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Old 19-03-2015, 11:29   #1
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Outboard dies when throttling up from idle

I have a less than 1 year old Tohatsu 3.5 hp 4 stroke that use for my dinghy. Most all of the hours on it so far are just break-in hours that were done according to the service manual and have only had a chance to actually use it on trips a few times. It's stored in my garage on a stand and I use Seafoam treatment in the fuel (1 oz / gallon).

Something that is giving me fits is that about 90% of the time throttling up out of idle will stall. It will run fine at idle, and it will run at higher than idle but I almost always have to start the engine at something like 20% above throttle to use it. Very rarely I can gun it up out of idle and get it to make the transition, but most of the time it just stalls. Going slowly out of idle will stall it 100% of the time. Once it's out of idle it runs perfectly (both in neutral and under load), and in full idle it will also run perfectly.

Any ideas?
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Old 19-03-2015, 11:45   #2
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Re: Outboard dies when throttling up from idle

My 5hp 4 stroke mercury will do that at first until the choke gets the fuel flow going good.

Once the flow of fuel to the carb is normal it will throttle up. This process takes maybe 30-40 seconds after startup. Then I turn off the choke. Actually sometimes I ease off the choke in the process.

Try throttling up with a little choke on.

Also, I just noticed that when on external tank the vent cap on the internal tank needs to be closed.

I'm pointing to a fuel drawing problem as you can see. Are you running on an internal tank or external with a long hose?

One more thing. My engine worked much smoother the more hours it got on it. You need to run that engine some after you get this problem fixed.

Also, I do use fuel stabilize (when I know my fuel will not be used for a month or two) since folks kept recommending it so as not to clog up my carb with 10% ethanol.
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Old 19-03-2015, 11:46   #3
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Re: Outboard dies when throttling up from idle

No wisdom bstowers, but my identical motor exhibits similar symptoms. Maybe not as severe and yours, but similar.

A cold start requires the throttle to be ~25% open (with choke of course). Warm starts are no problem.

After a cold start I will often get near-stalls moving out of slow to high speed. It has never actually died, and I find if I give it even more gas it usually gets past the stall. The issue also seems to go away once the engine is warmed up. But it is a bit annoying.

I've assumed it had something to do with the engine oil warming up, since most of my time has been spent on the very cold Lake Superior. But maybe there's something else going on.
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Old 19-03-2015, 11:56   #4
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Re: Outboard dies when throttling up from idle

Running on internal tank only, vent cap open. I've run it for about an hour this afternoon here (Florida in the 80+ degree sun) and behaves exactly as it did when cold. Slowly moving out of idle dies every time, rolling it as fast as I can from idle to about 35% - 45% throttle (physically on the throttle scale, not RPM-wise) will usually get it over the hump and running if I'm in neutral. Throttling back to idle from there is no problem no matter how slowly I do it. If I go quickly from throttle to idle to throttle to idle etc, it is fine so long as I don't really let the idle fully engage. That is, I go back down to idle but I can tell the RPMs are still falling off and not fully back into a true idle. If I do let it go to true idle, then its the same old behavior.

Very, very annoying. I can probably get use to it if I had to, but the wife is going to never be able to deal with this.
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Old 19-03-2015, 12:05   #5
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Re: Outboard dies when throttling up from idle

most outboards have a "secondary" set of jets for the transition from slow to fast.. When you "POP" it to a higher RPM fast, it jumps past the transition jets and opperates off the FAST set... so you can go slow, or you can go fast but nothing in the middle.. could be a bug in the carb..
Time for a rebuild..............or at least a cleaning..........
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Old 19-03-2015, 12:15   #6
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Re: Outboard dies when throttling up from idle

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Originally Posted by bstowers View Post
Running on internal tank only, vent cap open. I've run it for about an hour this afternoon here (Florida in the 80+ degree sun) and behaves exactly as it did when cold. Slowly moving out of idle dies every time, rolling it as fast as I can from idle to about 35% - 45% throttle (physically on the throttle scale, not RPM-wise) will usually get it over the hump and running if I'm in neutral. Throttling back to idle from there is no problem no matter how slowly I do it. If I go quickly from throttle to idle to throttle to idle etc, it is fine so long as I don't really let the idle fully engage. That is, I go back down to idle but I can tell the RPMs are still falling off and not fully back into a true idle. If I do let it go to true idle, then its the same old behavior.

Very, very annoying. I can probably get use to it if I had to, but the wife is going to never be able to deal with this.
One thing I've noticed with all the small engines including the outboards I've had over the years is that if fuel is left in the carb for any length of time over a couple days is that the modern fuels will clog small jets. I now run the carb dry after every use in all the small engines and I don't have that problem.

Fuel stabilizers are good and help a bit for some engines but not for all and any fuel with ethanol will not be good for your engine if left in your carb.

I get the engine running, shut off the fuel and wait for it to die before storing the engine. That's just a suggestion for you after you get your carb rebuilt and your outboard is running again. Can you take it back to where you bought it and get them to get it running right under warranty?
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Old 19-03-2015, 12:48   #7
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Re: Outboard dies when throttling up from idle

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Originally Posted by SkiprJohn View Post
One thing I've noticed with all the small engines including the outboards I've had over the years is that if fuel is left in the carb for any length of time over a couple days is that the modern fuels will clog small jets. I now run the carb dry after every use in all the small engines and I don't have that problem.

