hello was wondering if anyone can help me i have a 81 gs850g iv kept it stock for a long time but know id like to put a header on it 4 into one do i have to jet the carbs for this and how do i know what jets to buy thanks

 

If it were me I'd rejet the carbs, and also change out the air box with a free flowing design such as a K&N setup. Basic rule of thumb is if you put free flowing pipes, or intake on you r bike you will create a "lean" fuel to air ratio, and you will need to rejet to get a richer ratio. And you'll be surprised at how much better your bike will run. It will really wake it up...

As for which size jets to use, usually going up 1 size larger for your slow jet, the main jet should be adequate as it is.

FBR