CB 650c -Go back to Stock Air Filter??

I have a 1980 Cb 650 custom with currently 4-K&N s independant air
filters, with a 4 into one header. The bike needs re jeting as lean,
My concern is that rain can easily get into filters and ruin a good
day.
Even if I kept the K&ns and re-jetted the carbs am I really going to
notice a great improvement in performance over the stock air filter
set up?
I have just purchased a complete 1981 Honda 650 for parts and can use
the air filter from that bike to return my 1980 to the stock air
filter set up, I could also use the carbs from the 1981 which are
different, and wondering if these old stock air boxes were any good
and if they made lots of rattling noises etc...HELP what should I do??
Question:
1980 carbs or 1981 with stock air or K&ns on which carbs?

Thanks in Advance for any info here....
Tim C.

 

You might want to go up one or two sizes on the mainjets, but don't
jump into re-jetting without studying mainjet sizing. The round
mainjets used by Mikuni and Keihin are based upon a 1.0 millimeter
diameter hole in the jet, so they call it a #100 jet. Same thing with
the pilot jets, but they are smaller than 1.0 millimeter so the sizes
are in percent of one millimeter, like #30. #40, etc...

If you go from a #100 main jet to, say a #120 mainjet, thinking you'll
get 20% more fuel flow, that's *wrong*, you'll get something like 200%
flow, since increasing the jet's orifice size slightly increases the
area as to the formula pi times the radius squared...

And there may be EPA plugs blocking off your access to the idle mixture
screws, I know that Mikuni was installing blocking plugs in the early
1980's, don't know when Keihin started doing that...

The plugs can be carefully pilot drilled, a sheet metal screw threaded
into the hole, and then pulling the sheet metal screw out with a pair
of pliers removes the plug so you can screw the idle mixture screw out
half a turn or so to richen your idle mixture...

If you get a little bit of water passing through the filters, it turns
into steam in your combustion chamber, blowing out the carbon build
up...

But, if you get too much water into the engine, it can cause a
hydraulic lock in the cylinder and you can bend a rod...

I wouldn't ride in the rain if I didn't have to...

notice a great improvement in performance over the stock air filter set
up?

You will probably get about 6 or 7 horsepower improvement with K&N
filters and a well-matched 4-into-1 that was developed specifically for
your motorcycle, using a dynomometer for testing. You won't notice much
at the first power peak, engines have a power curve with two humps like
a camels back. But the second power peak will be at
a higher RPM than the stock power peak, so you'll have to ride harder
to enjoy your extra 6 or 7 horsepower...

And there will be a big trough between the two torque peaks. It will
occur between 5000 and 7000 RPM. It's called "the dreaded flat spot"...

The flat spot isn't much of a problem when you're accelerating on the
straight, the flat spot gets you in the curves. You might find that
you're comfortable taking a certain curve in 3rd gear, but that gear
drops you into the flat spot and you have to shift down to 2nd gear and
buzz the engine a little to keep out of the flat spot...

In the mid-1980's, many performance-minded riders and racers were
removing the stock airboxes, and they discovered that they had the
dreaded flat spot in their power band. So the smart guys put their
airboxes back on...

An properly designed airbox acts to smooth out the pressure
fluctuations in the airbox. The box doesn't make any more power, but it
smooths out the trough between the two peaks in the torque curve...

If you want to learn more about airboxes, google for "helmholtz
+resonator"...