Motor Boggs down when Hot and WOT

[Skoot]

Okay guys in need some help

Ever since owning this 1998 90hp Tohatsu (M90A) I've had this bogging down problem. It only happens when the motor has been run for awhile/fully warm and at wide open throttle. I'll run Wide open(5200rpms) for less than 1 minute and then she'll bogg down to what feels like 1/2 throttle or less. I shut it down for a few mins, lift the trim/put it back down and im off and running again until I floor it, then bogg city. I can Run 3/4 throttle(4200rpms) all day and it will not bogg down.

Also a new issue has started. Again when the motors fully warm and Im idling(900rpms) the motor will cough and die. I can start it back up, raise the idle arm to (1200rpms) and the motor will still cough, but not die. When returning home to flush the motor and still warm she will cough and a cloud of blue smoke exits the hub.

I have a brand new fuel tank, lines, primer bulb, and the carbs have just been cleaned.  Compression is 135 on all cylinders. I recently adjust the throttle linkage, carbs, oil pump, etc to factory specs according to the Manual.

Im at a loss!

[fitz73222]

Hey Skoot,

1) Make sure your hot horn is working and that you are not overheating at full throttle.
2)Sounds like you have developed a lean condition that is showing up as an idle cough and a high speed lean issue. Dispite your best efforts you may have introduced bad fuel or some kind of restriction like trash in the main jets or fuel nozzels in the carbs.
3) The best way to identify a high speed lean issue is to run the engine at full throttle until it drops rpms, run it for a minute or so and turn the key off without pulling back on throttle. Take out the plugs and look for white tips or blistering or any indication of plug overheating. If you pull back on throttle before shutting the engine off the cylinders will recieve a rich shot of fuel due to the throttle butterflies closing and mask the plug condition. This is an old race engine trick to understand whats going on in each combustion chamber. If you spot a white tip condition in one or more cylinders, chances are you have plugged or restricted jets in the carbs.

[Skoot]

dam that was fast.

How the heck do I sake sure the hot horn is working?   I will sound if no water is coming from the pee-hole.  Other than that I have no idea.

Im going to take her out today and will do check the plugs as suggested.

Thanks

[fitz73222]

Locate the wire the runs to the cylinder head. This is the temp switch, Turn on the key switch, (Engine not running). Take a pin or small nail and pierce the wire and touch the pin to the cylinder head to make a ground connection from that wire to the block. The horn should sound. There may be a connecter that is covered with plastic tubing that you can pull back and touch the connection to the block to accomplish the same thing. I really think there is a connection between the idle cough and the loss of power at WOT. One other misnomer is the relationship between the engine overheating and water coming out of the pee hole. Normally, the pee hole water is tapped in the water flow before the thermostat. That means that you can bake your engine alive and have plenty of water coming out of the pee hole if a thermostat sticks closed. A hot horn or temp gage is the only way to know if you have a thermostat problem.

[Skoot]

Okay im going to try that this afternoon. Thanks.  My plan is to be home and working on the motor by 3 today.  As for the thermostat, I've heard you can take some tongs and a pot of water on the stove. When it heats up the thermostat should open then when cool, close.  Does this trick actually work?

I am a little worried about the water pressure (visual inspection only) out the pee-hole.  I just recently changed the impeller, but the water pressure remained the same. At idle(900rpms) water pressure seems normal, when I shift into gear(6 - 700rpms) the alarm sounds and I have only a trickle of water coming from the pee-hole. I then give it a bit more gas(1000-1200rpms) and the alarm goes off and water pressure appears normal. During 3/4 to full throttle the stream of water turns into more of a half stream half spray.

[fitz73222]

Engine height to high!!! You`ve been playing with your engine height and got her jacked up to where you may be sucking air into the intakes. You may have actually cooked the water pump and you have flow issues at idle now.

[Skoot]

Engine height to high!!! You`ve been playing with your engine height and got her jacked up to where you may be sucking air into the intakes. You may have actually cooked the water pump and you have flow issues at idle now.

I dont think so.  The water pressure has been the same since owning the boat. Even with moving the motor up the anti-cav plate is approx 1" above the keel. The height seems perfect now.

