Jetting Question... 40 pilots, 122.5 mains?

[manofthefield]

I'm finally considering rejetting my bike... after two years of ownership.  The warmup, especially in cold weather is killing me.  I figured I would go with the standard 40 pilots and 125 mains as recommended in the jetting matrix (bike is stock), but I was wondering if I could just change the pilots and leave the mains alone. 

I understand I would not get a boost in performance, but in trade I  am hoping I can maintain my high fuel efficiency.  Anything wrong with this idea?

[manofthefield]

ttt

[Cal Amari]

Well, since you're still waiting for a reply... I think you should change the main jets at the same time; I don't think you'll see any substantial change in fuel economy, but you'll most likely improve throttle response by installing the larger mains. If you're REALLY concerned about saving fuel, make sure your tires are properly inflated (decreases rolling resistance), keep your drivetrain aligned and lubed, and decrease the warm-up period (the larger pilots will help there).

One benefit of a slightly richer mixture (from increasing the size of the main jets) will be that the engine should run slightly cooler; not enough to effect oil flow, but just enough to lower the temps at the piston crown and valve stem area (IOW, in the combustion chamber). Slightly lower temps there means less chance of bad things happening to a good engine over an extended period of time.

Just my opinion, others are bound to Buddha Loves You and disagree...

[Alphamazing]

The GS is rich on the top end anyways, as evidenced in this thread. Changing the pilot jet will richen up the low end and smooth out the power band, allowing much better control. You shouldn't notice a drop in fuel economy.

[scratch]

I tried the same thing that you want to do, just change to 40 pilots and maintain fuel economy, but had frequent, mild hesitations in the mid to upper rpm's and balky roll-on .  Then I thought to just raise the needles; first 1 washer (.5mm), then another, to no avail.  Then I finally broke down and bought 125 mains, and dropped one washer, and the hesitations and the balky roll-on stopped!

Now, I live in a valley, where the oxygen is more dense, that may be a factor as well.

[Alphamazing]

Ahh yes, climate differences can call for much different jetting settings. Good call, Scratch.

[Cal Amari]

I remember reading that thread you linked to when it was first posted, but I was on the road and didn't have time to reply. dbarile doesn't tell us that the valves were properly adjusted just before the dyno run, in fact, he specifically states that the ONLY maintenance he'd done was to change the oil and plugs, so valve clearance is a BIG grey area for that dyno run.

Also, normal (factory assembly line) cam timing differences come into play here; with a stock engine (clean stock air filter in an unmodified airbox, stock jetting, stock pipes, with a proper valve adjustment, clean plugs, engine at operating temperature before testing), I think his results would have been somewhat different, and he wouldn't have seen the engine running so rich in the upper rev range. He was testing a twelve year-old motorcycle with only 7K miles on it, which is better than testing an engine close to the end of the line, but I refuse to believe that his valves just HAPPENED to be properly adjusted (nothing he posted contradicts me), and that makes the entire test invalid for establishing a baseline as to how a stock engine performs.

If anyone wants to dyno a TRULY stock engine that is in a proper state of tune, then post the results here, that would be fine with me, but simply using that single dyno run to evaluate carb jetting, without knowing the condition of the valves, is ludicrous. FWIW, I'd also like to see a stock engine dyno'd after degreeing the cams; that would be a MUCH better baseline to determine which types of jetting changes to make for his particular engine.

Cam timing is the single most difficult task to accomplish on the assembly line; the cams are dropped in, clearances are checked, and if nothing is seriously out of alignment (valves not hitting pistons, etc.,) the engine is assembled. Dialing-in the cams would provide optimal valve timing, but there is no time for that on the assembly line, so we have to accept the compromise of nothing seriously misaligned in the valve train and live with the results.

Bottom line is, that dyno run is NOT something to base your carb-tuning opinions on; my own shadetree work and experience tells me those valves were probably not within spec, and I can't accept those results as truly typical of a stock engine.

Thanks for your input on this, scratch; I didn't want to get into atmospheric and temperature differences here...