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BF90 CDI 'safe' mode?

J

JonnyP

New member
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?
 
This is basic, but worth a check. Check to see you are getting full throttle. There are two places on the lever where you can connect the throttle cable at the engine. It should be connected to the inner hole, not the one at the end of the lever.

The soot makes me think timing, thermostat, flooded carbs or too much choke, too much oil, valve clearances out of adjustment. You seem to have most of them covered except for maybe valve clearances. Also be sure that the oil is well inside the marks on the dipstick.

Sorry all that sounds kind of basic considering what you have already done.

Where did you see the thermister? I only know of a thermal switch (that goes to the red light and to the CDI) that has to do with overheat. You could just disconnect it and see what happens.

There is also an oil pressure switch that does the same thing (that goes to the green light and to the CDI) for oil pressure.

If either is giving the CDI a false signal, you will go into safe mode.

Make sure that it is not overheating and the oil pressure is good.

It is not unusual for the CDI to get a signal from one of these and not cause the warning light to change states. An steady audible alarm is normally heard at the same time.

How high did you mount the motor on the boat? Due to the weight, I usually mount the motor so the cavitation plate is about an inch above the bottom of the boat on a whaler. I also move the battery up from the rear to under the helm. With this is mind make sure that the exhaust reliefs are well above the water line. This is probably not your main problem, but something to note.


Also due to the weight of the engine compared to the ficht, the boat will probably not go as fast as it did before anyway, but it should be closer than it is .

Mike
 
Mike,

Thanks for the response, I've answered inline:

hondadude said:
This is basic, but worth a check. Check to see you are getting full throttle. There are two places on the lever where you can connect the throttle cable at the engine. It should be connected to the inner hole, not the one at the end of the lever.

Yes - checked and good

The soot makes me think timing, thermostat, flooded carbs or too much choke, too much oil, valve clearances out of adjustment. You seem to have most of them covered except for maybe valve clearances. Also be sure that the oil is well inside the marks on the dipstick.

Timing is good, oil is in limits, valve clearences and thermostat I have not checked, thanks for the reminder.

Sorry all that sounds kind of basic considering what you have already done.

Nope, it's usually something basic or obvious in my opinion!

Where did you see the thermister? I only know of a thermal switch (that goes to the red light and to the CDI) that has to do with overheat. You could just disconnect it and see what happens.

That's what I mean by Thermistor (that's what the workshop manaul calls it) but 100% right, it's just a switch. I didn't think of disconnecting, becasue without any guidedance from the factory manual, I'm not sure if a disconnect defaults to a 0 state which kicks it into safe... This is a 'new' secondhand motor to me, so I'm being safe by replacing the switch and t-stat, that way I have a known baseline, plus it's not too expensive.

There is also an oil pressure switch that does the same thing (that goes to the green light and to the CDI) for oil pressure.

If either is giving the CDI a false signal, you will go into safe mode.

That's what I think is happening, but I guess I'm concerned because most engines I know with a 'safe' mode limit revs way down in the 2500-3000 range, certainly not enough to hit plane and 25 mph....

Make sure that it is not overheating and the oil pressure is good.

Check & check

It is not unusual for the CDI to get a signal from one of these and not cause the warning light to change states. An steady audible alarm is normally heard at the same time.

I suspect that is the case here.

How high did you mount the motor on the boat? Due to the weight, I usually mount the motor so the cavitation plate is about an inch above the bottom of the boat on a whaler.

Yes, that about where it is, I'm down one hole.

I also move the battery up from the rear to under the helm. With this is mind make sure that the exhaust reliefs are well above the water line. This is probably not your main problem, but something to note.


Also due to the weight of the engine compared to the ficht, the boat will probably not go as fast as it did before anyway, but it should be closer than it is .

Agreed, I'm thinking a bare minimum of 35-38 mph at WOT should be reasonable.

Mike
Click to expand...

Thankds for the input - much appreciated.
 
The overheat switch sends a ground when there is an overheat. The oil pressure switch removes a ground (If I remember correctly) when it is good pressure.

Of course, it is possible that you still have an issue in one of your carbs. One easy way to check, once you get the motor started, remove the black plastic choke linkage that connects all the carbs together. Then while running at full throttle, have someone back by the engine flip (operate) the choke lever on each carburator. Just operate each for a couple of seconds. You will have to pull out the manual choke lever for the bottom carb. If the motor picks up, you have found the problem carb. If the motor boggs down a little, it is probably ok.

Mike
 
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