Transam1 wrote:
What kind of parameteres to the engineers make for cross country
riding going from sea level to mile high on carbs ?
Fuel injection.
You're only on some of the overlap from
main on the low end ckts, aprox 3/4 throttle it's all main. I did some experiments a while back with some one else, we both went well over the norms on main jets, he went to a 205, I went with a 190 as long as we did a steady roll-on to 65mph (cruise speed, also the speed Mikuni uses for their cruise mileage testing)loss of mpg's was very slight, if you look at throttle position while cruising, usually you're below 1/4 throttle (
pilot ckt). (best way to see this is to use witness markings)yes, there is some over lap off the main, but not so much to lose mpg;s. It doesn't take much twisting of the throttle to start dumping gas from the main and eating a lot of mpg's, same thing from a stop, most usually hit the throttle hard and you'll be getting tons of gas from main ckt. I've probably read over the past 5yrs, not on this forum tho, at least 10k postings on carbs, the majority are using 1 of 3 main jets as well as the Mikuni brand, usually a 167.5, 170, 172.5. Imo, the 170 or 172.5 for high elevations would be fine, pilot ckt tuned for high elevation will be richer on lower elevations, it could end up being to rich low ele. and cause some stumbling, since the engine can't burn all the gas, probably a slight
pms adjustment. The cruise speed is defined by the pilot ckt, needle jet, and the diameter of the straight portion of the needle (just below the grooves)I use the 42hsr
carb, just changing the needle one size effects gas consumption since the only difference between the 4 available needles is the straight portion, and the difference from 1 needle to the next is only
.
01mm, but I have seen a difference in mpgs about 6 to 8 mpg. As you can see, just the slightest bit of difference can make a big difference in gas consumption. I also prefer to tune a carb by AFR (air /fuel ratio) and here, there is no right or wrong, there is a spread in AFR that you can tune to, depending upon if you are tuning for performance or economy, I'm at 14 for cruise yields 50mpg, at 13 (richer)40mpg, I'm willing to sacrifice a bit of performance to gain some economy tho. I don't care for the stock carb myself, the 42 has a better separation of ckts, so much easier to tune. The pilot ckt also has a better advantage on the 42 vs stock carb. stock carb pilot screw adjusts the gas flow, richen it up....uses more gas. 42hsr pilot screw adjust the air, adjusting this has way less effect on mpg since the pilot jet size hole controls the gas flow and is a constant. Probably the most important thing to do is to ensure there are ZERO air and or vacuum leaks on the intake, (manifold to heads, manifold to carb, and air kit to carb joints) even the slightest of leaks will play mind games when tuning, I fought for quite some time trying to find my leaks, wd40 would never show leaks, at one point only a physical look at the boot from carb to manifold, I'm using a
Nemesis manifold and the boot ripped, but at idle and sitting still, the rip was completely sealed, not carb cleaner nor propane showed this particular leak, was not until I took it all apart and inspected did I see the tear. I know now that I am completely sealed, in fact, using a smaller main then what is considered standard for the 42hsr and confirmed by an AFR reading. I used this articles AFR table to tune, though most of the article is geared towards HD bikes, an engine is an engine, AFR is AFR. I did use the leaner afr for each of the ckts they show the spread in.
http://www.daytona-sensors.com/tech_tuning.html
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