MOPED ARMY

How does your cylinder look? Specifically inside around the different ports.

picture of your motor might help. So if its a 102 then the motor is solid mount and doesn't move, but you have the special edition sp, hmm. well if its piston port intake off the cylinder, then timing could be off, or if its case inducted like the 103 then your reed block might be jacked.

So I was basing my previous statement on the assumption that you have tried jetting correctly. Or maybe you just have way too big a jet? What size jet are you running with the 15SHA? What does your spark plug color tell you? Is the plug wet?

Check that the ignition timing is spot on .

Wow thank you all for the help! The relevant suggestions are:

1. piston improperly spaced/bad ports (over-bored?)

2. timing

3. carb/jetting

I think #1 is the last thing I'll check because it requires the most disassembly. If numbers 2&3 don't fix the problem I'll return to 1. I will be making sure the timing is stock when my flywheel puller comes in the mail this week. As to the carb/jetting/spark, my spark plug looks good, it is the stock requirements, is gapped correctly, not too wet, and coffee color on the white ceramic. Seemingly no signs of running too lean or to rich. My jet is a #60. I have not messed around with the jets, because I wanted to fix this problem first. I try to follow the rule of only changing one thing at a time so that you can figure out which thing is the culprit. The bike has definitely been messed with by the previous owner because the exhaust, and carb are aftermarket, and the timing had sharpie marks on it when I exposed the flywheel. Sorry this is kind of a brain dump.

Here is more description of the problem. When idling there is a slight puffing sound from the carburetor. When the throttle is opened up this gets louder and turns into a muffled pop that shoots sensibly cold (as in not hot like exhaust) fuel/air mixture out. This problem is very bad when there is resistance against the motor, and is accompanied by bogging down. However when the bike is on its stand it can "work though" the puffing by getting to higher revs. The same effect can be achieved by pedaling furiously and variating the throttle on and off on level ground to get to a decent speed with no puffing or popping. Unfortunately though, there is little to no power.

UPDATE: I pulled off the flywheel and I have the five coil magneto. I reset the points to .4mm and set the timing to stock. The problem is exactly the same. There is however a new finding, IT RUNS BETTER WARM BUT NOT HOT. Once hot it will not run because of so much blowback out the carb at all rpms. It is running rich but this is due to the air passing the emulsion tube twice as much as it should (both directions, out and back in) and therefore picking up twice the fuel. I ended up disconnecting everything and pulling the engine off the frame and taking the top end apart. Could this be port timing? Is it that there needs to be a thicker gasket between the crank case and top end? I'm happy to take pictures let me know.

Thanks!

Good you have the five coil. Always post pictures, that would tell a lot about your setup without you even attempting to describe it. In the beginning you said your emulsion tube holes were drilled out, is that correct? Do you have an air filter and box and metal filter screens? What do you expect to acxomplish by taking the motor off the bike?

so looks like a stock cylinder, i wouldnt worry about your port timings. Did you say compression is good? I see some scoring on the exhaust side of the cylinder.

Anyways, stock jetting for a 15 sha im guessing should be a little bigger than 60. Are you using the airfilter/cap as having it on/off will affect jetting

Triple check your timing with a gun. As rebel said earlier, too advanced and itll blow fiel outta the carb.