Hello, I have a 1990 route 66 250cc California model with less than 5K on the clock. The machine runs fine, with good throttle response through the gears to 45mph..acts like you think it should...however, the machine has difficulty reaching beyond 52mph, and cannot maintain this speed on a gentle rise. Several variables...In Oregon, we are fed 10% ethanol, and I have changed the rear sprocket to a 37 tooth. When running the original 45 tooth sprocket, it would, as one would expect, achieve a slightly higher top speed, but seemed to be over-revving at 55mph, and would still lose rpm on a slight rise. The Yamaha shop suggested opening up the main jet (probably a good idea with the ethanol), but could not give me a wire size or dimension. If opening the main jet will help, what would be the recommended diameter? Also, the bike is getting only about 60mpg....As this is the first small bike that I have ridden after almost 50 years of larger road bikes, maybe I am expecting too much...if anyone has any experience on the 250cc Virago, and the performance is similar, I would appreciate this information, also. Thank you
You can get to 55 with and old Honda 90cc. Your 250cc should do 80 with no problem. Your gearing is backwards. The larger the rear sprocket, the slower you go. You have tried to compensate a loss of engine power with a gear reduction demanding more from the what little power the engine is putting out. Put the 45T sprocket back on and do some work on the cylinder head. Ethanol is famous for corrosion and your valves are proof of it. All your power and gas mileage are going out the exhaust pipe. Pull the cylinder head and re-seat the valves. This should get back the power that is missing. You didn't mention any smoking so the piston rings must still be good. The stock main jet is #110. The higher the number , the richer the mix. Go to a #112 or #115 if you need to but only after the valve job. Right now the main jet is not the problem and changing it will only mean even worse mpg.
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http://www.motorcyclecarbs.com/carbs101.... Go to this site and you can get the basics. Google "carburetor jet sizing" fo more info. I have no idea what the measurements are in thousandths. I believe the standard is in "drill sizes" so that everyone just goes by the numeric size i.e. #110 or #145. The gear progression is fixed, you can't change it. All that the smaller sprocket is doing is forcing you to wait while the engine slowly builds revs. Let me know how it runs after the valves are done.
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Thanks for your help. I wasn't trying to increase top end performance with the sprocket change, just the 1-4th gear progression.... Will do a compression check and then address the valves if required. Thanks again. P.S. Do you have the orifice dimensions for the 110, 112, and 115? I would appreciate the info.
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