115 HP Johnson OB cranks and wont turn off

[britton101]

My dad has a 1999 115HP OB. A fellow came to his house yesterday to replace his water impellor (which by the way has never been changed before). The fellow took an inordinate amount of time to get the lower unit off and spent a lot of time at the front part of the power head.

Anyway, it flooded to begin with but when it did crank, it went to full throttle and could not be turned off by the ignition switch nor by removing the spark plug wires. It ran like this for about a minute then shut off by itself.

Please advise...I know something about OB motors and any help I can get will be appreciated.

 

[crb478]

Re: 115 HP Johnson OB cranks and wont turn off

Without seeing the motor it would be hard to say exactly what made it happen. Most run away engines happen because of excessive carbon in the cylinders, and it continues to fire from heat in the cylinders like a diesel. You need to choke the motor to stop it, cutting off as much air as possible.

Changing the impeller may or may not have contributed. To drop the lower unit he had to get to the shift lever under the carbs. He may or may not have removed them to do so. If he did not remove them he might have changed the adjustments.

I would do a link and sink and follow that up with a good decard. I would also check for compression on all four to get a base line now and then again after the decarb.

 

[britton101]

Re: 115 HP Johnson OB cranks and wont turn off

Without seeing the motor it would be hard to say exactly what made it happen. Most run away engines happen because of excessive carbon in the cylinders, and it continues to fire from heat in the cylinders like a diesel. You need to choke the motor to stop it, cutting off as much air as possible.

Changing the impeller may or may not have contributed. To drop the lower unit he had to get to the shift lever under the carbs. He may or may not have removed them to do so. If he did not remove them he might have changed the adjustments.

I would do a link and sink and follow that up with a good decard. I would also check for compression on all four to get a base line now and then again after the decarb.

Thanks CRB!

I don't mean to sound ignorant, but when you say "link and sink"....do you mean adjust the linkages and then test the motor? Also, are you saying that he may have removed the carbs or changed the adjustment of the carbs? My dad has a manual somewhere, I just have to locate it. One last thing, what is the easiest way to perform a de-carb of the cylinders?

Thanks for your help.

 

[ezeke]

Re: 115 HP Johnson OB cranks and wont turn off

You can't stop the runaway on that motor with a choke because it does not have one. You may stop it by flooding it using the primer solenoid, but it is a bad bet; the best bet is to cut off the gasoline supply.

You may have had the primer solenoid in manual mode which would feed too much gas and then flood out the engine; Be sure that the red handle on the primer solenoid is aligned with the cylindrical part of the solenoid.

As mentioned, the most common cause of a runaway is red hot carbon in the firing chambers and the best way to help eliminate that cause is with an annual de-carbon treatment.

Edit: Use the BRP Engine Tuner with the 763644 Injector Valve and Hose Assembly for the easiest de-carbon treatment on your motor. The Assembly fits the Schrader valve on your primer solenoid.

 

[JB]

Re: 115 HP Johnson OB cranks and wont turn off

Thermal runaway happens when an engine with no load on it is allowed to rev up beyond about 1500rpm. It will not happen to an engine that has a load on it.

 

[britton101]

Re: 115 HP Johnson OB cranks and wont turn off

Thanks to all for the help. I took compression readings and they were 130, 136, 130, and 140. We put water to it and cranked it. It immediately went to full throttle though the gear shifter was in the idle and neutral position. I pulled the air filter and looked at the butterflies and all were in the idle position. We were able to immediately shut it down with the switch. Any ideas where I need to look to solve this "instant full throttle" problem.

Thanks,

Dave

 

[ezeke]

Re: 115 HP Johnson OB cranks and wont turn off

Look at your spark advance when the warm-up lever is in the run position. The advance may be stuck fully advanced.