Older Honda 9.9 oil light cuts out with throttle

[jpostal]

Hello, first post here. Brand new to boats and not really mechanically minded. I have searched this forum and the internet regarding this problem, but can't seem to find a similar situation.

Backstory...traded my ATV for a little fishing boat, trailer, and motor. I believe that the motor is a mid 90's BF100 but not exactly sure. It has a white cover with a yellow, light blue, and dark blue stripe. Serial number is B100S - 1708263 I believe. Anyway, the fellow I traded with seemed like a pretty straight shooter and claimed to have used the motor recently with no problems. I took his word for it without seeing the motor run...my bad. Got the motor home and got it in a bucket and surprise, surprise it wouldn't fire up, or if it did I died after 10 seconds. I pulled the carb and cleaned it, cleaned the tiny fuel filter, cleaned the gas tank and got new gas, re-cleaned carb and now it runs pretty good from what I can tell.

Next I emptied the gear oil from leg...it looks like some water is getting in somewhere so thats a project for another day.

Finally drained the motor oil...it looked not too bad overall. Filled up the oil and fired her up again. Good steady idle, steady stream of water coming out of the little hole, green oil light on and bright. Now for the problem...when I give it throttle the oil pressure light starts to flicker and eventually, as the RPMs increase, it goes out completely. Everything sounds good, but obviously there is a problem somewhere. Since the motor is probably only worth $500 I dont really want to dump much money into it at a shop so I am trying to diagnose and hopefully fix the problem myself. From what I have read, my first thought is that the oil pickup tube could be plugged which would require pulling the powerhead to clean it. As mentioned, I am not real mechanical but maybe with a repair manual I could figure it out. Any other ideas of what it could be?

Thanks in advance for your help.

Jon

 

[sam am I]

Hmmm, well few low cost ways to go about it suppose.........

Could determine first if it's a faulty light and/or associated wiring and/or supply voltage and/or pressure switch.

Check wiring first of course, bullet connectors abound in those Honda's

The light bulb (to work) gets its voltage (high side) from the CDI unit that supplies an AC voltage to the light bulb (Neon, 90'ish Volts AC to ionize the gas) when the magneto/engine is turning. It then also gets it's ground (low side) from the oil pressure switch that when pressured starting at something like a few PSI, completes a circuit to ground.

Could put a volt meter first on the high side of the bulb and fire it up, see if the voltage stays up (least 90VAC), if so, go on the low side of the bulb. Fire it back up, if it doesn't stay at roughly 0V, either the oil pressure is truly dropping out (does the engine knock a bit/sound different when light goes out?) or the pressure switch is flaky (opening up under higher pressure's)

At that point, ya have to pull the pressure switch and pop in a mechanical air/oil pressure gauge (any old car oil gauge or air compressor gauge laying around will do) and see if the oil is truly dropping out to confirm motor or pressure switch.

Or

Could also just pull the oil pressure switch first and pop in a gauge and skip the electrical tests.......Depends how handy you are with a voltmeter. Think i'd do the electrical first cuz that would take me (you) like less than a minute as opposed to removing the pressure switch which is a tad bit more involved due to it's tight to get too and remove with linkage, wires and housings that all tend to be in the way.

 

[hondam]

Make sure that its not overfull of oil, also make sure your using a 10W-30 FCW rated marine oil.

 

[jpostal]

Thanks for the info guys!

I guess it's possible that I overfilled the oil.

I didn't think to check the light since it is on at idle, but goes out under throttle. It would be great if it were just too much/too little oil or the light.

I'll start there.

Is there anywhere I can buy a repair manual for this motor?

 

[Igyjastabay]

Personally i wouldnt worry too much about it. The motor runs well, wiring will not affect its performance. I would guess a cdi issue. They are a pain in the butt. You have a cdi because you have an external plug for a battery. Its the motherboard for the external plug. Ignore the oil light, i would

 

[sam am I]

Personally i wouldnt worry too much about it. The motor runs well, wiring will not affect its performance. I would guess a cdi issue. They are a pain in the butt. You have a cdi because you have an external plug for a battery. Its the motherboard for the external plug. Ignore the oil light, i would

Ya know.....I might be inclined to agree with you to some extent and you could certainly be right BUT, shouldn't he first NOT "guess" AND determine beyond some what of a guess that it is fact the cdi unit/wiring before "not worrying too much about it"?

If the motor "runs well" now and his oil pressure is in fact dropping out, how long ya figure it'll run well with the oil dropping out? Especially at higher RPM's?

So perhaps is it just the cdi unit dropping the light out (this would be my first clue to replace the CDI before THE major failure), what if he does someday go low on oil for real then? Sure....you should always check oil levels before running but, if something breaks or leaks out during a run, he has no light/way of knowing he's running out of oil......

.....I just don't think like that I reckon.

And BTW, he has a CDI because the engine has an electronically controlled ignition not because of a external plug to charge a battery

 

[jpostal]

Thanks again for the feedback guys. My dilemma is this...spend $400+ to "hopefully" fix a motor worth maybe $600 in good operating condition, or "risk it" and run it as is hoping for the best.

I guess the smart bet is to check the light and oil pressure with a gauge. If both are good...just run with it???

 

[sam am I]

If the oil pressure tests good at the point where the light is shutting off, then sure, run it. But....Your indicator (the light on) of normal oil situation won't be working, if you can live with that, no problem.


However, if the oil pressure is good and the wiring is all good, then it's the CDI that provides power to the light failing, right? My personal thoughts are?......I'd replace the CDI!!

Justification if needed is......Since the flywheel's rotating mag also provides power to the CDI via a dedicated winding (see schematic above, the brown wire) and the light's power source (and spark plugs discharge capacitor source, internal to the CDI) is obviously then derived from this power winding, something is breaking down in the CDI and is dropping out the light control. That is, to me, in and of itself, an indication of the CDI issues. It's failed/failing and will strand you at some point when the CDI all together shuts down.....

I think the CDI units sell for around $150

 

[tommarvin]

Got to keep the idle jet clean, for a smooth idle, we get the largest drill bit that will fit in idle jet to clean it.gently.

Water in the lower unit, drain and fill with 100% synthetic marine gear lube tested with 10% water contamination.
Make sure you use the hard plastic seals on vent, fill, drain, you may not have a vent screw. Never use rubber O-rings.

Please get a service/repair manual so you can do all the repairs/maintance yourself, you need to change the water pump, its a rubber impeller on an outboard.

I would not pull the power head,to clean the oil pickup tube.

When old ignition parts get hot/overheat, random strange things happen, Very common problem, you cant re start motor after turning it off to fish, after running a WOT for awhile, ignition needs to cool down before motor will restart.

 

[isaksp00]

I agree it is important to keep the idle jet clean, as mine (8HP) clogs easily and ruins the idle - tends to stall when put in gear. Not sure I'd use a drill bit though to avoid scratching the orifice. I use a plastic piece - a split floss threader (you can get these at drug stores) and carb spray.