Thread: re-jetting for different backpressure-why?

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Sure, I can tell you why, well, maybe. When you lessen the back pressure via changing the exhaust system on your ride, this then leans the mixture.

This is because the cylinders are filled with less fresh air because of the lack of the scavenging effect. One would think that if less air is charged into the piston area, then one would lose power. Well, yes and no.

In actual fact, the leaner the mixture, the faster you go. However, if you go too lean, at some point the pistons would melt or bit the walls, so the manufacturer always errors of the fat side. So, one can safely change the exhaust restriction to some degree without even richening the mixture.

Just a side note, in recent years the manufacturers have started using hypereutectic pistons. These wonderful creatures don’t swell as their predecessors did. This in turn has allowed engine builders to use tighter tolerances and at the same time, run hotter thermostats equalling better fuel economy.

When one has slightly less vacuum, it has the effect of filling the cylinders with less air which is why one gets better fuel economy. This only works to a certain small degree and we are talking from a stock factory preset mixture. Keep in mind that the carb is calibrated for the predetermined airflow.

So, if you free up too much exhaust, the mixture will be far too lean and you will have hesitation, coughing, backfiring, all because your mixture is too lean. A little change will give you a little more horsepower. A big change will give you less horsepower until you re-calibrate your carb and jet it fatter to accommodate the higher flowing numbers. Once you do that, you will have even more horsepower, unless you go too fat. Hehee. Have fun.

Hey, my explanation works for me. Hehehe!

And now you know.