Tohatsu 3.5 HP - Hard To Start / Fuel Economy

[dwedeking2]

bwiencek,



I changed the oil on Saturday (after which I cleaned the carb, and started my hard start issue).



I'll look into adjusting the valves this weekend.

[Mike OReilly]

Originally Posted by dwedeking2

...My runs are pretty consistant right now (to and from dock to go to work every day) so the fuel burn rate was easy to notice. It's a wide open run, so I'm usually full throttle for 90% of the trip.

Yes, if there’s been no change in your throttle practice or speed choice then there is clearly something going on.

Quick question though (a bit of a drift, sorry): What is ‘normal’ mileage for an outboard engine? My quick calculations puts dwedeking2’s original efficiency at around 7.5 nautical mpg (8.7 mpg or 3.7 km/litre). What do other outboards get?

[bwiencek]

Originally Posted by dwedeking2

bwiencek,

I changed the oil on Saturday (after which I cleaned the carb, and started my hard start issue).

I'll look into adjusting the valves this weekend.

When you changed the oil - did it smell like you were draining fuel out or did it have no noticeable fuel smell to it? I'm going to guess you didn't notice anything which is good in this case.



One thing you said is that you cleaned the idle jet which is good - but did you remove the idle mixture screw and clean it's passages? They are usually "sealed" or have a cap or plug over them to prevent adjustment (Emissions reasons) but you can usually pull the plug, cap or plastic limiter and then turn the screw in noting how many turns in it was so you can put it back and use that as a baseline adjustment. Then pull it out and clean the passages with carb cleaner and compressed air. You can also richen it up a bit and see if the idle improves or it's easier to start.



After you set the valves and make sure everything is good - as a test I usually dribble a little fuel down the carb and then give it a yank - if it starts right up I know that there is a fuel delivery issue still.

[dwedeking2]

I didn't notice a gas smell when I changed the oil and I would have. I hang the motor on the companion way wall which puts the dish I use to catch the oil at head height. so it would have been obvious.



No I didn't adjust the pilot screw. I will look at that this weekend.



Thank you for your help.

[Jammer]

A fact to consider is that, with 2 cylinder outboard motors, an ignition problem affecting one of the cylinders will cause a 2:1 deterioration in fuel economy.


Engines vary in whether they will run well, or at all, on one cylinder.

[CSY Man]

Originally Posted by Jammer

A fact to consider is that, with 2 cylinder outboard motors, an ignition problem affecting one of the cylinders will cause a 2:1 deterioration in fuel economy.


Engines vary in whether they will run well, or at all, on one cylinder.

These Tohatsu/Mercury 3.5s are single cylinder..

[a64pilot]

Originally Posted by Mike OReilly

Quick question though (a bit of a drift, sorry): What is ‘normal’ mileage for an outboard engine? My quick calculations puts dwedeking2’s original efficiency at around 7.5 nautical mpg (8.7 mpg or 3.7 km/litre). What do other outboards get?

I don’t know on the little motors, but the bigger ones it’s real close to 1 GPH per 10 HP if run at full throttle and rated RPM.
I’d assume the little ones would be close to that, so a 3.5 if run full bore 1/3 GPH, does that sound right?

It’s pretty much a given that all spark ignition motors that the fuel system is properly designed go to a much richer fuel mixture at full throttle, this is done for cooling, there is a surprising amount of cooling from a rich mixture, plus cooler makes detonation much less likely, and max power is at a richer mix than max economy.

A well designed fuel system for a four stroke has it running its leanest at mid throttle, at idle too lean and it idles poorly, while at full bore you need full rich to protect the motor.
Running an engine at 2/3 throttle will often burn half or less than full throttle, and yet often the speed difference isn’t really all that much, just a few knots

[dwedeking2]

Originally Posted by a64pilot

I don’t know on the little motors, but the bigger ones it’s real close to 1 GPH per 10 HP if run at full throttle and rated RPM.
I’d assume the little ones would be close to that, so a 3.5 if run full bore 1/3 GPH, does that sound right?

