The Magnum 70 four stroke has a bit different carb in that the idle jet screws out to a stop vs in for all others that I've seen. Idle jet setup can affect the high end.
You need to break in the 70 a bit before flying it. The third tank is enough break for flying, but the engine will not be at it's peak for at least a gallon of fuel
The fuel I use in everything is Omega 15%. It has a castor mix and the Magnums manual recomends that the fuel have either full castor or a castor mix. It works so I don't worry about it. I do know that Cool Power 15% did not run well in my Magnum 90.
Set up both needle to the manual's starting point. If memory serves me, it's 2 1/2 turn from closed on the high speed and the idle one is two from full out. Start the engine and let it run a bit with the glow stick still on. It should be running ragged at this inital setting, ans smoking like crazy. Now go full throttle and slowly lean out the top end for max RPM then go back to the rich side by at least 500 rpm, 800 would be bettter. Now set the throttle to as slow as the enging will go and keep running. Don't worry about the speed at this point. Lean the idle very slowly for maximum RPM at this throttle setting. Now go back and do the high end again. Back on one more time on the idle side, this time the rpm's should be a lot slower that what you started with. When get this setup, adjust the throttle for 2400 rpm. Now, remove the glow igniter. Check out top and idle one more time.
Get the engine well warmmed up and run it up full throttle for a cound of 10 or so then go to full idle. Let it idle for a count of 30, it should putt putt along without any detnotation. Now jam the throttle full open. It should jump to full RPM without hesatation. If it stumbles and slowly gains RPM, the idle it to rich, lean it by 1/16 turn and try again until you can jam it and have it jump to life. If the engine just dies when you jam the throttle, the idle is to lean, back it out 1/16 turn at a time untl it jumps to life,
This should give you a good setup. My 70, cranking a 12-6 is good for almost 11,000 on top and will putt along at 2200 for a long time. It is mounted on it's side.
If you can get the engine running right with the above setup, you may have a fuel tank/line issue. Check for bubbles in the line, not good. Also make sure the tank centerline is level with the spray bar. If it is to high or to low, it can give you fits.
If all these things don't resolve the problem, I would pack it up and send it back to Hobby People for a check out. They are good people to work with and all it will cost you is the postage. Don't mess with valve lash, it is set right from the factory and if it is wrong, let Hobby People fix it.
Last, when you get the engine running right, don't mess with the needles until the engine starts runnig bad. We have a couple guys at the field whose every day ritual is to burn a tank of fuel screwing with the needles and they still end up dead stick. Get the set and the wait for the engine check lite
to come on. You will probalbly reach this first point about 1/2 gallon into the break in. Maybe once more at the end of the first gallon. Then maybe once every case of fuel you will have to tweek the adjustments. After the firs couple gallons, you can experment with different fuels, by then you will know how your engine runs and can really tell what the different fuel is doing for or to you
Don