Thread: re-jetting for different backpressure-why?

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I always hear people saying (regarding bikes mostly) that if you change an exhaust you have to re-jet the carb because of the different backpressure.

My next question is always Why? but noone ever knows.

Can anyone enlighten me on this.

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.

I couldn't really think of any reason it would require jet changing either, but everyone says it does, but I should add it's only been when talking about bikes. I don't see why the principles governing their engines would be any different though.

A question relating to the post above denny's: why would REDUCING backpressure REDUCE scavenging? I would think it would help scavenging.

Even so, if there is more EG remaining in the cylinder wouldn't it just act as inert gas since it has no more oxygen (or very little) so it wouldn't contribute to AF ratio.
The fresh charge still goes through the carb, just a little less or a little more. . .right.

Originally posted by TyphoonZR


Just a side note, in recent years the manufacturers have started using pneumatic pistons. These wonderful creatures don’t swell as their predecessors did. This in turn has allowed engine builders to use tighter tolerances

First time I've heard of pneumatic pistons.

Could you enlighten us please?

tcodi,
when folks talk of rejetting bikes, they are usually talking about a two stroke engine. Two strokes dont have a compression stroke separate from the exhaust stroke (they fire every stroke) so the backpressure affects the cylinder filling. Less backpressure equates to better filling (less inert exhaust) so you need more fuel to compensate.

Hey Typhoon, did you mean hypereutectic pistons maybe????

You rejet the carb every time you change the exhaust to get the benifit of the new exhaust. It is always done on all american V twins. Running lean on an air cooled motor is not good and turns your new pipes a wonderful dark blue not to mention all the other good things that happen when your bike is running way to hot.

Originally posted by tcodi
I always hear people saying (regarding bikes mostly) that if you change an exhaust you have to re-jet the carb because of the different backpressure.

My next question is always Why? but noone ever knows.

Can anyone enlighten me on this.

Remember (leaner is meaner)Until you blow holes in the tops of your new buckets.

Originally posted by C9x
First time I've heard of pneumatic pistons.

Could you enlighten us please?

i think he ment hypereutectic pistons.... and i really dont reccomend them... they're harder but tend to be brittle... i've heard way too many stories of spark detonation causing a piston to fragment and tearing the engine to pieces.... go with forged and you'll be happy

Originally posted by Dave Severson
Hey Typhoon, did you mean hypereutectic pistons maybe????

Ha aha, sorry, yes hyper pneumatic ones. I have no idea where pneumatic came from. I will change my previous post. Thanks. Obviously most knew what I meant, luckily!

The issue was definitely regarding a 4 stroke because we were talking about the guys harley right outside.

I still don't see how the backpressure affects the mixture ratio, the intake charge has to flow through the carb no matter how much backpressure there is.

I'll accept the fact that you need to re-jet when changing exhausts, but I still don't see WHY it leans out or richens up depending on the exhaust.

Ok, let me try again. When you open up the exhaust, the pulsation changes and the signal even lessens at idle. This causes less air to be drawn through the carb.

That’s my story and I’m stickin’ to it.

Here's something else that is unique to bike carburetors, thay dont have accelerator pumps or circuits in the carburators, mainly because the engines dont lug arount as much weight as cars. Just my 80 cents worth. S

Originally posted by southerner


Here's something else that is unique to bike carburetors, thay dont have accelerator pumps or circuits in the carburators, mainly because the engines dont lug arount as much weight as cars. Just my 80 cents worth. S

Not to be picky or step on anyone's toe's, but actually Harley factory carbs and some aftermarket carb's do have accelerator pumps. I disconnected mine and it ran like crap. Typhoon had the best explanation about the signal strength. One underlying cause is the fact that Harley's factory setup is very lean. An attempt to meet emission regulations I was told. As a rule Factory Harley's are very restricted on the intake side (air cleaner) and the exhaust side. Significant gains can be made with free flowing air filters and exhaust. Just my experience.