Monday, April 12, 2010

restoring Yamaha RD250

I am restoring a 1973 Yamaha RD250. This is a twin-everything bike; twin cylinders, twin carburators, twin tail pipes, etc. I love Japanese motorcycles, food, etc., but perhaps Nipon manufacturing is not so perfect after all. The bike has original 1973 Mikuni carburators, model VM28 (with throttle round valves at 30 mm; yes they're not VM30 carbs, they are VM28's with 30mm valves; strange but that's what it is).

The funny thing is that the beast gave me a lot of trouble to tune in the bottom 1/8 of the throttle span. It's great to have twin parts so you can compare! Like if your left ankle hurts--may be you fell from the bike--the first thing to do is to compare it with your right ankle to see if one is swollen. Well, I compared the carbs and there you have it. Mikuni forgot to drill the bypass hole in one of the carbs. The bypass hole is a tiny hole that bypasses the throttle valve when the valve is shut, thus feeding fuel from the pilot hole right ahead of the valve to the intake right behind the valve. The hole is so tiny that can't be made with the drill bits I have. I sort of solved the problem by adjusting the fuel level 2 mm up in the float chamber, to get a bit richer fuel mix out of the pilot hole. The fuel goes under the almost-closed valve and seems to work. Still I have less than hoped for torque when starting from a standstill (i.e, traffic light). At least it does not accelerate out of control like a Prius :)

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