Re: Big jets = fast?

 Hi!

On Fri, 18 Apr 2008 19:00:50 -0700 (PDT), Craig wrote:

I my opinion, seems to be that this is the consequence of a series of
misunderstandings:

Normally, the first thing "people" do, is mount a racing airfilter, open up
the exhaust, intake, perforate the airbox like an emmentaler (swiss
cheese) and/or mount a $$$ aftermarket titanium/carbon muffler that adds
another so-and-so much horsepower. Because those people need every single
bit of their bike's power delivery since they always operate it at the
utmost maximum all the time, no matter at what noise level. So it's
perfectly clear that also the jetting has to be "opened up", regardless of
what the status quo is/was. "Opening up" is, as they heard from a friend
of a friend and as everybody knows, increasing every jet size and moving
up the needle a number of notches.

Then there is a number of people who are afraid that their bike might run
lean as the result of a rejetting process and fry its piston. So they stay
"on the safe side" and keep it way rich. Others might have jetted it for
15°C and sea level (pretty much stock setup) and don't care when it gets
"a little rich" at 30°C and 1000 metres. Four strokes anyhow seem to be a
bit more forgiving when it comes to wrong jet sizes. Additionally, staying
a tad on the rich side mellows out the power delivery curve a bit - at
least with my Dellorto-driven 1998 KTM 620 SC, the snappyness seemed to go
away when I hung the needle a notch higher.

Also, I see a tendency that people overoil their filters, because "much
helps much", in a way that the oil clogs the pores and starts dripping
from the lower part of the foam after a while. This will also result in a
richer-than-desired mixture without even touching a jet.

There also might have been an airleak as the result of a damaged intake
manifold, an errorneously mounted carb, etc. etc. (add a variety of
twostroke problems here) that was "compensated" with larger jets by
mistake. Then, after some maintenance, the orignal problem was cured
(probably even unknowingly) and the jetting kept as is, because the bike
"ran fine" with it.

Last thing I can think of is a mis-adjusted accelerator pump. Without the
"BK-Mod" (a pump rod actuator stop mechanism), the FCR-MX carb in my Yam
426F would inject for a whooping five seconds (no kidding) after whacking
the throttle which also makes the bike run way rich. After the mod, I was
down to some below one second which helped a lot regarding response and
out-of-berm-handling.

Moral: Jetting is not an easy thing to do and folks that are unsure about
the where and what should probably thoroughly read some information and
consult a friend with some knowledge before digging into their bike's guts.
Writing down the changes is also a mandatory thing to do. Or buy a fuel
injected scoot and rid themselves of permanent rejetting hassle. ;-)

Cheers,
Volker


--
@:  I N F O at B A R T H E L D dot N E T
3W: www.bartheld.net

 wrote:


http://www.crocker.com/~mwilliams/Derestricted_path.htm

Nice write up.

M


--
Mike W.
96 XR400
70 CT70
71 KG 100 (Hodaka-powered)
99 KZ1000P (training)
99 KZ1000P (rider)
00 Beta Rev-3

 

Yabut bragging about my engine being so powerful that it needs REALLY
SMALL JETS just doesn't carry the same weight...

Craig

 

Thanks Mike.

My KTM 300 is getting 22mpg, whereas my ATK 260 got 30mpg.  Arrgghhh....
I'm already running toward the lean-end of jetting recommendations.  I've
got to wonder how much the V-Force reeds changed the baseline.  As I'm
headed to Utah this week, and will be riding deep sand at a lower elevation,
I'm probably not going to experiment with leaner jetting.  I'll just carry
extra gas.  Mountain riding this summer will be a learning curve in jetting.

-Jeff-

 On Mon, 21 Apr 2008 13:52:21 -0600, "HellSickle"


My pleasure. Someone has to make up for Jay.


I'm not sure if you remember it, but Gordon wrote a really nice piece on
the experimental process he followed in arriving at the famous Gordon Mods
back in the day. He posted it either here or in the XR400 group. I'm sure I
could track it down if you really wanted it. Gotta pack... later.

Mike


Mike W.
96 XR400
70 CT70
71 KG 100 (Hodaka-powered)
99 KZ1000P (training)
99 KZ1000P (rider)
00 Beta Rev-3