Stock moby 50v mod question r/t blunderbuss

Ok guys excuse me if this is an old topic!!! BUT...

I'm trying to make the most of the stock parts on my 50v before I decide to put down the money for performance parts. The only upgrade I've really bothered with is replacing the gurtner with a dellorto 14.12/shorty intake. Right now I'm jetted at 57 (if it matters). I'm planning on doing some mild porting to the cylinder, but not till further (farther?) down the road and I'm wondering if I should even bother derestricting my blunderbuss until then. I have the version with the 2 baffle plates. I'm topping out at 32 on flat ground and it would be awesome if i could just boost it a few mph. Will derestricting with my current setup do that for me? Or am I wasting my time (albeit learning a really valuable skill) and should look into porting first?

The other question I have is if it WILL make the difference I'm looking for, do you guys have any recommendations as far as how much to enlarge the transfer holes on the baffle plates in relation to the widened outlet hole. I know that it's relative to whatever outlet width works best for my setup but I'm looking for ballpark ratios here.

I'm using info I've learned from rebelmoby's discussion on this (and super helpful diagram!!) from this thread: http://mabecanemobylette.freeforums.org/blunderbuss-exhause-modification-t1394.html

Thanks in advance dudes. (edited)

Re: Stock moby 50v mod question r/t blunderbuss

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

Re: Stock moby 50v mod question r/t blunderbuss

I have seen a few french writeups that I couldnt easily translate, but it seems like they get some good performance outta derestricting those. loud tho. I got rid of my moby before trying anything out tho

Re: Stock moby 50v mod question r/t blunderbuss

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

Re: Stock moby 50v mod question r/t blunderbuss

If you ever see anything french which looks interesting I might be able to give a better english translation then anything online... unless you find my english incoherent then it would be pointless.

Basically you would not want the sum of the hole areas to be less then 60% of the carb cross section area. The stock exhause you are more concerned about derestriction then reflection of the shock wave to give you that wonderful reversion shot to over fill your combustion chamber. There is not much tunable about the exhause length, chamber size or bulkhead location on the blunderbuss.

For sound deadening you would want many small holes to make up the 60%, if you are unconcerned about the sound then just get some goodly holes in the bulkhead.

Focus on the size of the final outlet by experimenting with a block off plate - somewhere around 11mm or 7/16 inch is a goodly size, if you have a 12mm carb. (edited)

Re: Stock moby 50v mod question r/t blunderbuss

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

Re: Stock moby 50v mod question r/t blunderbuss

@rebelmoby When you say 60% of the carb cross section area... Could you please be more clear about what you mean by cross section? Are we talking about the diameter? I'm looking for information about this in the wiki but either cant find it or am looking for the wrong topic.

@mel thanks for the specs on your mods! Super helpful, especially bc I wasn't sure what kind of pipe to use for the outlet tube and where to place the additional holes.

From what responses I'm getting (and seriously, THANKS GUYS!), I definitely want to derestrict... I just want to be 100% informed on what I'm doing before I do it. Thus far I understand how the exhaust flows through the blunderbuss and why widening the holes will help, still just fuzzy on the math. (edited)

Re: Stock moby 50v mod question r/t blunderbuss

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

Re: Stock moby 50v mod question r/t blunderbuss

Rebel, if one tacked varying sized washers to the end of the exit pipe, would it work as well? Would it make any difference which end of the pipe?

Re: Stock moby 50v mod question r/t blunderbuss

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

Re: Stock moby 50v mod question r/t blunderbuss

Rollo Tomassi /

Great thread

Re: Stock moby 50v mod question r/t blunderbuss

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. :)

Re: Stock moby 50v mod question r/t blunderbuss

David Gabert /

What about just cutting out the baffles and attaching a silencer? What would this do? Any negative effects?

Re: Stock moby 50v mod question r/t blunderbuss

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

Re: Stock moby 50v mod question r/t blunderbuss

David Gabert /

Okay, cool! Thanks reb!

Re: Stock moby 50v mod question r/t blunderbuss

David Gabert /

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?

image.jpg
image.jpg

Re: Stock moby 50v mod question r/t blunderbuss

Exhause goes out the blue pipe, all the way back towards the header via the black holes and then exits through the grey pipe. Reversing through the two black holes in the bulkheads is what gives the quiet aspect to the silencer. You can make those bigger as needed. The grey pipe is the one that needs to be enlarged for better exit and less containment or back pressure. That is the pipe you would want to replace with 7/16 or 11mm ID tube. You can experiment with the exact right size by making a blanking plate to replace the plate and long tube and drill a 11mm or 7/16 hole and see how she performs before getting and bending a new inner pipe.

Shortening the inner pipe you are replacing will also have an impact on performance as the route to get out is shorter, but also noisier.

Re: Stock moby 50v mod question r/t blunderbuss

David Gabert /

Perfect! Thanks Reb!

Re: Stock moby 50v mod question r/t blunderbuss

So would it just simply be doing something like this and varying the length of that tube to control noise level? Otherwise, I don't see how you would drill the holes in the baffle plates to accept the larger diameter tube. Maybe I just haven't seen drill bits or extensions or something long enough to reach that far down in the exhaust.

BBnewAutopsy-revised.jpg

Re: Stock moby 50v mod question r/t blunderbuss

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.

Re: Stock moby 50v mod question r/t blunderbuss

David Gabert /

How in the heck do you get a drill bit inside there to drill? Lol

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

Login or Create Account