MOPED ARMY

I have a million blunderbusses i want to sell if you want any cheap to experiment with.

I would be totally interested in buying an older style blunderbuss if you have one along with a newer style. Pm me! And thanks!

@will yeah I've read theyre loud but that you can get some silencing if you replace the outlet tube correctly. I don't mind a loud ped. There are so many kids who ride around with loud ass 4-stroke dirt bikes at all hours in Philly, the noise coming from my exhaust is the least of anyone's problems hahaha

I've done this to both my 50V's with exelant results both times . I enlarged the bulkhead holes to 3/8" and added 2 more 1/4" holes near them. I used 1/2" emt conduit for the exit pipe. Not the best pipe to use but cheap. It is a little louder but not bad , a richer, deeper sound. You might try Reb's method of more, but smaller holes for quieter sound. Gain was very noticable on both bikes. setups- #1) mild port job from wiki on stock 30mph cylinder, shorty phbg intake, amal 15mm carb,=40mph. #2) airsal 74cc kit with stock head, 19mm phbg with shorty intake, =45+mph. Be sure to drill both bulkheads! good luck

The cross section area of the carb is basically the area of the narrowest part:

SHA14:12 is necked down to 12mm inside so that is 113 sq.mm (area of a circle is 3.14xradiusxradius) multiply the area by 60% gives you 68 sq mm. Reverse the area formula to find the target size (diameter from an area is 2xsquare root of (area/3.14)). That gives you 9mm as the starting point which would be optimum as a expansion chamber but for a stock exhause it is design more as a silencer so you would need to go for containment management rather then reversion.

To experiment with your set up then you would make a temporary metal plate to replace the silencer section of the exhause and drill a 9mm hole as an exit. Ride around and the performance would be slightly better then stock but lots noisier. Make the exit hole slightly bigger and ride again to check performance. Go like that making bigger n bigger holes until there is no more improvement in performance.

Measure your new exit hole size and source some tubing with that inside diameter and make a permanent back plate or modify the original plate to accept the new exit pipe. You want at lease 40mm or 1.5 inch inside the blunderbuss so it can deaden the sound and not get neighbors hostile every time you ride. If you need to hush it more then you can cap the inside portion of the pipe and turn it into a piccolo by drilling multiple small holes in the side wall of the tube so it diffuses the sound.

I found with 12mm carb on a stock AV7 it likes approximately 11mm or 85% as a outlet on the blunderbuss. Strangely enough a 14mm carb seems to like 10.5mm on a stock motor which is 56% so you have to play with size a bit.

Back to the maths, no one likes to do long calculations so here is a site to calculate circle areas...

http://www.rkm.com.au/calculators/calculator-circle-sphere.html

Much easier to drill 4 rows of 4 small holes with a 1/8 drill bit then to stack washers big-small-big-small-big on to the end of the pipe, you also would not want the small washers inside diameter to be smaller then the inside diameter of the pipe.

You could cap the inside end of the pipe with a washer though, it does not need to be a solid cap but the larger the hole the more noise.

I know a lot of people are not so concerned about the noise but some neighbors are real hostile about noise. No one wants their bike accidentally totaled while parked by Ms.Irate and her scoccor-mom-mobile

Ok UPDATE:

So I opened up the blunderbuss to take a look at the baffles. Guess what? SOMEONE ALREADY TRIED TO DERESTRICT THE THING! The exit tube is 1/2", which leads me to believe that someone tried to mess with it, but the transfer holes seem to be unchanged. The plan now is to enlarge the transfers to 3/8" and add two 1/4" holes.

I just want to say thanks SO MUCH for all the help, especially Rebel and Mel.

Next step now is to port the stock cylinder, slap a 15.15 on it, and maybe upgrade the variator. :)

Completely gutting the blunderbuss makes it VERY LOUD with no noticeable improvements over derestricting the orifices and adding some small holes to baffle the exhause noise

Hey rebel, so doing the math for a stock AV7 with a dellorto 15.15 carb and a malossi 15mm intake (not port matched) I keep coming up with a starting point of 10.8 or basically 11mm. Now the drilling is where I get a bit confused. I have the two baffled blunderbuss exhaust. So I drill the exit hole where the exit pipe would go when assembled and then drill two extra holes with a 1/8th bit to make more baffling? Or the two baffle holes which I circled in red? Then add a larger piping for the exit?

Perfect! Thanks Reb!

Docking the length of the interior portion of the exit pipe will give you better performance at the expense of noise. Stock the exhause has to double back through the two bulkheads, which will dampen the noise, before it gets to the exit pipe. The longer course and passages it must pass will take away some of the performance.

Best plan for modification on the exhause is to make a new back plate with the larger size exit pipe and then experiment with length. This way of you do not like the results you can go back to stock or make another test pipe for experiments rather then cut up something and then have to mess around with trying to reverse the experimental modifications.