Why Rejet when changing exhaust? - ATVConnection.com ATV Enthusiast Community

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Old 01-12-2001, 02:27 PM
Range Rover
Thread Starter
Join Date: Oct 2000
Posts: 105

Can anyone give me a reason why you would need to rejet the carburetor when changing the exhaust?

My theory is that if the carburetor is jetted PROPERLY with the stock exhaust and filter you would not need to rejet the carb when putting an aftermarket exhaust on it.

I do understand that changing the air filter to a better flowing sytem will change jetting because of the fact that the stock airbox restriction may be causing a vacuum or low pressure area before the carburetor which actually lowers the air density.

Bartolo proved this when Big Gun told him to remove the airbox lid when he had the 160/165 mains in and it was running poorly with the airbox lid in place.

Carburetor's cannot account for changes in air density and that is why you need to rejet at different altitudes(air densities).

The exhaust, on the other hand, does not change the air density of the air going into the carburetor.

This is my theory and would love to hear any thoughts that would prove my theory wrong.

The next question I have is: Why are people changing their pilot jets so quickly?

According to the the carb tuning guide idle mixture screws and pilot jets are some of the last things to adjust.

Just some food for thought.
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Old 01-12-2001, 03:39 PM
Pro Rider
Join Date: Nov 1999
Posts: 508

i think it is because you are moving more air out thaen you are in
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Old 01-12-2001, 03:55 PM
Join Date: Dec 2000
Posts: 83

An aftermarket exhaust system works by scavaging the cylinder better due to less back pressure. You see when the piston makes the exhaust stroke if you dont have a good flowing exhaust some of the burned gas is left in the cylinder. Then you have less room for new gas and air on the intake stroke. If with the new exhaust you have more room in the cylinder on the intake stroke then it will try to suck more gas and air in. This is where the problems start. With stock jetting,the same amount of fuel is sucked in as with the stock system but it also sucks more air in and provided you have a good air source,which it doesnt look like the Raptor does,then it will run lean. I personally think the raptor has limited air which might be a big part of the restriction.
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Old 01-12-2001, 04:24 PM
Pro Rider
Join Date: Sep 2000
Posts: 236

If you buy an aftermarket exhaust you probably did so to improve performance.Your aftermarket exhaust will move exhaust gas out of your cylinder more efficiently like Digs said,now this will allow you to put more fuel into the cylinder and more fuel is what gives you more power.If you install an exhaust and dont have to rejet you are gaining nothing but noise and there are exhausts out there that dont give you any gains because they haven't reserched and tested their products enough or think their customers like louder but not faster.Hope I helped.

88' 250R many extras
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Old 01-12-2001, 04:29 PM
Range Rover
Thread Starter
Join Date: Oct 2000
Posts: 105

A carburetor adds fuel based upon a "volume" of air passing through it.

If you reduce the exhaust restriction it will increase the "volume" of air passing through the engine and also the carb. The carb will automatically add more fuel when the volume of air increases because that is what a carburetor does. If you had to rejet because your changing the "volume" of air passing through the engine then you would have to rejet for different engine speeds. (example: I would need a different main jet for 7000 rpm than I need for 9000 rpm.)

What the carburetor does not know is how dense the air is coming into it. In other words it doesn't now the "mass" of air coming into.

This is one of the many reasons modern car companies have gone to fuel injection. Modern fuel injection systems have Mass Air Flow meters that tell the engine controller the "mass" of the air going into the engine and thus automatically compensate when you drive at different altitudes.

Basically what I am saying is that an aftermarket exhaust system can change the "volume" of air going through the engine but will not change the "mass" of the air going through the engine.

However, removing a restriction on the intake side before the carburetor can increase the "mass" of the air going through the engine and the carburetor will need to be rejetted.

Again, please review bartolo's experience about changing to richer mains with the stock airbox and the improvement he got when he removed the airbox lid.

Thanks for the reply's and keep them coming.
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Old 01-12-2001, 04:58 PM
Pro Rider
Join Date: Oct 2000
Posts: 806

This is the answer...previous post...

With stock jetting,the same amount of fuel is sucked in as with the stock system but it also sucks more air in and provided you have a good air source,which it doesnt look like the Raptor does,then it will run lean.

The carb doesn't compensate for more air coming in. If it did, There wouldn't be ANY companies selling jets.
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Old 01-12-2001, 05:26 PM
Join Date: Nov 2000
Posts: 56

Before we get started lets state some ground rules.

1. The engine is stock and jetted right, before we modify it.

2. The plug is being checked by the tuner when changes are made to assure proper plug color, thus signaling rich/lean conditions when appliable.

3. In a fixed volume environment, adding heat to the system will make the pressure increase even though the volume in constant. Just like heating a can of beans on the stove. Eventually the can will explode because the expanding gases have no where to go but out.

Changing the pipe on your four stroke to a better flowing system will allow the exhaust to evacuate
more exhaust gases from the piston cylinder, we agree on that.

