Tohatsu 5hp unreliable!

[Stratocruiser]

My five year old Tohatsu 4st 5hp engine ran perfectly last season but this year would run for 15mins before rapidly slowing misfiring and stopping (opening the throttle had no effect). It would easily restart but after another 5 mins or so die again etc. Starting does not seem to be an issue.
I replaced the fuel in the tank as it was probably stale (definitely) and changed the spark plug for good measure. The vent in the tank is open and not blocked. The primer bulb pumps fuel through the fuel filter to the carb, so again no restrictions there. Unfortunately I left the engine over winter without draining the carb so is it the the most likely suspect with a 'gelled fuel' blockage in a jet or one of the galleries inside the carb? Would a pinhole in a fuel pipe cause this problem? Is there anything else I can check? Any suggestions or advice please.

 

[C08]

I had a stopping problem with my 8 year old 20HP Tohatsu. The labrynth pipes at the top of the carb had a white hard calcious layer firmly stuck also a white powdery type of deposit. I thought of getting it US Cleaned at my local tohatsu Dealer. He said he would do it but coulld not guarantee it would be gung free as US was good with gummy deposits but not so good with the hard stuff I had. The 20HP carb is probably more complex than a 5HP but the same reservations probably apply. It is probably worth getting the carb US cleaned anyway.
It sounds like a fuel problem, is the tank vent letting air in as this would cause the symptoms you describe. other than that it is worth taking off the carb and inspecting it for gummy deposits that may be affecting the fuel level.
I bought a new carb as I was on holiday and had not the time to mess around.

 

[VicS]

My five year old Tohatsu 4st 5hp engine ran perfectly last season but this year would run for 15mins before rapidly slowing misfiring and stopping (opening the throttle had no effect). It would easily restart but after another 5 mins or so die again etc. Starting does not seem to be an issue.
I replaced the fuel in the tank as it was probably stale (definitely) and changed the spark plug for good measure. The vent in the tank is open and not blocked. The primer bulb pumps fuel through the fuel filter to the carb, so again no restrictions there. Unfortunately I left the engine over winter without draining the carb so is it the the most likely suspect with a 'gelled fuel' blockage in a jet or one of the galleries inside the carb? Would a pinhole in a fuel pipe cause this problem? Is there anything else I can check? Any suggestions or advice please.

It does sound as though it is "starving of fuel" as the 'mericans would put it.

If you were to keep pumping the fuel bulb would it keep going. If so suspect a fuel pump problem or air leaking into the fuel line. The prime suspect for air in leakage is the fuel line connector(s)

Does it have in internal fuel tank you could try ?

Double check the tank vent by taking the cap off

 

[Stratocruiser]

Thanks C08. Fuel tank vent seems clear as on a hot day when I open it, it Is expels air from the plastic tank with a hiss. A new carb sounds like a robust solution if I find solid deposits in it.
It died too quickly for me to pump the bulb VicS but it would have been a good thing to try. The engine is now in my garage and no, there is no internal fuel tank although I have connected a one gallon can and proprietary fuel line and connector to the engine for testing. I will strip the carb down, it does look fairly simple...

 

[geem]

If you have the remote tank option then check the fuel cock that isolates the integral tank. Some models have a mesh in the fuel cock that blocks easily and would give your symtoms

 

[VicS]

Thanks C08. Fuel tank vent seems clear as on a hot day when I open it, it Is expels air from the plastic tank with a hiss. A new carb sounds like a robust solution if I find solid deposits in it.
It died too quickly for me to pump the bulb VicS but it would have been a good thing to try. The engine is now in my garage and no, there is no internal fuel tank although I have connected a one gallon can and proprietary fuel line and connector to the engine for testing. I will strip the carb down, it does look fairly simple...

Exploded diagram and parts list for the carb here:- Tohatsu 2015 MFS5C - 4-Stroke Tohatsu OEM Parts Diagram for CARBURETOR, FUEL PUMP | Boats.net

when you reassemble make sure the little clip (#10) between the float arm and the needle is in place or you will have similar trouble all over again.

 

[C08]

Thanks C08. Fuel tank vent seems clear as on a hot day when I open it, it Is expels air from the plastic tank with a hiss. A new carb sounds like a robust solution if I find solid deposits in it.
It died too quickly for me to pump the bulb VicS but it would have been a good thing to try. The engine is now in my garage and no, there is no internal fuel tank although I have connected a one gallon can and proprietary fuel line and connector to the engine for testing. I will strip the carb down, it does look fairly simple...

