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Old 18-06-2007, 13:50   #1
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Power Loss in Tohatsu 5HP Outboard

I figure I'll ask on here to see if anyone has had this problem with a 2 stroke outboard:

My Tohatsu 5HP, 2 stroke outboard has a vexing problem. It starts right up and develops good power for approx the first minute or two of operation, until the outboard warms up. After this time, the outboard produces somewhere around 1/2-3/4 of the normal power it would produce.

You can run it for an hour, but as long as it's warm, it produces 1/2-3/4 of the power it does when it's cold.

If I drop the dinghy at a dock for a few hours and start it from cold again, the normal power returns. Any time the engine is cold, it runs perfectly. Any time it's warm, it gets sluggish.

Things I know:

Impeller is producing a good jet of water and cooling the engine (it isn't heating up or producing any temps inside the housing)

Lower unit gear oil was replaced this spring, as normal.

Spark plug was replaced a few minutes ago as a possible root to the problem. That didn't change the behavior of the outboard.

Fuel isn't an issue since it has been doing this on more than one tank of fuel. I've burned 2 tanks of different fuel with the same results.

Thinking outside the box:

Is it possible that my rings are shot or something? I mean those things expand when heated (or the cylinder does), which may make some spacing issues as the engine is warm that don't take place when cold, right? Just grasping at straws here. I'm not quite sure what to start looking at next.
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Old 18-06-2007, 14:15   #2
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2 strokes are very simple, yet scary for most as we vary rarely get to play in one.
Check the reed valve. Look for fouling under the valve. It must close cleanly. Check for cracks in it.
The only other area is the power pack. This produces the spark. But usually these fail with no regard to temperature.
Ensure you are using the correct two stroke oil. There is outboard oil and motorbike oil. Both are very different. Ensure you have the right ratio of oil. I recomend a good synthetic, as it reduces smoking and you can usually use a 100:1 ratio instead of a 50:1 ratio. Although in hard use, I feel it's good to run even synthetic at 50:1

I doubt very much it is a wear problem. Heat expands and takes up loose rings. Cold allows less compression and usually makes the motor hard to start.
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Old 18-06-2007, 17:30   #3
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Thanks for the input, Wheels. I'll have at the engine early next week, since it's our only mode of transportation to shore at the moment. It runs ok, but we can't plane for our 2-3 mile ride (one way).

I'll be sure to inspect the reed and I will likely pop off the power head just to be sure about rings/pistons/cyclinders.
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Old 18-06-2007, 17:53   #4
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Power loss

This is a real long shot.....check the exhaust port for carbon buildup.

Good luck

Bob....ZS6FB
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Old 18-06-2007, 18:48   #5
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just in case.....

Have you tried draining the carb? Both myself and mates have had little 2-stokes with the gas tanks mounted on them and they all seem to get water into the fuel. The motor will start-up fine and run ok for a bit, then dog-out and want to die as fuel fills up the carb bowl a bit. This gets some water into the air/fuel mixture.

If you throttle down and it runs fine, but then try to plane and it wants to go tits up that might be it. It gives you a little warning before the point of no-return if water is the problem and you can usually make it somewhere close by before it goes all together.

We finally got into the habit of covering the engine with a plastic bag if it was raining at all and/or draining the carb once a week. We tried just closing the cap screw-vent, but I swear rain water would drain down the control face and work its way into the carb throat! Our outboard wanted to swing face up when tilted and that didn't help. If you look behind that control face plate you'll see how easy it is for water to get in (if your Tohatsu is like the one we had).

