Variable Back Pressure Exhaust

Cannonball3

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This is for all you experimenters out there. This is on my low end Minareiil that runs a stock exhaust. Its gutted and has three 1/4” additional holes. After going thru the jetting/needle adjustments it runs very well. With one exception-off throttle/low throttle surging. I finally concluded
it was probably a back pressure issue or lack there of. So I modified the cap to be adjustable. After much experimentation this is what I arrived at.
With all holes open it was prone to WO 4stroking. I stuffed the three holes with coarse steel wool and that improved the 4stroking, and ran great overall, but still surged at cruise/ light throttle. After adding the cap and a bunch of test riding, the ideal setting was 3 partial holes, with none of the original opening. Was quieter, smoother and low and mid improved, no surging. Amazingly it still had 35 top end. It will top around 42, ports wide open. I ride thru a lot of quiet neighbor hoods to get to town so smooth and quiet is important. The cap is retained by the mufflers bolt and has a stout compression spring to hold settings.

Just throwing my experiences out there. If you ride 3/4 throttle all the time surging wont bother you. It suck's at light cruising. All builds are different, so you are on your own figuring out yours. This worked for me.
 

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2speedboi

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Can you get a close up of this muffler? I like the diagnosis you came up with. A couple months ago I was dealing with similar issues, to much back pressure made the 15-19mph real crummy. A little less and I had great torque. Suffers a little at the very top end but I still hit 30 no problem and very quiet.
-2sp
 

Cannonball3

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Also, is that two crush washers behind the bolt?
Its a stock CG muffler. There is a spring between the washer and the cap to hold the plate firmly against it, but still allow adjustment. This type of arrangement regardless of muffler allows quick determination/adjustment of exhaust changes. i just spent a good bit of time trying different settings. Its amazing at least with this engine set up how much difference it makes. I have found my sweet spot for it. Closing it a bit has added smoothness and great torque, at the cost of only a few mph. Its easily regained by opening it up.
I was having to maintain 15-17 mph minimum on a 36t sprocket to keep the engine happy. It now will pull contentedly at 12 with no surging. This is an easy way to experiment with exhaust dynamics.

Thinking of adding a movable ring washer similar to the old 60s Snuf-R-Nots to my Athena Minarelli to be able to tune its ranges. Imagine a system linked to the throttle. Restricted on low end and opening at high throttle. Hmm, I just
happen to have a twin pull throttle cable!
 

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Its a stock CG muffler. There is a spring between the washer and the cap to hold the plate firmly against it, but still allow adjustment. This type of arrangement regardless of muffler allows quick determination/adjustment of exhaust changes. i just spent a good bit of time trying different settings. Its amazing at least with this engine set up how much difference it makes. I have found my sweet spot for it. Closing it a bit has added smoothness and great torque, at the cost of only a few mph. Its easily regained by opening it up.
I was having to maintain 15-17 mph minimum on a 36t sprocket to keep the engine happy. It now will pull contentedly at 12 with no surging. This is an easy way to experiment with exhaust dynamics.

Thinking of adding a movable ring washer similar to the old 60s Snuf-R-Nots to my Athena Minarelli to be able to tune its ranges. Imagine a system linked to the throttle. Restricted on low end and opening at high throttle. Hmm, I just
happen to have a twin pull throttle cable!
Man that cap design is absolutely genius 👍 I'll never forget that one. I'm running an eBay gorilla gasbike mz65 clone that I've refabbed a little that's working so good for now. But I'm sure someday I'll be messing with one of those stock pipes again fo some reason or another. So this one is in the bank!
 
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