@Matthew Johnston As Greg suggests, check for spark, whilst doing so it might not hurt to try gently turning the lead clockwise in the CDI, the wire literally just goes over a screw and mine came loose after I'd had it disconnected once, though in my case it manifested as what sounded like 4-stroking.
I'm guessing you tried starting with the choke lever in different positions? As the weather changes the position it wants to be in to get the motor running will also vary.
If going down that route leads you nowhere, my next stop would be the carb, I guess, such as making sure the air cleaner isn't clogged up, that there isn't a load of crud or varnish in the bowl or blocking the jet. Beware that if you take the bowl off the bottom, you
might have to replace the gasket depending on how long its been there.
Similarly if you find that spark and fuel aren't the problem, and if you find that the air cleaner has been getting blasted with oil, there may be a buildup of crud in the exhaust, but you probably will have to replace the exhaust gasket if you take that off to clean it, as they usually leak slightly if you put it back on with the old one.
@Goblin Evidently you saw my thread. As my motor is retiring in the next week or so, I've been tampering with it and found a few things in the process. Firstly, due to the load on my engine I run more oil than most here, but I did discover that lowering the amount of oil actually makes it perform worse and have some theories as to why.
It may be worth dumping more oil into your gas, maybe double, and giving it a try, maybe when you're at half a tank so you can always dilute it with gas again if it ends up making things worse and want to go back to the old mixture. Obviously you might have to play with the needle again to make it run right with the increased oil. Whilst I don't remember the exact science behind it, the fuel I have access to is known to be trouble and there's some relation somewhere between octane level and engine compression, as this is an engine with fairly low compression my guess is that the oil is somehow making the fuel more or less stable in some way, because if I use less of it the engine runs worse, at 50:1 I would be hearing constant pinging, at 24:1 the engine performs horribly, as I approach 16:1 it improves, though if I keep adding more oil beyond that it starts to run worse and smoke like it's running on coal, as one would expect. Once I have the new bike and engine, I may try using different fuel in this one to see what effect it has.
Secondly, I discovered that the gasket for the intake port was impeding said port slightly, I took a spare which would also have done this, but made the hole larger, so that it was the same shape and size as the port on the cylinder, whilst this didn't gain any more speed, it
did make the engine kick noticeably harder when accelerating and make achieving the top speed much easier to do. It seems that between that and slowly drilling out the baffles slightly, I did at least gain more torque if nothing else.
- Of course, I'm not expert in these matters, so you should fully expect my ramblings to lead you nowhere, especially as my engine is a cheap piece of trash made up of parts from two different versions and running on sub-par gas, who knows how much this has skewed my observations.