Hey,
So basically most of the stuff i've read about shorty mufflers with cb350's is that it won't work because of the back pressure, meaning you need more resistance on the air that is outputting from the pipes. Since stock pipes are longer than shorty's, they don't have this problem.
Well I love my shortys. They are loud and cool as ****. Sooo, i read somewhere that welding a washer to the end, or having the washer pivot on a bolt can add more resistance to the airflow out the exhaust.... Has anyone tried this????? Seems like an easy fix to a problem that has been haunting bobber/cafe customs for a bit.
This guy knows a lot and mentions this:
The airbox is a resonant chamber that helps the mid range flat spot if it is
designed correctly. But old time dirt track riders used to strip stock
airboxes off and clamp a piece of women's nylon stocking over the carburetor
mouth.
They might even oil the nylon to keep the dust from getting into their engine.
But, after every heat race they would find mysterious holes in the nylon,
caused by reverse air flow through the carburetor during the camshaft overlap
period when a pressure wave from the exhaust pipe would come back through the
open valves and out the carburetors.
If you stand off to the side of a motorcycle without an airbox and the
afternoon sunlight is right, you can actually see fuel/air mixture OUTSIDE
the carburetor mouths as the tuner rev's up the engine.
Hot rodders called this mist "standoff". If they didn't at least run a low
air filter, the standoff mist would ignite when the engine backfired through
the carb and they'd have an under hood fire.
A well-designed airbox keeps that fuel/air mix inside and sends a positive
pressure wave of about 1/4 of a pound towards the carburetors, working
against the adverse pressure wave from the exhaust pipe.
You can google for "helmholtz resonator" to read about airboxes, but
basically the volume in the airbox controls the frequency the box resonates
at, and the box has to have an inlet neck of a certain size and length or it
won't work at all.
HR's work over a wide range of engine RPM, but they mainly need to be
designed to overcome the flat spot that occurs around 5000 to 8000 rpm.
Dirt track and road racers thought that the way to get the most power was to
run open carburetors with velocity stacks, but they had the problem of all
that dust being sucked into the engine and wearing it out, so that's why K&N
filters and oiled foam pods were invented.
Dirt track riders used to take their mufflers off and just run a straight
pipe.
When the exhaust gases got to the end of the straight pipe, they would expand
rapidly, making a lot of noise, but a weak pressure wave would still come
back to the carbs, making the air flow the wrong way.
The factory road racing teams were experimenting with long duration camshafts
and open megaphone extractor exhaust systems, but the 'phones were very loud
and the engines had such a terrible flat spot the riders had to slip their
clutches to go around slow corners in 2nd gear.
Then the factory teams started putting a short section of reverse cone on the
end of the megaphone to help the flat spot and make the motorcycle more
rideable.
The earliest Honda CB750 four cylinder motorcycles had four exhaust pipes
that were *styled* like reverse cone megaphones seen on grand prix racers of
the 1960's, but they had baffles inside to quiet the noise down, and riders
weren't trying to race around using 10,000 RPM all the time, they were
putting around town and cruising on the highway using 4000 to 5000 RPM and
they
weren't even getting into the flat spot that occurs between 5000 and 7000 RPM.
The two mufflers that came stock on your CL350 were shaped like the old "pea
shooter" mufflers that came on 1960's British motorcycles like Triumph.
But Honda figured out that what they really needed to do was add a balance
tube between the two separate exhaust pipes before the pipe widened out into
the muffler. The balance tubes underneath the engine were actually larger in
diameter than the exhaust pipes they were welded to.
These balance tubes gave the engine more midrange torque, and the Japanese
motorcycle factories, especially Suzuki, were using balance tubes to good
effect even up to a few years ago to smooth out the flat spot in the power
band.
And, tuners like Yoshimura were making the balance tubes into resonant
chambers that helped their highly tuned motorcycles accelerate out of corners
better by providing a bit of "back pressure".
You would see one of the chambers behind each pair of exhaust pipes on their
4-cylinder machines.
>Here is the complaint when im riding the bike and the
>motor hits around 5000 6000 rpm the motor starts to give this popping
>back fire and eventually bogs down.
