This is a page that explains what to expect with each engine modification.
Lets start with you have already Un-Corker the bike….
If not start here à
Bob’s
Un-Corking Page (send him an ‘E’
mail telling him to finish it)
You need to feed that giant bucket of a cylinder more food if you want your Oinker to run wild.
Remember, more air
into the engine you need to flow more air out. With more air through the engine
you need to add more fuel to keep the 15:1 (or close to it) fuel to air
mixture. Re-Jet. Chart at the bottom and Eric has one also, Here: à
Eric’s jetting page. Over in 
Ok, you want more air
so, you can add more fuel. Start with a good flowing air filter like a Uni or a
No-Toil. I used the Uni for the longest time and really believe in it. I used
Golden Spectro Filter oil. I washed my filter after every long ride and after
every other day ride. I would wash it several times with dish soap and then
wash it in regular gas. You can wash it seven times with dish soap till it
looked clean…….wash it in gas after that and the gas turns dirty blue with silt
and oil. I have started using No- Toil and what a difference in clean up. Much
easier and faster….no need for gas. I don’t use a back fire screen…….if you
dump the bike and don’t look to see if you flooded the air box with fuel you
are looking for trouble. If you have a stock carburetor it will flood the air
box with fuel very easily. So, look out.
Next, drilling holes
in the air filter side panel is a big improvement! Some that like to ride under
water might not like this modification. 
There
is screen behind the holes. Barnum’s and others sell kit’s that makes it look
real professional.
Now you have more air flowing into the engine, now it’s time to move it out. On the stock engine I went with the XR’s Only Header to give me more power in the higher RPM’s and keep some of that great low end torque and capped it off with a White Brother’s R4 Muffler to save weight.
This
was the ticket for me! I lost a little too much bottom end with the spark
arrestor screen out and it was just too loud. This was a good compromise of
power and weight savings. I like the White Brothers “E” series Muffler also,
with twelve disks and no back plate. It’s loud but, if you need to get quite
quick, you throw the back plate back on and you are quieter. Eight disks (with
the back plate) and you’re legal! The Moriwaki is the best buy out there right
now and it is the best made system, period. You will lose some bottom end (on a
stock bike) but, it’s worth it. $399 from Big Valley
Honda. It was $411 to my door. With the quite insert and jetted right you
lose little bottom end and gain some at the top.
Now that’s a perfect weld! And the biggest
diameter header you can get. This system will flow some exhaust! Moriwaki makes
an awesome product. 
Ok, you have the air moving in and out but, you have to make the air to fuel mixture right! You can make the stock carburetor work and that dead spot when you wack the throttle will never go completely away. Jetting for these big air flow modifications is: 70’s Pilot, b53e Needle (forth clip down), and a 185 main for sea level to about 4,500’ (getting rich here). Go to this page to do the stock carburetor fix’:
If you really want to fix the fuel system get a Edelbrock Pumper! If you are going for the over bore 680cc get the Edelbrock bored out to a 40mm X 42mm. Or a Mikuni TM40! What a difference. The pig will squeal right now when you twist it’s ear as fast as you can.
For top performance and keeping a bullet proof bike I went with the 680cc over bore. They always say there is no replacement for displacement………In this case it’s true. The price is about the same as having the stock bore redone. The steel sleeve is about the same price and the Wisco piston is cheaper then a stock piston. If you wore out your bike or something has gone wrong it is worth the money to go 680. While it is apart you might as well throw in a few goodies like; Barnums Pro Mid cam (higher lift, longer duration then the HRC race cam), stainless steel valves with 3mm over sized intake valves, heavy duty valve springs and have the crank balanced.
Well, you want to know is it worth the time and Money? Well here is a round up of what to expect when you do different modification’s to your swine.
I will round all the
numbers up to the next Horsepower to make it easy. All the numbers will seem
low because Honda threw out horsepower numbers for this bike that are from the
crank. These numbers I show are Dyno (rear wheel). Un-Corked they say this bike
puts out 55bhp that is brake horse power measured at the crank.
The
difference of Brake Horsepower and rear wheel Horsepower is real close to25%.
It would go like this: Rear wheel Horsepower is 44hp on a dyno so, add 25% of 44hp to the 44hp and you get 55bhp that Honda shows. It’s not perfect but, real, real close. It’s funny it’s the same all the way up to race engines for cars. Car on the Dyno puts out 315rwhp and then the engine is checked giving 405bhp.
A
stock bike is about 35hp. About 44hp Un-Corked. This chart shows an
approximant. The stock bike has a lot of dynamometer charts out there so I made
an average. (They are a lot lower on some dyno’s). These horsepower gains only
tell part of the story……How the power is delivered is what counts. How much
torque you develop and when the toque comes on in the RPM range. Some of the bikes I have Dyno number from had
Barnum’s Exhaust. It has a header that goes around both sides of the cylinder
and comes to a collector under the carburetor. It gave a smoother Horse Power
curve at the bottom of the RPM curve. The 680cc 11:1 compression piston,
Edelbrock pumper carburetor bored out to 40mm X 42mm had a Barnums Pro mid cam
(more lift & longer duration then the HRC cam) with 3mm oversized intake
valve’.
Horsepower to modification chart:
Here in this next chart is shown the torque numbers in relation to RPM.
More flow moves the Power forward in the RPM curve. There is more torque everywhere but, less in the beginning compared to the amount you gain over all. Torque is the number you feel do the job.
Horsepower &
engine strength click here: Strong Engine
& Horsepower 
If you would like to see who many cc it tacks to make a horsepower from different engines click here: cc’s per horsepower
Jetting chart for stock carburetor Kehin 40mm
Mikuni TM 40
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http://www.borynack.com/Bruce/XR650R/xr650r.htm