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Running better in cold air?

  • Thread starter Thread starter bwanna
  • Start date Start date
B

bwanna

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Ok. So I'm having a brain lapse. My 82 GS450 is running better in cold air. I've changed the exhaust to something that breathes better. Stock air box, up'd the main jet 2 steps and the pilot jet one set. Original needle and setting.

This is still a work in progress, but I've noticed on these colder mornings that the bike is pulling slightly better at WOT in getting to 80+ mph.

Does this mean my main jet is too big for warm air running? (Idea being that the colder air is more dense and the larger main jet is now properly mixing well.)

I'm sure this will make sense as soon as I hit the Enter key.

Thanks!!!
 
Depending on how much "better breathing" your new exhaust is, you likely have a bit more jetting to do. Two sizes on the main might be enough if the system is still rather restrictive. If the system breathes a bit "better" than that (meaning it's likely also louder), you might need to go another couple of sizes.

Pilot jets don't usually need to go up with an exhaust change, and, ironically, they are what is in use a great majority of the time you are riding on the street. Because they are now on the rich side, it will hurt proper running in warmer air.

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Thanks for the quick response. Re: the Pilot. I up'd it one step because it seemed to be running hot and also seemed to be thin during starts and town riding. It appeared to improve in both of these areas with the change. I could be wrong and getting fooled by things, but it does feel better.

On the main, I can get another set or two and experiment. Those area easy enough to change on the fly.

What's your take on why it's pulling better in the 75+ mph range in cold air?
 
Adjusting the pilot mixture can be done in much finer steps if you use the mixture screws, rather than the large steps provided by jet changes.

As a rule, most of these bikes came from the factory with the mixture screws set between 1 1/4 and 1 3/4 turns out, then sealed. Because they were set for emissions, not proper running, we have found that they work much better between 2 and 2 1/2 turns out. If you have to turn the screws out past 4 or 4 1/2 turns, then is when you go to the next-larger pilot jet and start with the screws about 1 to 1 1/2 turns out.

Have you done any plug chops to verify your jetting? Reading the spark plugs is the best way (unless you have a wide-band O2 analyzer) to judge what is happening inside your cylinders.

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I did use the mixture adjustment procedures partially to help determine the need for a larger pilot jet. Haven't had a chance to chop plugs yet to verify the main jet changes... just been going by performance.
 
Cold air is more dense than warm air so there is more oxygen in it. More oxygen equals a more intense combustion process equals more power.
 
bwanna said:
I did use the mixture adjustment procedures partially to help determine the need for a larger pilot jet. Haven't had a chance to chop plugs yet to verify the main jet changes... just been going by performance.
Click to expand...
You don't need to chop up any plugs, just hold a constant throttle position (how much depends on which jet you are checking) long enough to color the plugs, then 'chop' the throttle, hit the kill switch, pull the clutch lever and coast to a safe place to stop. Once you are stopped, remove the plugs to check the color.

Mark your throttle so you can see how far it's open, and so you have a repeatable setting.
Up to 1/4 throttle will check your pilot settings.
1/2 throttle will check the needle jetting.
Full throttle will check your mains.

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