90hp Mercury -idles perfectly at 650 rpm, occasionaly idles up to 1200 - Stuck float?

[Imjus4u2nv]

I have a 1986 mercury tower or power, 90 hp inline six.
I have it tuned to idle at around 650 rpm's which is just about perfect from what I can tell.
I will sometimes run the boat for part of the day, and sometimes (doesn't always happen) the rpm at idle goes up to about 1200. I am planning on taking the carb's apart, as I think it may be a cracked/sticking float, but wanted to reach out and see if anyone had other ideas before digging in.
Thanks in advance!

 

[flyingscott]

Make sure the throttle linkage or a carb plate is not binding.

 

[ondarvr]

The floats have nothing to do with it, it doesn't mean they're good, just that they won't be the problem.

 

[Imjus4u2nv]

The floats have nothing to do with it, it doesn't mean they're good, just that they won't be the problem.

I am pretty handy with this outboard, but may not know all the terminology so my apologies in advance.
I will be sure to check the throttle linkage - hard to check until the issue arises (which is usually on the water). What is the best way to do this? wait for issue to happen, then disconnect all the linkage and see if still at 1200rpms?
As far as the carb plate- what exactly is that?
Thanks again fellas!

 

[Imjus4u2nv]

The floats have nothing to do with it, it doesn't mean they're good, just that they won't be the problem.

I guess my logic was that if the float is stuck in the up position it could be giving to much fuel and upping the rpms, but don't know much. Any ideas what it could be beside what flyingscott said.

 

[flyingscott]

Carb plate is the throttle plate inside the carb make sure they all come back to closed. When it happens push the throttle linkage towards the idle screw see if the idle drops.

 

[ondarvr]

I guess my logic was that if the float is stuck in the up position it could be giving to much fuel and upping the rpms, but don't know much. Any ideas what it could be beside what flyingscott said.

Too much fuel will make run rough and idle slower, or die.

 

[Texasmark]

Too much fuel will make run rough and idle slower, or die.

+1. Gotta have more air aka throttle plate (butterfly in carb venturi) opened more with more fuel for the engine to run. about 12% fuel to air mix is what engines run on as I recall reading somewhere.

Explain what you are doing when, or what you were doing before the 1200 rpm idle happened. Did the engine really rev up or did the tach tell you it was running at twice the previous rpms? Did it happen when you were coming off planing speed? Did it happen after you revved the engine above 1200 rpm?

 

[Imjus4u2nv]

+1. Gotta have more air aka throttle plate (butterfly in carb venturi) opened more with more fuel for the engine to run. about 12% fuel to air mix is what engines run on as I recall reading somewhere.

Explain what you are doing when, or what you were doing before the 1200 rpm idle happened. Did the engine really rev up or did the tach tell you it was running at twice the previous rpms? Did it happen when you were coming off planing speed? Did it happen after you revved the engine above 1200 rpm?

That all makers perfect sense. It only happens after I would shut the engine down after it was running fine and when i restarted would be at 1200 rpms (the tach read 1200 and the engine you could tell was at higher rpm's by the sound and by the clunk into gear). So basically always happens when i start the engine after running it for part of the day (I believe it has happened once when the boat was sitting cold on the first start). Generally turning the engine off and restarting after it is a 1200 rpms does not solve the issue, can restart five times in a row at it stays at 1200 rpm's then it just kinda goes away on it own. When a 1200 at idle, if ii run the boat she drives perfectly normal at full throttle just the idle is high so i get the cluck into gear.

Thanks again!

 

[racerone]

Look into adjusting the throttle cable at the motor.---Make sure it pulls the throttle arm against the stop.-----Even preload it a little.

 

[Chris1956]

The throttle should be pushed against the idle stop, when the motor is in neutral. Preload it as was suggested, as it will help you find neutral.

If that doesn't fix it, maybe the idle stabilizer is acting up? Just a guess.

BTW - That idle speed of 650 was in gear right? That is still pretty good for those motors. Normally it is a bit higher, even when tuned right.

 

[Texasmark]

If the remote interconnect linkage isn't sticking as the other guys stated, possibly your spark advance mechanism or linkage is sticking in too far of an advanced condition when you have that idle run away. Either under the flywheel or between the flywheel and the throttle control linkage from the remote inside the engine.

 

[Imjus4u2nv]

Thanks everyone. I tried getting the issue to happen the past few days without any luck and had to winterize the boat yesterday. When I get back at it in the spring I will be sure to post the results in case it helps someone else in the future. Thanks again and happy holidays.

 

[quicktach]

A binding advance?