I am questioning the charts I have seen for determining which circuit is providing fuel. Some base the fuel circuit by throttle position, others by RPM.
If the throttle position charts are correct, than whats the point of vaccum lift slide.
If the RPM charts are correct, Then going down a steep hill at 7,000 RPMs with the throttle nearly closed would have the same slide position as wide open throttle acceleration at the same RPM.
My guess is that there are more variables at play on a CV carb. and neither chart is correct all the time.
So, when testing main jets, how can I be sure the slide is all the way up?

Craig

 

Fuel delivery is controlled...relatively...by the throttle (butterfly) position. Obviously the engine has to be turning over to create pressure/vacuum to draw anything through the carb in the first place, but I'm not going into every bit of the minutia involved in the carb process. The point of a vacuum carb is to provide the cleanest, crispest, smoothest, most economical throttle response and with the cleanest emissions. When comparing cable pull slide carbs and CV carbs, assuming everything is set to optimum, the cable pull slide carb is almost always a better performance component. Still, the CV carb provides a more "no brainer" approach to bogging and even moderate elevation changes over the more conventional carb...and the CV carb is usually more econonomical in regards to fuel mileage.

Here's a good article about the general operation of the CV carb, and it applies to our KLX carb almost 100%:

C:\Documents and Settings\Administrator\My Documents\CVK Carb.mht

 

TNC, is that article somewhere on the web? That link appears to point to somewhere on your computer.

 

*Insert facepalm image here*


Me thinks TNC was "enjoying" a bit too much of the cheap tequila last night.

 

Hmmm...tried to send it as a document, but who knows. Here's the link to a site that has it too.

Care & Feeding Of The Keihin Carb

 

Thanks for the link TNC, but I'm not sure the article answered my question.

When the throttle plates are wide open, will the slide always be pulled all the way up?
Are there no rpm or engine load conditions where the throttle plate is full open yet there is not enough vacumn to pull the slide fully up?

Craig

 

Hi Craig,

Here's how I understand it. I think the article answered the questions, but you may have to read into it. Others please chime in for what I don't know or get completely wrong!

No, the slide won't always be pulled all the way up with throttle plate fully open. There is a lag between the butterfly valve and the slide movement. I don't think the slide opens all the way up at lower rpms, and maybe loads too. The vacuum has to develop to pull the slide up, and the slide doesn't move up instantly because this vacuum has to transfer through the little hole on the top of the slide. At low rpms, there is less vacuum. That's why the article talked about fanning the clutch to get the rpms up, so the vacuum can develop and lift the slide and to move the engine into a better power range for crossing the stream or lifting the front end.

Yes to your second question. And I believe that is on the low end of rpms. If the slide lifted right up at low rpm, there could be bog and inefficient fuel use, which is why the CVK carb is claimed by some to be better.

To your original questions
The charts probably don't perfectly apply to the CVK, especially when the vacuum circuit hasn't stabilized. Down hill at 7K, the butterfly is closed, vacuum low, and slide down (so the rpm chart isn't applicable, the throttle position chart is, and I bet this is the case for non-cvk carbs too). Under open throttle at 7K, the butterfly is open, and if the vacuum circuit is stabilized and vacuum adequate, the slide position should match the chart. To test your main jet, I suppose it seems best to have the throttle full open at a higher rpm for adequate time for the vacuum system to stabilize and fully open the slide. In practice, this could mean putting the bike on a dyno, or doing some kind of timed runs where you roll on the throttle in a higher gear at speed (say 50 mph, or maybe around 5K to 6K rpm), and measure your time to go a certain distance or reach a certain speed, or measure your speed after a certain distance. This is often done on a lonely highway with sufficient markers, with runs in both directions to help average wind and hill affects, at steady air temperatures, and with the bike fully warmed.

 

Very good clarification, Scott.

 

I concur

Here is more on the role RPM plays:

The higher the RPM at any specific slide opening, more fuel will be introduced into the engine for that specific opening. The higher rpm creates a higher intake velocity which in turn draws more fuel.

This is probably easiest to visualize with the main jet circuit. At wide open throttle the slide will ultimately rise all the way and you will be running on the main jet. Say you are going 50mph @ 5000 RPM when you pin it. You keep going faster (higher RPM), but the slide can't go any higher. Now you are going 90 MPH (with your 351 ). You will certainly need more fuel at 9000 RPM than 5000 RPM. Obviously more gas is being supplied even though the slide hasn't moved. Thank RPM for that.

Ron

 

Good points Ron.
Thank you both, Ron & TNC, for the compliments.