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115ETLG carbs

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a_leonard

Contributing Member
Customer complained of stumbli

Customer complained of stumbling and dying on acceleration. He had this same problem in the past and took it to a shop that rebuilt the carbs. He said that it ran fine for some time but the problem was back. I pulled the plugs and right away noticed they were scorched white. Compression was good and even on all four. Ignition tested good at 7/16 gap of the tester. Spark plugs indicated fuel starvation to me so I checked the tank pick-up and anti-siphon valve OK. I replaced the hose and bulb assy and fuel filter. I then disassembled the fuel pump expecting to find stiff diaphragms but they were ok as were the check valves. I went ahead and replaced the pump anyway because the fasteners attaching the check valves were showing signs of corrosion. Fuel pump pressure and flow was then checked and ok. Assuming the carbs had been redone I left them alone. The test run revealed the same problem. Next I removed and completely disassembled the carbs. They were clean and properly set up (float adjustment and all jet sizes per Yamaha) but I went ahead and purged all passages with carb cleaner followed with compressed air. The next test run revealed the same problem spark plugs still scorched white. I next pulled all four main jets and inspected them with an otoscope and found them free of any obstruction (no gum varnish nothing). I drilled all mains .003 oversize and that took care of the problem. Can any one offer an explanation as to what caused this problem?
 
"I accept the vacuum leak theo

"I accept the vacuum leak theory but what troubles me about that is the fact that the engine started and idled perfectly and would start to pass trough the transition phase between the idle and main circuit. But once on the main circuit it would lose power. It has been my experience that vacuum leaks are most noticeable when intake and crankcase vacuum is high (during idle). Also considering that each cylinder is sealed from the other once the fuel/air charge passes through the reed valves (these carbs have a passage allowing pressure or vacuum pulses between #1&2 cyl and between #3&4 cyl in the intake manifold only) I'm left wondering how all four cylinders could be affected. If the leak was between the mating halves of the crankcase wouldn't there be external evidence of leakage? During the intake transfer phase the crankcase pressure is increased, at some power setting, above atmospheric pressure which I would think would cause external leakage. The powerhead and the interior of the cowling is clean and dry.

Yes, I drilled out all four main jets .003 of an inch and the motor is running fine as last reported."
 
"My guess would be at the inta

"My guess would be at the intake manifold. That is the only place in the system that sees only negative pressure. I would look for a crack along the axis between the carbs. But, all of this is just theory.

I know that intake manifolds are high pressure at idle and low pressure at cruise but crack is the only thing I can think of.

BTW when you drilled this out .003 you only opened them by .16% (That's point one six of one percnet, not 16-precent). Or about the exact same amount of Co2 in our atomosphere. In other words, it really doesn't seem like enough of change to affect change this makes me wonder if tearing the carbs apart also didn't fix somekind of hidden problem."
 
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