mightytunacan Posted November 16, 2015 Share Posted November 16, 2015 Hey y'allI bought an old 79 Yamaha IT 250. It was in running condition (albeit poor) out the back of the tail pipe oil would drip out and the acceleration was gutless. The previous owner suggested maybe it was from an incorrect oil/fuel ratio. No big deal. I got it home and started to go to work on the carb, thinking that it was just an overly rich fuel mixture I looked at the needle jet position... it was adjusted to the next lean setting. So I moved the adjustment to the leanest position and fired it up. the problem persisted, oil coming out of the tailpipe and no balls.When I tried to tune the carb's throttle adjustment, the engine run away from me. I knew that the bike had sat for a year with old fuel in it, so I dismantled the carb and cleaned it up. I returned the fuel mixture to the standard setting, and reassembled the carb after cleaning. I read in the manual that the air screw should been turned out 1 1/2 turns from factory. So, I adjusted the screw to 1 1/2 put new fuel in it with a 32:1 oil fuel ratio and started the bike. The darnedest thing happened.... after the engine idled for about 20 seconds, the RPM's kicked up on it's own and steadily climbed until I killed the engine with the kill switch. I adjusted the air screw back to the original 2 1/4 turns and restarted the bike. The same thing happened. I checked the throttle cable and found that the throttle cable was adjusted all the way out meaning that even with the throttle grip at the resting position there was enough tension on the butterfly valve to be giving the engine fuel. After I adjusted the throttle cable back to make sure there was no tension on the butterfly I started it again. BINGO! The engine idled beautifully... but then it would sputter out and stall. I looked in the tank and realized that the fuel was sitting below the fuel cock. I switched the selector over to reserve and restarted the bike... fuel/oil LITERALLY sprayed out of the tailpipe probably about 2 feet behind the bike, and the engine ran away again. That was the point that I left it at. If have any suggestions on what to change or what component(s) could be failing that would cause a runaway engine, and the throttle having no balls, I would greatly appreciate any help.Points that I have looked at:air screw is adjusted to 1 1/2 turns as per the manualthrottle cable is adjusted to allow about 4mm of throttle turn before actuating the butterfly oil/fuel ratio is 32:1 as per the manualadjusting the throttle screw had no effect on the idle speed, and at numerous positions the engine would still run awayneedle jet setting is at standard positionreeds are in good shape and provide a good seal when in the resting positionspark plugs are wet with fuel after every time the engine runs awaycarb body, valves, jets were cleaned with a varsol solutionair passages were cleaned with compressed brake clean Link to comment Share on other sites More sharing options...
finnerz89 Posted November 16, 2015 Share Posted November 16, 2015 Maybe check the crankseals? Air leaks around the carb/airbox as well? Link to comment Share on other sites More sharing options...
blackhat250 Posted November 16, 2015 Share Posted November 16, 2015 That bikes filled with unburnt fuel [ in the exhaust and crankcase,] it need a good blast ,up the road, 1 Link to comment Share on other sites More sharing options...
mightytunacan Posted November 18, 2015 Author Share Posted November 18, 2015 Thank Finnerz for the suggestion... I have fashioned a compression tester hose with a gauge and female air receiver to see if I have any evident leaks. Will keep you posted on progress. BlackhatI know that when i bought the bike last month, the crankcase was full of fuel. I just flipped the bike upside down with the jug off and drained it. Have you got any suggestions on what could cause that? Bad rings? Or something more? I have new rings to install tomorrow. Think that could be the issue? Thank you both for you reply, this is indeed a puzzling fault for me. Link to comment Share on other sites More sharing options...
blackhat250 Posted November 18, 2015 Share Posted November 18, 2015 I would say repeated attempts to start her, and maybe the float level/valve in carb ,, just get her running then giv her a blast up the road, . new plug will help too , Link to comment Share on other sites More sharing options...
KevtheRev Posted November 19, 2015 Share Posted November 19, 2015 Leaky fuel petcock can cause the crankcase to fill with neat fuel . Link to comment Share on other sites More sharing options...
mightytunacan Posted November 21, 2015 Author Share Posted November 21, 2015 KevtheRevyou know what? That's exactly the problem, back when the float was stuck, I had left the petcock on the "off" position, came back the next evening and there was a puddle of fuel on the floor of my garage. When I pulled the carb to inspect the float, I noticed that the fuel line was leaking at a rate of about a drop every 5 seconds. So, until I can put a new petcock in, I use a rad hose clamp to seal off the fuel line. That makes total sense now. BlackHatOnce I have the bike running properly, rest assured I'm going to giv'er a good run no doubt. AlconI have run a leakdown test on the cylinder with the exhaust port and intake covered in an effort to find a catastrophic air leak, but the cylinder held at 10psi. That leads me back to the carburetor. I'm looking at replacing the jets, valves and main nozzle in an effort to sort this fault. Thank you for your input, and I welcome anything else y'all have to offer. Thanks again Link to comment Share on other sites More sharing options...
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