The mid range fuel air mixture is getting too lean, the spark plug is getting
too hot and carburetor tuning won't do much for the problem because you have
the reverse air flow trying to come back out of the carburetor.
There is an idle mixture screw on the side of each carburetor, but you can
screw on that idle mixture screw all you want, and it still can't do anything
about the adverse pressure wave blowing backwards through the carburetor.
A carburetor is a stupid device, it doesn't know which way the air is going,
it sucks gasoline out of the float bowl when the air is going the wrong way,
and when the air goes the other way, it sucks up another shot of gasoline and
the mixture goes back and forth from being too lean to being too rich.
>I did some research and the
>remedys were to check ignition timing which is perfect,
NO! What a silly "boi" you are if you think like that!
The ignition timing is perfect according to what it says in the book. The
ignition timing in the shop manual is based upon your engine having the stock
airbox and the stock exhaust pipes.
The ignition timing in the manual is also a timing that you set when the
engine is running without any load on it.
The spark will occur when the voltage at the spark plug can overcome the
pressure in the cylinder. This happens some milliseconds AFTER the points
open.
If the spark plug gets too hot, the spark occurs *before* the points open,
and the engine pre-ignites.
Sustained pre-ignition will heat up the piston and melt a hole in the middle
of it.
Don't ask how I know that.
To get the ignition timing right with your K&N filters and low back pressure
exhaust system, you're going to have to experiment with retarding or
advancing the static ignition about 5 degrees and then do some acceleration
runs to find out how many degrees retarded or extra advance works best.
And this assumes that the spark plug you are using is the right heat range.
With open exhaust and no air box, you might want to run a spark plug that's
one or two heat ranges colder so you don't get pre-ignition from the spark
plug ground electrode glowing cherry red under lean mixture conditions.
If you run a colder spark plug to avoid preignition, don't blame me if the
engine is hard to start and runs crappy around town. We used to run hot spark
plugs for slow riding and switch to colder spark plugs when we went for long
rides at high speed on the highway.
> raise the needle which cant be done the 350 needles
Silly "boi". Of course the needles can be raised. Go to hardware store and
buy about 8 or 10 washers with 3mm holes in them and place a stack of washers
under the head of the needle and experiment with mid range mixture.
Of course the reverse flow problem still exists and the engine cannot suck in
any extra mixture with the air flow going the wrong way.
or put in a bigger main jets in so i did
>that. The stock main jet was a 115 so i put in 120 and i have 125s by
>the way.
Silly "boi". The main jets control the fuel air mixture when the throttle is
wide open and the the needle has lifted more than 75% of the way out of the
needle jet tube. Your problem is occuring at smaller throttle openings and
5000 to 7000 RPM. Mainjet size has very little effect on the problem you're
having.
I did some more research and found out that those small
>japaneese bikes need a lot of back pressure and i dont think that
>those shorty mufflers have enough knowing that the baffles are really
>thin so i made my own baffles i welded a fender washer to a bolt and
>bolted it inside of the muffler about a inch away from the opening on
>each pipe the bike sounds great but i am having the same problem but
>around 7000 8000 rpm.
In the late 1960's, riders who took the mufflers off their Honda 305cc
Scrambler were installing a device called "Snuff or Not". It was just a
baffle washer that plugged up the end of the exhaust pipes and reduced the
racket while they were riding around town.
Then they would rotate the washerinto the open position. It was like a
throttle butterfly in the exhaust pipe.
And computer controlled throttle butteflies in the exhaust pipe are used by
all the
Japanese superbikes nowadays. They are underneath the engine where the four
exhaust pipes come together at the collector.
>what should i do now any input is welcome please
The easiest thing to do is install the airbox with a new air filter, install
some original equipment peashooter mufflers, and return the carburetor
setting to stock.
Or, try to figure out what colder heat range spark plug you need now and play
with the static ignition setting, retarding or advancing the spark no more
than 5 degrees.
You can also weld a balance tube between the exhaust pipes, about 18 inches
from the cylinder head.
