The International CBX Owners Association

banjobill08 wrote: Wed Feb 26, 2025 3:36 pm I got mine a few years ago, but made the initial enquiry a few more years before that direct for schule, from the first contact I heard nothing back, so I just gave up, then a year later he contacted me saying he would only make them with a minimum of 8 and he put me on the list?
Then 2 years after that he contacted me again saying was I still interested, I said yes, he went through the options he had available, I went for the special version and I paid upfront.
Nearly 2 months later I took delivery of the best looking systems for the cbx and worth every penny.

By far the best system for a CBX. A friend of mine was the USA distributor. They were about $3400.USD when they were available here.
Best,
Phil

CBX-tras wrote: Wed Feb 26, 2025 7:34 am Not too much to explain.

If there isn't sufficient back pressure, they don't work and when you crack the throttle the motor stumbles and dies.

I’m by no means an expert on carbs, but I have a fair understanding how the CV carbs work, and the statement that CV are dependent on exhaust back pressure to work is not correct.
Just try to run the engine without exhaust headers, it starts and run with no backpressure…its not good for the engine, but the CV carbs work just fine.

As far as I understand the only direct “contact” between the exhaust and the air/fuel inlet, is during valve overlap (just before and after TDC at the end of the exhaust stroke/ beginning of the inlet stroke).

During this short period is when the scavenger effect takes place, sucking out the remaining exhaust in the combustion chamber above the piston at TDC, which is not pushed out by the pistons exhaust stroke.

Some call it back pressure, but it is negative back pressure or more correct vacuum created by the exhaust scavenging effect and has nothing to do with the function of the CV carbs.

This is a “free” force that cleans the combustion chamber at the same time as it fills the chamber with fresh AF mixture, even before the inlet stroke starts and is only possible during valve overlap.

The faster the exhaust flows out with as little backpressure as possible, the better the scavenger effect will be.

So theoretically a perfect exhaust system will have high exhaust velocity, low backpressure and good exhaust scavenging.

As I said previously, some modern sport bikes have a variable throttle type of blade in the exhaust to create back pressure at low speed and open up at mid-range to WOT.
And, how many times that people have to re-jet to compensate for an aftermarket exhaust?
Obviously, back pressure plays into it.

BINGO 512 ! But there's no arguing with xtras.
Most people struggle with re jetting CV carbs because they don't understand
what to do and what not to. For example the first thing they usually try to change
is the main jet, which is the last thing you want to change.
If necessary I can give a lengthy explanation of the experiences I've encountered
tuning road race bikes, what and how I learned and an example of how it paid off
not only at the track but in the day to day work at the Honda shop.
What xtras is referring to is Yamaha's variable exh valve in the collector that was
intended to get more low end/mid range power and preserve top end power as it
opens at higher rpm. It's like being able to change the exit dia. of the collector while
riding. Notice it was a short lived feature.
When you change the exh or air box/filter and understand how to compensate for
the difference in air velocity (quite simple) it's no big deal to get CV carbs to respond
correctly. Yor can leave the exh. sys. and air box/filter alone and simply change the
cam timing and you're likely to need to do a little carb work to get the motor to run
right and benefit from the change.

My five penny's worth.

FalldownPhil wrote: Thu Feb 27, 2025 1:25 pm By far the best system for a CBX. A friend of mine was the USA distributor. They were about $3400.USD when they were available here.

I've heard different things about the midrange with those exhausts. Did you ever own or ride one? What was your impression?

I'm trying to get some measurements off of one to do some modeling/analysis as I design my own header - if you have any pointers for me on where I might be able to find that kind of info I would really appreciate it.