Good luck....
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Old 18-06-2007, 19:22   #6
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I'm guessing the problem is the coil packs. Sometimes aging coil packs start failing and when they get hot they throw a weaker spark, intermittant spark or no spark to one cylinder if it's a twin.
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Old 18-06-2007, 19:26   #7
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It does sound more like the power pack than a carburetor problem. Sometimes the power packs get a bit screwy when they get hot but perform flawlessly when cool. But it doesn't hurt to tear the carb apart and check the reed valve and clean it all up. If that doesn't work try and swap out the power pack.
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Old 18-06-2007, 19:59   #8
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Love those Tohatsu engines

Hey Sean,
Everyone's suggestions are good. From what I have observed with Tohatsu engines (I have two different hp and observed many others) is that the coil ignition system is quite reliable as compared to others. The observation regarding the reed valve is good, yet, again, they don't seem to cause problems with those engines. The exhaust comment is worthy if you have run a lot at low rpms and low ambient temp. The carb comments are worthy if you don't ALWAYS turn off the fuel and run the carb dry every time you shut off the engine for more than an hour.

I doubt that you need to take it apart to the point of looking at the piston/rings.

One unmentioned problem could be a lower end bearing having a gauled (like the spelling?) race. Replacing it is a do-it-yourself as long as you have the shop manual with parts numbers, etc. and if you have the tools and are mechanical. These bearings can go bad regardless of how you have taken care of the lower end grease possibly because the housing is not always perfect regarding the bearing alignment when assempled. IF that is the case be very careful with insertion of a new one. As I recall you can tell by removing the lower end and then rotating the propshaft to feel for wear and binding (without the long shaft inserted between the power head and lower end.

Hope this helps!
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Old 20-06-2007, 00:25   #9
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Whatever you do follow a consistent cost effective troubleshooting method. Changing parts willy nilly gets expensive.

Considering "likelihood" start with cheap and work to more expensive. If you can also confirm diagnostically prior to changing any parts you will be way ahead.

One simple idea - is the fuel tank venting properly? Sometimes we think it's related to heat when it is really related to time. A restriction in the vent could be causing fuel starvation.

Here is an interesting site with a great deal of troubleshooting tips.

Outboard Boat Motor Parts Johnson Evinrude Mercury Mariner Force Yamaha
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Old 20-06-2007, 01:59   #10
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I forgot the most obviouse. A dirty fuel filter. Trace the fuel line inside the engine housing. Between where it enters and the carby, there should be a filter. It may not look like a filter, but a fuel line will go in and come out. Hopefully.
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Old 20-06-2007, 09:21   #11
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Good tips/ideas, guys. I do love the engine. Not a bit of trouble since it was made in 1997.

I just have to narrow down the cause. It's a difficult one to diagnose. I guess as suggested, I'll go with the basics:

Spark
Fuel
Air
and then Cooling

I posted to an outboard forum where some experts said degraded power when warm only is going to be either the spark or the mechanical parts of the engine. I'll start with spark, then the idea that something is heading up the fuel system and restricting it, then into the deeper stuff later.

Thanks for the tips. A sane, logical approach is always best, and sometimes it does take an outside tip to remind you of that.
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Old 20-06-2007, 09:38   #12
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I'll vote for dirty fuel filter/water in fuel.
Just my .02c

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Old 20-06-2007, 09:39   #13
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Quote:
Originally Posted by Ex-Calif
One simple idea - is the fuel tank venting properly? Sometimes we think it's related to heat when it is really related to time. A restriction in the vent could be causing fuel starvation.
Dan beat me to it. Your problem sounds like a classic case of a plugged fuel cap vent. Runs fine for a minute of two, then has no power. Try to run with the fuel cap partialy open to see if that is the problem.
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Old 20-06-2007, 10:38   #14
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Quote:
Originally Posted by senormechanico
I'll vote for dirty fuel filter/water in fuel.
Just my .02c

Steve B.
Thanks for the tips.

To restate: It has behaved the same way on 2 different tanks of fuel from 2 different fuel vendors.

As to the tank, it has a plastic screw vent that you open when you use the outboard. When it is open, you hear a hiss as the pressure is released (or increased) as it equalized with the surrounding air pressure. There is no chance of a closed air intake on the tank since you can audibly hear it hiss as you open it.

The filter though... could be. I'm taking a peek at that.
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Old 20-06-2007, 13:51   #15
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Aloha Sean,
Are you using ethanol fuel?
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