Power Loss in Tohatsu 5HP Outboard

[Alan Wheeler]

Make the spark issue last on your to do list. It is the most expensive and the only way to check it is to replace it.

[chris_gee]

Hmm in the recesses of my mind is that on 2 strokes they depend on some sort of spiral in the cylinder wall for lubrication I presume. This blocks. There is some stuff you can get from any boat shop. You put it in the air intake while the motor is running until it stops, then leave it for so many hours to eat the up the build up of ..... Run the motor for a while and bob's your uncle. Works. Water is common but it doesn't sound like that.

[Ex-Calif]

A good description of the symptoms is always helpful. Liek the doctor ask, "Can you tell me what's wrong?" Me, " I thought you went to med school for 12 years!"

BTW - My 2 stroke experience is primarily with dirt bikes but should be pretty similar.

1/ First start of the day - No problem, starts right up, idles fine and makes full revs in neutral, forward and reverse. It blows no smoke, burns no oil and makes appropriate sounds.

2/ After 2 minutes it still runs smoothly in forward, reverse and neutral with no smoke, stuttering or any other condition except it is power limited to approximately 1/2 to 3/4 of full power. It revs fully in neutral (no load)

3/ It will run as in condition 2 indefinitely although I have run it for 45 minutes to 1 hour in this condition

4/ If I park it for 2 hours it will restart cleanly with no trouble as in condition 1 and the cycle will continue as through condition 3.

5/ The engine is 10 years old has been trouble free until now and has never had any major overhaul work done.

6/ The ignition system, fuel system has not been worked on other than routine spark plug and wire changes and fuel filter changes.


Troubleshooting:

An engine needs Fuel/Air in the right ratio, a spark at the right time in the right place and compression.

Compression - I view this as a low possibility. Low compression indicates bad cyclinder sealing and I would expect smoke and oil consumption to be reported. If yuo have the tools a compression leak donw check is very cheap.

Fuel/Air of the right ratio - The carb is probably dead simple and the mixture should not change, however the time element does not make sense. Why would the fuel air mixture change over time or temperature.

Dirty fuel system - Very cheap to clean and inspect and flush the fuel system. There have been cases I know of where debris dislodges at rest and floats out of the way or sinks to the bottom but underway or after time the debris migrates to partially block the fuel system. A fuel filter and a cleaning of the fuel system is very cheap. It could be debris in the carb fuel bowl. Dirt in the carb may be harder to sus out requiring carb removal and disassembly but it is still virtually free.

I assume the carb has a float system. We have had partially sunk carb floats before. They allow the engine to actually flood as the fuel eventually fills the bowl and flows out the carb vent into the intake. This however seems to manifest usually as an engine that bogs down when throttle is applied off idle.

Reed Valves - I like this one very much. Again this is an inspection item.

Air filter and filter box clean? - Nuff said.

Spark at the right time -

Electrics can have all kinds of different failures and often exacerbated by heating.

Spark plug - Ceramics fail and sometimes can show up when hot "bleeding" a spark to ground or the spark breaks down and is not full strength.

Coils - I have had al kinds of weird failures with coils from low power to complete failure when hot and working fine again when cold. Coils can be checked for resistance when hot and cold but you will need a manual to get the specs.

Plug wire - Insulators and conductors can both have failures.

I guess that's about my brain dump. I don't post it to suggest anything. Simply hoping that something might spark a thought.

[Alan Wheeler]

Hmm in the recesses of my mind is that on 2 strokes they depend on some sort of spiral in the cylinder wall for lubrication I presume.

Ummmmm, Nope. Never heard or seen a spiral. Any marks in the cylinder wall is not good and a sign of scoring and that is bad.
2stroke is the main engine technology we teach at work to the first year students. It is a very simple and reliable engine. But because it is reliable, most of us don't get to play inside them, so they become a thing of mystery to many. Mostly left in the realm of the outboard mechanic. But for those who are into dirt bikes, even Kids learn to strip them down and replace cylinder linings and pistons and so on and so on.

[chris_gee]

Ok the recesses of my mind are a bit murky. The stuff is to decarbonise and degum the buildup of stuff. I gather some manufacturers recommend its use every 50 hours or so. Maybe it sticks in the grooves in the piston and makes the rings sticky. Dunno aint no mechanic, but it works and various manufacturers seem to have such products. It seems to me that 15 -20 $ on a can is a reasonable gamble v a mechanic making it worse.

[ssullivan]

Thanks for all the input, everyone. I have even more mystery to add, but have had a change of behavior after one of my fixes.

