Random thought - Star Motorcycle Forums: Star Raider, V-Max, V-Star, Road-Star Forum
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  • 1 Post By Shawn855
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post #1 of 7 (permalink) Old 07-19-2019, 01:08 PM Thread Starter
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Random thought

Why is it necessary to re-jet after changing your exhaust? The carburetor air/fuel mixture ratio remains the same entering the combustion chamber, so I'm confused how changing the exhaust would impact that? Just curious
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post #2 of 7 (permalink) Old 07-19-2019, 01:23 PM
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Air cooled bikes run hotter with mods so you have to richen them up. Same with updating air boxes. Liquid cooled you don't have to.
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post #3 of 7 (permalink) Old 07-19-2019, 03:52 PM
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My understanding has always been you donít have to re-jet if youíre just changing the exhaust. Itís when you change to a different air kit. Iíve always changed both, so I donít have first hand knowledge.
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post #4 of 7 (permalink) Old 07-19-2019, 04:56 PM
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the engine pulls air with with vacuum on the intake stroke, and pushes it out with pressure on the exhaust stroke

if you open up either side it will be able to suck in more air, or blow out more thru the exhaust

the stock exhaust on a motorcycle is already pretty un-restricted. The mufflers do not block much flow, so putting a loud aftermarket exhaust does not make that much difference in the amount of air 'left' in the cylinder after the exhaust stroke.

But the intake is restricted by design - that is the primary way the engineers set the max HP, by how much air will flow thru the intake and carbs, or thru the throttle body for fuel injection bikes.

So if you open up the intake you will get a significant increase in air flow - then a modified exhaust becomes more useful, to let that extra air out easier.

Its not so much air cooled vs water cooled that makes a difference, its carbs vs fuel injection (but most fuel injected bikes are water cooled).

A carb has no way to adjust the mixture for anything: more airflow, changes in altitude, temp, octane rating... the jets are fixed and do not vary. So any changes you make have to be matched with the right jet sizes, and adjustment to the Pilot Main Screws (PMS).

A fuel injected bike can compensate for changes in temp (no choke needed), air pressure (MAP sensor) and if the bike has a full O2 sensor closed loop controller, it will also compensate for different octane fuel, and changes to the intake and exhaust resistance.

Most fuel injected motorcycles do not have the full closed loop adjustment (like all cars) - I think only HD and BMW have that ability. So for Vstar bikes you either need to get your ECM reprogrammed (flashed) to match any changes you make on the intake or exhaust, or you need to get something like a power commander with knobs you can turn and adjust (or program) that increase or decrease how much fuel gets squirted for any given throttle setting.

to answer the original question: if you only change the exhaust, short of putting open pipes, the slight change in air/fuel mixture does not warrant changing the jets (but maybe open the PM screws a little bit) or re-programming the ECM on a FI bike - it will only be slightly lean, but you wont get any real performance change - it will just get LOUD.
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Last edited by KCW; 07-19-2019 at 05:02 PM.
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post #5 of 7 (permalink) Old 07-19-2019, 07:30 PM
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Exhaust = No adjustment needed
Intake = YOU BETTER ADJUST!

Rob - 18 Raspberry SVTC, 08 Pearl White Navi Goldwing
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post #6 of 7 (permalink) Old 07-19-2019, 11:08 PM Thread Starter
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Thanks everyone for the excellent responses. I now have a much clearer understanding on the subject.

2003 Vstar 1100 Custom

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post #7 of 7 (permalink) Old 07-20-2019, 05:21 AM
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back pressure, it scoops fuel from the carb while the valve is open. opening the exhaust reduces back pressure and creates a lean condition. drag pipes reduces low end torque
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