Why are the FP jets smaller than the stock?

[andrewsw]

I was perusing the parts catalog (thinking about replacing my emulsion tubes) when I noticed that the stock jets for GSF400P are a #105, but everyone seems to be running much smaller jets (like #99's or so) in their 32mm carbs. Why is that?

A

[tomacGTi]

Needle taper.

Most jet kit needles have a much smaller tip and a different taper in comparison to OEM so you don't need as large of a mainjet.

P, hm, thought 102.5 was standard across the board.

[andrewsw]

That makes some sense, but by the time you're running on just the main jet, the needle is almost completely out of the way. I can't see that the needle makes that much difference at that point. Certainly for mid-range rpms where the needle is a good way down the emulsion tube, sure, but once it's maxed out?

Makes me wonder...

[interfuse]

Mikuni 5ez67 (canadian & euro adjustable needle)
Total Length 55mm
10 mm - 2.60mm
20 mm - 2.55mm
30 mm - 2.36mm
40 mm - 1.85mm
50 mm - 1.79mm

FactoryPro
Total Length 56mm
10 mm - 2.56 mm
20 mm - 2.48 mm
30 mm - 2.33 mm
40 mm - 2.02 mm
50 mm - 1.75 mm

Dynojet
Total Length 56mm
10 mm - 2.50 mm
20 mm - 2.50 mm
30 mm - 2.30 mm
40 mm - 1.98 mm
50 mm - 1.75 mm

You can see the mikuni needle is all around fatter which means leaner (except for the 40mm, the mikuni needle has a strange double taper).  The needle can't come completely out of the NJ or else it would risk binding, so around the 50mm mark is where you'd be looking for WOT. You can see the difference. Also I believe having a smaller MJ makes the midrange easier to tune.

[andrewsw]

okay, I'll buy that, I suppose. I think I'm going to shell out the $ for a dyno run in the spring. There's a guy here who will give you 6-10 hours of time in his shop for $100 US. He doesn't actually do anything but run the bike on the dyno, and "help" with the tuning work. So you get a good bit of time to tune it up, I guess. I think it sounds like a deal and would get rid of all this silly "seat of the pants" stuff that I can't seem to work out...

[interfuse]

Can't go wrong for $100 an hour. Just as an FYI, the stuff you can't work out is usually wear and tear. I had a hell of a time sorting out my carbs until I replaced the o-rings, NJ, and needles. 10,000 km more and I'm in desperate need of new NJs again. You can move the needle clips, change the MJ, drill holes in your air box,  add more oil to your filter, but unless everything is in good shape you'll end up with problems somewhere.

[andrewsw]

Just to be clear, this is $100 for a minimum 6 hours max 10 hours. That is $100 for a day with a dyno.

Apparently this guy is a retired helo mech and he has taken up motorcycles as a hobby. He bought a dyno and does this to help pay for it, not to make money.

Yeah. I replaced the NJ's a couple thousand miles ago, but with some from a used set of carbs. I think they've gone out again. I get a slight rich stumble when I just barely crack the throttle over 1/8 at 5-6k rpm. Worse when hot... this rich. I'm planning to drop the needles this weekend just to make sure that's not it before I shell out for the NJ's. Otherwise I'm all good (except my top end isn't there... can't reliably get over about 100, not that I really need to, probably limited, and the resistor trick doesn't seem to work, but that's another story).

[mademiriam]

WOW for $100    I'd be at his door tomorrow morning!