Causes a runaway motor (bad Lower Seal?)

[johnmark]

What would cause a motor to go from idle to runaway and then even turning off the key would not stop the motor. someone said carbon burning and it firing from the carbon not the plugs thats why the key would not kill motor (thank God I had the hood off I cut the air off and it died) thanks Guys , John

 

[Silvertip]

Re: Causes a runaway motor

Re: Causes a runaway motor

Reving too high in neutral is a good way to ruin a motor. Failure of the ignition advance plate to return can cause this. A severe air leak in the induction system can cause it. Throttle linkage that has been messed with can also cause it. Let's have some details on what you were doing and what was done to the engine prior to this happening.

 

[aganser]

Re: Causes a runaway motor

Re: Causes a runaway motor

I would think revving a motor with no load (either in neutral or out of the water in gear) would be a major factor.

Bob

 

[johnmark]

Re: Causes a runaway motor (Bad lower seal ?)

Re: Causes a runaway motor (Bad lower seal ?)

My lower cylinder has fire , has 130 lbs compression(both clyinders do) But when I pull plug wire on lower motor dies. And the motor has ranaway on me three times now.when the motor is running and I give gas it just dies.(1983 50 hp johnson model J50BELCTA)
I'm lost Thanks John

 

[Willyclay]

Re: Causes a runaway motor (bad Lower Seal?)

Did the three runaway events occur on a boat underway in the water or on muffs?

 

[johnmark]

Re: Causes a runaway motor (bad Lower Seal?)

Each time out of gear and no load two times on muffs once idling to dock but out of gear. I'm wondering if its sucking air thru lower seal. today I will re-clean carbs and replace every fuel line on this motor. twice it ranaway it was idling kind of high but the time at the dock it went from low idle to runaway and would not die by killing the key switch,I had to cover the air and choke it out.

 

[Willyclay]

Re: Causes a runaway motor (bad Lower Seal?)

Tashasdaddy was working on someone else's 50 a while back that wouldn't take gas. Maybe we can get him onboard to share what he found.

 

[reelmess]

Re: Causes a runaway motor (bad Lower Seal?)

Either the throttle plates are opening on their own due to loose or disconnected linkage or if it's got a fuel primer, maybe it's leaking or powered up when it shouldn't be? (As if you had pushed in the key switch)?

For the carbs to draw fuel, they have to be drawing air to create the venturi effect needed, if they are not open, then it's getting fuel elsewhere. This may expain the bog when you give it gas. Or else, both carbs are not connected? Could be running on only one carb and the other is disconnected and able to open with engine vacuum?

 

[johnmark]

Re: Causes a runaway motor (bad Lower Seal?)

Carbs redone by a shop, motor runs good so far. has'nt ranaway again so far so good glad to be back on the water,thanks everyone for the input,John

 

[gss036]

Re: Causes a runaway motor (bad Lower Seal?)

I had an old Yamaha 125cc motorcycle do that on me and it turned out to be a lower crank seal letting air into the engine. It pegged the tach at 10,000 rpm. Thank goodness I was running synthetic oil and it saved the motor.
Could be that the base gaskets on the carbs were leaking, allowing extra air st suck in.