So basically most of the stuff i've read about shorty mufflers with cb350's is that it won't work because of the back pressure, meaning you need more resistance on the air that is outputting from the pipes. Since stock pipes are longer than shorty's, they don't have this problem.
Well I love my shortys. They are loud and cool as ****. Sooo, i read somewhere that welding a washer to the end, or having the washer pivot on a bolt can add more resistance to the airflow out the exhaust.... Has anyone tried this????? Seems like an easy fix to a problem that has been haunting bobber/cafe customs for a bit.
This guy knows a lot and mentions this:
The airbox is a resonant chamber that helps the mid range flat spot if it is
designed correctly. But old time dirt track riders used to strip stock
airboxes off and clamp a piece of women's nylon stocking over the carburetor
mouth.
They might even oil the nylon to keep the dust from getting into their engine.
But, after every heat race they would find mysterious holes in the nylon,
caused by reverse air flow through the carburetor during the camshaft overlap
period when a pressure wave from the exhaust pipe would come back through the
open valves and out the carburetors.
If you stand off to the side of a motorcycle without an airbox and the
afternoon sunlight is right, you can actually see fuel/air mixture OUTSIDE
the carburetor mouths as the tuner rev's up the engine.
Hot rodders called this mist "standoff". If they didn't at least run a low
air filter, the standoff mist would ignite when the engine backfired through
the carb and they'd have an under hood fire.
A well-designed airbox keeps that fuel/air mix inside and sends a positive
pressure wave of about 1/4 of a pound towards the carburetors, working
against the adverse pressure wave from the exhaust pipe.
You can google for "helmholtz resonator" to read about airboxes, but
basically the volume in the airbox controls the frequency the box resonates
at, and the box has to have an inlet neck of a certain size and length or it
won't work at all.
HR's work over a wide range of engine RPM, but they mainly need to be
designed to overcome the flat spot that occurs around 5000 to 8000 rpm.
Dirt track and road racers thought that the way to get the most power was to
run open carburetors with velocity stacks, but they had the problem of all
that dust being sucked into the engine and wearing it out, so that's why K&N
filters and oiled foam pods were invented.
Dirt track riders used to take their mufflers off and just run a straight
pipe.
When the exhaust gases got to the end of the straight pipe, they would expand
rapidly, making a lot of noise, but a weak pressure wave would still come
back to the carbs, making the air flow the wrong way.
The factory road racing teams were experimenting with long duration camshafts
and open megaphone extractor exhaust systems, but the 'phones were very loud
and the engines had such a terrible flat spot the riders had to slip their
clutches to go around slow corners in 2nd gear.
Then the factory teams started putting a short section of reverse cone on the
end of the megaphone to help the flat spot and make the motorcycle more
rideable.
The earliest Honda CB750 four cylinder motorcycles had four exhaust pipes
that were *styled* like reverse cone megaphones seen on grand prix racers of
the 1960's, but they had baffles inside to quiet the noise down, and riders
weren't trying to race around using 10,000 RPM all the time, they were
putting around town and cruising on the highway using 4000 to 5000 RPM and
they
weren't even getting into the flat spot that occurs between 5000 and 7000 RPM.
The two mufflers that came stock on your CL350 were shaped like the old "pea
shooter" mufflers that came on 1960's British motorcycles like Triumph.
But Honda figured out that what they really needed to do was add a balance
tube between the two separate exhaust pipes before the pipe widened out into
the muffler. The balance tubes underneath the engine were actually larger in
diameter than the exhaust pipes they were welded to.
These balance tubes gave the engine more midrange torque, and the Japanese
motorcycle factories, especially Suzuki, were using balance tubes to good
effect even up to a few years ago to smooth out the flat spot in the power
band.
And, tuners like Yoshimura were making the balance tubes into resonant
chambers that helped their highly tuned motorcycles accelerate out of corners
better by providing a bit of "back pressure".
You would see one of the chambers behind each pair of exhaust pipes on their
4-cylinder machines.
>Here is the complaint when im riding the bike and the
>motor hits around 5000 6000 rpm the motor starts to give this popping
>back fire and eventually bogs down.
