Tohatsu 90A Boggs after hot/full throttle

[Skoot]

Hey guys, first time poster here looking for some help.

I have a 1998 Tohatsu M90A motor that a purchased in Nov 2009. Ever since owning this motor 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(5000rpms) 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 never bogg down.

The fuel tank/lines/filters/bulbs etc were replaced Feb-2010. I fully cleaned and synchronized the carbs and installed new plugs. With all this the problem still exists.

Any Ideas?

 

[TOHATSU GURU]

Re: Tohatsu 90A Boggs after hot/full throttle

Run the ignition checks from the service manual....When they all check good go back to the carbs and clean them again.

 

[Skoot]

Re: Tohatsu 90A Boggs after hot/full throttle

So are you guessing its not an electrical problem?

Let me give some more info/background and a new problem

Besides the above, Compression is 135 on all cylinders. I recently adjust the throttle linkage, carbs, oil pump, etc to factory specs according to the Manual. With this I gained 200rpm.

The motor appears to have weak water pressure from the pee hole. 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 spray. I changed the impeller and the results are the same.

Now a new issue has started. Again when the motor 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 and still warm she will cough and a cloud of blue smoke exits the hub while im flushing the motor. Im guessing a Coil???

Again, both the Cough and and loss of power at Full throttle only happen when the motor is fully warm and been run for about 15 minutes.

 

[TOHATSU GURU]

Re: Tohatsu 90A Boggs after hot/full throttle

Run the ignition checks from the service manual....When they all check good go back to the carbs and clean them again.

The point is that you are not approaching the problem logically. You have to test the engine's systems and by process of elimination, determine what is wrong. Otherwise it's just random, sometimes educated guessing. The fuel system is the problem on 90% of outboards so you can start there or go there last.

 

[pvanv]

Re: Tohatsu 90A Boggs after hot/full throttle

The point is that you are not approaching the problem logically. You have to test the engine's systems and by process of elimination, determine what is wrong. Otherwise it's just random, sometimes educated guessing. The fuel system is the problem on 90% of outboards so you can start there or go there last.

Yep. Keep in mind that your problem seems thermal... So, you need to run the checks when it's hot and acting up. With some intermittent thermal problems (trouble when hot), all the checks will be OK when cold.

 

[Skoot]

Re: Tohatsu 90A Boggs after hot/full throttle

Thanks guys for your help, but im starting to get confused!

So the first step is check the fuel system. Okay, here goes. I know the fuel/fuel tank/lines/primer bulb are either brand new or fresh, carbs have just been cleaned and adjusted. The only thing remaining is the pump. Am i on the right track?

pvanv, I completely agree with you about running the checks while hot, just haven't had a chance, yet. The whole thermal issue is what got me thinking it has to be something electrical/breaking down. But, i guess it could be a fuel system issue as well.

 

[TOHATSU GURU]

Re: Tohatsu 90A Boggs after hot/full throttle

Your kind of on the right track. If your confidant that the fuel system is correct then you move on to the ignition system. If that checks good you start looking at the block. If that appears good then you go back to the fuel system again. You have those three systems on your engine. One or more are wacky, so if they all check good you missed something. That's why outboards can be so expensive to work on...You have to test and you have to guess. Sometimes you pick the right system first, sometimes you pick the right system last.

 

[pvanv]

Re: Tohatsu 90A Boggs after hot/full throttle

Sounds like you're on the right track. Verify all the fuel system bits; pay particular attention to the fuel pump, since that's still a wild card in the mix. It's always possible that a minor detail in a carb was overlooked.

Is it just as likely an electrical problem? Sure. That's where the Factory service manual and a good analog ohm meter with DVA capability comes into play. Don't bother with a cheapie digital unit, as it won't give you the results you need.

 

[Skoot]

Re: Tohatsu 90A Boggs after hot/full throttle

TOHATSU GURU,
Im confident on everything except the fuel pump and the filter assembly.

Is it just as likely an electrical problem? Sure. That's where the Factory service manual and a good analog ohm meter with DVA capability comes into play. Don't bother with a cheapie digital unit, as it won't give you the results you need.

You got that right, I had a chepie digital unit on hand and our results were all over the place. (i thought I was doing something wrong).

 

[Skoot]

Re: Tohatsu 90A Boggs after hot/full throttle

UPDATE

First, I inspected the fuel system and all is well. 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 another 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. Wooowhoo

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