That does not sound right - when you open the tank cap I am not sure there should be a pressure difference. It does sound as if the vent is blocked. have you tried Vics's suggestion of running it with the cap off?

 

[Slowboat35]

If it runs for 15 minutes before it goes rough it is not an obvious carb problem. Carb problems are immediate or almost immediate and tend to be permanent in the particular throttle/choke combination where they are found.
" Fuel tank vent seems clear as on a hot day when I open it, it Is expels air from the plastic tank with a hiss."
Sounds quite the opposite to me -that sounds exactly like a blocked cap breather. There should be no pressure differential across a properly vented fuel cap. Try running with the cap loose and see what happens. Riddle out cap breather or drill a 1/16 in hole right through it.

That's yer problem!

 

[chris-s]

We had a very similar problem with our new 6hp Tohatsu where if would run for 30 mins then die out. We found air was getting into the fuel line. Quickly priming it before it cut out would restore the revs and it would carry on for another 10 mins or so before needing priming again. Clear fuel line revealed the air in the line.

 

[Stratocruiser]

Slowboat 35, the plastic portable fuel tank has a brass knob in the cap so that it can be closed for transport to the boat. When I open the breather prior to starting the engine it releases vapour with a hiss suggesting that the vent is not blocked.
Chris-s, I will retrieve the portable tank and fuel line from the boat and check it all for air leaks. I have heard that the connector onto the engine can let air in.
I have found quite a few fine brass filings beneath the carb but also tiny flakes of brass coloured metal higher up. I cannot for the life of me see where where they have come from apart from when I drained fuel from the carb. There are no metal filings in the fuel filter so it couldn't have come from the fuel pump. There are brass components in the carb but I cannot see how fine shavings could be produced. This may be a red herring but none the less has anyone any ideas where this could have come from? I was waiting for a carb gadget set before stripping it down.
I tried to upload a photo but when I tried to insert the image it appeared momentarily before disappearing with an error message. Perhaps the JPEG file is too big?

 

[VicS]

Slowboat 35, the plastic portable fuel tank has a brass knob in the cap so that it can be closed for transport to the boat. When I open the breather prior to starting the engine it releases vapour with a hiss suggesting that the vent is not blocked.
Chris-s, I will retrieve the portable tank and fuel line from the boat and check it all for air leaks. I have heard that the connector onto the engine can let air in.
I have found quite a few fine brass filings beneath the carb but also tiny flakes of brass coloured metal higher up. I cannot for the life of me see where where they have come from apart from when I drained fuel from the carb. There are no metal filings in the fuel filter so it couldn't have come from the fuel pump. There are brass components in the carb but I cannot see how fine shavings could be produced. This may be a red herring but none the less has anyone any ideas where this could have come from? I was waiting for a carb gadget set before stripping it down.
I tried to upload a photo but when I tried to insert the image it appeared momentarily before disappearing with an error message. Perhaps the JPEG file is too big?

Cannot help with the brass particles but
I reduce pictures that are too big using Windows paint. If you reduce then to no more that 1150 pixels wide I think they should be OK
However new members cannot post pictures anyway

 

[Stratocruiser]

Thanks guys for your help and the exploded diagram was very useful VicS. The carb strip down did not reveal any 'monsters' within but I cleaned the jets and gave everything a clean and blow out with carb cleaner. The engine did still seem reluctant to start and I wondered if I was just over priming it as the spark plug looked rather wet. Infact the plug was not just wet but felt a bit oily too... I noticed that the oil level on the dipstick was a bit high (above full). Rereading the instructions again it clearly stated that this is as bad as too little oil! So I drained 100ml, until the oil level was mid-way between full and refil. I then drained ALL the oil out and measured it, 550ml not 450ml as stated in the manual.- confirming 100ml overfill. I refilled the engine with fresh oil to the correct dip-stick mark. The engine started first pull and has continued to do so, hot or cold. The crankcase breather is connected to the carb intake manafold (presumably to stop carb icing?) oil was present beneath the air intake which presumably came from the breather pipe and was getting into the engine. Formally there was some smoke on start up for a few seconds and the engine was reluctant to pick up. Now it not only starts easily but no smoke and fast acceleration. I'm keeping my fingers crossed that no further damage was caused. Meanwhile the engine has been uber reliable.
The engine was serviced prior to me buying it (with the boat) by a main agent and I have no reason to doubt this but looking on the web, over filling engines with oil is not an uncommon occurrence, so beware.