Fuel stabilizers are good and help a bit for some engines but not for all and any fuel with ethanol will not be good for your engine if left in your carb.

I get the engine running, shut off the fuel and wait for it to die before storing the engine. That's just a suggestion for you after you get your carb rebuilt and your outboard is running again. Can you take it back to where you bought it and get them to get it running right under warranty?
From inputs to this forum, it appears the carburetors on the 3 - 3.5 HP model outboards are smaller and more finicky than the ones for the 5-6 HP models.

I haven't had any problems leaving the fuel in or running it out, but I do run out the fuel if I know I won't be starting it for say a month or so like this past winter.

Here's a link to a guide for removing the Tohatsu 3.5 Carburetor:

Tohatsu 3.5 HP Carburetor
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Old 19-03-2015, 14:24   #8
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Re: Outboard dies when throttling up from idle

Just finished removing and cleaning the carb (found a good video at since I had no idea what I was doing) and it runs perfectly now. Thanks Randyonr3, you were right.

And yeah SkiprJohn, I will definitely run it dry from now on, stabilizer or no stabilizer.

On the bright side, I know how to tear it down now.
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Old 19-03-2015, 16:07   #9
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Re: Outboard dies when throttling up from idle

Good to know that Seafoam alone doesn't prevent things like this, if the gas is left in there.
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Old 19-03-2015, 16:30   #10
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Re: Outboard dies when throttling up from idle

I also have a Tohatsu 3.5 4 stroke. It also exhibits the same tendency to stall when accelerating past idle. Has done this since new. Have had the carburetor professionally cleaned twice after getting gummed up from ethanol and it still happens. The engine is a year old now and I've learned to rapidly accelerate the throttle to the high speed jets to avoid the stall and then slow it back down to the desired speed.


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Old 20-03-2015, 08:42   #11
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Re: Outboard dies when throttling up from idle

I have a 3.5 hp two stroke outboard that exhibits the reluctance to rev up. The solution is to put in fresh gasoline. Gasoline degrades with time (over winter). With a 4 cycle you could put old gas in an automobile gas tank without a problem. I had a small Stihl chainsaw that was so finicky, demanding fresh gas, that i sold it even though it was the right saw for limbing.
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Old 20-03-2015, 10:28   #12
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Re: Outboard dies when throttling up from idle

In my experience with this motor it is flooding when you give it throttle and it dies. Have you tried with the choke to see if it makes a difference in throttle up? There is a mixture screw in the carb that on some is covered with a brass plug.. I drill mine out so that I can fine tune it. Careful to get the drill centered on the plug I only go down about 1/4" and the insert a small tapping screw. Once the screw gets a bite you can easily wriggle the brass plug out. Turning the screw in will lean out the mixture. tied securely you can test the engine on the dinghy at the dock. In fwd close the screw in 1/4 increments until she doesn't stall. I then back it off a bit to make sure the mix isn't too lean. Check your plug within the first hour of use to be sure the mixture is burning correctly.
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Old 20-03-2015, 10:46   #13
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Re: Outboard dies when throttling up from idle

This is typical of a clogged carburetor jet. When alcohol in the fuel is allowed to dry inside of the carb, a residue is left behind that can clog the jet. There are no additives that are effective that I have ever found. The best thing to do is to is to disconnect the fuel line and let the outboard burn the remaining amount of fuel out of the carb when you are done with the engine.
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Old 20-03-2015, 11:41   #14
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Re: Outboard dies when throttling up from idle

Bstowers, All fuel has some water in it. There is a way to drain your fuel out of the engine and then running it dry. When you are not going to use your engine for three or more weeks. There is a fuel funnel that will polish old fuel. I keep an empty 5 gallon tank handy for friends who keep there boat here but go home up north. We siphon their fuel or pore it through this funnel to a clean tank. The funnel will hold the water in the bottom we throw that water away and put the fuel back into their tank through the filter again probably not necessary but we do it anyway. It polishes it or takes the water out and then the fuel can be reused. This filter in the funnel can be purchased at any number of places I found it at West Marine for about $34.00 Port Supply. I carry two, one for the gas in my dinghy and one for the diesel fuel in my sail boat. I filter all fuel it doesn't take that much longer and I know I have the best fuel I can have. When you get gas at the pumps at the street in most cases there is a big cylinder by the hand grip, that is a water separator filter. I also use the water filter Clearly Filtered for polishing my drinking water. Almost same process it takes out 99% of heavy metals, and chemicals from the water.
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Old 20-03-2015, 11:48   #15
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Re: Outboard dies when throttling up from idle

Quote:
Originally Posted by bstowers View Post
Just finished removing and cleaning the carb (found a good video at since I had no idea what I was doing) and it runs perfectly now. Thanks Randyonr3, you were right.

And yeah SkiprJohn, I will definitely run it dry from now on, stabilizer or no stabilizer.

On the bright side, I know how to tear it down now.
I'm really glad you got it sorted out. There is no feeling like success after tinkering with an engine.
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