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1998 Merc 150 EFI @ WOT, RPM increases when activating trim switch

1

1998 150 EFI

New member
My 1998 Merc 150 EFI has 350+ total hours. It is mounted on a 22 foot tritoon, and has run great until a few weeks ago. At full throttle, with one person on board, it would run about 30 mph. Three weeks ago, it started missing badly at all throttle settings, from idle to WOT. I replaced the ignition trigger assembly with JETUNIT aftermarket unit. The motor runs strong now with no missing, but it will not reach full rpm at wot until I touch the trim button. With the throttle lever full forward, it runs well, but not at wot. As soon as the trim switch is touched (up or down), the motor immediately increases to full throttle. My tach is not working, so I am going strictly by sound level and speed, but I would estimate I am gaining roughly 3-4 mph when I activate the trim. This happens instantly, not because of any change in the angle of attack of the motor. It has to be some kind of electrical issue. I have checked the battery connections, and any ground I can find on the motor, but everything seems clean and tight.
I have a video I took last weekend that shows what is happening -
It is hard to hear anything at first because of wind noise, but toward the end of the video, I moved the phone down closer to the throttle and you can hear the engine speed up as I touch the trim. I am open to any suggestions or ideas.
Thanks for your assistance, John
 
The issue will be in the switch boxes..... A damaged Bias Circuit will eventually damage the engine... Switch Box PN is 7778A12
The Bias circuit controls timing... If the circuit is failing the added ground through the trim will advance timing. Don't ask me the particulars on this... it just helps with the field. There are sets of these boxes for sale on Ebay for less than 100.00... CDI brand.

Copied this from another page... Pretty good explination

The switch box monitors trigger voltages for each ignition circuit. As the magnet approaches a trigger coil, trigger voltage for that coil rises, and peaks as the magnet as almost over top of the coil. Its voltage reverses as the magnet travels away from the coil. The rising voltage is key. The switch box sets a threshold, and when that threshold voltage is met, it discharges a capacitor to the ignition coil. Timing is variable because the trigger rotates with throttle position, of course. It is also adjusted electronically. The idle stabilizer and advance modules (if the motor has them) can RETARD timing. The advance module is actually a "retarding" module. It biases the trigger voltage. By lowering the trigger's output voltage, it makes the switch box wait longer for the voltage to rise high enough before it fires the coils. When the modules are not biasing the voltage, the timing remains at the max setting determined by trigger position.




More info to read....
http://www.marineengine.com/boat-fo...ance-wire-do-between-the-2-CDI-switch-modules.
 
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