Moto 7 released from its shackles

Gromosaurus Rex - Brad Pitt /

Found this lil guy chained to a telephone pole on a dirt road, 100 miles north of detroit. 913 miles. Purchased for a song. It cleans up all purty like.

3.jpg
4.jpg

Re: Moto 7 released from its shackles

Gromosaurus Rex - Brad Pitt /

stripped cleaned and waxed. ready to go. Just waiting on treats to bring new tubes and tires. Its my first moped. Any ideas on near-stock upgrades/mods. Got it up to @26/27 with my fat ass(235). would like to crest 30-35. Attainable ?

IMAG0178.jpg
IMAG0191.jpg

Re: Moto 7 released from its shackles

Aaron Summers Effler /

that was my first moped, too. when it comes to performance, what you can expect without much work depends on which bike you have. assuming this is post 1978, the "C" is the fast (30mph) model. the VIN plate at the front of the frame should indicate which you have. the difference is in the carb, the intake, the porting, the head, and the rear sprocket (the 30mph one had 35 teeth, the 20 and 25mph ones had 39 teeth).

i put a Dellorto 15mm SHA carb. on mine and ported it to the spec's published in the wiki. i also had a squish band milled into the head. when i was done it ran 31-33 pretty consistently. what you're after is attainable. i've also read that putting an old style motobecane "blunderbuss" exhaust on there will add speed (mostly top-end), as it's less restricted than the newer ones (like the one on the model 7). lightening up your clutch would help with acceleration, too. if you have a slower model, you'll want to replace the intake and rear sprocket. replacing the intake would be tricky, as it's actually part of the motor mount. your best bet is to find a model 7 intake from a 30mph model. you could also reset the timing so it's 1.7-1.8mm before TDC. of course you can also kit it, but try these things first.

Re: Moto 7 released from its shackles

Gromosaurus Rex - Brad Pitt /

Awesome. Thanks for the suggestions. I checked out the wiki and Treats is also sending me a 35T rear sprocket with my tires. I assume i have the "B" model, but i suppose i should have checked that out before i bought the sprocket.

How does it indicate the model on the VIN ? Does it actually state "A, B or C" ?

Re: Moto 7 released from its shackles

Gromosaurus Rex - Brad Pitt /

its a "B", confirmed

Re: Moto 7 released from its shackles

Gromosaurus Rex - Brad Pitt /

Re: Moto 7 released from its shackles

Yeah, that is is it, but it will not fit well if at all on a "7" due to clearence issues. That exhaust fits the 50v which has bigger wheels and thus more room underneath. Great looking bike.

Re: Moto 7 released from its shackles

Gromosaurus Rex - Brad Pitt /

can the stock "7" exhaust be derstricted ?

Re: Moto 7 released from its shackles

Aaron Summers Effler /

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.

Re: Moto 7 released from its shackles

You prob could, but motobecane exhausts are pretty free flowing stock, unless it is all carbonized, you would prob have best results cleaning what you have and then going with the "c" jug and intake and sproket. It is no header don't get me wrong, but will do nicely with mild mods. My 2 cents.

Re: Moto 7 released from its shackles

Gromosaurus Rex - Brad Pitt /

@Aaron Thanks for pulling that up for me. I will play w mine...start w a good cleaning and maybe a mild derestriction as it is currently stock. Big ups again to RebelMoby..having all this knowledge..doing all the work..and then sharing it w the lay masses.

@SK. I am very green with this stuff. What is a jug ?

Re: Moto 7 released from its shackles

Aaron Summers Effler /

there are four different stock exhausts from motobecane. the two piece is not very restrictive, but it was only put on AV88's and AV89's, out of the ones imported into the US. it is designed differently, but performs similarly to the least restrictive "cashew" style exhaust made for the imported 40's, 50L's and 50v's in the early 70's. they had a "40" stamped on them (between two motobecane symbols), and only one crimped rib perpendicular to their length (like the one in that ebay auction you linked). these are the pipes that people like stephen keller likely have, they will run up to nearly 40mph on a ported stock 50V. i've also seen them going 45+ on a parma kitted 50V.

in the latter part of the 70's motobecane started replacing the cashew exhausts with the newer, quieter model. they are stamped "40 - 85" between motobecane symbols, and have two crimped ribs instead of one (running perpendicular to the length of the pipe). if you swap one of these onto your 50v that previously had a two-piece or fast cashew on it, you'll loose 5-9mph instantly. unfortunately the design of this pipe was very similar to the design of the 7's pipe. both were meant to run quieter, and meet environmental restrictions in some countries.

so, you've got a couple options. 1: modify a fast cashew or two-piece to work with the angle of the seven's motor. 2: modify your 7's stock exhaust to open it up a bit, and see if that helps (see my post, above). 3: buy an after-market motobecane exhaust, which would likely have to be modified to work with the 7's mounting geometry (like the homoet in this post). 4: buy one of these and modify it (if it's needed). this pipe came on the most recent MBK's and the AV-10 model 51's sold in europe and imported into canada. it's not restricted the way the slow cashew is, and it may just bolt right up to the seven (but you might need to make a bracket, or re-shape the header). 5: put a VLX pipe on it, but you'll want to derestrict it first, and modify the mounting hardware a little. it performs similarly to the 7's stock pipe. i have one i'd part with, if you're interested, and it's already setup for a model 7.

good luck, and post pics when you're done!

Re: Moto 7 released from its shackles

has any one tried to put that origonal motobecane black performance pipe on there model 7 looking for a pipe for my bike

« Go to Topics — end of thread

Want to post in this forum? We'd love to have you join the discussion, but first:

Login or Create Account