Motorcycle Repair: Power Loss on Yamaha 74, yamaha rd125, blown fuse

Question
Chris,
I recently purchased a 1974 Yamaha RD125 as my first and learner bike. The bike has only 4000 miles on it, but had been sitting in storage for possibly 10 years or more. The guy I bought it from took it to the shop before I came to pick it up and had general maintenance done on it. After getting comfortable riding the little bike a fuse blew out, which scared me because the bike wouldn't turn on at all after the first day. Naturally I felt I had been ripped off, but upon inspection I found the blown fuse, replaced it and got the bike started again, running fine. Once I got out onto the road, and into higher speeds, I found that there is extreme power loss on the bike once I get into higher rpms in first gear and on. The bike will start accelerating perfectly, but then just slows down significantly after a couple of seconds, until I shift into 2nd gear, either the same thing happens, or it just will not accelerate much at all. On flat roads it takes an extremely long time to get to even 25-30 mph. Since I am completely new to motorcycles, I can't determine if something is wrong with the engine, fuel or exhaust, with the way I'm riding it (no one has showed me how to ride properly, although I feel that I know what i'm doing enough to get it past 25 mph) or if it's just to be expected on a bike with such a small engine. Anyone's help would be greatly appreciated as I do not have a truck or the money to bring the bike to a mechanic yet, and have the desire and ability to learn about these things they call motorcycles. It also dies after idling for more than 7 or 8 seconds... bad traffic problem.
The bike has a brand new battery and spark plugs. Thank you so much. -McClain

Answer
Hi McClain.
 Call the guy that you got the bike from and find out if the general maintainence included pulling and properly cleaning the carb, valve gap and timing.  It may have just been a mechanical once-over to make sure that the bike runs and nothing more.

Next get the repair manual.  It has specs, wiring diagrams, exploded views and troubleshooting proceedures as well as a listing of any specialized tools that you may need to eventually get.

 What you have sounds like a clogged fuel system and possible need of valve timing and adjustment and/or a hot ground in the ignition system wiring or bad ignition coil.

 Since the bike sat for so long, the fuel in the system evaporated and left what we call "varnish" in the fuel system.  The effects of this varnish can be anything from loss of power to not running at all.  You will need a carb cleaning and the fuel tank will need to be drained and resealed, then replace the old fuel that you had with fresh fuel or you will re-contaminate the system.

Check that and see what you find.
Good luck.
FALCON