Seems I have mixed up the ouboard with the one I recently sold. This outboard is a 50:1, while the older one I sold is a 100:1. I had mixed it 100:1, as per the older outboard I recently sold.

Possible Theory: Rings weren't getting enough "bite" due to lack of lubrication oil. As they warm, tolernaces change to some extent, and there was some blow-by.

Now that it has run with a 50:1 mixture, the behavior has vanished (for now). I am using ethanol fuel (10%, I think). I don't like it, but they don't sell anything else around here.

I will update this thread if the behavior returns. All I have done apart from the oil mix was:

*Spark plug change (looked properly worn)
*Cleaned up contacts between capacitor, spark plug, etc...
*Prayed a lot! ha ha

I will add to the post if the behavior returns, but for now, it looks like if you have this symptom, that in some cases it is caused by either an old spark plug, dirty contacts or improperly fuel to oil ratios.

[SkiprJohn]

Aloha Sean,
Good that you sorted it out. The reason I asked about the ethanol fuel is that we have similar problems here on our island. Our only cure is to drain the float after a day's use (or run the engine out of fuel) and decant the fuel in the tank if we don't use it up on a regular basis.
Kind Regards,
JohnL

[ssullivan]

Uh oh, John. I hope that's not the problem, because it's back.

The motor worked well for about 24 hrs and then started up with the same exact behavior again. Loses power (and RPMs) to about 3/4 of what it would normally do at full throttle.

The annoying part is now I can't plane with my wife and I in it. I can just barely break to a plane with myself alone. Ugh... time to regroup and figure this out.

Theory was WRONG. Had nothing to do with fuel/oil ratio, unless I permanently damaged the rings.

[SkiprJohn]

I think if you cured it once for 24 hrs you might have hit on the problem. All clean fuel and tank (no water)? All clean fuel lines? Pressure bulb clean? Vented cap working properly?
There is usually a bit of drain screw on the bottom of the carb float bowl that you can loosen to get some water and debris out of the float bowl. Have you tried that?
JohnL

[Alan Wheeler]

No Sean, it won't be rings. That will show up from cold, not after it gets hot. I suspect a fuel issue as well. But still on the lines of some dirt or water somewhere.
When the reed valve breaks, you lose power as well. You can often get it to idle, but have little power which also equates to no or poor revs.

[ZS6FB]

When the fault appears try removing the fuel tank filler cap. I suspect a partially blocked breather hole.
Bob ZS6FB

[Pura Vida]

Sean,
There are two things you might try that are diagnostic. First time how long the engine takes to run the float bowl dry with the fuel off and the engine under load. This will give you an idea how long the engine would perform normally prior fuel flow related prolems becoming an issue. Second get a can of WD40 or some other spray penetrant that will repel water. Spray the ignition system components after the problem starts. If there is an improvement in performance the you know that the ignition parts are bad and shorting out.

I don't know if your model has a seperate coil or an integrated powerpack but I had similar problems with a motorcycle years ago. I called my father who built racing engines for motorcycles as a hobby and he diagnosed it as a coil with cracked internal insulation. I changed the coil and it corrected the problem.

pv

[ssullivan]

Those are some good ideas, PV! I'll try both tomorrow.

[CaptainK]

Hey Sean!!

I really do hope that you can get that problem solved. With your "old" outboard engine.

By the way!! I have been trying to respond to your PM, for the past few weeks. And your PM box is full.

[ssullivan]

PV, your test has put me onto the right track! Thank you, thank you, thank you, thank you, thank you, thank you, thank you, thank you, thank you, thank you for coming up with an idea that didn't involve taking apart the motor we rely on every day to commute. Getting stuck on board (or worse, having to get a dock!) would have been difficult and expensive.

I found that the loss of power conincided EXACTLY with the amount of time it took to burn through the fuel sitting in the carb (using PV's test).

This can only mean one thing (I think):

The fuel line from the carb to the tank is not allowing full flow through. As suction builds during operation, it isn't allowing enough fuel into the carb.

I am going to start by swapping out the little fuel priming bulb and attached fuel hose. The filter is squeaky clean, so that's not it. I also have run with the gas cap OFF the tank, not just the vent open, so the vacuum isn't caused by that. I'm going to replace that fuel hose in the AM and see what happens.

Thanks again for the help everyone, and PV - a special thanks for an insightful test into what could be causing the power loss. Thankfully, the problem is on the TANK side of the carb, which makes fixing it a breeze!