Hawk 250 vacuum line

[Matt6288]

Hey,

So I've got a brand new '24 hawk 250, and just installed the 28mm Nibbi carb and intake manifold. Unfortunately, I didn't notice until after the install, the new replacement nibbi manifold does not have a vacuum port on it like the original intake manifold does. The original manifold has a vacuum line running to what looks like some kind of valve (maybe the PCV system?). It has two larger hoses connecting to it. One leads to the air box, the other leads to the engine head. What do I need to do to finish this job? Can I just leave this vacuum line unconnected to anything, or could that cause issues?

[Thumper]

It is possible that you can simply plug the line to the Vacuum system. It is EGR related, I think. You should still be able to jet and tune without that system connected.

Give it a shot. You might be OK.

[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.

[Thumper]

All good points Jocko. I am not in favor of removing EPA equipment. I maintain the cat systems on both my son's 2002 cars (a Civic LX and a Suzuki Vitara). In fact, I bought and installed the primary and secondary converters on the Vitara (secondary cat theft) along with a complete exhaust system recently. And before that, I replaced the manifold/converter on the front of the Civic to OEM standard as well.

On my Templar however, I fell over in dried weeds and the red hot converter lit a fire under the bike in less than a second last Summer. My right foot was trapped under the rear fender. By the time I struggled out, dragged the bike away (fortunately and clay and dirt patch nearby), I was barely able to stamp out the fire, but it melted the insulation on my spark plug wire and the flames were rising up past the carb while I was pinned. I haven't panicked like that for a long time.

Forest Service will tell you that Cats are responsible for car fires in campgrounds, dirt bikes too. It is a significant fire hazard.

[superjocko]

The fire hazard concern is a valid point. I'm not all that familiar with the Templar, but is it safe to assume that its cat is located in the headpipe? I much prefer the location of the cat to be inside the muffler as that way there is a significant buffer around it due to the much larger diameter of the muffler that prevents the unit as a whole from reaching the same surface temperature as a headpipe mounted cat.

[Megadan]

Originally Posted by Thumper

It is possible that you can simply plug the line to the Vacuum system. It is EGR related, I think. You should still be able to jet and tune without that system connected.

Give it a shot. You might be OK.

There is no egr.

It is either for EVAP, or the pulse signal for the air pump on the PAIR system.