Hi.im having a few stalling problems with my 1/10 truck.the truck had previously run great,then i had broken my muffler assembly so instead of buying a replacement i decided to make a cool looking shiny aluminium one at work(im a cnc engineer).The item i made is similar to the original except its 10mm wider 14mm longer and the tail pipe is +5mm dia, the manifold is same.At the same time i also instaled a 2speed gearbox.my problem is that i struggle to get even more than a few feet on grass before it suddenly dies,its not bad on flatter concrete(if a little slow on pickup)the problem is more noticable at sudden full throttles and rapid trigger pressings,and as soon as it touches medium grass it struggles then when i try to 'give it some' to keep it going it dies within 2feet.this also happens a little on concrete when its faced with a decent size hill.this is what made me think this may be a 2speed engagement problem,as it used to romp over grass and be able to power up hills easily before the mods when it was a 1speed.However sometimes as i said before at certain throttles it appears to have a bit of lean-bog,im not sure how because ive been from ultra rich to ultra lean with the needle and its the same(single needle carb).No air leaks,New glow plug,good fuel,good weather,plenty of white smoke and within temp range,drive train also pretty much checks out.can anyone offer advice on how to trouble shoot this,i want to know what i should be lookin at first and if this custom muffler is doin the job ok,i dont want to get a new muffler and find out it was the 2speed all along and vive versa.any help on how to find what and how is going on would be greatly appreciated.thanks

First thing I'd do is lose the custom pipe and put a proper tuned pipe back on. By going up in length , width and outlet size you've probably lost everything the pipe is/was suspposed to do for you.
The mufflers are more properly called tuned pipes. They are designed so that a protion of the exhaust wave is reflected back into the combustion chamber. That return pulse is timed (or tuned) to hit the cylinder exhaust port just before it closes off to force any unburnt fuel that has made it through back into the combustion chamber. Altering the pipe alters when that pulse hits and is a very big factor in low, mid and high end performance and response. Many people choose different pipes simply because they want better low end torque or top speed.
I cannot guarantee that a stock or performance pipe will completely solve the problem but from what you've described I think the pipe is the most likely culpret.

I have to agree with DaveG55.