Tohatsu 3.5HP 4 stroke OB problem

[ProDave]

This engine is only a few years old and has been reliable. But now it has a fault.

Starting it today, it started and ran at idle, but as soon as you increased the throttle it died.

After leaving it to idle for a few minutes to warm up a bit, it would work with throttle and seemed okay. So before setting off, I thought we would try it under load. So boat still tied to pontoon, put it in gear and reved it up a bit.

It ran fine for about 2 minutes then stopped dead and has not started again since.

Plug out, looks okay, plug wet so looks like fuel getting through. Plug out and it is seen to spark, so electrics okay.

Someone had some easy start so we tried a squirt or 2 of that but no sign of life not even a cough.

Will try a new spark plug (could not get one today) but any other ideas?

 

[AndrewB]

Water in carburettor? For some reason the Tohatsu 3.5 seems prone to this. Would have come through from fuel and so stopped motor. There is a drain plug, but it is not difficult to take the carb out altogether and clean. This would also sort any possible problem with the fuel jets.

 

[greeny]

Is the fuel old? Last years? Change it.
Clean the carb and get any dirt / residue out as said previously.

 

[ProDave]

Fuel has been changed. Water in carb sounds possible, will drain the float chamber and test, along with the new spark plug.

 

[VicS]

Fuel has been changed. Water in carb sounds possible, will drain the float chamber and test, along with the new spark plug.

Check the strength of the spark with a spark tester. You should get a good strong blue spark which will jump at least 1 cm. Just checking across the spark plug is not good enough. Improvise some sort of tester if necessary.

Also worth while checking the compression ....... especially if you can find out what it should be !

I think this may be Lakesailor's homemade spark tester

 

[coopec]

A lot of people have has problems with this motor (Not saying it is a bad motor). Have you tried "Tohatsu 3.5HP 4 stroke OB problem"

 

[ProDave]

New plug has solved the problem. Just goes to show, a plug that when removed from the engine appears to spark is not necessarily "good"

 

[sparkie]

New plug has solved the problem. Just goes to show, a plug that when removed from the engine appears to spark is not necessarily "good"

Still worthwhile checking the spark as Vic suggests. Weak spark could be due to faulty coil pack/CDI unit and lead to fouling of new plug. Spark testers are available on Ebay for £4-5, and work pretty well.

 

[Boatwilliam]

A lot of people have has problems with this motor (Not saying it is a bad motor). Have you tried "Tohatsu 3.5HP 4 stroke OB problem"

Is the fuel old? Last years? Change it.
Clean the carb and get any dirt / residue out as said previously.

Had fuel problem with my Tohatsu 3.5 4 stroke, stripped carb down, ran good for short while and same problem as before, noticed today black rubber piece inside integral fuel tank. This has been blocking fuel intermittently. Rubber has come from bottom of fuel cap arrangement and is due to rubber vent arrangement perished and rubber falling down.

 

[Stemar]

I had an obscure problem on a 2T version of the same motor. Eventually, we discovered that the connection of the coil earth lead to the block was corroded, causing an intermittent weak spark. A good lean and a bit of grease and all was well.