Questions on performance in changing temperatures.  By  tiopedro (Read 1,178 times)

Change in temperature does not require a change in carburetor tune up, but when you do, you might get some extra power out of it. Remember to change it back when the temps increase.

Performance exhaust will more than likely cause almost no improvements, especially not on a 50cc.
The only improvement might be that the performance exhaust is lighter in weight... That's all..
but if you need to change it anyway, better get a performance exhaust (though I don't know if they make them for 50cc's). Any exhaust with an exhaust tip where you can put your index finger in the hole, is good enough for a 50cc exhaust system.

You might want to consider changing oil to a lighter oil. If you put 10W40 oil in it, try switching to 5W30. 20 degrees colder justifies lighter oil.

Also put some Chemtool B12 in the tank, to clean it out a bit.

So you adjust towards richer in cold, and leaner in hot weather ??

To expand on the temp change issue:

Air density is affected by temperature (as well as elevation but that's not the topic here).

The ideal (stoichometric) mixture is a 14.7:1 ratio of Oxygen to Gasoline*. The atmosphere contains 'X' amount of oxygen along with various other gasses at any given volume.

If you seal off a container of air (say, a tin can), and heat it, the gas will attempt to expand, but being trapped in a rigid container, it will only be able to build pressure...the density remains the same because the volume remains the same. Place the same amount of air in a flexible container (say, a balloon) and heat it, and the container will change volume allowing the gas to physically expand (lose density).

Inversely, if you run the same experiment but chill the containers instead of heating them, you get the opposite effect on the balloon...it shrinks (reducing it's volume) as the density of the gas inside increases.

Now, to apply this to a carburetor.

The mechanical fuel metering system in a carburetor relies upon air pressure differentials to pull fuel into the airstream as it passes through the throat of the carburetor. The carburetor has no way of measuring air density, only air volume as a byproduct of the Venturi effect of the air passing through the carburetor.

For any given volume of air, the density (read: concentration) of oxygen molecules will decrease as temperatures increase, and vice-versa.

So as temperatures drop, the concentration (density) of oxygen molecules for any given volume of air passing into the engine will increase, resulting in a 'leaning' effect...more oxygen molecules added to the same amount of fuel (with the opposite happening as temperatures increase). The greater the temperature change, the greater the effect. Eventually the distorted air/fuel ratio will become noticeable as a change in engine performance.

Generally this temperature induced distortion can be fixed with just a twist of the mixture screw. You shouldn't have to re-jet unless you run into severe seasonal temperature extremes...like my cabin in Montana which can swing from +38c in the height of summer to -40c in the depths of winter.


Too-long, didn't-read version: Yes, temperature CAN affect mixture ratios (carb tune), but in general it's nothing a little twist of the mixture screw can't resolve.


* I have always found it interesting that the Stoich. ratio for gasoline is essentially the exact same number as the atmospheric pressure at sea level (in PSI)

While all of the above is "technically" true- You would have had to be running hella lean to begin with for performance to decrease with a temp drop like that. The air does become denser- with more oxygen in it- in cooler temps but IDK if that is your issue. Adjusting the air/fuel on your carb should be a simple process- I would definately try that first. There are instructions on this forum (and all over the internet) on how to do it if you dont know. If that IS your issue- you should be happy because you may have just saved your motor from an untimely demise. =) I usually run mine just a hair rich and cool mornings my bike flies- probably because the mixture is running correctly. =)
On the sport exhaust issue- If your bike needs a new exhaust, I'd absolutley go for a performance replacement. They are as cheap or cheaper than a factory type replacement. You most likely wont notice any performance gains, but it might sound cooler and you'll feel cooler and hence- it will seem to go faster. =) If you want more pickup but the cops are stingy in your area go for variator weights. They are dirt cheap, albeit a pain in the to install if you dont have the correct tools. Lighter weights plus maybe a stiffer contra spring can get you up and moving quite a bit faster.
Back to the exhaust- a lot of "performance" exhausts you find online DO require light modifications to be done to install them. This would most likely consist of drilling a hole or holes into your frame for a mount. Not a big deal but you should just know in advance if it's a direct replacement or what.

Sorry no they run better when cold because the mix is leaner. Google your own data or read an owners manual. Less dense means less air by volume and less fuel period simple physics you should of got the data in high school.