Runs better in cool, humid weather?

[XJkid]

I took Galaxy out for a night spin on the levee road that winds along the Sacramento River, temps in the high 50's, humidity(certainly up there, being on the river). It seems that she runs better in these conditions, and a buddy of mine with a '79 XS says the same thing. Are we imagining things, or is this the case? Does the cool air allow more fuel to be atomized? How would humidity factor in?
Enquiring minds want to know.

[DiverRay]

Cool air is denser, so you have more oxygen per cycle, and it pulls in the fuel needed so yes, more HP and better running. Out here at the coast, I almost always have cool air.

[red bandit]

Humidity is water. Water is H2O, more oxygen. And with the cool dense air engines tend to run great and don't over heat.

[XJkid]

Would this pertain only to carbureted motors, or would it also affect direct injected as well?

[MaximPhil]

Would this pertain only to carbureted motors, or would it also affect direct injected as well?

Hi XJkid,
as explained in the above post . it is the density and humidity of the air that makes the " charge" taken into the cylinder contain more Oxygen which makes the fuel burn better. So yes Injected motors would also make more HP in the cooler humid air.
Phil

[Jerry]

You might be running a bit rich. Cool and moist air (affects air density) richer mixture is fine. Warmer and drier air generally required leaner jetting for best performance. Are your carbs are running stock jets? You might want to consider going leaner if that is the case.

Opens a new can of worms about carb tuning!

From experience, when it gets hot in the summer here in western Colorado and the humidity drops into the single digits performance suffers!

[DiverRay]

From experience, when it gets hot in the summer here in western Colorado and the humidity drops into the single digits performance suffers!

Jerry, The Density Altitude when hot at your place is probably 9'000 feet! I know from experience that most bikes will not run well at 9K altitude. My one exception was the 650 Seca Turbo I owned, it didn't care, too much.

[XJkid]

Yes, it has stock jets. From what I have read on here, a slightly rich setting will help keep the motor cooler on really hot days. Here in the Sacramento area, the summers are dry and hot with the humidity dropping to the single digits sometimes, with the winters rainy and cool (when we are not in a historic drought), rarely going below 32. Maybe have a carb set for summer and one for winter rather than taking them apart every 6 months and re-jetting/tuning?

[BA80]

Cool dense air, engines love it and go faster, bullets hate it and go slower.

[FranksBMW]

cool dense air: nature's supercharger

[Jerry]

Maybe have a carb set for summer and one for winter rather than taking them apart every 6 months and re-jetting/tuning?

That would be good, or convert to fuel injection. Most of us end up with something in the middle, slightly over-jetted for "average" summer conditions, and let it go at that. Just makes us look forward to early summer and and late fall when conditions are 'right' for best performance.

[FranksBMW]

... Maybe have a carb set for summer and one for winter rather than taking them apart every 6 months and re-jetting/tuning?

While temperature has an effect on performance, a bigger culprit is elevation. Change your jets to match the altitude where you do most of your riding. There's a big difference in air density (and resulting carbed engine performance), between near sea level and 6,000ft.

On a cool crisp morning riding to work, my bike has more umpf than when I'm riding home in the warmer later part of the day. I'm not going to switch jets or carbs to match that temp difference.

[motoman]

While temperature has an effect on performance, a bigger culprit is elevation. Change your jets to match the altitude where you do most of your riding. There's a big difference in air density (and resulting carbed engine performance), between near sea level and 6,000ft.

On a cool crisp morning riding to work, my bike has more umpf than when I'm riding home in the warmer later part of the day. I'm not going to switch jets or carbs to match that temp difference.

Actually, altitude changes don't have that much effect to justify main/pilot jet changes. About only thing noticeable is the 3% horsepower loss for every increase of a 1000ft. in elevation. The Mikuni carbs, being vacuum slide design IS the advantage with increasing altitude. Reason being, the higher altitude increases, inch vacuum decreases. For example, at sea level vacuum levels are in the 21-22in.vacuum range. Here, at my location, in.vacuum levels are at 18in.vacuum. Also, at 10,000ft. elev., vacuum levels are at 15-16in.vacuum. The vaccum drop with elevation holds the vacuum slides/metering rods farther down, restricting being over-fueled.

[FranksBMW]

Motoman, I can only speak from my experience with elevation. 3 summers ago when I did my 'Round the US trip on my 1975 Honda CB750, while in Colorado I thought there was something seriously wrong with the bike which had been performing flawlessly until then. It lacked power; my impression was that about 1/2 of the horsepower disappeared. I could not maintain speed in top gear, or even second from the top while riding up a hill. I had to downshift and maintain high revs to keep going. Finally I figured out that it was elevation. Perhaps the Honda has a different type of carb? All the HP came back once i got lower in elevation.

I'll stick with my assertion that jetting for a bike ridden by someone living near sea level is going to be different than the jetting for a bike owned by someone living at much higher elevation.

Now, it wouldn't make any sense for me to change jetting due to elevation change because it was only a temporary thing while passing through. If I were to move to Colorado, I would need to change the jets to fix the performance issue.

[motoman]

Motoman, I can only speak from my experience with elevation. 3 summers ago when I did my 'Round the US trip on my 1975 Honda CB750, while in Colorado I thought there was something seriously wrong with the bike which had been performing flawlessly until then. It lacked power; my impression was that about 1/2 of the horsepower disappeared. I could not maintain speed in top gear, or even second from the top while riding up a hill. I had to downshift and maintain high revs to keep going. Finally I figured out that it was elevation. Perhaps the Honda has a different type of carb? All the HP came back once i got lower in elevation.

I'll stick with my assertion that jetting for a bike ridden by someone living near sea level is going to be different than the jetting for a bike owned by someone living at much higher elevation.

Now, it wouldn't make any sense for me to change jetting due to elevation change because it was only a temporary thing while passing through. If I were to move to Colorado, I would need to change the jets to fix the performance issue.

Again, not true since these carbs being vacuum operated slides/metering rods are known to a certain extent as 'altitude compensating' carbs.......and unless you bought that xs11 new at 6,500ft. elev. and rode that elevation daily for some 10yrs., plus add in 95% of that riding was above and up to 12,500ft...........your speculation and opinion of this matter is of no consequence.