Topic: Exhaust opening - where's the real gain?

[Buber]

So, as I can't keep my hands off my bike...  I had a normal exhaust. Just for fun (and not to be "outgunned" by my wife's Virago ) I drilled couple of holes. All was well. Then, after I changed the camshaft, it went louder, so I closed the holes. And I noticed, that on the idle it was much more prone to dying. So, to take the experiment further, I took the last baffle off for today, and ... surprise! It went at extremely low idle when cold - I don't know, maybe 500?, but didn't die.
So - what's your input on that? I mean, I'm totally not after "roaring" sound - actually I would preferr it to just "purr", but if there's technical gain in it, then I'm all for it.
Is really "restrictiveness" of the exhaust so visible? What about low, and mid-range power, that is supposed to be better with little restriction?
Aha - it's CB650 Z european version, with aftermarket mufflers 4-2 looking like original ones, some italian brand.

Just stirring the pot... 

[Tower]

Sounds like you're running too rich at idle - are you getting black smoke?  Lean the idle mix and it should run with the original configuration. 

As for power related to pipes - yes, the more balanced and less restricted the flow the better, as long as the pipe diameter is within a certain range (75% - 90%) of the exhasut port diameter.

[Buber]

No, no, actually I think I'm too lean now. I never had black smokes, etc.

BTW - any suggestions what jet size should I go, if I have 98 on CV carbs now? 108? I want to ride whithout teh air filter cover to improve the flow, but if i do it now, it bogs the thing dow - way too lean of course.

I just wish i could keep it open & quiet. Anyway - Trial is on now, let's see what happens....

And any suggestions for a shop that sells the jets cheap? I know they can be had for like 3$ a piece, whereas here, it's more like 7$ per piece... Thanks!

[Tower]

Did I misinterpret your original post?
When you take a baffle off or unrestrict the pipes (drilling holes), that typically makes an engine run leaner.  So if it runs better with the leaner condition, then it was probably running too rich with the baffle in place.  Hence, to keep the baffle in place (or plug the drilled holes), then lean out the idle to compensate.

[hapsh]

Is really "restrictiveness" of the exhaust so visible? What about low, and mid-range power, that is supposed to be better with little restriction?

Exhaust back pressure actually increases your low to midrange power.  Your exhaust valve timing is set to achieve power at your higher RPMs, so they open a bit before BDC to speed your exhaust scavenging (even more-so on performance Cams).  At lower rpm you loose part of your power stroke when the exhaust valve opens before BDC letting part of the explosion out the exhaust.  By having some back pressure on your exhaust pipe it helps keep some of the charge in the cylinder, in part delaying its exit out the exhaust valve.  Yamaha has a pretty cool power valve (EXUP) on there FZR's that increases back pressure at low rpm ranges which dramatically increases low to midrange power.  I took mine off my FZR and hated the loss of midrange power and MPG (50MPG down to 36MPG!!) so I put it back.

[Buber]

Yep, I know about the exup thing, and so on. But - I was really baffled when with open exhaust=solid tickover (not rpms, mind, this I can always put up, but a solid, not-dying tickover), and proper exhaust - tickover only good when engine warm....

Still - I'm going for a ride tomorrow, so I will see how this affects consumption. Fuel is going higher & higher here... right now its 7$ per gallon. No kidding... 

[hapsh]

I think what you stumbled on with the open exhaust was a leaning out of the idle mixture just enough to balance out the cylinders just right.  I think that if you are shooting for the lowest possible tick-over idle speed you will have to be very acurate with your carb vacuum balance AND adjusting your idle mixture screws individually per cylinder to get the smoothest idle when together.  This means one cylinder may have the airscrew 1.5 turns out, another 1.75, another 1.25, and the last at 2 turns out.  I have done this in the past using an electronic tach (dwell/tach).  First you adjust for a stable idle with stock airscrew settings and vacuum balanced carbs.  Then adjust the airscrew for one cylinder using 1/8th turns until you have the maximum idle speed.  Do this with each cylinder until they are all adjusted.  Then go back and drop your idle back down as low as it will run and vacuum balance check the cylinders.  You will have to do both vacuum balance and airscrew adjustments a few times to get everything as close as possible because each time you adjust your vacuum balance it can change the idle mixture needed for each cylinder, and vis-versa.  Once you are done you should be able to achieve the same low idle that you did without your exhaust baffle.  However this will only be the case when fully warmed up.