot exhaust header and is ignited.  Try riding in a manner where you roll off the throttle when you shift - but you still keep the throttle opened a tiny bit.......and do the same when you are decelerating.  As you slow down roll the throttle open just enough to make the noise go away.

I purchased a 150 main jet but in a city of 1.2 million I could not find a 50, 47.5, 52.5 or 55 pilot jet. I ordered all 4 from fort nine who were clearing them off for $1.63 each. But.....I have to wait a week for delivery. Disappointing but I do have a second bike to ride in the meantime.
I will post my experience once I receive the pilot jets.

Thanks for your input Dave. I was thinking along the same lines. Since I have to wait a week for arrival and they were so cheap I thought I'd get 4 different sizes in case I need to change. If I could have found one locally, I would have got a 50.
I appreciate the advice.

Seipgam wrote on 05/15/21 at 17:44:41:


I don't agree with installing a larger pilot jet to add more fuel when the throttle is closed.  If you do that - the engine will be running too rich at idle and at low throttle settings.  You cannot properly adjust the idle fuel mixture screw when you have too large of a pilot jet installed.  If your idle fuel mixture screw can be turned all the way in without the engine going lean - the pilot jet is too large.

Seipgam wrote on 05/15/21 at 19:34:08:


I agree to that.  The Mikuni manual procedure for choosing a Pilot Jet states you should achieve the smoothest idle with the screw out 2 - 2.5 turns.  Less than 2 turns you should be installing a smaller jet, and more than 2.5 turns and you should be installing a larger pilot jet.  (At 3 turns out the screw is maxed out for flow....there is no reason to try and turn it out any farther).

The stock carb has a TEV valve that is supposed to add more fuel under high vacuum conditions to reduce the lean condition that exists when you close the throttle.  As the bike ages the diaphragm becomes stiff and it doesn't function well anymore.....and even at that it has proven to be somewhat ineffective (perhaps the E10 fuel requires a bit more fuel than the pure gas that was around when this engine was designed).  I need to look and see if there isn't some way to modify the TEV to flow a bit more fuel.

When I have my little Ninja 250 it developed an issue where it had an occasional lean surge when you were cruising at a steady speed at constant throttle.  It was the weirdest thing......the bike would run fine at idle or when accelerating and even at cruise - but if a gust of wind hit you head on the bike would begin to stumble!  If you added a bit of throttle it would run just fine.  I started to change jets and move the slide needle to see if I could correct the issue......it just needed a bit more fuel when the wind caused the bike to slow down and the vacuum slide opened slightly.  In the process I screwed up and got the mixture a bit too rich - and although the bike ran OK....it lost the crisp throttle response it had when jetted properly.  Eventually I got a set of carbs from the next generation of the Ninja and the issue was resolved, and the bike was so much more responsive with proper jetting.  It is amazing how sluggish running an engine can be when it is jetted too rich....that is why I don't recommend installing jets that are just too big!  Some folks install big jets and are satisfied that the engine runs OK.....and in doing so they often get crappy MPG and sluggish performance.  

My fist round of carb changes:
- Changed the pilot jet from the original 45 to a 50
- Changed the main jet from the original 145 to a 150
- Replaced the white spacer with 2 brass washers which equals 1/2 the white spacer thickness. ( I had to file down the outside diameter of the brass washers a little bit to the same diameter as the white spacer)
- I checked the float height and found it to be a bit low at 25.6mm or 1.01" (spec is 27 to 29mm or 1.06" to 1.14")
First round results were:
- I think the main jet change to a 150 is probably perfect. Good steady power on the throttle. No surging and no harder pull when I roll off the throttle.
- I achieved my best idle at 1 3/4 turns out on the idle mixture.
- I'm still getting a lot of popping when I roll off the throttle hard. I don't mind a bit of popping (actually kind of like it) but the frequent backfire is annoying. There has got to be a further alteration to eliminate the backfire. I tried turning the idle mixture out another 1/2 turn to 2 1/4 turns. Even though it smelled rich it made no difference in the popping or backfire.

I'm thinking maybe going down to no spacer or 1/4 spacer. Or maybe go to a 52.5 or 55 pilot jet (which I have both)

What would be your next moves???

Thanks a bunch for anyone's input.