Motorcycle Repair: Lost power and re started with a bang, 1100 vstar, oxygen molecules

Question
After a long ride, 150 miles, with a climb to 12,000 feet elev. in the last 25 miles. Air temp. was cool at that elevation.
My 1100 VStar started loosing power, on level road,  I downshifted several times until I was in 1st gear, still wouldn't pull itself.  I pulled over to the side of the road. Checked to see if I was getting gas. Everything seemed OK.  I cranked it over a few times and it backfired REALLY loud. one time.  Started up. and has ran perfect every since.  Could I have had a valve stick open?

Answer
Hi Grant,

I suspect the problem has to do with cahnges in carburetor air-fuel mixtures as altitude increased.

I cannot say with absolute certainty, though, because there's no indication of a base altidude with a destination of 12k feet.

Usually, the carbs are affected with about every 1k feet change in elevation.

I doubt there is any problems with the valve train.

Respectfully,
Mark Shively



The Effects of Temperature, Altitude and Humidity on Jetting
By Canadian Dave

Once your jetting is set it’s not necessarily set for life. Changes in air temperature, altitude and humidity can have an effect on how your engine runs.
If you captured a measured volume of air on a humid 90° F day at sea level or a cool dry 40° F day at 10,000 feet both would contain about 22% oxygen. The density and therefore the total number of oxygen molecules however would differ enough to affect the performance of your engine.
Temperature- For most of us changes in air temperature will have the greatest effect on our jetting. As the air temperature gets colder the air density increases. The air molecules become less active (move around less) and therefore take up less space. Because they take up less space more air, and therefore more oxygen, can fit into a measured volume of air as the temperature decreases. As the temperature drops the engine will begin to run leaner and more gasoline will need to be added to compensate. As the temperature increase the engine will begin to run richer and less gasoline will be needed.
Altitude- Again this is an issue of air density. At sea level atmospheric pressure is around 15 psi and as the altitude increased the atmospheric pressure decreases. Because less pressure is exerted on a measured volume of air as the altitude increases the air molecules are able to relax and they take up more space leaving less space for additional molecules. The higher the altitude the less air in a measured volume and therefore less oxygen present so jetting will have to be leaned to compensate.
Humidity- Humidity is a measure of how much water vapor is in the air. The higher the humidity the less space there is for additional molecules of air and therefore oxygen. As the humidity increases there is less oxygen and therefore the engine runs richer. Jetting that may have been spot on in the cool dry morning air may start to run rich as the temperature and humidity increase over the course of the day.
Correcting for Changes in Temperature, Altitude and Humidity
Correction Table-You can use a correction table to roughly determine the appropriate jetting changes to compensate for changes in temperature, altitude and humidity. I’ve included a typical correction factor chart that has been modified specifically for use with the KDX. To use the chart go back to your log book and record what jetting is presently installed in your carburetor then determine what altitude you’ll be riding at and the temperature. I'm assuming here that you've already optimized your jetting.  I’ve used my present jetting as an example. You’ll need to slightly modify the table to fit the specific requirements of your bike but I’ll go over that in the example.
Example- I’m presently running a 45 pilot jet with the air screw 1.25 turns out, an 1173 jet needle in the second from the top clip position and a 152 main jet. This jetting was optimized at 20° C and 2240 ft above sea level. For this example lets assume I’m going riding in the mountains where the temperature is 20° C at 9600 ft. The first thing I do is adjust the bottom of the table so that it reflects the condition where my jetting was optimized. Using the illustration below as an example I draw a straight line from 20° C horizontally across the graph until I hit the line that represents 2240 ft., and then draw a line vertically to the bottom axis on the graph. This point becomes 1.0. Adjust the work sheet by subtracting 0.02 for each increment to the left of this point and adding 0.02 for each increment to the right of this point. My graph now looks like this: (AllExperts servers does not display images)...

Now using my personalized graph I can calculate what jetting I should install before making the trip to the mountains. I draw a horizontal line from 20° C over to 10000 ft and then vertically down to determine the correction factor of 0.95. To find the correct pilot jet size I multiply 45 by 0.95 and the new jet size would be 42.75. The closest available size is a 42 and I’ll fine-tune the pilot circuit with the air screw once I get there. I then multiply my main jet size, 152, by 0.95 and the new jet size would be 145.  Now remember this is intended to give you a rough indication.  
Using a correction table should allow you to closely meet the requirements of changing conditions. It is however intended to be used as a guide. You should always carry an assortment of jetting in your toolbox and check any jetting suggestions you receive. At a minimum do a plug reading at WOT after changing your jetting to insure you aren’t running lean. Jetting recommendations that work well for one bike may not necessarily work for another even if it is being ridden in the same area with identical modifications.