Your up pretty high, so it doesn't surprise me that you are only getting 46 mpg.  You could try adding a washer over the spacer to lower the needle about 0.5mm.  The compression is very low, so in stock configuration these engines aren't very fuel efficient.

Your main jet (155) is too large for 4-7.5k’ altitude.  Drop it to a 150 main jet and see if that doesn’t help your mileage and power in the higher throttle range.
Are there any washers in the carb that replaced the white spacer?.  When the previous owner removed the white spacer he hopefully installed 2 or 3 washers in its place.

Just read the PO notes again. He has 1/2 a white washer for the needle. I'm thinking he means he has washers that are the equivalent . f 2-3 washers Looks like I have a fun Sunday afternoon project.

I average about 52 and live in Denver at 5280 feet. I actually get better mpg on my mountain rides, like today, where I was at about 8500 feet. I get closer to 60 on those days.

sounds like your pilot jet is too big.

Just PM'd Lance for jet kit. I'm, unfortunately, an instant gratification kinda guy. I was hoping for a miracle by twisting a screw. Not to be.

miatamarty wrote on 07/19/23 at 14:41:13:


The pilot jet has little or no affect on mileage.  Nobody rides slow enough to be using the pilot jet as the main source of fuel/air while riding.

The fuel mileage while you ride is dependent a lot on how you ride.  If you ride at 70mph and are using 3/4 throttle or more all the time, then the main jet is likely controlling the majority of the fuel flow.  If you ride 1/3-1/2 throttle the majority of the time the needle position is likely contributing to your mpg and performance.

Jetting for how your engine runs with the throttle open is more important than jetting to eliminate backfires when you close the throttle.  Closing the throttle while slowing down or while shifting gears causes the slide to drop and the needle to close most of the fuel flow from the needle jet....and this causes the mixture to do lean as the majority of the fuel is provided by the idle circuit.  If you make your idle circuit richer to reduce or eliminate the exhaust popping - your engine will be running rich when you are idling or opening the throttle.  If you learn to roll the throttle slightly open when the noise begins you can ride quietly while still having the engine jetted correctly  Roll the throttle open only enough to reduce the noise - not enough to cause the bike to accelerate.

A 155 main js just tooooo big for the elevation where you ride, and you may even require a #47.5 pilot jet.  An engine that is too rich can run smoothly and seem just fine - but when you get it running at a proper mixture you will be surprised how much crisper and more responsive the engine is.  When jetted properly you should feel the bike idle a little slower and feel a bit of a lean surge for a few minutes as the engine warms up.....if the bike runs smoothly when cold it is likely jetted too rich.