Small Power System

Having problem with my Tohatsu 5hp 4 stroke . It came with Athena for her dingy, from day 1 it ran great until you give it full throttle then it cuts out and dies. Then it will start up and run at low rpm's until you give it to much throttle.
So I'm thinking lack of fuel, so I went there the fuel system from tank, fittings, carb jets, etc. No change. Then I pulled the plug so I could turn her over easy and pull the fuel line off the carb after the fuel pump and turned her over.
No fuel pumping! Pulled the pump off and found oil behind the diaphragm, so I cleaned it out reinstalled and Its pumping fuel. Problem fixed right, No problem may be worse? Any Ideas. The motors only worth about 6 or 7 hundred so don't what to drop to much into her, but she fits the bill nice.

See if there is a mixture adjustment for high speed running.

Try a new spark plug and reduce the gap a bit.

A intake gasket leak or the float set too low could also be a cause.

Change the plug. Check tank vent. Check fuel line for interior delamination of membrane.

There are only fixed jets and adjusted the float. I have a carb kit coming with new gaskets and a fuel pump. I will check into the plug, it did not cross my mind that it could be a plug problem. I will get a new hose and primer as well. that will make the fuel system all new!

I bet you'll be fine after you rebuild the carburetor. Small 4 stokes are infamous for their finicky carburetors. I wouldn't be surprised if the previous owner stored the OB with gas in it. We always run ours dry if we aren't going to use for a several days or more.

Many of those carbs (like OMC) have tiny...I mean really small jets frequently picking up fuel from the bottom of the bowl. When you rebuild the carb run a small wire about the diameter of a hair through those jet(s). You may not even see anything in there but any obstruction what so ever will screw up the running. Blowing and solvent do not always move solids.

I had the exact same problem on a 3.3 Mercury which is a Tohatsu as well. After doing almost everything mentioned above I gave up and took it to a local shade tree mechanic in FL. $40 later it was fixed. Turned out to need a new o-ring that seals the little fuel shut off valve. It's the little handle on the side of the engine that shuts off the fuel supply from the tank. When revved up, it would suck air through the bad o-ring.

Hope this helps.

I have had good luck with disassembling the carb as much as possible and soaking overnight in carburetor cleaner. The small passages get plugged with ethanol gunk and soaking cleans them out.

Also check the fuel line, strainer and valve as well as the tank for gunk buildup. You can drain the tank, fill it with carburetor cleaner, run the cleaner through the fuel line and leave it sitting for a while. But as others have noted, sometimes the sealing membrane in the tank fails.

David

Great advise, got some parts coming. Ill let every one know how it go's.

I had trouble last summer after not draining the carb after my winter trip.

4 times I dissassembled and cleaned and soaked in 3 different carb cleaner solvents.

I hand one of the mechanics at the marina soak it in his solvent and run it in the ultrasonic cleaner 2X.......problem solved..

Good to know
I just found a jet that is behind a brass plug, I'm sure the plug is for emitions control. But when removed I found yet another passage that seemed to have gunk in it. I hate the factory set jets, they run when you buy them but after some hours they start running bad with now way of fixing it.

Some 4S Tohatsu carbs have a plug over the idle mixture screw screw. Drill a little hole in the plug and pull it out with a self tapping screw. Then tweak screw with a little screwdriver. Can get a nice idle then.

One common hassle is a rubber tipped float bowl needle .

These can swell and not pass enough fuel for high speed operation.

I think you found your problem. Tiny fixed jet so we won't cause any more air pollution by adjusting our own engines.

"Tiny fixed jet so we won't cause any more air pollution by adjusting our own engines."

IF the EPA finds out you may join VW in being hung in public!

I will keep a low profile and claim I don't remember. Works for politicians.

Ok new carb kit and I it looked like I found the problem. the float jet did have a swollen tip compared to the new one. installed kit and it ran great until it warmed up ,then same problem. So fuel must not be the problem I turn to electrical. I had originally clean and gapped the plug, but put a new one on and problem fixed. thanks for the help .