Strange Correlation: Main Jets Go Down, MPG's Follow Suit

[IanDMacDonald]

So, in the Spring, I dropped the Main Jets from 140 to 137.5. I also upped the Pilot jets to 45 from 42.5. I did not like the response I got from the 42.5's, so I dropped them back down to the 42.5's. I tuned the bike with the 137.5's. I've noticed that my MPG's have dropped almost similar to a Special's, if not worse: 90-100 miles till Reserve. Granted, I was not getting stellar mileage with the 140's (130-140 on a good day), but nothing this thirsty.

Pulled the plugs, and they do not look rich. Positive to no leakage, not 100% positive to some kind of drag, although the tire spins okay on it's own.

Any ideas?

[Bonz]

Could be that the smaller mains are causing you to use more throttle, which would lift the needles more, but that's about a 40% loss in mpg. Something else is amiss.

Go back to the 140's would be the next thing I would try.

[BA80]

Are you running stock exhaust Ian?

[Bonz]

Signature line says stock exhaust. Given no other changes, even after market exhaust wouldn't dump MPG in the toilet with a jet change on a decently tuned bike.

Something else is at work, assuming everything is sealed up (no intake air leaks) and nothing dragging I am at a bit of a loss. That's where I think going back to the 140 mains puts him back where he was, and maybe something will show itself during that process of on/off with the carbs.

Ian, what tuning did you do to it when you put in the 137.5's? "Normal" would be tweaking fuel screws based on my experience. With everything else back to the same now, next step is back to the 140's and see where it is. Did you notice the MPG go down with the larger pilots, or didn't leave them in long enough to find out? The fact you have the smaller pilots back in would mean its a moot point, but thought I'd ask. I see you have the vac adv deleted, so nothing there that could be going on.

[IanDMacDonald]

Hey Greg- Yes, stock exhaust. And, I know I could get better fuel mileage without the vacuum advance running locked-out, but I actually went back to stock setup before, and saw no change in mileage.

Mr. Bonz- Yes, regular tune and sync. Nothing out of the ordinary. And, I could tell going upto the 45 pilot was too much, so I dropped it back down well before I tracked my mileage. I'll put the 140's back in, but drop my float height to see if I can save a few drops of gas.

Thanks for the insight.

[Bonz]

Had you adjusted floats during the jetting swap? Are you meaning to lower the fuel level, or lower the float height?

Lowering floats will raise fuel level, makes it go toward the rich side all things equal, especially in throttle response at lower rpm as fuel is pulled up from the bowls when you whack the throttle.

[IanDMacDonald]

Had you adjusted floats during the jetting swap? Are you meaning to lower the fuel level, or lower the float height?

Lowering floats will raise fuel level, makes it go toward the rich side all things equal, especially in throttle response at lower rpm as fuel is pulled up from the bowls when you whack the throttle.

25.7mm is what I have always ran. Whoops, honestly that always confuses me. Do I want to go more than 25.7mm, or less than 25.7mm to save gas?

[Bonz]

A higher float measurement, thus more than 25.7mm, will lower the fuel level in the bowls, however....

-Float height keeps fuel level in the bowls at an ideal level to time fuel coming into the main jet to mix with incoming air when you open the throttle so you aren't too rich or lean. Lower fuel level (raising float height) delays fuel into the main jet because it has to travel a bit further (lean throttle response) vs a higher fuel level (lower float height) that keeps the fuel closer to the main jet (richer throttle response). Does very little or nothing to control overall gas mileage, that is the job of pilot circuit, needles and main jet.

[MPittma100]

Mileage

Are you using Mikuni jets?

[IanDMacDonald]

Are you using Mikuni jets?

Give me some credit MP, lol. You betcha! I think it's what Bonz said, I'm overworking the bike with the smaller mains. I may look into putting a '78 mechanical advance on sometime to see if I can increase the mpg's. Maybe a project after the XS rallys.

[Bonz]

Good question on Mikuni brand jets. Jets named as direct replacement equivalent but not Mikuni brand have a reputation of being way off in size. "We're the 137.5's genuine Mikuni"? (that one goes out to Motoman)!

Ian, once you get back to the same set up you had that was working, see if it still works like it did before the jetting stuff you did.

Then we can be a whole lot more sure the problem is somewhere else.

[MPittma100]

Mpg

Maybe I am confused. You had bigger than stock jets with everything else stock. You went to the stock jets and MPG went down? You are right, logic has left the building. Something else has to be the contributor.

I've had the best luck with the highest spec setting (tried all of the spec range) on the float heights - best MPG and no fuel seepage.

I do give you credit for sure. Just a good idea to always ask the obvious questions. Things like needle type and clip setting also come to mind. I'm thinking that you should get 38-42 MPG with everything correct. At 38 you should get 148-152 miles before reserve on a good day. Does that sound about right?

[IanDMacDonald]

Maybe I am confused. You had bigger than stock jets with everything else stock. You went to the stock jets and MPG went down? You are right, logic has left the building. Something else has to be the contributor.

I've had the best luck with the highest spec setting (tried all of the spec range) on the float heights - best MPG and no fuel seepage.

I do give you credit for sure. Just a good idea to always ask the obvious questions. Things like needle type and clip setting also come to mind. I'm thinking that you should get 38-42 MPG with everything correct. At 38 you should get 148-152 miles before reserve on a good day. Does that sound about right?

Well . . . only thing stock would be the exhaust and jetting. The biggest factor on the lower mpg's would be my locked-out advance. I'm going to put the 140's back in, and try to run the leanest mixture I can within reason.

When you say the highest spec range, do you mean 26.7mm? I believe it's a +/- 1mm if I remember correctly. Or, 24.7mm?