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Author Topic:   90 Yamaha Rough Idle

jtms posted 06-16-2014 12:29 PM ET (US)
My 90 Yamaha 2 stroke is running very rich and fouling plugs which is causing poor idling and operation while in gear. When in idle the engine constantly cuts off. Once on plane the engine runs fine and only runs poorly at idle or in gear.
When I removed the spark plugs (I've changed plugs twice) they had a good amount of black oil on them. I changed the fuel pump thinking that I might have a bad fuel diaphragm and this did not resolve my issue. Also, I put a new thermostat on the engine last year. I removed the carb cover and manually squeezed the primer bulb to see if fuel was entering the carb throat (it was not) because a valve or jet was clogged in the open position.
I was thinking that I needed a carb rebuild, but I am starting the think that something else might be the culprit. What do you think?
jtms posted 06-16-2014 12:31 PM ET (US)
Prime Start System acting up, perhaps?
contender posted 06-18-2014 05:10 PM ET (US)
I would redo the carbs, sounds like you have a clog, The fuel sitting in the carbs the eathnol makes a varnish and ends up plugging your jets. And for some reason I have seen this problem more with the smaller Yamahas. The thing to do is everytime you return home from a day of boating, when you flush your engine diconnect the fuel line at the engine and let the engine run out of fuel, dries out the carbs and gets all the fuel out
GRAND NUSSIE posted 06-18-2014 08:39 PM ET (US)
The linkage on the oil pump on my 90 Yamaha became disconnected thus giving the engine maximum oil at idle. However, this only caused extra smoke and did not cause stalling or fouling of plugs. You might want to check the linkage though. Speaking of plugs, my engine is an '86 and is on it's second set of plugs.....and I'm not sure why I bothered to replace the first set.
Dick
jtms posted 06-20-2014 08:04 AM ET (US)
I checked the oil linkage and everything is in order. I'm starting to think it is the carbs because I can't think of what else it could be. When I look down the throat of the carbs they look clean and the jets appear to be working properly when I increase the throttle.
tedious posted 06-20-2014 12:58 PM ET (US)
Having plugged jets would make it run lean, not rich.
Good call to check for a stuck float, needle valve, or leak in the primer circuit by squeezing the bulb - you know those are not the problem.
It could still be the prime start system though. Once you've got it started and warmed up, turn the motor off, then remove the electrical connection to the primer valve, start up again, and see if the problem is fixed. That way you rule out the primer circuit being engaged while the motor is running.
Is there an adjustable idle mixture on this motor? I am familiar with OMCs, and with them, it's actually an adjustable idle air bleed. If that was plugged or misadjusted, that would result in a rich condition.
Tim

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Author	Topic: 90 Yamaha Rough Idle
jtms	posted 06-16-2014 12:29 PM ET (US) My 90 Yamaha 2 stroke is running very rich and fouling plugs which is causing poor idling and operation while in gear. When in idle the engine constantly cuts off. Once on plane the engine runs fine and only runs poorly at idle or in gear. When I removed the spark plugs (I've changed plugs twice) they had a good amount of black oil on them. I changed the fuel pump thinking that I might have a bad fuel diaphragm and this did not resolve my issue. Also, I put a new thermostat on the engine last year. I removed the carb cover and manually squeezed the primer bulb to see if fuel was entering the carb throat (it was not) because a valve or jet was clogged in the open position. I was thinking that I needed a carb rebuild, but I am starting the think that something else might be the culprit. What do you think?
jtms	posted 06-16-2014 12:31 PM ET (US) Prime Start System acting up, perhaps?
contender	posted 06-18-2014 05:10 PM ET (US) I would redo the carbs, sounds like you have a clog, The fuel sitting in the carbs the eathnol makes a varnish and ends up plugging your jets. And for some reason I have seen this problem more with the smaller Yamahas. The thing to do is everytime you return home from a day of boating, when you flush your engine diconnect the fuel line at the engine and let the engine run out of fuel, dries out the carbs and gets all the fuel out
GRAND NUSSIE	posted 06-18-2014 08:39 PM ET (US) The linkage on the oil pump on my 90 Yamaha became disconnected thus giving the engine maximum oil at idle. However, this only caused extra smoke and did not cause stalling or fouling of plugs. You might want to check the linkage though. Speaking of plugs, my engine is an '86 and is on it's second set of plugs.....and I'm not sure why I bothered to replace the first set. Dick
jtms	posted 06-20-2014 08:04 AM ET (US) I checked the oil linkage and everything is in order. I'm starting to think it is the carbs because I can't think of what else it could be. When I look down the throat of the carbs they look clean and the jets appear to be working properly when I increase the throttle.
tedious	posted 06-20-2014 12:58 PM ET (US) Having plugged jets would make it run lean, not rich. Good call to check for a stuck float, needle valve, or leak in the primer circuit by squeezing the bulb - you know those are not the problem. It could still be the prime start system though. Once you've got it started and warmed up, turn the motor off, then remove the electrical connection to the primer valve, start up again, and see if the problem is fixed. That way you rule out the primer circuit being engaged while the motor is running. Is there an adjustable idle mixture on this motor? I am familiar with OMCs, and with them, it's actually an adjustable idle air bleed. If that was plugged or misadjusted, that would result in a rich condition. Tim