Motor rebuilt 2 years ago and hardly used. I have gone through the carbs 2 times in the last year, everything clean and not sticking. I have installed a new fuel pump. Symptoms are very rough idle with a hard miss, sometimes hard enough to make it die. Also, while trying to plane off once given full throttle, it will sometimes bog around like it has no power. Cylinder pressures are good and withing spec. I am convinced this is an electical issue. I have a manual and attempted the pulsar, lighting and primary/secondary coil resistance tests. I am unsure of the readings I got with my multimeter (maybe i'm not using it correctly?). Which of these electronics is the most likely to cause my symptoms. Almost forgot: 1989 Pro50LF - I have removed the oil injection.Thanks for any help guys,Ryan
I am convinced its fuel, but its easy to test use a peak reading type meter and check coil primary input voltage on all 3 at the miss range. my bet is a fuel restriction at the cowl or tank, bad fuel pump or the carbs still have issues. that miss that kills the engine, is it a big cough when it stalls almost like a backfire ?
Hi rodbolt, thanks for the reply. I will try the coil test as you mention once the rain clears up around here. The miss that kills the engine sounds more like a quick spit rather than a cough. I should also mention that the engine is capable of WOT without running out of fuel, it just isn't very smooth getting there.
on a yamaha carb all the carb circuits are involved at all speeds, sounds like the idle and itermedeate circuits are partially plugged or the rubber idle fuel caps inside the fuel bowls may be missing. the air/fuel metering screws on that carb do not alter the ratio they just control the amount of air and fuel entering at idle. the ratio is determined by the pilot jet and the idle fuel jet. the spit normally indicates its lean, if the fuel pump is weak it may not keep the fuel bowls full at low speeds but can at higher RPM if you can get it there.
The last time I had the carbs apart I distinctly remember all 3 rubber caps going back into place after I cleaned the gadgets underneathe them. I called a local mechanic who has helped me in the past. This guy seems to help me find the problems then I fix them, which is good for both of us. He pretty much confirmed what you said rodbolt, about the lean mixture. Since he has all the facts of what I've done to the motor recently, he suggested I look for a vacuum leak and check reed valves. I pulled the carbs and manifold to find the reed valves "look" correct and sit flat. I have not yet checked them with a feeler gauge. I did notice the gasket that seals the assembly to the crankcase was wet with fuel in between the first and second reed assembly. In addition, the gasket looked to be slightly bowing out from the manifold. I'm guessing the fuel on the gasket in this area is a possible vacuum leak? Right now my plan of action is to replace this gasket and see if the problem persists. If it does I suppose I'm back to disassembling the carbs for the 50th time and possibly looking at the brand new OEM yamaha fuel pump I installed and used once so far.
Update after gasket fix: yesterday I noticed that when I pulled the plug off of cylinder 1 the miss would quit. Today I pulled the #1 carb and thoroughly blew it out with compressed air. Reinstalled - same miss. Pull the plug and miss still goes away. I then swapped coil 1 and 2. I let the engine run and waited for the miss to start then pulled plug 1, and as expected the miss was still there. I plugged back in #1 and pulled #2 and as UNEXPECTED the miss was still there.
make sure the CDI primary wire are attached to the correct coil. mark TDC on the flywheel for cylinders 2 and 3 with a magic marker, if they are not already marked. use a timing light and look for crossfire. whatever plug wire you have the pickup lead on is the only number you should see. the TDC mark for 2 will be 2 and 3 will be 3. ign issues with that motor are rare but do occur, fuel issues with all motors are common.
Even though, the fuel problem would be the culprit in the majority of cases with these symptoms, the three cylinder CDI's have been the least reliable of the normally bullet proof Yamaha electronics and do fail. Use of a timing light as rodbolt suggested may give you the answer. Be syre the motor has warmed up good since I have seen them be intermitent.
Yamaha 50 very rough idle & bogs
Linear Mode
