Hi,
Not a total expert but I have come along way to understanding the 18RG beast through reading the following thread;
http://www.toymods.org.au/forums/showthread.php?t=222
Based on the description you have already provided, you will need to check valve clearances and compression to make sure you dont have burnt/leaking valves. If these are in spec, then you will need to check carby synchronisation, ign timing advances (mech and Vac), cam shaft timing.
I am sure there are others on here that can add to this based on their experience.
Good luck
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