Engine breathless at higher throttle openings

It's that fast idle cam or "chock" on the end of the choke rod that's
causing the really fast idle, plus the rich mixture from the starting
enrichener.

Hopefully, you have clean new spark plugs in the engine and all
connectors and the wiring to the ignition unit is in good condition.

When you remove the air filter and the intake hoses, the engine will
run a little richer at idle because there's no pressure drop across an
air filter and it will run too lean at full throttle, when the jet
needle has come up out of the needle jet about 7/8ths of the way.

It's very hard to tell where the needle is at on a constant vacuum
carburetor when you have the throttle butterflies fully open as
indicated by the fact the throttle twist grip won't turn any further.

Do-it-yourselfers get all confused by this main jet, needle jet, jet
needle, pilot jet business. People who JUST DON'T KNOW ANY BETTER will
say, "Oh, your main jet is dirty, you need to clean it out."

But you are very rarely ever running on the main jet if you're a
street rider. You're mostly running on the idle jets and the straight
part of the jet needle.

I'm not saying that the main jet NEVER gets dirty. I've seen carbs so
filthy the main jet and everything else was plugged up.

Amateur tuners think that the various circuits of a carburetor work
only at certain throttle openings. That's not true at all. The circuits
are all working at the same time, except for the enrichener, which you
always turn off.

Amateur tuners get the idea that the idle jet only works when the
engine is idling and that's not true. The engine is supposed to get ALL
of it's fuel through the idle jet up to about 1/8th throttle, but it
keeps sucking fuel through the idle jet until the throttle is wide
open. It gets less and less fuel through the idle jet as the throttle
is opened.

downstream of the throttle butterflies come into play to help the
engine accelerate from idle. A little bit of fuel starts flowing
through the main jets, but most of it is coming from the idle jets.

Then, from 1/4 throttle to about 1/2 throttle, the diameter of the
untapered part of the jet needle influences mixture the most. More fuel
is coming through the main jets. But fuel is still coming through the
idle jets.

out of the needle jet and the area is increasing rapidly, but it's
still less area than the main jet's area, so that's what's controlling
the fuel, not the main jet. But the fuel has to come through the main
jet before it can pass through the needle jet. And fuel is STILL coming
through the idle jets.

Finally, when you get the throttle open 7/8ths to full throttle, the
engine is running on the main jets. But you have to remember that the
main jet is IN SERIES with the needle jet/jet needle orifice. The area
of the main jet ALWAYS has to be bigger than the difference in the area
of the needle jet minus the area of the jet needle or the engine will
starve for fuel at large throttle openings.

Amateur tuners go blindly guessing and buying a lot of expen$ive bra$$
jets at around
$5.00 each, or they ask other people that just don't KNOW how the jet
sizing numbers work. I've explained how the numbering system works
dozens of times in this NG.

But, you probably don't need to run out and buy a bunch of main jets
anyway. Just hook up the air hoses and install a new air filter, after
you've thoroughly spritzed out the carbs with B-12.

 

The air filter is disconnnected because the bike didn't come with one
(one is on the way) and that I wanted to get the carbs in decent order
before I wrestled with the air piping.

 

Does using a K&N filter make an appreciable difference compared to the
standard paper one? Will I run too lean with it also? I ordered one
because I actually got it cheaper than a stock replacement would have cost.

 

A K&N filter will flow more air or less air than a stock replacement
filter, depending on how recently you oiled it. Follow the instructions
and let the filter set to drip out any excess oil before you put it in
the airbox.

Amateur tuners who have over-oiled their K&N's and tried to re-jet
their carbs thinking the mixture would be too lean found that it was
too rich and the experts recommended not even trying to tune the carbs
for a few days until the oil spread out evenly.

K&N's are a very economical replacement for OEM filters because a K&N
won't wear out, you can just keep washing it out over and over again. I
use kerosene, not the special K&N washing liquid.

Then I don't oil the filter because I don't ride in a dusty area.
That's no problem with a motorbike that has chrome plated rings. I
wouldn't do it if I had a more modern bike with cast-iron rings and
electroplated cylinder walls, the soft rings would wear too fast.