Motorcycle experts, I have a question

[michaels]

My dad bought a Triumph America a few months ago and he was wondering if he takes the factory pipes off and puts some 4 inch turn downs on, will he have to rejet the bike?

 

[Kalvin00]

You don't have to, but it will not be at its full performance potential if you don't (don't know if that's what you're after). It might run rich or lean after you put those on.

 

[JoLLyRoGer]

If it is carb'd then yes he'll need a re-jet and probably a different intake to get the engine set up properly. If it is FI then it will need a new fuel map. Also, when opening up the exhaust you have to open up the intake too so that you can re-tune for a correct fuel to air mixture. Otherwise you're just removing backpressure and ultimately hurting your torque curve.
IIRC, 14:1 is about right Air:Fuel.

 

[michaels]

thanks

 

[CombatChuk]

Depending on the Altitude you're at it might be ok to just put on more open pipes.

I'm at 6,000 ft above sea level so the stock setup on my '95 VFR runs rich. So when I added a Two Brothers exhaust system, K&N air filter and I took the snorkel off. The carbueration on the bike is perfect, no fluffyness or surging it's just right. And I gained a few HP doing so (not enough to offset the 10% loss of the high altitude)