The reason the engine will need more air/fuel is this. On a exhaust restrited motor like the Raptor, the more hot expanded spent gases you get out of the cylinder on the exhaust stroke, the more air/fuel can come in, thus making a lean condition. Here is why...

When the piston comes down for a intake stroke,
the combustion chamber is pulling an increased vacuum until the piston drops below the intake ports. This is when a cool charge of fuel/air gets sucked into the motor for a compression stroke.

Because of assumption 3. above, we know that the hotter the gas, the more pressure it makes.
So it stands to reason, that cooler temp. will result in less pressure.

After the exhaust stroke, the less cylinder pressure you have, the more vacuum you will pull on your next intake stroke, thus pulling more fuel/air into the cylinder for the compression stroke.

This why the stock tuned Raptor with a better flowing exhaust will probably need to be jetted richer.

The Raptor carbs are twin Mikuni 33mm BSR CV carbs. You are right in saying not to change the pilots, before tuning the main jets.
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Old 01-12-2001, 05:41 PM
Range Rover
Thread Starter
Join Date: Oct 2000
Posts: 105


I respectfully disagree with you.

The jets are required because of changes to air density.

You said "The carb doesn't compensate for more air coming in."

How does the carb deal with different engine speeds? There is MORE AIR COMING IN at 9000rpm than 7000rpm for example.

What your saying is that we would need a different jet for every engine speed. And that is not correct.

7000rpm on a Raptor moves 163 cfm and 9000rpm moves 209cfm. An aftermarket exhaust will not change the flow through the engine as much as that.

Again, here is my point

I think you need to change jetting if you improve the flow before the carburetor because if there is a restriction across the filter(you are in affect causing a vacuum)the air density will drop same as if you went to a higher altitude. But the exhaust does not have an affect on air density(mass).

Carburetors can deal with air volume changes but cannot deal with air mass changes.
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Old 01-12-2001, 06:33 PM
Pro Rider
Join Date: Jul 2000
Posts: 439

Your reasoning's pretty good, (I've a little trouble with your username and its arcane significance, if any); however, your conclusion, a stock main jet can produce an optimum air/fuel mixture with a modified exhaust, appears incorrect.

While a stock jet compensates to some degree for varying air flow, its response to venturi-induced partial vacuum is not entirely linear ("leans out" at both high and low end); stock jetting itself is a compromise design decision.

The main jet dominates composition of the air/fuel ratio only on the upper end of throttle opening and air intake fluid flow; the greater volumetric efficiency enabled by a high-performance, low backpressure ("scavenging") exhaust results in greater flow-rate at high rpms the stock jet can only partially accommodate; thus, a bigger jet is required at the higher flow rates for production of an optimum air/fuel mixture.

I guess I might more simply say to you, "Carburetor main jets "lean out" at the high end. The increased airflow from a trick exhaust should be compensated for by a larger main jet; otherwise, the engine will run lean at high rpms, and the engine will be unable to take advantage of the maximum power mixture ratio in that area of operation."

A good illustration of the relationship between air/fuel ratio and throttle valve opening is found in "Motorcycle Carburetor Manual," by Pete Shoemark, page 6, Figure 1.1, "Typical air/fuel ratio curve for slide carburetor." (The main jets lean out at the low end, too, as shown in the referenced illustration; thus, the "pilot system" exists for providing an appropriate mixture at low throttle openings with consequent low rpms and airflow.)


P.S. Well, this post doens't make much sense to me, either; regardless of the poor writing, increased airflow, resulting from exhaust modification, through a carburetor at higher throttle openings requires a larger main jet to produce an optimum air/fuel mixture.
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Old 01-12-2001, 07:12 PM
Range Rover
Join Date: Oct 2000
Posts: 120


More exhaust flow makes the bike run leaner. Less restrictive exhaust systems mean the bike needs more fuel to maintain the correct carburetion. Without re-jetting the bike may "pop" on deceleration and may run hot.

Re-jetting is modifying the carburetor fuel outlets to change how much fuel is released under various circumstances. This controls how "rich" or "lean" the fuel mixture is.

Generally re-jetting is only required in combination with other performance enhancements, or perhaps for people in high altitudes where the air pressure is lower. Just putting more fuel in the mixture will make the bike run too rich and cause problems like sooty deposits on the spark plugs. The Raptorís standard carburetor setup is excellent and doesn't need modification unless you have replaced the pipes or made some other performance change which may have affected the fuel/air mix.

Keep in mind that Big Gun only recommended removing the air box lid after I had gone up 8 sizes in the jets (assuming they are 2.5 increments). The stock setting is 140/145 and 24.5 (I think?) and they recommended going to 150/155 and 27.5 with the complete system and NO AIR MODIFICATIONS. Since I went richer than they originally recommended, I needed to compensate for it by pulling in more air.

Hopes this helps...
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