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my 1994 250 only gets 2.35 miles per gallon. it only hits on a couple of cylinders at idle causing a very rough idle. The motor smooths out at speed and will turn 5700 rpm at top speed of about 47. I have been told that yamaha produced an engine that on idle fired 2 then 4 then 6 cylinders for fuel economy and shifting ease. this makes sense for an injected motor but not for a carb model. Does anyone know if this is true? Also i am told that the fuel pumps(?) can some how pump fuel into the exhaust without passing through the intake whihc would cause excessive fuel consumption. Is this possible? when the motor is running flat out it stumbles as though starving for fuel.
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Missing at idle is most likely dirty carbs. Start with cleaning them and see if it improves. Be sure to set everything to the factory settings, including adjusting the carb linkage per the manual afterwords. On my motor various people had tweked things here and there trying to fix what was ultimately an ignition problem so they were all adjusted "off". Ran much better when I set everything back.
I've gone through a bunch of effort tracking the mystery symptoms on my twin 250's. The hardest one to find was the plug caps. They contain resistors to supress ignition noise and I was told (by a retired Yamaha mechanic) are a common failure. To check them unscrew them from the end of the coil wires and measure the resistance. New ones are about 5000 ohms. I had some showing 100 to 1000 times that and the motor still ran, but poorly. I would never have found them if a he hadn't told me to check them.
Next on the hard to track list was fuel system vacuum. "T" in a vacuum gauge just ahead of the motor mounted water seperator and measure the vacuum in the fuel inlet line at full throttle under load. I was told it should be less than 5". Mine was right at 5". I did some fuel system re-rigging and was able to drop it to 3" and the motors did run better. Also look for foaming/bubbles in the fuel coming into the seperator. I had thought that was normal (boat did it since I got it), but after I replumbed the fuel system they went away (the mechanic had also told me to look for any bubbles as they are also a problem).
Finally, if you run it under load with the intake silencers off you can actually see when the main jets start flowing fuel and the high speed system kicks in. On my motors even after having cleaned the carbs once some were not. A re-cleaning of those carbs and my motors went from topping out at 5000 RPM to pulling to 6000 RPM at the same trim setting.
-Greg -
the carbs have been rebuilt as the motor was . my machanic not a yamaha dealer told me that the engine was designed to run on only 2 cylinders at idle i find this hard to believe. do your motors run on all cylinders at idle? thanks for the tip about the plug wires. this i can check ... carsonComment
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Mine are 1993 and yes they run on all cyls at idle. They use a timing retard at idle to slow the idle down, timing is actually about 10* "after" TDC.
-GregComment
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If you think it is getting too much fuel, make sure the little emergency bypass valve in the fuel lines is closed (if not it will pump fuel directly to the intake behind the carbs).
-GregComment
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