[superjocko]

That vacuum hose goes to the pulse secondary air injection system, an exhaust emissions control device. It utilizes negative pressure pulses in the exhaust to draw fresh air into the exhaust system to aid in burning off carbon monoxide and hydrocarbons. It requires no engine power to operate, as it uses pulses in the exhaust system that are already there as a part of normal engine operation. It also supports the proper operation of the catalytic converter, if your bike has one. If your bike has a catalytic converter and you intend to keep it, then you should run a vacuum hose from your new intake manifold to the secondary air injection valve/reed assembly. This could be accomplished by drilling an appropriate size hole and then tapping it to accept an 1/8" NPT fitting with a nipple for the vacuum hose. If you don't, the catalytic converter could become restricted with carbon over time, particularly if you richen your carburetor to improve performance. The alternative is to either gut the catalytic converter in your stock exhaust system, or install an aftermarket exhaust that doesn't have a catalytic converter, then remove the air injection system altogether and cap off the air injection port on the head with a steel plate and gasket.

Removing the air injection and catalytic converter seems to be popular modifications. However, other than a very small weight savings, there is really very little, if any, performance advantage. I believe that most of what people perceive as a performance increase from removing the catalytic converter is little more than additional noise that creates the impression of improved performance. The metallic substrate catalytic converters used in these bikes offers very little restriction when allowed to function as originally designed (with the air injection system intact and functioning). That said, the tailpipe diameter and the end of the muffler (depending on the outlet size of the pipe on your particular model bike) may very well be an unnecessary exhaust restriction. A simple, cheap, and effective solution for that issue is to cut out the stock tailpipe and weld in one with a larger pipe I.D. back in place. Per plenty of readily available internet based exhaust system calculators, a 20mm I.D. tailpipe is more than adequate to flow enough for a moderately modified true 250cc engine and, of course, a stock displacement 229cc engine.

The bottom line is that "emissions control devices = bad" just simply isn't accurate in a lot of cases with these bikes. Yes, they come jetted too lean for best performance, and with a carburetor that isn't capable of flowing enough to support some mild mods, but the air injection doesn't hurt performance at all and the catalytic converter doesn't affect performance anywhere near as much as people seem to think it does.

My background as an automotive instructor, teaching courses in engine performance and emissions controls, has led over the years to my thinking on this subject not necessarily being in line with the mainstream. Take it for what it's worth.