The mid range fuel air mixture is getting too lean, the spark plug is getting
too hot and carburetor tuning won't do much for the problem because you have
the reverse air flow trying to come back out of the carburetor.
There is an idle mixture screw on the side of each carburetor, but you can
screw on that idle mixture screw all you want, and it still can't do anything
about the adverse pressure wave blowing backwards through the carburetor.
A carburetor is a stupid device, it doesn't know which way the air is going,
it sucks gasoline out of the float bowl when the air is going the wrong way,
and when the air goes the other way, it sucks up another shot of gasoline and
the mixture goes back and forth from being too lean to being too rich.
>I did some research and the
>remedys were to check ignition timing which is perfect,
NO! What a silly "boi" you are if you think like that!
The ignition timing is perfect according to what it says in the book. The
ignition timing in the shop manual is based upon your engine having the stock
airbox and the stock exhaust pipes.
The ignition timing in the manual is also a timing that you set when the
engine is running without any load on it.
The spark will occur when the voltage at the spark plug can overcome the
pressure in the cylinder. This happens some milliseconds AFTER the points
open.
If the spark plug gets too hot, the spark occurs *before* the points open,
and the engine pre-ignites.
Sustained pre-ignition will heat up the piston and melt a hole in the middle
of it.
Don't ask how I know that.
To get the ignition timing right with your K&N filters and low back pressure
exhaust system, you're going to have to experiment with retarding or
advancing the static ignition about 5 degrees and then do some acceleration
runs to find out how many degrees retarded or extra advance works best.
And this assumes that the spark plug you are using is the right heat range.
With open exhaust and no air box, you might want to run a spark plug that's
one or two heat ranges colder so you don't get pre-ignition from the spark
plug ground electrode glowing cherry red under lean mixture conditions.
If you run a colder spark plug to avoid preignition, don't blame me if the
engine is hard to start and runs crappy around town. We used to run hot spark
plugs for slow riding and switch to colder spark plugs when we went for long
rides at high speed on the highway.
> raise the needle which cant be done the 350 needles
Silly "boi". Of course the needles can be raised. Go to hardware store and
buy about 8 or 10 washers with 3mm holes in them and place a stack of washers
under the head of the needle and experiment with mid range mixture.
Of course the reverse flow problem still exists and the engine cannot suck in
any extra mixture with the air flow going the wrong way.
or put in a bigger main jets in so i did
>that. The stock main jet was a 115 so i put in 120 and i have 125s by
>the way.
Silly "boi". The main jets control the fuel air mixture when the throttle is
wide open and the the needle has lifted more than 75% of the way out of the
needle jet tube. Your problem is occuring at smaller throttle openings and
5000 to 7000 RPM. Mainjet size has very little effect on the problem you're
having.
I did some more research and found out that those small
>japaneese bikes need a lot of back pressure and i dont think that
>those shorty mufflers have enough knowing that the baffles are really
>thin so i made my own baffles i welded a fender washer to a bolt and
>bolted it inside of the muffler about a inch away from the opening on
>each pipe the bike sounds great but i am having the same problem but
>around 7000 8000 rpm.
In the late 1960's, riders who took the mufflers off their Honda 305cc
Scrambler were installing a device called "Snuff or Not". It was just a
baffle washer that plugged up the end of the exhaust pipes and reduced the
racket while they were riding around town.
Then they would rotate the washerinto the open position. It was like a
throttle butterfly in the exhaust pipe.
And computer controlled throttle butteflies in the exhaust pipe are used by
all the
Japanese superbikes nowadays. They are underneath the engine where the four
exhaust pipes come together at the collector.
>what should i do now any input is welcome please
The easiest thing to do is install the airbox with a new air filter, install
some original equipment peashooter mufflers, and return the carburetor
setting to stock.
Or, try to figure out what colder heat range spark plug you need now and play
with the static ignition setting, retarding or advancing the spark no more
than 5 degrees.
You can also weld a balance tube between the exhaust pipes, about 18 inches
from the cylinder head.
