Effect of humidity and air temperature on carb performance

[156 D]

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[drunkmunkey6969]

[firekdp]

It has often been noted that things like race tuned cars and bikes go a little better when it is a cold crisp morning. I know that Mikuni and Dellorto both do a sliding chart showing which way to go for the jets. Same thing for alltitude.

Thanks for that "T5-190"........(you won`t know what`s going on)........but you saved me saying it!!.......

Been here before haven't we?

Colder air = weaker mixture
Warmer or damp air = richer mixture
So racers use the chart to match the jets to the conditions to always maintain the optimum mixture for their engine before each meeting. Road going cars/bikes are jetted by the manufacturer as a compromise for the average conditions it will be used in.

[tony]

yep, dense air, more charge. With modern ecu's mixture is taken care of in 4 strokes. If you think about it all a supercharger does is feed the charge under pressure to get more in to the cylinders. This dense air is a very mild effect similar to that.

[storkfoot]

Thanks for the replies, lads. I'll drop the pilot and see how it goes

[tony]

You mean increase the pilot- i'd look at richening everywhere at these temps not just the pilot mate.
Then again i'm too soft to ride at these temps

[storkfoot]

Damn, just as I thought I understood something Surely, I need to weaken the mix If it runs well when the mix is weaker ie. cold and non-damp weather, and if it runs less well when it's damp and the mix is richer, surely I need to look at weakening the mix?

[tony]

bloody ell yer confusing me now!
Is it running rough at mo? Does the bike sort of die when you open the throttle?

[156 D]

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[storkfoot]

bloody ell yer confusing me now!
Is it running rough at mo? Does the bike sort of die when you open the throttle?

Generally, it's running fine. It's just too bloody cold at the moment to do another series of plug chops .

When it's damp, moist air it dies somewhere on the pilot. You pull the clutch, drop the revs and then rev it whilst letting the clutch out and it sort of clears. The carb has a RamAir through the panel. It only does this on damp days. Conversely, when it's cold and not damp, performance is so much better than usual.