hmmmm. to be so lean that things start melting...it shouldnt be able to run at all. or starting is so hard you would have given up and whinged that it wont start...
you really have to baby the throttle when theyre lean, else they cough and splutter and die. you cant just give them full throttle when taking off.
a blocked jet will do this.
an air leak can behave rather differently, with it only really being an issue at idle. the throttle slide is closed at this point, so maximum "suction" is placed on the seal. and, at idle, the excess air is a large proportion of the total airflow, making what does pass through the carb leaner. it idles high, theres even more suck, even more air, even leaner, even faster, even more suck again, etc... pretty easy to pop the intake off and give the gasket some sealant. and make sure the carb is pushed completely onto the intake. the slots that let it "clamp" sometimes dont quite cover the end of the intake tube...check!
crank seals...make for hard starting, and really erratic mixtures...usually on the lean side. the mess under the magneto cover is a dead giveaway.
if you hit the killswitch does it try to keep running when its hot?
notice any strange "cracking" noises when its under load? runaway detonation. quality! (not really but is amusing)
well, it could be a sign that you got lucky and its got a really hi compression head...
which leads to...
im suspicious about the plug. get an ngkb(6or7)hs. try it. ive found some china plugs go into self destruct mode. the first thing is to get rid of that china plug! (yeah, i also got one thats 10 years old and going strong!)
post a pic. have a look at the electrode. if its brown, its not the mixture being lean, or the plug.
if its all white and blistered looking...we have an issue somewhere. start with the sparkplug and airleaks.
checking the thing with a timing light is simple, but has a few issues. you need to mark the magneto, so you know where TDC is, and you need marks for 5, 10, and 15 degree advance. you need to put these on yourself. so you may be doing some research first to figure out how...
with no definite physical guide for when its triggered, unlike a pitbike CDI or a points ignition, these engines pose some challenges to the uneducated mechanics to figure out when its firing when turned slowly by hand...
ie, you need to have it running to time it. so, it needs the marks...and a timing light.
and then, the biggest question of all is, whats the STANDARD idle advance in degrees anyway? NOONE seems to have ever said what it is ,exactly. how peculiar...
they just say that its wrong wrong wrong.
8 bdtc is a standard norm at idle on most engines... but the exact value depends on...compression ratio, rpm, cylinder design, head design, octane of fuel, what pipe you running, blah blah blah. some are 2. some are 12.
but as a basic guide...if you did time it and found 35 degrees advance at idle... somethings not right.
could be a faulty magneto, badly keyed. that key position is critical. could be a bad CDI.
and finally...dont bother wasting money on super expensive 2stroke oil, just buy whatever you grab. the engines cheap. run it on 50:1 if the oil is useable in 50:1 (100:1 is a lil too low), forget the 16:1 BS thats based on super high quality pig fat or recycled sump oil for lube.
it will also be slightly richer if you use less oil, and should run slightly cooler.
in fact, it might just be the 16:1 causing the issue. cus maybe you miscalculated on the last mix and its way too much oil, and suddenly way too lean?
