I'm about ready to put on my Elmer Fudd suit, take down the 12 ga., and go fix my carb.  I cannot seem to strike a balance on this thing.

I KNOW this bike will make 52 mpg steadily and over 60 if I baby it.  I've done it.  

Right now I'm getting 42 mpg every tank.  My math makes that around a 20% decrease.  I have not changed my driving habits.  What I have changed is the pilot jet, the air filter, and the muffler.  #50 pilot from 47.5, Nufoam filter as per Old feller's post, and opened the stock muffler WAY up.  I still have the stock 155 main jet installed.

The bike was running too lean with the intake/exhaust changes, so I changed the jet.  I had the screw out three turns before rejetting.  Now it is running too rich with the screw all the way in.  Header is ugly, rear brake arm is turning black, occasional (rare) cannon shot after low RPM (like through town at 35 mph in 4th gear) run.  The bike doesn't like to idle when it's hot out, but starts great without choke when it's cool in the mornings.

My Jardine muffler is backordered and will be 2-3 weeks before it gets here, at least.  I had hoped to wait until I installed it before fooling with the carb again.  Tomorrow, we're going up to Glacier Park.  I'll need to put the rear plate back on the muffler to keep the spark arrestor police happy, and I'll be riding at about a million feet above sea level.  

I have the bowl off the carb right now, but don't really know what to do with it from here.  I don't want to change the jet today and then change it again tomorrow night.  Am I missing some "middle ground?"  Is there an overlap in the two jet sizes combined with the idle/air screw setting that I haven't found?  Most importantly, why did my mileage suffer so just by changing that jet?  I know it isn't supposed to affect anything but the low throttle performance, so what's going on?

I thought maybe the stupid thing fell out of its hole into the bowl, but that is not the case.

I could really use some input from those who know.  

Thanks,
Sandy

EDIT:  Further research has revealed that these two jets don't look alike.  The 50 has eight tiny holes (four pairs of two) drilled in the side of the shaft.  The 47.5 has no such holes.  I have no idea what this signifies, but could it be a difference that matters?

  When I order a richer pilot jet I had to take out the pilot so they knew if it had holes or not to order the jet.So I would say put back in the pilot that came in it.I don't remember which one I have but its an 06.

bill67 wrote on 08/02/08 at 11:43:31:


Ahhh... Thanks Bill.  I never paid attention to the holes before.  If they make a difference, I'll go get one without.

verslagen1 wrote on 08/02/08 at 11:49:47:


The next size pilot, right?

I'm sorry if my attitude is getting skanky, but I'm so frustrated.  I've read and reread all of the tech articles, along with the countless other threads on this very topic.  I have a completely dismantled carb from the parts bike to look at and understand how this all should work.  I know this stuff (I thought) but it is not working out.  By opening up the air and exhaust, I should be getting leaner, shouldn't I?  Not richer?  I haven't changed the main jet, so it should be fine, or else getting leaner at WOT, shouldn't it?

I do appreciate the help.  I just can't figure out why the common wisdom isn't applying to my bike.  Maybe the aliens are screwing with it at night while I sleep.

  Anyone know what the holes are for.

For better or worse, the new, non-bleed jet is installed.  It seems to have changed things.  I left the screw all the way in and took it for a short run.  In other words, the only thing I changed was the jet.

On decelleration, I now have the old familiar backfire.  It also gave a little fluff on shutdown.  I haven't heard either of those in weeks.  I then set the idle up a little and tuned it by ear.  I ended up about 5/8 turn out, although the change in RPM is less distinct with the turning of the screw (much as it was with the old 47.5 jet I started with when I bought the bike.)  If my performance or mileage change, I'll let you know.

It does appear that I now have a little wiggle room with the adjustment screw, instead of having to be all the way in and still not starting to lose RPMs.  Time will tell, I guess.

I couldn't find an explanation of the difference in "bleed" and "non-bleed" jets online.  Just looking at them side by side with my uneducated eye, I've formed a hypothesis.  The bleed jet is metered at the threaded end, then turns into a bigger tube with holes on the sides and end.  The non-bleed jet is large throughout its length and then is metered at the downstream end.  I'm thinking that this nozzle at the end delivers a mist as the gas leaves the jet, whereas the bleed jet delivers a mist inside itself, which then has a chance to condense and become liquid again before it oozes out the end.  I'd be interested to hear if my idea has any merit, if someone knows.

Well, I called up two of my friends who are motorcycle mechanics today to ask about these "bleed jets."  It seems that some carburetors use the pilot jet across a wider throttle range than ours.  This calls for the pilot to allow more fuel through.  I can't remember all the lingo.  My head is full of "flat slide, square slide, CV, yabba dabba doo."  Suffice it to say that the BS40 is not a carb designed to use the bleed jet.  When that jet is used in the BS40, it allows more gasoline into the engine than an equally sized non-bleed jet, as the engine is drawing on both the pilot and the needle jet at times.  

This explains (to my satisfaction, anyway) why there was no overlap in the two jets with all the adjustment available in the pilot/air screw.  The one is just too much "larger" than the other.  I have hope that I may get my mileage back.  Wouldn't that be nifty?

I'm pretty much just parroting back what I was told today, so I can't really defend it.  If someone knows more or different info, I'd still like to hear it.  I just won't be able to argue that my opinion is correct, so don't ask me for an intelligent discussion of the matter.  I'll let you know how the bike runs with the new jet.  That will settle the matter in my own mind.

It's the next day now, and I just finished a 200 mile run at varied speeds up and down steep mountains and around lots of twists and turns.  I got 59 mpg!  I still can't explain the whole thing, but there is absolutely no doubt in my mind that those two #50 jets are not interchangeable.  They both fit the same hole, but they don't do the same thing.  The bike is backfiring again, which tickles me pink.  At least now I'm back on familiar tuning ground, and know what to do.  The return of 17 mpg is pretty nice, too.