Thoughts of using 2 part putty as a way to decrease stinger tip diameter on f2 exhaust

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Nov 9, 2023
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New Iberia, La
So I’ve been playing around with this exhaust mainly because I already have it. I know there are better exhausts out there but for some reason I’m stuck on seeing what I can get out of this one. I sanded down the welds on the inside of the flange and removed metal in the exhaust inlet to match the exhaust port. I noticed a difference in torque right away. Last night I was trying different ignition timing settings and I decided to put a lot of some 2 part epoxy in the stinger tip. I then got it smooth using sandpaper so the only thing providing back pressure would be the diameter of the hole instead of the roughness of the surface of the epoxy. I opened it up little by little. I have a ways to go but so far I am running a 32 tooth sprocket, ports cleaned up and moved a bit, inner lip of the transfers shaved a bit so the fuel/air mix will be directed up towards the spark plug, and a delorto clone carburetor. Mid-range torque is an absolute blast! I would however like more powet in the upper rpm range. I don’t really want anything over 7000rpm max because I want the motor to last. Any suggestions on what else I can do to modify this pipe or am I in unconquerable territory because everyone else buys better pipes?1st pic is where I started. Not much back pressure at all. 2nd pic is where it’s at currently.
 

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Do you have a picture of the whole pipe? Then I can make some recommendations. What's the outside diameters of the header at the flange and the stinger where it comes out of the convergent cone?
 
I did this on a bike before and it never did. This stuff gets hard enough to grind on. I rough sanded the inside of my pipe before applying too so it really sticks
 
Get yourself a pipe/tube that slips over the end of the stinger. Wrap electrical tape around the stinger. Shove your pipe on and ride it till the tape gets a little mushy. Then slide the pipe back and forth until you like the way it feels. Once you find this point, weld it.
 
What you are after is back pressure. A longer stinger will provide that in addition to possibly giving room for another wavelength.
The ideal inner diameter for the stinger on an 80/66cc is 15mm. I make mine at 16-ish but that is because a piece of 3/4"/19mm seamless has that for the inner.
 
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