DT150SS Idles Too High

[rowlandct]

I have an 87 DT150SS (Super Six). Until recently, it ran great. I'm now experiencing a high idle condition.

At startup, it idles around 4000 RPM and never drops down. It has an idle speed selector switch that has no impact on the RPMs when I move it to either of the 5 positions. I just replaced the throttle valve sensor with no improvement. I also replaced all fuel lines to ensure I didn't have a cracked fuel hose introducing extra air into the system. I placed an ohm meter across all five idle switch positions and got the following readings from low to high (open circuit, 20 ohms, 6 ohms, 2 ohms, 0 ohms).

The motor idles too high to even attempt putting it into forward or reverse gear. I don't want to blow up the lower end.

QUESTIONS:

1. Do the ohm readings on the idle switch seem consistent with a properly working idle switch?
2. Can anyone provide any suggestions that could cure my high idle issue and get me back on the water?

Any help an/or discussion would be extremely appreciated. I'm 60 miles from the nearest dealer and want to fix this on my own.

Thanks - Craig

 

[ondarvr]

Have you looked at and inspected the throttle cable, plus checked to see if the throttle is fully closed?

 

[rowlandct]

Yes. I should have mentioned that in the original post. The throttle valves are fully closed in neutral, and the throttle cable is working as intended.

 

[ondarvr]

Verify that the timer base is rotating back to the stop when the throttle is at idle, the timing is what controls the RPMs in this range.

 

[rowlandct]

Ondarvr - I'm going to apologize early, because I'm not familiar with all the outboard parts and pieces lingo. But I'm willing to learn, and I appreciate your patience.

I'm not sure what you are referring to as the "timer base." I do have a Suzuki manual and looked up "timer base," but it doesn't indicate a timer base on models larger than the DT85.

I will note that moving the throttle, when the motor is not running, will open and close the throttle valves as intended.

My thought is that I'm getting a bad signal to the CDI unit, or the CDI unit is processing the signals incorrectly.

I originally replaced the throttle valve sensor because the manual indicated I should read 200-250 ohms at the sensor when the throttle valves are fully closed. My meter indicated an open circuit. The new sensor also indicates an open circuit at this setting. I doubt the new sensor is broken, because it was a new part.

By timer base, are you referring to the pulser and condenser coil assembly? Based on the diagram in the manual, it looks like this assembly has multiple inputs to the CDI.

 

[ondarvr]

I haven't looked at one of these motors, but most motors of that vintage have a rod that connects the throttle and timing, as you advance the throttle, the rod advances the timing, normally before opening the butterflies in the carbs. The timing is what sets and adjusts the idle RPMs, the rod moves the timer base, if the timer base sticks, then it may not return to an idle correctly.

 

[rowlandct]

I looked on line and found a video that shows a stuck timer base for an old Johnson. My DT150SS does not have a similar rod that advances the timing. I did find the following paragraph in the manual:

On models equipped with Integrated Circuit (IC) and Micro Link ignition (which mine does), the ignition timing advance is controlled electronically with no means for mechanical spark advance adjustment. The IC and Micro Link systems process information provided by various sensors and switches and automatically adjust timing advance according to engine speed, throttle opening and engine temperature.

My next thought is to determine how engine speed is determined and sent to the CDI.

Does it sound like I'm on the right track?

Appreciate your thoughts.

Thanks,

Craig

 

[ondarvr]

Yes, either an air leak or a sensor failing.

 

[rowlandct]

OK - I've ordered a new Tachometer Control Unit and will check for air leaks. I'll post next weekend once I try the new part.

 

[rowlandct]

UPDATE - Changed out the Tachometer Control Unit and saw that the water pump was not working after I started the motor. Ordered a new impeller and installed. Went to the lake this morning and the water pump worked great. Unfortunately, the motor still idles way too high to shift into gear. I'm now at a complete loss.

Any final recommendations before I take it into a shop?

 

[ondarvr]

Might be the only way to find out the exact problem, but they may not work on it, and even if they did the parts may not be available for it.