not always a bad thing
this is not always a bad thing.. WARNING.. this is gonna be quite a dissertation.
basically the compression of the ENGINE (it is NOT a motor damnit) has been lowered ever so slightly so that it is becoming hard to tune in.. if what I'm thinking is correct then you will only get it to run if you richen it up to the point where she nears hydrolock and then she will start and run very boggy and then quit pretty easily/won't idle and so on..
Now I just wanted to throw this out there (i know this is a pretty old post and nobody will probably really care)
In a case such as this, believe it or not you're engine WANTS to run VERY badly.. I have had MANY engines do this exact same thing.
My solution was thus..
A) Take engine completely apart and make sure everything is VERy clean. (crank, bearings, rod, piston, and MOST importantly.. the carb.
B) Dissassemble and clean every bit of the carb.
C) Take a dremel and lightly turbo port the crank (little swoosh at the piston of the crank) The turbo port should curve with the bell of the curve towards the rotation of the engine. Using and end mill type dremel bit (or any multi-fluted metal shaving bit.. work away a small amount of material and make sure the edges are smooth. Then come in with a polishing compound and polishing bit and just touch up your work.
D) Using the same bits and technique, very lightly smooth the outer edges of the intake ports, the vacuum ports (on the sides) and DEFINITELY port match your header if possible. On the intake port take away just a SMALL tad bit more material on the ends of the lower edge. (Matching vallery ports here help atomize fuel further and create a more efficient burn as the mixture is slightly more turbulent entering the cylinder.
E) Lastly mix a small amount of liquid overhaul (from Pep-boys etc.) into your fuel and/or after-run oil.. also use this to lubricate everything heavily when putting the engine back together.
If these techniques are done correctly the same engine that seemed as though it was on it's last legs can be COMPLETELY refreshed and feeling better than new. I have done this with notoriously bad engines. (a trx .15 that literally felt like having NO compression and would ALWAYS overheat immediately if it ever started turned into a full on killer that ran more consistently than ever and had top end that put my stock O.S. .18 CV-R to shame.
The only reason this works is because alot of the time with these engines they are just trying to burn too much fuel with not enough compression. When you speed up the intake charge and allow for a thicker mixture the engines compression does not need to be as high (these are the same basic principles used in a turbo setup) Of course, sometimes changing to a hotter plug can have the same effect and when done with the porting and polishing above can work wonders as well.. but sadly a hotter plug alone doesn't always help because you are not getting enough fuel AND air fast enough to keep it burning all the way to TDC.
IN SHORT.. DON'T GIVE UP ON YOUR ENGINES SO QUICKLY.. I SEE PEOPLE CONSTANTLY PAYING 200.00 - 800.00 FOR THEIR R/C ENGINES ONLY TO THROW THEM AWAY AS SOON AS THEY START TO FALTER ONE BIT..
This is one of the reasons why the industry continues to expand so fast.. and while that is not really that bad a thing.. in some cases.. the prices are going right along with them...and really there is no need to produce and then throw away so many great engines..
What I wouldn't give to have even a fraction of the engines the people on this forum have thrown away in the last two months.. I'd be set for the next few years..