175 sportjet uncontrollable idle

[Dieseldad72]

Hello, first time on the forum. I have a 2005 mercury 175 sportjet in an Alumaweld boat. I have a particular problem, I have an engine that runs a high idle and climbs upwards on it's own. I need some ideas on what could be wrong with it. Thank you in advance.

 

[Cobra Jet Steering LLC]

The probable answer is you have an intake vacuum leak even if you shut the engine off it can continue running because the carbon on top of the pistons is hot and acting like a diesel engine igniting your fuel even with no spark from your ignition, they call that dieseling , If that happens, To shut it off if it is screaming wide open is to open the throttle allowing air in that will cool it off inside the cylinders. With no load on the engine like running on the trailer out of the water it can really scare you . This happens occasionally with 2 stroke engines because fuel travels through the crank case then to the combustion chambers.
Usual causes are bad or loose gaskets or crank case seals gone bad. Look at the top of the pistons they should be chocolate brown , white or lite colors usually mead a lean running condition, and obviously air leaking in when you don't want it to will make it run lean.

 

[Dieseldad72]

Thank you for your Input. This problem came about after I rebuilt the 3 carbs. The engine starts right up and then the idle keeps going up and up. When I shut off the ignition it kept on running at a high idle. It died when i pulled the small vacuum lines from the cold start solenoid. Could the solenoid be leaking fuel thru it feeding fuel behind the throttle plates?

 

[Cobra Jet Steering LLC]

No it is running lean you have air leaking in. when it does it hit the throttle wide open and the engine will shut off . I know it sounds odd but it is common with a vacuum leak. Some gasket going into the crank case or under the carbs or intake is leaking air.

 

[Dieseldad72]

I have eliminated any vacuum leaks. The engine has been hard to start and one of the reasons why the carbs were rebuilt. During my checks I found some burned up wiring going to the trigger plate assembly. I pulled the flywheel and found the trigger plate assembly had gotten hot and was cracked in to places. I have one on order. The magnets on the flywheel are solid and the stator just has some surface rust on it but wiring is not cracked or split.

 

[Cobra Jet Steering LLC]

Well honestly if it does what I described ,runs wide open with no load will not turn off when you turn the ignition off it's a vacuum leak. I experienced that with 2 stroke motorcycles and jet skis several times , the first time scared me because I even pulled off the plug wires and it just kept screaming wide open. Later on I learned to open the throttle to stop it, just exactly the opposite of what you would think to do when an engine is running wide open with no ignition. So beyond the advice I gave you I am out of suggestions . Hopefully someone else can be of more assistance.