Ok.. there are a couple things that i can think of. First, about not going over 2500 RPMs. This sounds like either bad ignition and/or bad fuel delivery, change your spark plugs and get your carbs cleaned (this could be many many other things but Im just puttign the simplist possiblilties). But then you mention that once it gets to 2500rpms it backfires and cuts out (does the engine stall at this point or just drop RPMs?) The backfireing i would think is a improper ignition timeing OR maybe your catalytic converter is full. If it is absolutely full it may create enough backpressure to stall and backfire the engine at 2500 rpms but below that engine speed not enough pressure is created. If your cat is full then it will smell like rotten eggs usually. However about the ignition timing, if it is a conventional distributer with points (not an electric distributer) this is a high possibility. The points will wear down and must be filed and re-spaced.
<span style='font-family:Geneva'><span style='font-size:14pt;line-height:100%'><span style='color:red'>Fox</span></span> <span style='color:gray'>the Pirate</span>
Diplomacy is saying "nice doggy" until you find a rock.</span>
I live my life one quarter mile at a time, and for those 23 or more seconds I'm free.