Firstly this up and down reving is a sign of the engine running [ too lean ] so don,t run the engine until you have checked and done the following; is the fuel tank vent pipes and main feed clunk set up correctly ? check to make sure there are no air leaks in the fuel tank stopper cap as well as the fuel lines where they meet the fuel tank, tip [ block off all vent lines and blow into one only line to see if the tank builds up in pressure ] if you have only 2 lines which is 1 for carby and 1 for pipe pressure make sure you re attach the fuel line to the muffler pipe before starting and if you have 3 lines make sure the third line is also blocked off when starting and running. if all is fine with the above its time to look at the Carby, disconnect both lines from the remote needle valve to fuel tank and remote needle to carby then take the needle valve out completely, with some spare fuel tube connect this to the remote needle unit and blow through to make sure any crap is dislodged also do this to where the needle valve goes. now open the carby to wide open position and connect the spare fuel; line to the carby and blow through as before, if all seems clear re-connect all fuel lines and re fit the remote needle valve by screwing it all the way in [ but not tight ] then back out this needle to 2 & 1/2 turns [ not 2 turns ] now its time to re start the engine; fill up the fuel tank [ with good fresh fuel ] open the throttle to full with radio on and flick the prop over a couple of times only whilst having a finger covering the carby vent, now bring the radio stick back so the carby barrel is only open about 1/16 to 1/8 th of an inch max. now fit the glow plug starter and start up the engine, let the engine warm for about 45 seconds then open up the throttle to full and go from there with the recommended run in specs that you have in the O.S. booklet. i am sure you won,t have any problems with this fantastic engine, take your time.