Re: Moto 7 released from its shackles

Aaron Summers Effler /
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i would assume that you can follow the same instructions as you would with the 50V exhaust. they are pretty similar inside. you may need to wing it a little bit, but if you're working with the same concept the result should be the same. make sure you drill out the holes a little at a time, test, and then repeat. as soon as speed stops increasing, you're done. here's rebel_mob's description of the process:

First I will show some modifications on a cashew exhause. Foremost step would be to clean the exhause, which is posted here - http://mabecanemobylette.freeforums.org/viewtopic.php?f=27&t=1390

The cost of having a quiet motor is performance. Some people prefer the look of the stock exhause for their own reason but it is bothersome to be slowed down by the exhause. If you have a slightly ported motor and a larger carb then stock you would benefit from liberating the restriction.

The muffling portion of the exhause system is to reduce the noise from the exhause to a level required by law. The norm these exhause were desined for were to have the noise below 85dBA. The cashew has a diffuser and end cap which would provide between 68.5 and 73dBA.

Best performance is found when the cross section area of the final outlet is 50-65% of the exhause port window area. Measurements will be in mm or for areas squair mm and are rounded to one decimal. Stock Moby exhause is 21mm x 9.5mm (not accounting for the rounded corners) which gives a cross section area of 199,5 so the final outlet should be between 99,8 and 129,7.

To determine the cross section area of a round pipe use the formula PI*radius*radius. By measuring the orifaces in each bulkhead in the exhause you can determine the total area for each baffle in the system. The target of this excercise would be to ensure none of the baffles cause a restriction to the flow except the final outlet.

I have measured a stock cashew and have come up withthe following figures...

Header 19,8mm = 307,9

Cashew outlet 6,5mm x 8 = 265,5

Center baffle 5,5mm x 9 = 213,8

Silencer inlet 5mm x 6 = 117,8

Silencer outlet 10mm = 78,5

You can see the area for the exhause is slowly being restricted more and more as it travels through the exhause to the outlet.

For a completely stock motor you would only need to concentrate on the silencer portion of the system. To determine the ideal size of outlet you would want to make a blanking plate to replace the silencer portion of the exhause. As each motor and setup has its own characteristics it is best to have the motor find its own sweet spot for the exhause outlet. The target outlet area of between 99,8 and 129,7 gives us a diameter of 11,3mm - 12,9mm. If you start with a blanking plate with the original 10mm and start opening the outlet hole incrementally using larger drill bits or reaming the hole with a file to slowly bring the size larger as you test ride for each change. Note the size of the change and then take a test ride.

Warm up the motor to bring it to temperature and ride around to see how she feels, sounds and runs. Then open the outlet a bit more, note the size and try it again. The sweet spot is the last size before the test ride when you feel no change or a decline in performance.

Stock jetting is set up slightly rich, but after modifying the exhause you may find you need to go one size richer on a stock setup. You may need to go more then one or two sizes on a ported motor or one which is fitted with a different carb then stock. After rejetting you may want to check if your sweet spot has changed and you need a larger or smaller outlet, it will not change by much though.

Once you have determined the optimum size you can fabricate a permanent end cap or modify the original silencer to the optimum size.

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