Gentlemen:
I have a two-stroke question that isn't exactly chainsaw related. I hate to clutter the forum but I don't know a where a more knowledgeable and practical bunch of 2-stroke gearheads hangs out. If I'm out of order I simply ask that the mods to move/delete the thread. Thanks in advance.
The 2-stroke devil in question is a 1996 4hp Yamaha outboard. It runs great at higher speeds (once the high speed jet kicks in) but struggles at low speed. Carb is a very typical Japanese made 2-stroke carb with the idle-air low speed jet circuit on the right side of the carb. Because this carb operates vertically it DOES have a float/bowl at the bottom. Carb is clean as a whistle. Every passageway, main jet, pilot jet, needle valve, air mixture adjusting screw, etc--including the two small holes behind the welch plug--seems in good repair and clear of obstruction. Welch plug is sealed and doesn't leak. Float is adjusted correctly (22mm to the top of the float makes it level) and the diaphragm pump has no trouble keeping the bowl full. Air mixture screw opened to factory spec (1-3/4 turn out, +/- 1/4 turn).
Symptoms:
It will run at low speed but if you try to give it throttle all at once it usually bogs down--sometimes to the point that it stalls. You have to "nurse" it through the transition from the low speed to the high speed jet. To me this generally indicates a lean condition in the low idle circuit. However, adjusting the low speed air mixture screw (to improve throttle response by enriching the low speed fuel circuit) has no effect on the problem. I can open and close the air mixture screw until the motor dies w/o having any positive effect on the throttle response.
Usually starts easily. But can be hard to restart if you let it run out of gas. [Hot from being lean?]
I have not checked the compression, but the engine doesn't look like it has enough hours on it to have poor compression--at least not from normal wear. I'm in process of borrowing a compression gauge from my father, but I'm fairly certain it will confirm good compression. Engine runs very strong on the high speed jet.
I may be getting a little bit of fuel puddling between the carb and the reed valve assembly. It's not much and it may be normal for this engine. The single piston operates in a horizontal plane. In fact, the spark plug is at the "back" of the motor and the carb is at the front. If you remove the carb and reed valves (which are in perfect condition) you are looking at the crank/bottom con rod. This makes me think there is a crankcase leak which is affecting velocity through the carb. Seems to me that would be ONE BIG HONKIN' AIRLEAK though. But at low speeds in a engine this small, perhaps not. I've considered buying a new carb but I'm not convinced there is anything wrong with this one.
I'm at the point where the only thing left that I can think to do is check the crankcase for leaks with a vacuum/pressure tester. Easier said than done since it requires removing the power head from the lower unit to plug off the exhaust port with some sort of plate. Plugging off the crankcase at the carb is easy enough but the exhaust port will be a bit harder. Any ideas from the knowledgeable fellows here would be appreciated. I can post pictures if it will help.
I have a two-stroke question that isn't exactly chainsaw related. I hate to clutter the forum but I don't know a where a more knowledgeable and practical bunch of 2-stroke gearheads hangs out. If I'm out of order I simply ask that the mods to move/delete the thread. Thanks in advance.
The 2-stroke devil in question is a 1996 4hp Yamaha outboard. It runs great at higher speeds (once the high speed jet kicks in) but struggles at low speed. Carb is a very typical Japanese made 2-stroke carb with the idle-air low speed jet circuit on the right side of the carb. Because this carb operates vertically it DOES have a float/bowl at the bottom. Carb is clean as a whistle. Every passageway, main jet, pilot jet, needle valve, air mixture adjusting screw, etc--including the two small holes behind the welch plug--seems in good repair and clear of obstruction. Welch plug is sealed and doesn't leak. Float is adjusted correctly (22mm to the top of the float makes it level) and the diaphragm pump has no trouble keeping the bowl full. Air mixture screw opened to factory spec (1-3/4 turn out, +/- 1/4 turn).
Symptoms:
It will run at low speed but if you try to give it throttle all at once it usually bogs down--sometimes to the point that it stalls. You have to "nurse" it through the transition from the low speed to the high speed jet. To me this generally indicates a lean condition in the low idle circuit. However, adjusting the low speed air mixture screw (to improve throttle response by enriching the low speed fuel circuit) has no effect on the problem. I can open and close the air mixture screw until the motor dies w/o having any positive effect on the throttle response.
Usually starts easily. But can be hard to restart if you let it run out of gas. [Hot from being lean?]
I have not checked the compression, but the engine doesn't look like it has enough hours on it to have poor compression--at least not from normal wear. I'm in process of borrowing a compression gauge from my father, but I'm fairly certain it will confirm good compression. Engine runs very strong on the high speed jet.
I may be getting a little bit of fuel puddling between the carb and the reed valve assembly. It's not much and it may be normal for this engine. The single piston operates in a horizontal plane. In fact, the spark plug is at the "back" of the motor and the carb is at the front. If you remove the carb and reed valves (which are in perfect condition) you are looking at the crank/bottom con rod. This makes me think there is a crankcase leak which is affecting velocity through the carb. Seems to me that would be ONE BIG HONKIN' AIRLEAK though. But at low speeds in a engine this small, perhaps not. I've considered buying a new carb but I'm not convinced there is anything wrong with this one.
I'm at the point where the only thing left that I can think to do is check the crankcase for leaks with a vacuum/pressure tester. Easier said than done since it requires removing the power head from the lower unit to plug off the exhaust port with some sort of plate. Plugging off the crankcase at the carb is easy enough but the exhaust port will be a bit harder. Any ideas from the knowledgeable fellows here would be appreciated. I can post pictures if it will help.
Last edited:
