4-1 vs. 4-2 fuel and air mixture

[grifff]

How critical is the air and fuel mixture going to 4-1 exhaust or vise versa. Are new jets needed?

[chevy45412001]

generaly

i have not found a need to rejet with exhaust alone. Now if you change intake tract different story.Usaualy if you open up the air side ,you would then want to bring more fuel so as not to run to lean (white plugs ) there is a jetting guide somewhere if you scroll down to (i think modifications) below and look in carbs and fuel . this could shed more light on the subject.

[TopCatGr58]

Back when I was young and ignorant(now I'm just older!), I put a 4-1 set of pipes on my bike, never did rejet, rode it for 7+ years that way, of course I also didn't really check my plugs that well either! But, I had all the power I needed/wanted, rev'd to 8.5+ and ran great, no holes in pistons when I did my rebuild! But you should monitor plugs, get a new set and if they are bleach white, then you may need to rejet, but if they show some light tan/gray color after a good sustained mid/high rpm run and chop, then you should be good to go!

[Dennyz]

It is really about what you want from your XS. The XS seems to be over built enough to run just fine under a variety of conditions. If smooth gently power with excellent fuel economy is desired, the run it lean. If every little bit of power possible is desired, the jet it up.

My experience has been that a 4:1 will lean a motor out a bit compared to a 4:2. I believe it is due to improved scavenging (each cylinder assisting the other by pulling exhaust out of the combustion chamber).

I found that any change to the intake or exhaust can warrant a jet change, but may not be worth the trouble. I have been able to modify my "F" at both ends and jet accordingly. The end result has been great power and great fuel economy (35-38mpg).

I do believe that jetting is required when any change is made to the intake or exhaust if maximum tune of the engine is desired. This is the only way to obtain the proper balance between drivability, power, and fuel economy for the given configuration. I also believe that if the XS is bone stock, jetting will not gain a thing UNLESS! Altitude is a factor.

[wiredgeorge]

This is a good question and the real answer is that it depends on what 4 into 1 you use. My experience is that V&H 4 into 1 will flow a lot better than a MAC and somewhat better than a Kerker on most inline fours. Most four into 2 systems flow about the same as whatever came on the bike or even worse. I suggest you ride without rejetting and do this...

#1... note if the bike pulls OK up to red line in 3rd or 4th gear... If it does, then your mains are OK...

#2.... riding 60 mph, crack open the throttle and note if the bike accellerates briskly (like it did before the pipe mod?) If it rolls on OK, then you don't need to mess with your jet needle. If it accellerates a bit slow, you might want to shim or lower the eclip on your jet needle to raise your jet needle to richen the mid range.

If you take the baffles out of the 4 into 1 you will want to bump the pilot jets up one size or it will probably backfire when you chop the throttle on decelleration.

Changes in the airbox, as has been pointed out, affect jetting much more than pipes alone.