I say this as counter to "logic" or experience with other bikes VTRs have this odd habit of going rich when you improve flow. This is counter intuitive to most folks but there is a logical reason
Before I get into what I see going on I'll give you a bit of my back ground.
While I have been messing with bikes for longer than I want to admit, back when I did go to work everyday I was a vacuum tech.
No not the rug suckers but large chambers and helium mass spectrometers were my specialties and work at Lawrence Livermore National Laboratory.
So I have the unfair advantage of actually knowing how things work inside CV carbs.
So with that lets tackle the issue of why they go rich when other carbs would go lean.
This has to do with the size of the carb.
Being so much larger than a "standard" carb does change the rules.
Basically you are dealing deal flow "states"
In order to keep this short and readable we will only deal with 2 for now.
The first is "turbulent" flow.
It is easier to picture it as water flowing, this type of flow is just like the name sounds.
It is moving but rolling and twisting around.
Then you have "laminar" flow.
This is when everything lines up and flows in a smooth straight line.
Again like water, it is the spot in the stream where everything straightens out and the water flows smooth and fast as the speed increases exponentially when this state is reached.
Then just to keep things interesting, there are "transitional" states between these 2 flow rates.-
So we together so far
The next bit that we have to understand is that "vacuum" (or partial pressure) does not "pull" anything. It is the positive pressure on the otherside pushing that causes things to move,
With those two concepts, lets look at the issue once again.
What is happening is these carbs are so large that they live in the "turbulent" flow state.
When you start cleaning things up in the airbox, replacing the stacks and even as in my case add a good full exhaust then you start hitting laminar flow through the carbs.
Common thinking with this info would be and is, more air flowing through the carbs then I need bigger jets to flow more fuel to keep up......
On bikes with small carbs, yes as they are already in laminar flow.
On this set up things are a bit more tricky......lol
What I see happening is the mods I listed start bringing things into the "laminar" flow range.
With one more concept we'll take a look at this.
The last bit of the puzzle is to keep in mind the pressure increase is the square of the speed increase.
With all that out of the way time to figure this out.
We are not to worried about the amount of flow through the carb venturi but the effect of this flow on the carb circuits.
In the stock set up, again with turbulent flow, the vacuum signals are also turbulent and while everything does flow and work, it is not at its optimum rate.
To compensate, you just run a little larger jet. Not optimal but acceptable for most situations.
Now the OCMD folks get in there and mod stuff and get that laminar flow....what does that do to the carb circuits?
They also are now getting a good, strong vacuum signal. They respond by now flowing at their optimal rate.
What does this do...... you go rich and need to lean out to compensate.
There in also lies the problem with tuning for mods. You really don't know how much you have improved things.
Remember I said there are transitional states between the different types of flow rates..... that is what we are really dealing with.
So there is no set formula once you start modifying, it all depends on the flow rates you achieve and even more than stock, each bike will be different.
Yes there is also a point where you will start going lean and need to jet up if you do enough mods but that is usually only for engines with pistons and cams that I have tuned.
Hope this helps some and yes I was bored today.....lol
And yes I am getting old and did mess up the thread title at first if anyone is wondering....lol
