Low-Medium speed vibration (4-stroking?). Removing muffler cap fixes

[wahtermelon]

I was getting a ton of vibration at all speeds and I noticed during some runs I could get it to accelerate stronger, vibrate much less, and sound different by adjusting the gas. After doing some reading here I guessed it might be 4-stroking so I adjusted the carb to the leanest setting even though the spark plug looked fine with a toasty powdering (it was previously 1 notch away from leanest).

In the same breath I tried removing the muffler cap. Wow. What a difference - power and noise wise. The most major power difference was the strong pull from low speeds with no vibration and a very different sound than before. My top speed seemed improved too, though I didn't have a speedo. At the very least vibration was much less when I got there.

I had to put the muffler cap back cause that was LOUD. Now I'm almost back to square one. Low speed pull is terrible and jutters really hard until its running I'd guess 20mph.

Does anyone have any suggestions? A tuned pipe seems like the way to go but I'm looking for a cheaper short term solution. Maybe I could modify my stock muffler?

Please, I'm missing that low end torque.



Thanks,
Darren

 

[Powertool]

might try something like this
http://motorbicycling.com/picture.php?pictureid=7699&albumid=1264&dl=1373337548&thumb=1

 

[maniac57]

Drill a few holes in the large part of the pipe near the bottom close to the seam where the cap seals. Two or three 1/4 holes will open it up without being too loud. Tune with number and size of holes.

 

[wahtermelon]

Thanks for the suggestions. I've been playing around with loostening the muffler cap so it hangs a bit which really helps. I think the cap was tightened down too hard before. I'm going to try drilling some holes. Maybe I'll do a dB test with my phone to get some perspective on the noise difference.