Modifying carbs, will I need larger jets?

Hey all --

I have a 1984 KZ700, 9000 miles. Right now it's pretty much all stock
except for a k&n filter in the airbox. I have a needle and jet kit that
I am planning to install along with k&n pod filters. My question is,
since I am already changing the jets with the kit I bought, will I need
to get even LARGER jets because of the pod filters?

Thanks
James

 

Nope. If you bought one of those silly Dynojet "stage III" kits, the
jets that they give you are plenty large. They would be large enough
for high speed riding on a race track.

The problem with Dynojet's home made jets is that they aren't all that
precision made, and the numbers don't correspond with Mikuni and Keihin
jets.

Your KZ700 probably came with about #100 main jets and your engine
probably doesn't need anything bigger than a #115 or #120 main jet, but
how can you even tell what the Dynojet numbers mean?

They have some tiny little being with a hammer and a set of number
punches marking each aluminum jet with their own secret number, but
Keihin and Mikuni make precision *brass jets* and we KNOW what the
numbers on the jets mean.

The jet numbering scheme of Mikuni and Keihin jets is based upon a #
100 mainjet having a precision hole that is 1.000 millimeters in
diameter. A # 120 main jet has a hole that is 1.200 milimeters in
diameter.

The # 120 jet doesn't pass 20% more fuel, since the area of the hole is
pi time the radius squared. If you do the math, you will see that the
#120 jet has 44% more orifice area.

It's very hard for me to imagine that your KZ700 is going to need 44%
more fuel just because you switched to Pods. You would have to build a
radical high compression, high RPM engine with radical camshafts to
ever come close to needing that much extra fuel.

 

Thanks for the info.

The kit I actually bought is a K&N jet and needle kit, would this be the
same as the Dynojet kit? And given what you've said, should I buy some
Mikuni jets as well?

 

Are the jets in your kit made of aluminum or brass? Dynojet makes their
home made jets out of aluminum. Keihin and Mikuni make their precision
jets out of brass. If you have a Kawasaki, it probably has Keihin
carbs, so you would buy Keihin jets.

The difference between Keihin and Mikuni is that Keihin doesn't make a
"half sized"
jet. Both Mikuni and Kehin make a #100 jet, but the next Mikuni size
would be a #102.5 and the Keihin size would be #102.

The difference in hole size would be about 2/10,000ths of an inch...

Go ahead and try the kit to see what it does. You get the effect of
bigger mainjets at full throttle, and if the vacuum is too low to pull
the needles all the way out of the needle jets, you never get to use
the main jet at all.

If you can get the engine warmed up and find a long open stretch of
road where you can run full throttle for two or three miles, you can
get a good feel for whether the main jet is too large or too small by
rolling the last 1/8th of a turn on and off. If the
engine howls at 7/8th's throttle, but you start hearing invidual bang
bang banging of the cylinders, the main jet is too large, you need
smaller mains.

OTOH, if the engine howls at full throttle, you might be jetted just
right. That's when you want to do a "plug chop". You run the engine at
the highest speed the motorcycle will go for a few miles with new spark
plugs, then you push the kill button and pull in the clutch and coast
to a stop and pull out the spark plugs and "read" them.

You're lookign for a narrow black ring around the center insulator,
deep inside the spark plug. The ring might be 1/8th of an inch wide. If
there is more soot than that, the engine is jetted too rich.

OTOH, if there is no soot on the insulator, or you see little black
specks that look like pepper burned onto the spark plug, that indicates
the engine is jetted too lean.

The next possibility is little shiny silver balls of piston material
melted onto the spark plug. That means that the jets are really too
small for full throttle use.