Learning something new on a twice barn-found beater

Nice, I have an old YG1 80 (72cc)

Are you going to port this bike and do some more tuning on it? Look forward to your progress!

Eventually, yes, that is my plan. What I MAINLY want to do is build a pipe for it, but some light porting will probably happen, just have to see what looks like it will work the best.

Man, I love sketchup.... Digitized my port measurements, heres what I came up with:



Been reading the Two Stroke Tuning book by A. Bell. I think the only internal things i'm going to change are the exhaust port width from 30.5 to 36 or 37 mm and possibly change the rear boost port timing by filing a tad off the top of the piston so they open a bit sooner. Need to degree my engine before I decide to change any timing though. Next step will be figuring out the pipe. Pretty interesting stuff the more I read about it!

Rainy day, so I played a bit more. Had the top end off to hone and re-ring, figured that was a good time to degree my engine. Here's what I came up with:

printed a degree wheel of the internet, cut it out, and bolted it on to the crankshaft. Used a cheapo laser pointer set on the engine to light the degree, set the wheel to zero with the piston at top dead center. Kinda hard to see, but if you look close you can see the red dot at the 0.






Rotated it through, noting the degrees that ports opened and also the duration. Had to take the cylinder and piston back off to see the intake to get the degrees for the rotary valve.


Heres my digitized version. Not sure if its right, seems a little wonky compared to some that I've seen:

Honed the cyl, put a new piston and rings in, still had pretty anemic compression... Been playing with my home made baffle cone and think I finally got it close to where it should be. That, or my rings are breaking in. Got an extra 10 mph out of it now, so it goes a whopping 55 mph on the flat!

Nice!

I would build a trail machine out of it. But it's your bike. You can do whatever you like Nice going on the tuning!

Your not the only one who has said that... but i've already got SEVERAL old, underpowered, underbraked, and undersuspended bikes that will beat you to pieces when you ride on the trail. This one is doing road duty- for now.

been playing with my welding "skills"... Blah. monkeyed around some more with Sketchup and got a decent model of what the pipe is 'sposed to look like:



I didnt build the first looonnnng cone. got the second cone, belly, baffle cone, and stinger fabbed up out of some junk steel i scrounged up. Used the box and pan brake to bend the cones and then finished them off on the stake table. Wanted to practice my oxy/acet skills, so I "welded" it all up with the torch. Came out so-so, not to bad for a first attempt at an expansion chamber.





No. The welds are not pretty, but they should hold!

Going to weld it on to the stock header pipe which I still need to measure to see if its the right length. Fingers crossed!!!



That would be kadepunch lending a helping hand. Hope to get the rest of it tacked / welded tomorrow. No school!

No school today. Whacked off the old pipe and got the new one tacked in and fabbed some mounts for it.







Just need to weld it all up and test it out. (fingerscrossed)

Nice job!

If you don't like your welds you can grind em up some.

That looks too small, that chamber would be good for a model aircraft engine. Your power is going to be at 13,000 revs, pity it will only rev to about 3,000 with that pipe . With the stinger on that I wouldn't be surprised if it punchs holes in your garage wall at idle, which will be at around 10,000

Yup, this was def. a learning experience. It idles fine but sure runs like crap until you get to the higher rpms. It was pulling 50 mph in 3rd gear when before you'd be lucky to get that in 4th, but it didnt have the low end to get 4th gear up into the power band. I was using a free program and since this was truly a practice run, I'm not to tore up about how bad it sucked. Fun learning experience though. It also sounded LOADS better than the open megaphone it had on it!

I broke down and bought an expansion chamber design program, seems pretty nice. Its called 2 stroke wizard, came with a second program for designing headers for 4 strokes as well. It was a royal PAIN to get installed on my computer, but seems to be working well now.

The COOL part is they have a free cone layout tool- type in the measurements and it PRINTS a FLAT layout cone you can trace onto your metal and cut out!!!! Talk about a LOT of work saved!

Find it here: http://www.buildandclick.com/html/utils.html

Took POJ for another spin this afternoon, and the pipe actually works pretty good. The first trip I guess I didnt have the collar tight and the pipe fell off the head and I lost the gasket... I put it back on today and it definately has WAY more top end. It easily holds 50 mph in 3rd, where before you were lucky if you could touch 45 in 4th! To bad it doesnt have enough low end to get it up to speed in 4th, but hopefully pipe #2 will fix that. The second design is roughly the same diameter on the engine and stinger, but the two cones and belly are almost twice as big.

....cool thread! My first dirt bike was a Trailmaster.
My dad and I made a 'tuned pipe' for it as well....oh, and yes the headpipe constantly falls off. Safety wire for sure!
I think we had about a 3"+ diameter x 6" cone on each end of about an 18" length chamber, maybe a 6" stinger.
The power wasn't alot in gain, but the loss of weight from the muffler alone made it all worth it...and the sound was a big plus!

That thing with the two springs on the end of the rotor is a governor. It retards your timing as it spins. If you want more go you should disable it.

pretty sure its not a governor, its your timing advance. Disabling it will most likely result in either very crappy running at idle or very crappy running at high rpm. Either way, not gonna make it go faster. I've been wrong before though!

It is the advance. Removing it will hamper performance, but hey its your bike.

Time for an update. I've been working on POJ a lot, but I started a seperate thread for what I was doing... Started on another expansion chamber. See the entire thread here:

http://www.advrider.com/forums/showthread.php?t=655547

Some pics of the build:











Thats about a tenth of the build pics... Still workin on it too. :huh