Exhaust reversion problem with Mikesxs commando exhaust system?

[Tnrfd]

Freshly rebuild stock 1972 XS2 motor with Mikuni VM34 carbs and BB ignition. Header pipes from Mikesxs with a 1 3/8" I.D.

With the commando style mufflers mounted the engine bogs down and looses all power at around 3-4k rpm. I can see a cloud of fuel/exhaust vapor standing in the carb inlet when revving in neutral so suspect the problem is exhaust reversion. Tried running with open mufflers and then the bike pulls much stronger over the enire rev range so confident it's an exhaust problem.

Feels like it's running rich, but I've tried smaller main jets (all the way from 190 to 120), but that doesnt solve the problem, only masks it slightly. Still has really poor mid range power.

Anyone else experienced reversion problems with Mikesxs commando exhaust system? Should I try torque inserts? I know the stock header pipes are smaller (1.25" I.D?) and that the XS1/XS2 has a slightly hotter cam than the the later model so maybe that contributes?

 

[Tnrfd]

Tried with som stupid loud open mufflers I've had lying around and that worked like a charm. Pulls strong through the rev. range so def. problems with mufflers/backpressure.

 

[DogBunny]

I'm not an expert, but I believe the following:
Reversion as an open pipe thing. Exhaust systems that include baffles and/or that make the exhaust travel through varying diameter pipes, just aren't prone to reversion. Are the Commando megaphones just straight-through pipes? If not, I think you're over-jetted for the Commandos -- not just the mains, but the pilots and jet needles all need to be addressed.

 

[Tnrfd]

The commando mufflers have baffles etc. Ok, sounds like your are correct about the open pipe thing. Then I should think about re-adressing the jetting. Start fiddling with the pilot/needle jet and needle...

 

[gggGary]

where are your needle clips set?

 

[Tnrfd]

Current jetting:
- 180 mains. I've tried anything from 120 to 195. Same problem only less noticeable the leaner I go, but no real power gain.
. 30 pilot jet
- 2.5 slide cutaway
- Needle jet: 159-P5
- 6F9 needle, clip in the middle position.
- Air jet removed.

 

[gggGary]

Those VM float settings can be a bit tricky, have you confirmed your fuel level?
PS there are several rather different float types used in various VM34's
while my set up isn't exactly like yours I currently have my needles all the way down, top groove. I also just installed torque inserts. I have run this 750 with both straight through Borla cans and commandos. Are the mufflers commandos or those emgos with 7 small exit tubes? Hint; after several years of mucking around I s-canned a set of emgos, best thing I've ever done.

 

[5twins]

I have to agree, those new "Commandos" are nothing like the 1st original ones they used to sell. It wouldn't surprise me if they're nothing more than re-shelled EMGO Dunstall replicas, and those do suck.

 

[member28833]

I have to agree, those new "Commandos" are nothing like the 1st original ones they used to sell.

Dang, that is a bummer. The one pair I heard in person lacked the stock standard pipe personality and tone.
There just has to be pipes out there that sound good !

 

[TwoManyXS1Bs]

...With the commando style mufflers mounted the engine bogs down and looses all power at around 3-4k rpm. I can see a cloud of fuel/exhaust vapor standing in the carb inlet when revving in neutral so suspect the problem is exhaust reversion. Tried running with open mufflers and then the bike pulls much stronger over the enire rev range so confident it's an exhaust problem.

Feels like it's running rich, ...

Those are the signs.

The 256 cam has a torque drop spot around 4krpms (red circle),
And a torque peak around 6-7krpms (green circle).


That torque curve can be flattened, or s-shaped, by the exhaust design.

A final confirmation is to check power output at the anti-node rpms, say around 6,000 - 6,500 rpms. Accelerate in a lower gear, WOT, perhaps 3rd. Squeeze past 3-4krpm, see how it performs beyond that.

If power picks up, dramatically, it's reversion.

If not, poor flowing, choked mufflers...

 

[Tnrfd]

Those are the signs.

The 256 cam has a torque drop spot around 4krpms (red circle),
And a torque peak around 6-7krpms (green circle).
View attachment 137318

That torque curve can be flattened, or s-shaped, by the exhaust design.

A final confirmation is to check power output at the anti-node rpms, say around 6,000 - 6,500 rpms. Accelerate in a lower gear, WOT, perhaps 3rd. Squeeze past 3-4krpm, see how it performs beyond that.

If power picks up, dramatically, it's reversion.

If not, poor flowing, choked mufflers...

Interesting. Power picks up dramatically at around 6k.. Behaves almost like a tuned two-stroke.

Is it possible to further investigave the "inversion theory" without mounting torque inserts / anti reversion inserts / exhaust porst optimizers.?

 

[TwoManyXS1Bs]

Sure. It helps if you understand the phenomenon.

Overly simply put, exhaust pressure pulses are bouncing around in the exhaust system, and at certain rpms a reflected pressure pulse will actually travel backwards thru the exhaust valve and intake valve during valve overlap, stall induction, and disrupt carburetion. At other rpms the effect reverses, boosting induction during overlap, a desired thing. With fixed geometry exhaust systems, many compromises.

These are sonic engines. The exhaust system acts like a musical instrument, from the brass section, like trumpets and pipe organs. You can change resonant frequencies, shape the waves,...etc. Exhaust design and geometries. A wonderfully deep/dark hole, pursued by many, also a source of madness.

To scratch the surface, do Google searches on "Exhaust scavenging", "Exhaust tuning", Exhaust reversion", "Tuned exhaust pipes", "Sound wave theory", "Speed of sound temperature".

The simplest changes are exhaust pipe diameter and length, muffler choices...

 

[grizld1]

Pay attention, y'all. The above post by 2M illustrates the only meaningful use of the term "back pressure." Guys often use "back pressure" to mean "restriction"--not the same thing at all.

 

[gggGary]

This is why Yamaha used EXUP https://www.yamahapart.com/exhaustultimatepowervalveexup on some motors. Similar schemes are also used on many two strokes that have a hard time getting an accurate air/fuel charge just in the combustion chamber.
Gotta wonder if the torque inserts also reduce the high RPM power....... When I originally got it Madness was a high RPM rocket, when it hit 5K rpm it launched. I think I have gained on the low RPM but may have lost some up top. It had VM38's a martec ignition and straight through with packing chambers mufflers on 1 3/4" headers. Overall I think the perceived trade off is worth it. Just not that often I go for redline on the XS650.

 

[TwoManyXS1Bs]

... the only meaningful use of the term "back pressure." ...

Haha, should we start installing these?