BF90 CDI 'safe' mode?

[JonnyP]

Hi there,

Long time lurker, first time poster

I recently acquired a 'rebuilt' BF90 for my Montauk, it's a 97 in reasonable condition. When I got it rigged and out on the water, I couldn't get it to go above 4000 RPM and 25 mph. When I pulled her out, there was significant soot around the exhaust port. For comparison, with my previous (now dead) Ficht 90 I could get around 43 mph.

I ran through the engine per the workshop manual, good spark, great compression (185, 185, 185, 183). New filters/Oil/plugs, brand new fuel system and tank with a 'pre-bulb' Racor. Timing is good and I synched the carbs and adjusted the mixture at max vac. I also went through 2 different props and used the 'vanilla' Honda aluminum factor prop borrowed from my dealer.

This did not solve the problem, so I tore down the carbs - no varnish, I replaced all the o-rings and seals, new needles and main jets, soaked and cleaned the slow jets.

Back on boat, synch and set, same problem. Won't run above 4000 rpm/25mph. I have a brand new OEM tach and harness, all settings are correct. I've worked with my local Honda dealer and they are stumped.

I spoke to a full-time mechanic who specializes in four strokes and he said that there is a thermistor that connects to the controls and runs in to the CDI (I checked the wire diagram in the workshop manual and this is correct). He said that if it is old or corroded it can cause an errenous overheat state, which is picked up by the CDI and kicks the motor into some kind of 'safe' mode. He said that is retards the timing to limit the revs to prevent you toasting the motor.

While this could plausibly explain the current symptoms, I looked in the workshop manual and I can find nothing about this, I also searched on the web and can't find any detail to validate.

Has anybody heard of this or experienced anything similar?

 

[cp]

Re: BF90 CDI 'safe' mode?

Good detailed info on what you've already done.

Don't know about the thermistor but believe that the "limp home" mode would limit you to significantly less that 4000 rpm, so not ready to buy into the CDI safe mode theory yet.

Might the remaining problem be caused by your control cable not fully advancing the throttle? Have you checked that your throttle cables have the correct adjustments at your throttle levers and that your throttle lever contacts the full throttle stop when the helm control is at the WOT position? Have you verified that the #4 carburetor throttle valve lever is coordinated to contact its stop screw when full throttle is reached? Your shop manual should contain the procedures for doing this.

Just some thoughts; hope they help; let us know, especially Jim Briggs (not sure where he's at in his troubleshooting; his last few posts have been gibberish). Good luck.

 

[JonnyP]

Re: BF90 CDI 'safe' mode?

Thanks for the response.

You've echoed my #1 concern. When I had my Ficht go into 'safe' mode once, it was a 3000 RPM/10MPH deal. The boat certainly would not have planed.

I did check all the throttle controls and setting, engine and control side. I verified WOT mechanical operation and validated that the butterflies were 90 degrees. I also checked the mechanical and electical choke operation to make sure that it was not stuck. I also checked the vacuum diaphram and dashpot operation (when the engine was running)

I have a new thermistor arriving today, so I will test tomorrow and publish results.