Jump to content

Air density modifier for jetting

Recommended Posts

I've been reading... finding information on air density corrections. Most of this applies to racing and the aviation community, but there is relevance in any carbureted motor.

There are a lot of calculators and charts to correct for air density and smart phone apps that provide the relevant data to plug in to them. We all have adjusted for elevation and temperature, but I never realized how relative humidity is to all of it.

I've been using an iPhone app called Auto dens and getting altitude and barometric pressure from the device sensors or from local airports. I don't know how much to trust it, but it's a start. I then apply the apps results to web based calculators and apply those modifiers to current jetting. I have not utilized it as far as swapping out jets based on those parameters but there is quite a difference in the current jetting and what I have calculated. It would be great for an app that would include this data and apply the calculations and provide recommended jet size based on those calculations as well as catalogue jet notes, if you know of one let me know.

Does anyone else apply this type of computation to your baseline jetting, who really needs to do all of this for a 230f?

 

Edited by NEODS

Share this post


Link to post
Share on other sites

Terry Miller and I have spoken on this subject and while adjusting jetting to air density is fairly easy, determination of actual air density is not.

The problem in our case is that there is no "best" main jet size due to wild fluctuations in head temperature.  This is more suitable to a modern IC engine with liquid-cooling and full closed-loop control systems.

Edited by VortecCPI

Share this post


Link to post
Share on other sites

We use a software/tool for drag racing that gives us a corrected density altitude. It combines temp, altitude, pressure, and humidity. Assuming our drag bikes are properly running, we can usually guess within about one or two hundredths of a second what it will run for 1/4 mile ET. It's a 1500 dollar setup, so not exactly for the budget - minded trail rider.

  • Like 1

Share this post


Link to post
Share on other sites

Honda provides charts and guidance in the Service Manuals for adjusting jetting for temp and altitude, which seems close enough from my experience with XR carburetors.

Basically it starts with mixture screw adjustments, then needle clip position, and finally a main jet change.

From my own experiences the mixture screw is easy to adjust and fixes most problems with minor altitude, ambient temp, and engine temp changes.

Edited by Chuck.
  • Like 2

Share this post


Link to post
Share on other sites

Based on the title alone and thinking on what this thread would be about prior to reading your first post I concluded you were talking about......

news__n-73.jpg

 

Kinda crazy how every so often we discover a new aspect of how things work and quickly determine that the way we have been doing it all along is just barely possible and it's difficult to understand how our engines even run with a carburetor.  This can apply to food, water, the universe, bumble bees, etc.

Edited by MetricMuscle

Share this post


Link to post
Share on other sites

As temp and altitude go up jetting may need to be adjusted, standard jetting is for sea level and 15C (approx 60F). And I've found stock jetting works OK up to 5000ft on a warm day with minor blubbering, and to avoid main jet changes can be fixed first by about 1/4 turn in on the mixture screw, I do the same for hot days near sea level. If that doesn't work then a leaner needle and richer idle mixture can get you thru the day;  move the needle clip up one groove (leaner) and turning the mixture screw OUT by 1/2 turn from stock (richer).  A main jet change can be used instead but I've found it easier to just ride or use one of the above.

Honda publishes tables of correction factors for temperature and altitude in the Shop manuals. The temperature correction factor for a 40C (104F) day at Sea Level is .965, and would be only a main jet change; 110 main times .965 equals 106.15, so use a 106 main.

If the correction factor is .95 or lower, such as for 4900ft and 80F then the above needle clip and mixture screw adjustments are also used along with a main jet change. Here are Honda's instructions:

Honda Temp Altitude How To.jpg

And the table:

Honda Main Jet Correction Factors.jpg

Honda Slow Jet Correction Factors.jpg

Edited by Chuck.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:


×