carb jets for 250cc Honda twin.

 I'm getting ready to rebuild the carb on my wifes 86 Honda Rebel 250,
and was looking for some info. There used to be a yahoo group (it may
still be around but i can't find it) that was dedicated to 250cc
motorcycles (not just the rebel) and someone had posted about a jet
change for the carbs in the Honda 250 twins (Nighthawk, Rebel) that
gave better performance that the stock setup. It was referred to as
"the way Honda should have done it in the first place"

I'm trying to find that info again, so if anyone could help it would
be much appreciated.

Thanks

 Justice wrote:

Somewhere over the years, Honda reduced main jet size from a #128 to a #108,
which is an area reduction of about 29%.

It's amazing that Honda could miss the original main jet sizing by that much.

However, I don't see where changing the main jet would make that much
difference unless it was obvious the engine was gargling excess gasoline at
wide open throttle on the larger jet.

You engine only runs on the main jet when the throttle is wide open and
there's enough vacuum for the slides to completely raise the jet needle out
of the needle jet.

Your engine runs on the pilot jets or the needle jet/jet needle variable
orifice 99%
of the time. You can make the engine more responsive to small throttle
openings by carefully drilling out the EPA anti-tamper plug and turning the
idle mixture screw about 1/4 to 1/2 a turn counterclockwise.

The plug I am referring to is the bottom thing in the item #1 square on this
diagram. The parts list is referring to the little rubber o-ring when it says
it's part of the gasket set.

http://houseofmotorcycles.bikebandit.com/partsbandit/ShowSchematic.aspx?deptId !0753&machineId724


Jet sizing is based upon a #100 main jet having a precision drilled hole that
is 1.000 millimeters in diameter. A #35 pilot jet has a hole that is 0.350
millimeters in diameter. You can work out the area formula = pi X radius
squared to see the difference in area, just for comparision.

www.bikebandit.com > oem parts > honda > year?

1986 Rebel
35: JET (#128)
36: JET, SLOW (#35)

1996 Rebel
36: JET, MAIN (#108)
37: JET, SLOW (#38)

You can also check the part numbers of the needle jet sets and needle jet
holders to see if there was any change over the years.

--
Message posted via MotorcycleKB.com
http://www.motorcyclekb.com/Uwe/Forums.aspx/bike/200708/1

 On Wed, 22 Aug 2007 19:45:54 GMT, "Albrecht via MotorcycleKB.com"



Thanks for all the responses guys!

 Justice wrote:

I assume by "better performance" you mean 'better throttle response'
and not a significant horsepower gain, because that engine is never
going to make a whole lot more power than it does now.

Before changing jets, have you tried opening up the idle mixture
screws an additional half turn or so?

If you insist on changing jets, go up _one_ size on the pilot jets.
Resist the temptation to fiddle with main jets (they're usually too
rich anyway) or needle position.  The pilot jets supply the idle
circuit and the transition ports, which is where almost all the fuel
comes from at moderate throttle openings.  My departed CM400T ran
a lot smoother with larger pilot jets but it did cost a noticeable
amount of gas milage, YMMV.

--
'01 SV650SK1  '99 EX250-F13  '98 ZG1000-A13
OMF #7

 Mark Olson wrote:

Go with fuel injection, it's less trouble and hassle.