Tohatsu 5hp 2-stroke problem.

Harry_

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Hi,

I recently bought a 1998 Tohatsu 5hp for my dinghy and I'm having a couple of issues with it and would really appreciate any help I could get.
On the first and only outing it would run fine anywhere up to half throttle, once you pass half throttle it would cut out without any warning after about 30 seconds.
After this has happened it becomes a pain to get started again and seems to backfire occasionally when trying to start it. Eventually it made a loud bang on of the pulls and it managed to fire out one of the bolts holding the lower unit/gearbox onto the leg.

Does anyone have any idea what could be going on? I'm a bit reluctant to get it looked at professionally as I can't really afford any extra expense at the moment. I'm going to rebuild the carb soon as I suspect the cutting out is to do with some gunk in there, fuel tank, lines and filter are clean.

Cheers
Harry
 

Graham376

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First off I would check the tank fuel filter isn't partly blocked. When it cuts out, have you dropped the carb float chamber to see if it's dry?

Leg bolt dropping out is weird. I would be questioning why the previous owner sold it - buyer beware.

P.S.Just noticed you've already checked filters etc.
 

VicS

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Check compression if you have a gauge or can beg borrow or steal one .

Clean carb

Check spark. It should be possible to draw a spark the best part of a centimetre from the plug lead.

Check fuel system. Assuming you have a remote tank, can you keep it going by pumping the fuel line bulb. If so suspect fuel pump or air in-leakage. The connector is the most likely place for the latter.
 

Iain C

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You say you've recently bought it? Are you using fresh fuel mixed to the correct ratio? My 3.5 can be very tempramental if there's too much oil in. I accidentally put a little bit too much in once, and thought it would be OK. It wasn't...and had almost identical symptoms to yours (apart from the bolt). New plug and the correct fuel and it runs sweetly now.
 

Cuan

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You say you've recently bought it? Are you using fresh fuel mixed to the correct ratio? My 3.5 can be very tempramental if there's too much oil in. I accidentally put a little bit too much in once, and thought it would be OK. It wasn't...and had almost identical symptoms to yours (apart from the bolt). New plug and the correct fuel and it runs sweetly now.

I agree with Iain C, new plug and new fuel fresh and mixed correctly might well fix it. If it doesn't clean out the carb and go from there.
 

VicS

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You say you've recently bought it? Are you using fresh fuel mixed to the correct ratio? My 3.5 can be very tempramental if there's too much oil in. I accidentally put a little bit too much in once, and thought it would be OK. It wasn't...and had almost identical symptoms to yours (apart from the bolt). New plug and the correct fuel and it runs sweetly now.

It should run satisfactorily with twice the normal quantity of oil, i.e. 25:1, as that is specified for the break in period for new engines
 

gus

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The beauty of 2-stroke engines is their simplicity. The right fuel mix, compression and a spark at the right time are usually all they need. The one little known problem I have found is that if the engine starts fine and runs but then refuses to restart when it is warm, the crankshaft seals need replacing. If they leak then the mixture being passed into the cylinder is weakened.
 

ghostlymoron

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Most outboard problems seem to be caused by carburettor blockages once you've eliminated obvious things like old fuel, air leaks, HT leads, faulty/old plugs. A good clean of the jets with carb cleaner/soft wire would be my first try.
 

Harry_

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Thanks for the amazing response, really appreciate all of your help.

I have just stripped, cleaned and re-built the carb, it was pretty immaculate already so I don't think that was the problem.
Spark plug seems fine, fuel lines, filter, pump, tank and vent are clean and working and have been running it on new fuel correctly mixed at 50:1.

It's all a bit strange to me as I've had two stokes with fuel issues before and they usually bog down and then cut out where as this just cuts out with no warning at high rpm and it starts first our second pull and idles fine. Is there any chance this could be to do with the exhaust/backfire and the leg filling up with water? After taking of the lower unit it seems to be missing a couple of parts 15 - 16 in the attached image. Also there's quite a bit of oxidisation inside.

Thanks again for all the help.

15967928
 

VicS

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Thanks for the amazing response, really appreciate all of your help.

I have just stripped, cleaned and re-built the carb, it was pretty immaculate already so I don't think that was the problem.
Spark plug seems fine, fuel lines, filter, pump, tank and vent are clean and working and have been running it on new fuel correctly mixed at 50:1.

It's all a bit strange to me as I've had two stokes with fuel issues before and they usually bog down and then cut out where as this just cuts out with no warning at high rpm and it starts first our second pull and idles fine. Is there any chance this could be to do with the exhaust/backfire and the leg filling up with water? After taking of the lower unit it seems to be missing a couple of parts 15 - 16 in the attached image. Also there's quite a bit of oxidisation inside.

Thanks again for all the help.

15967928

If it drops dead suddenly I would be looking for an ignition problem rather than a fuel problem. You say the plug "looks fine" but have you tried a new one ?

The parts, # 15 and 16, you mention are only fitted to the long and ultralong shaft models according to the official parts lists

p5pb15967928.jpg
 
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Chainplate

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Hopefully, the OP has found a solution to the problem by now. I, too, own a Tohatsu 5hp 2-stroke. Even though the engine has very few hours on it, we've had a couple of annoying problems. A Would-Not-Start issue turned out to be a problem with the emergency kill switch. The second problem, Starting-And-Then-Dying, turned out to be a carburetor problem. I had taken the carburetor apart several times, cleaned the jets as thoroughly as possible and the problem persisted. In frustration, I took the engine to the Tohatsu dealer who couldn't solve the problem either. He told me to buy a new carburetor. I took the carburetor off once more, completely disassembled it again, used boiling water in an ultrasonic cleaner and then used an aerosol carburetor spray. Then the engine ran like new.

Fair winds and calm seas.
 
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