Exhaust and carb tuning

[wildfire]

This is more of a good to know question.

I am not intending to make any change at present to my exhaust system  but I am curious as to how do you know what carb jetting changes to make when you change your exhaust.

[FJmonkey]

This is more of a good to know question.

I am not intending to make any change at present to my exhaust system  but I am curious as to how do you know what carb jetting changes to make when you change your exhaust.

If you still have your stock air box then you will not likely need to rejet. If you do not have the stock air box and have more free flowing air filters then you will need to rejet to a larger size and raise your needles up a touch. The exact size of jets and how far to raise the needles will take a little testing. However, there are few here like Randy that can recommend what size jets to start with and get you some needles with clips so changing the position is easier.  Every bike and set up is a little different, no precise wall chart to refer to on this kind of upgrade. The good news is, if you get rid of the stock air box and use the UNI dual pods like I did, getting the carbs on and off is much easier. And since your are that far down the rabbit hole, get the SST fastener kit from Randy. Another guge improvement when working on your carbs.

[Ed Ramberger]

This is more of a good to know question.

I am not intending to make any change at present to my exhaust system  but I am curious as to how do you know what carb jetting changes to make when you change your exhaust.

I've tuned many motorcycles over the years, carbureted and EFI, using a dyno and wide band sampling equipment.

One thing to remember is that in 1989 (my particular FJ vintage) ethanol wasn't a factor.  The bikes were jetted for straight gasoline, and now run leaner with the addition of ethanol.  Many carb bikes like the bump to the next size on the slow speed jets.  On my FJ, I bumped the slow speeds 2 sizes and the mains one.  Running with a K&N and E10, the bike is very happy.

A great rule of thumb is that if you have to turn your mixture screws out 3 turns, go to the next slow speed jet.

When the bike was bone stock it love a slow speed jet bump of one size. 

Just some observations.