Im going to check the alarm wire and thermostat first.  Check for any obstruction in the water line/hose and if all checks out good im going to the lake.   I'll check the plugs as suggested and I also have a In-line Spark tester to use once the motor starts acting up..

Thanks again for all the tips, ill report back later.

[fitz73222]

For the thermostat test, open the thermostat enough to get a piece of sewing thread between the plunger and seat. Note the temp on the thermostat, usually 110-130 degrees. Hang it a pot of water with the thread tied to something. Have a pyrometer or some digital accurate means of measuring water temp. The thermostat will fall of the thread at the temp that is stamped on it. Ive tried this and it works.

[Skoot]

Okay, got the thermostat out and preformed test according to manual.  I suspended thermo in water and slowly heated to verify thermo opens at 140ºF.   Well it didnt.  It opened slightly at 145ºf and what "appeared" to be fully open at 150ºf.  Im gonna get a new one.

Unfortunately I couldn't get the hot alarm to sound.  The manual states that the key switch must in the on position and control lever in either forward or reverse. Well when its in gear, and on dry land that will also activate the water pressure horn so im going to have to try again when im on the water or with the muffs connected.

Now I did follow the Overheat sensor wire back down to where it exits the motor out toward the control box/horn.  I unhooked the bullet connector at the furthest point and check for continuity from there back to the sensor and all was good.  Question, can I use that unhooked bullet connector and touch that to the engine block to get the horn to sound?

[Asport-Rog]

I had an issue similar to this on a 1977 Johnson 85hp and it ended up being the Timer Base.

[GoneFission]

I think Fitz had it right the first time with a lean situation.  Check the carbs for dirt and clean the main jets.  You can also try adding more oil to the gas mix, as what might be causing the bogging is the needle bearings getting hot.  I used to have a motor that would scream at WOT until the bearings got hot, then bog, then scream again after cooling off.  I went from 50:1 to 25:1 oil/gas ratio and the problem disappeared.

[Skoot]

Okay, finally an update!
First off I got the hot horn to sound according to Fitz's instructions.  Second I changed the thermo and gasket.  Then took her for a test and....  the water pressure was still low. :(   I took her back home and disconnected all water pipes and the water nipple.  Blew compressed air into all water pipes/passages to clear out anything that may be stuck.  I also took a drill and bore the water nipple a hair larger.  Took her back down to the lake, started her up and She was Peeing like a race horse, blasting through surface of the water, Just like it should be.   I was thrilled already.

Took her for a run and she was a bit sluggish, some hesitation, coughing and even died at idle a couple times.  I change the plugs and fired her back up. Let me tell you what, I ran that girl for approx 2 hours straight varying between 3/4 and Full throttle without a hitch. At one point I had her wide open 5200rmps for 30mins straight and she performed flawless.   I checked the plugs and they looked like a toasted marshmallow.  

To end it all off, the old girl even towed another boat back to the docks.
     

[flounderpounder225]

Great job Skoot..  

[Skoot]

Hey, thanks,

Now I just need to get a buddy to call in sick and we can go fish.

[MarshMarlowe196]

Man that is awesome.  I'm happy for you.   8)   Happy to hear you got it worked out.  I bet you'll have several hours of running that motor without any problems.  (hope I'm not jinxing you...     )

You know, back when I first acquired my 19-6, I had the option of hanging a 4cyl OMC 90HP or an older 3cyl Nissan 90HP (same as your Tohatsu).  Because the boat already had OMC controls, and I was more familiar with the OMC power, I decided against the Nissan.  

Well, the OMC died on my second trip with it in the middle of a real bad storm  ... I think I'm still dealing with PTSD from that experience :)  

I sold the Nissan to a neighbor and good family friend for $700 who is a commercial shrimper.  This was before I even finished rigging my OMC. He hung that motor on a 22ft Carolina Skiff and runs it hard (4000RPMs) all day, 5 days a week, pulling a big net off Morris Island.

That was over 2yrs ago now, and the motor is still running strong and putting money in his pocket.

I love my Suzuki, I really really do.  BUT- If I had it to do over again, I would have kept the Nissan.  Enjoy that engine