Expansion chamber didn't add top speed

[noah9988]

Zeda 100, E3.32 spark plug, 40:1 gas mix, Speed carb w/ #75 jet (#70 was causing 4 stroking at high speed), needle clip at leanest setting, MZ65, 44t sprocket, 270 pounds.

33mph top speed. I thought the MZ65 would add some but it didn't. Of course it did add tons of low end torque, but no top speed at all. Does this sound right or is something off? I did have to cut and reweld the pipe back together to get the chamber out away from the bike because it was hitting the frame. Maybe that messed something up?

 

[DAMIEN1307]

To get just a little more top end speed, but less low end torque, you would need to have a rear sprocket of between 32 to 36 tooth.

Based on your weight of 270 with 44 tooth sprocket etc. and getting 33 MPH, that sounds about right...DAMIEN

 

[YetiWoodz]

Sounds kinda right, but you could still try jetting up. I went from a 61 with a cdh66 (mz65 knockoff pipe) to a 70 jet with a real pipe and that seems to work best so far.

 

[Karl Snarl]

You need to port to get more rpm. Porting determines rpm. Simple as i can place it. Its more than that. But you need to port to get more rpm.

 

[YetiWoodz]

I did have to cut and reweld the pipe back together to get the chamber out away from the bike because it was hitting the frame. Maybe that messed something up?

Yes this would be exactly what happened. Lengthening the pipe makes the pipe hit at lower rpms. I made my pipe 38" (measuring from flange to beginning of stinger pipe) and that seemed to be effective for low to mid rpms I wanna say 3500 to 6500rpm. My top rpm before and after the pipe is 7500. Going 1" shorter can change it by very roughly, 200rpm just to get an idea.

 

[Karl Snarl]

Yes this would be exactly be what happened. Lengthening the pipe makes the pipe hit at lower rpms. I made my pipe 38" (measuring from flange to beginning of stinger pipe) and that seemed to be effective for low to mid rpms I wanna say 3500 to 6500rpm. My top rpm before and after the pipe is 7500. Going 1" shorter can change it by very roughly, 200rpm just to get an idea.

Without porting you are very limited in the rpms where power is produced on these engines. The balance between exhaust porting and the pipe timing is critical for power. With your long pipe, it will keep rpms down, and the power band lower, the reversion wave is slower to get there. If you shorten it, it will move the reversion wave up, not necessarily give you more rpms. Most likely just burn the side of the piston because the timing is off. an keep power down as well. The height of the exhaust port in concert with the pipe is the main contributor to rpm( keeping it simple, shape of the pipe does too, etc etc), but has to be balanced with the transfer too or the transfers will then be the limiting factor. Which all has to be balanced with case compression and intake. the intake should be roughly 60% of your exhaust port size. Not that how you have it set up is bad, I set most mine for lower rpm power bands. I think everyone here should read the work from the two stroke gods. Will really help you understand the dynamics behind these engines and how to really bring them to life. There's a few guys here that think they know everything, they really should read these books two. I spent $600 bucks buying 4 books. These guys won race after race, were the best in their classes, etc. I think they knew what they were talking about, yet some of these guys say they are wrong. I still believe the 2 stroke gods over their advice. I did not create any of this info. It is a culminations from decades worth of racing from the best teams in the biz. I just share it freely, not everyone can afford $600 for some books. lol

 

[YetiWoodz]

Yes this would be exactly what happened. Lengthening the pipe makes the pipe hit at lower rpms. I made my pipe 38" (measuring from flange to beginning of stinger pipe) and that seemed to be effective for low to mid rpms I wanna say 3500 to 6500rpm. My top rpm before and after the pipe is 7500. Going 1" shorter can change it by very roughly, 200rpm just to get an idea.

I have another pipe, a kx65. Its 34" long. I put it on and went for a ride today. I hit 8250 rpm as my max. I'm liking this better than my 38" one.

 

[SirLurksAlot]

Got Gordon's two stroke tuners book which is $300-$400. Cash me $100 and all 156pages are yours

Also available to download for free....

https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://www.vintagesleds.com/library/manuals/misc/Two-stroke%2520Tuner%27s%2520Handbook.pdf&ved=2ahUKEwji9bj7tMiBAxVnJUQIHSxwAHgQFnoECCkQAQ&usg=AOvVaw3n45m6GrIhMQiAmkuTygpP

And

https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://vintagesleds.com/library/manuals/misc/jennings/2stroketunershandbook.pdf&ved=2ahUKEwji9bj7tMiBAxVnJUQIHSxwAHgQFnoECCoQAQ&usg=AOvVaw3V0UYeM5DpBOHglbshAIX7