I'll try to keep this short but I also want to give as much info as I can. I bought a 1996 Yamaha 9.9 MSHU 2 stroke that had been sitting in storage for over ten years. Before I put it on the boat I did the normal maintenance, rebuilt the carb/ fuel pump, changed the water pump and gear oil, cleaned the fuel filter and lubed the grease fittings. I had a hard time starting it the first time and when it finally did fire off, it had a terrible sneeze/ spit at idle and would die without some throttle. I ran a can of engine tuner through the airbox, spraying until the engine bogged down and then letting it soak for 15 mins. Did this several times and it helped a little. I adjusted the pilot screw and throttle stop, this made the sneezing subside some but brought on an erratic idle, it would vary between 800 to 1200 r's, never staying stable. It also jumps around a little bit, not unusual for a 2 stroke but more than you would expect. I ran the motor like this all last summer/fall because at mid and full throttle it has no issues and I hardly ever idle or troll. At WOT it is an absolute screamer with excellent throttle response and it jumps the boat on plane as soon as you open it up.
So last week I pulled it off and put it in my shop. Recleaned the carb, pulled the reeds and inspected them, replaced the fuel tank hose, cleaned out the fuel lines, pulled the cylinder head and removed the carbon buildup from it and the piston crowns (quite a bit of build up.) Replaced the thermostat, cleaned, as much as I could, the water passages (it had been run in brackish water.) The plugs were almost fouled wet, so I went to the next higher heat range plug. Adjusted the timing and all the controls. To get the timing on the flywheel marks I had to use up every bit of the adjustment length of the magneto control lever. I don't know if this could be a part of the problem because it is still in range of the adjustment but I wonder if the flywheel may have jumped a notch or something like that?
I put it in the test tank, it fired right up and the sneeze/ spit were gone. But the erratic idle was still there, running between 800 and 1200. I played with the pilot/ throttle screws, no change. Sprayed all the gaskets with WD-40 and then soap/ water, saw nothing (an air leak would be a constant lean condition, would it not?) Top plug slightly wet/ oily, bottom plug clean and dry.
While the engine was running, I noticed that the exhaust wasn't blowing out the hub but instead coming out the relief holes, I tilted the motor up to inspect this and when I did it revved up on it's own to about 1500 r's and the exhaust started coming out the hub. When I tilted it back down, the idle smoothed out to 850 and held for about 3 minutes before it started to hunt again. This made me think that something in the carb is building up pressure and then spraying gas when it gets to a certain point.
Cleaned the carb one more time, paying special attention to the idle passages, put the motor back on the boat, much better. Screamed across the lake, put it in neutral, let it idle, nice and smooth at 850 but after 3 or so minutes it would start to rev up a little before going back down, less severe hunting as before. Also figured out that if you rev it up in neutral it seems to "clear out" the condition and you get another 3 minutes of stable idle before it revs up. It still jumps/ shakes at idle but not much more than you would expect from a 2 stroke.
So, after all that, my question is - what could "load up" and release fuel after 3 minutes? Is the float bowl fuel leaking up into the idle passages? What about unburned scavenge gasses recirculating into the intake and the higher revs burning them off, returning the fuel/air back to normal? I don't think the CDI has anything to do with it, it doesn't have a "start/ advance circuit," does it?
So last week I pulled it off and put it in my shop. Recleaned the carb, pulled the reeds and inspected them, replaced the fuel tank hose, cleaned out the fuel lines, pulled the cylinder head and removed the carbon buildup from it and the piston crowns (quite a bit of build up.) Replaced the thermostat, cleaned, as much as I could, the water passages (it had been run in brackish water.) The plugs were almost fouled wet, so I went to the next higher heat range plug. Adjusted the timing and all the controls. To get the timing on the flywheel marks I had to use up every bit of the adjustment length of the magneto control lever. I don't know if this could be a part of the problem because it is still in range of the adjustment but I wonder if the flywheel may have jumped a notch or something like that?
I put it in the test tank, it fired right up and the sneeze/ spit were gone. But the erratic idle was still there, running between 800 and 1200. I played with the pilot/ throttle screws, no change. Sprayed all the gaskets with WD-40 and then soap/ water, saw nothing (an air leak would be a constant lean condition, would it not?) Top plug slightly wet/ oily, bottom plug clean and dry.
While the engine was running, I noticed that the exhaust wasn't blowing out the hub but instead coming out the relief holes, I tilted the motor up to inspect this and when I did it revved up on it's own to about 1500 r's and the exhaust started coming out the hub. When I tilted it back down, the idle smoothed out to 850 and held for about 3 minutes before it started to hunt again. This made me think that something in the carb is building up pressure and then spraying gas when it gets to a certain point.
Cleaned the carb one more time, paying special attention to the idle passages, put the motor back on the boat, much better. Screamed across the lake, put it in neutral, let it idle, nice and smooth at 850 but after 3 or so minutes it would start to rev up a little before going back down, less severe hunting as before. Also figured out that if you rev it up in neutral it seems to "clear out" the condition and you get another 3 minutes of stable idle before it revs up. It still jumps/ shakes at idle but not much more than you would expect from a 2 stroke.
So, after all that, my question is - what could "load up" and release fuel after 3 minutes? Is the float bowl fuel leaking up into the idle passages? What about unburned scavenge gasses recirculating into the intake and the higher revs burning them off, returning the fuel/air back to normal? I don't think the CDI has anything to do with it, it doesn't have a "start/ advance circuit," does it?