2000 Carbed FourStroke 90 (Yamaha F100 powerhead) routine maintenance

[pcrussell50]

So I have this 2000 Merc branded Yamaha power headed FourStroke 90hp on my 17 Whaler fish boat. (I shop for powerhead part as a Yamaha F100. The parts are cheaper before Mercury takes it's cut, I guess). After learning some things about what it likes, like how to balance the carbs, it's been running great every season for the past few years. And it's still running great this season. All I do is drain the carbs and leave the drain plugs out, when I take her out of service for the few months I take off from boating.

Still, other than water pump changes and draining the carbs in the off season, I really haven't done anything to it. I run it maybe 30 hours a year in a big year. So it has me wondering...

...Should I be changing something out? Fuel pump maybe?

I don't suppose too many people here have these, but some of you have experience fixing them. I'd like to get out ahead of some known maintenance issue before it becomes a problem.

-Peter

[25two.stroke]

Nah theres plenty of them out there. Yamaha stayed with a real similar design for a while even though merc moved a different route in 2005. I had one. I work on plenty of them. You must be doing something right cause them carbs don't normally last more than a year between rebuilds. Their jets are so damn small. Maybe your gas is better than here in northern Cali. I wouldn't do any more routine maintenance other than block zincs and spark plugs. Those belts seem to last forever and same with the fuel pumps. Maybe run some fuel stabilizer/some carb cleaner/storage oil or something through the fuel pump before you drian the carbs and store it. But honestly if you have it lasting then don't worry about it.

If you use it in salt then flush it good: they have the same corrosion problems as yamhas and hondas...salt eats the heads and adapter plates through and before you know it theres water in your oil. Yamaha and honda use that trash aluminum that doesn't hold up in salt. For some reason Merc and Suzuki actually last...

[pcrussell50]

If you use it in salt then flush it good: they have the same corrosion problems as yamhas and hondas...salt eats the heads and adapter plates through and before you know it theres water in your oil. Yamaha and honda use that trash aluminum that doesn't hold up in salt. For some reason Merc and Suzuki actually last...

I moved it out to Lake Mead (fresh water) back in 2013. So it gets flushed every time I use it ;-)

I use a bottle of Techron and an ounce of two of StarTron in each 14 gallon tank. Don't know if it does anything but my (now deceased, died in a boat racing crash) mechanic likes/liked that cocktail. Not sure if it does any good, but it doesn't seem to have hurt.

-Peter

[25two.stroke]

Then I would keep doing that. And check you block zincs/thermostat periodically for corrosion.

[25two.stroke]

Just serviced another one of these this weekend. Found out something new...thats not in the merc book...I seem to learn something new every day when I am working on engines.

Guy brought it in (75 hp carb fourstroke, 90 hp is identical except for the carb bore size, even the ECMs are the same). He wanted an oil change and said it ran perfect the day before fishin. So I didn't even bother running it before the oil change. After I changed the oil it was super hard to start and ran like garbage when it did start. I immediately wen to the carb mixture to try to dial them in and found #2 and #3 were not running. Pulled the plugs and found no spark in those cylinders. Swapped coils and still no spark. Took the feed from the 1/4 coil and plugged it into the 2/3 and it fired them. Figured the ECM was dead and not firing that side. I looked all through the book to see if there was anything external that would kill those two cylinders only, and nothing. The rev limiter an all that sort of thing kill 1 and 4, not 2 and 3. Onle last thing I tried shifting it in gear and immediately ran better. The shift interrupter switch was what was killing those two cylinders...Cable was a tiny bit out of adjustment to where sometimes when it came back to neutral it was still depressed. Quick cable adjustment and it was fixed. BUT the book doesn't mention at all that the interrupter switch only kills those two cylinders. It doesn't say a thing about what it kills...now I know and so do you.

[pcrussell50]

Onle last thing I tried shifting it in gear and immediately ran better. The shift interrupter switch was what was killing those two cylinders...Cable was a tiny bit out of adjustment to where sometimes when it came back to neutral it was still depressed. Quick cable adjustment and it was fixed. BUT the book doesn't mention at all that the interrupter switch only kills those two cylinders. It doesn't say a thing about what it kills...now I know and so do you.

Damn! Some of the insidious ways things can go wrong are scary.

Where is the shift interrupter on these motors? In the remote or under the cowl? Now I'm going to look at mine.

-Peter

[25two.stroke]

On the engine and it sits right next to the shift cable

[pcrussell50]

Thanks mate. In advance of that problem, I'll take a look at mine.

-Peter

[pcrussell50]

Couple new things:

1) The UP trim relay clicks, (so it's getting signal power) from the switch on the cowl and the switch on the throttle lever. BUT it does not always trim up. I have to hit the UP switch a few times and then sometimes it takes. Is this a sign that the relay itself is bad? Can it be opened and cleaned and brought back into service?

2) Had a rectifier fire out on the lake yesterday. The wires coming out of the bundle underneath the rectifier/regulator. bunch of the insulation right where the wire bundle enters the rectifier caught fire and burned off leaving copper wires to short against each other. It's a long story of good luck how we spotted it early, but I shut down the motor and master power, waited till it cooled off, then separated the wires with the burned insulation so nothing touched anything else. The motor started right up again and ran fine, though we slow rolled it back to the ramp. Anyway, has anyone who services these motors encountered this before, or was my case probably an oddity? I have no idea how durable these are, but the motor is a 2000 (Yamaha powerhead) Merc 90 carbureted four stroke and as far as I can tell it might be the original rectifier regulator. Also, it is hot where we operate, high desert, huge lake, Lake Mead. Maybe these things are sensitive to operating on hot day? Here is a pic of what it looks like:


Oh and obviously I'll be needing a new one. The genuine Yamaha ones are close to $300. But I see ads all over the place for rectifier regulators for $50-$70. Is it safe to assume the cheap ones are junk?

Also, the tach has been intermittent to inoperative for the past few years. All of a sudden yesterday before the regulator rectifier wires shorted and melted, the tach was working normally. And it continued working normally after the smoke/fire.


-Peter