newbie has engine problem

[motorjake]

hi all, iam very new to this, ive just bought a brand new 70cc motorized bike kit off ebay,when i first started it up it wouldnt idle for more than a second, i changed the carby over from the standard NT carb to my spare speed carb, then it was idleing and seemed to be running smoothly. i took it out after that and it seemed to have plenty of power but after it was running for a good 10 mins it started to have no power at all and was making a loud earpeircing ping noice when i open the throttle under load. to me it seemed like it had overheated so i shut it off with the kill switch but it did nothing the bike still ran, then i unplugged the CDI from the magneto, it still ran! i shut it off by letting the clutch out slowly. ive read heaps of other forums and most of them said the 16:1 ratio is too much oil, seeing as oil holds heat i thought by changing the ratio to 25:1 it will fix the problem. i rode it today but its still the same, i was thinking maybe an air leak but i thought id get a second opinion. help would be appreciated! cheers!

 

[Ludwig II]

That's pre ignition, where something hot inside the engine acts as a glowplug, like a model plane engine. I suspect you'll need a new piston and rings at least, once you take the head off.

If you take the barrel off as well, lift it slowly, and as you do so, stuff a rag or cloth or something into the crankcase mouth to prevent any debris falling back into it.

Hopefully it's only carbon build up and you stopped the engine quickly enough for scraping it out (only use plastic scraper, don't use metal and damage the engine) and replacement of the barrel and head on the engine. Do NOT reuse old gaskets if you can avoid it, it's best and safest (cheapest) to use fresh ones.

 

[motorjake]

Thanks for the info, it all makes sense, I'm hoping I will get a replacement engine from the seller