2 stroke modding ?

vwfan79

vwfan79

Active Member
#1
#1
I have a 1978 Sears moped w/ 2hp 2 stroke motor and would like to get a little more out of it. I really need some better take off power and would like a little more top end, currently it does between 30-35mph. What are some basics on modding a 2 stroke motor, are there some simple things that can be done. The plan is to tear it apart this winter and give a complete redo but if there are some things I can do in the meantime without a complete tear down please share. Any info on modding a 2 stroke would be great.

I checked out a moped forum and wasn't able to find my answers and wasn't happy with the forum setup.
 
W

WLB

Active Member
#2
#2
Two strokes are very easy to modify but you will have to remove the head and cylinder. A big difference can usually be make by increasing the compression (don't go over 165 or it will inhibit flow through the transfer ports), by raising the exhaust and transfer ports, and by lowering the intake port or cutting the intake side skirt of the piston. You really need to put a degree wheel on the engine with the head off and determine where the ports are opening before you start modifying. It is easy to go too far. The right expansion chamber with dimensions that are compatible with your porting will really wake up a two stroke. A slightly larger carb will help also.

What increases you add to the top end will likely be lost on the low end. The timing of the opening and closing of the ports on a two stroke is equivalent to the opening and closing of the valves by the cam on a four stroke.

Two strokes were always my favorite engine. With just a lathe or milling machine to raise the compression (you can also do it with coarse sandpaper and a piece of glass if you have patience), a die grinder to raise the ports (you can use a file to raise the exhaust port on many engines), and the die grinder or a file to cut the piston skirt you can usually get 10 to 20% increase in power. No cam, stiff springs, valve guides, valves, titanium retainers, high compression pistons to buy.
 
W

WLB

Active Member
#4
#4
That 70cc kit is definitely the way to go. It is the same kit I used to build a 70cc Tomos moped engine. My kit also came with the expansion chamber and big carb.

To make it work well on the Tomos I had to TIG weld the transfer port area up and then port to match the Puch cylinder. That engine has set at the shop for several years waiting on me to put it in a moped.
 
Txpowdercoater

Txpowdercoater

New Member
#6
#6
A ton of good info already in this thread. I happen to own a T-500, 1969 2 stroke suzuki and I have the book the factory race team put together showing their race trim 2 stroke mods. Its definitely a science like WLB explained well.
 
vwfan79

vwfan79

Active Member
#8
#8
There is a lot of great info so far, unfortunately at this point in time I'm "TRYING" not to spend a bunch of money as I have multiple minibike projects going. Now when it comes time to redo this bad boy this winter the big bore kit and a carb and pipe might be in order. I have opened up the exhaust a bit and readjustment the carb and have gained a little in both acceleration and top end. I also put some cafe style bars on it and gave it a lil better look. My main goal here is to keep up with traffic a little better. Thanks for all the good info and keep it coming, I love learning new stuff.
Here's with the stock bar

Here's the new bars
 
paul_probus

paul_probus

Member
#9
#9
Nice, I have one just like it that is in rideable shape, but I want to eventually restore.

Another couple of good sources of moped parts (in addition to 1977mopeds linked to above) are:
Treatland.tv
themopedjunkyard.com

Paul
 
vwfan79

vwfan79

Active Member
#12
#12
I did it in stages and most of which is reverseable, the only thing is that it gets loud quick. Just a few adjustments and it sounds like Steve's kart. Pat seems that you know something about 2 strokes, I will have to pick your brain one day at the track. Maybe I'll bring it out this weekend, would be fun to run around the track.
 
shelby28

shelby28

Member
#13
#13
When your ready to order parts use these guys. There alot better then 77 mopeds you will have your parts in no time and you won't have any issues with them. I have a Puch magnum that's heavily moded. 84cc kit better crankshaft bigger carb race pipe cdi. I do 65 to 70. I'm a smaller guy weight wise. You can get that bike doing 50 with probobly 150 to 200 bucks, basically a kit and a pipe. You probobly have a 12 mm bing carb, you may have a 14 mm which is better. Have you tryed mopedarmy.com? That forum is just has easy to use as this one and there wiki has all the info you will need.
 
shelby28

shelby28

Member
#14
#14
https://www.treatland.tv/puch-70cc-cylinder-treat-kit-p/puch-treat-kit-70cc-the-people.htm

https://www.treatland.tv/puch-tecno...aust-pipe-p/puch-tecno-circuit-pipe-pinto.htm

That kit with that pipe will get you to 45 to 50 depending on your size and tunining. You may have to play with your gearing. But those two things will work good to get you going. That kit will work with your carb you may have to adjust the jet but that's it. I have this set up on a less moded bike and I do 50 with a 14 bing. On your carb there will be some stamping of numbers if there is a 12 then it's a 12mm if there is a 14 it's a 14 mm
 
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