When I first got this motor I did an approximately 15 mile run around Stock Island. Took me 3 hours and almost exactly 3 tanks of gas at .26 gallon per tank.

Wide open the whole way

[a64pilot]

Originally Posted by dwedeking2

When I first got this motor I did an approximately 15 mile run around Stock Island. Took me 3 hours and almost exactly 3 tanks of gas at .26 gallon per tank.

Wide open the whole way

That is a little lower than my estimate, but if the 3.5 and the 5 are the same motor it would make sense, cause derated motors often don’t go as rich cause the motor isn’t run as hard.
The 9.9 Suzuki and the 20 are the same motor, but I’d bet at full throttle the 20 burns three times as much gas as the 9.9.
Plus the scaling differences come into play, but I would have expected it to burn more than a quart at full throttle.
Scaling is big motors are often more efficient per cu in.

[Mike OReilly]

Originally Posted by a64pilot

I don’t know on the little motors, but the bigger ones it’s real close to 1 GPH per 10 HP if run at full throttle and rated RPM.
I’d assume the little ones would be close to that, so a 3.5 if run full bore 1/3 GPH, does that sound right?

Thanks a64pilot, translating from mpg to gph, I’d say yes, that sounds about right for my little 3.5 engine. The tank is about 1/4 of a US gallon, and I can run around at planing speeds (~full throttle) for an hour or more.

I know b/c last season before putting the engine away I had to run it dry. Some idiot (that would be me) had just filled the thing. Took me over an hour of running back and forth across Humber Arm before it finally sputtered to a halt.

So dwedeking2, you really do have something odd going on. Ah well, if you’re going to have a problem, it might as well be an interesting one .

[Bill O]

Originally Posted by a64pilot

That is a little lower than my estimate, but if the 3.5 and the 5 are the same motor it would make sense, cause derated motors often don’t go as rich cause the motor isn’t run as hard.

The Tohatsu 4, 5 & 6 (also Nissan & Mercury) are the same base motor w/different jetting and not the same as the 3.5


Bill O.

[tweitz]

I have the Tohatsu 3.5 because of the light weight for getting it on or off the dinghy, and I have found it to be a real pain. I am not a heavy user of the dinghy, and I find that when it is running well it is OK, but it gets off its feed far too often for my taste. I also find that the tiny fuel tank and mediocre fuel economy means that refueling is a constant effort. I looked into an auxiliary tank and it is simply not set up for it. I will keep it because of the cost, but would never buy another one.

[brianbeyyoung]

I had that engine on my J/22. I found it took 2 to 3 years for the engine to loosen up and get decent fuel economy. No problem starting it though.

[mowerandy]

Sorry to hear of your troubles. I cant help since I don't know 4 stroke outboards, but was wondering whether I should make the transition.

Nope, methinks not! - my ultralight weight 2 stroke 3.5 Toey is about 25 years old, has ridiculously high power to weight ratio, has never not started by the second pull, is still on its original spark plug, and runs for hours on a tankfull (maybe not at full gas but I never tried that). It also hasn't had a powerhead water jacket for 10 years, since it corroded right off due to lack of flushing at one point in time, and still starts first pull and runs just fine on 25:1 so long as I slow down to idle speed for a couple of minutes every now and then. Had its first proper failure last month when the plastic fuel tap centre fell out due to decades of use, and it dumped a tank of fuel in the saloon and cockpit. Looks like a little screw in the right place should fix it for another 25 years...

These are my favourite outboards by a mile - if you see a good one grab it.

For what its worth - I am with whoever said that you might have a high fuel level in the float chamber and hence starting/running way rich - maybe bumped the float level tab, misaligned tab pin, sinking float, or poor feed valve seal??

Hope you can get it sorted

[CAR54]

Bought my Tohatsu 3.5 HP new in 2007, started on first pull and ran smoothly for about 2 months. Then it wouldn't idle or run smoothly. Turns out Congress mandated Ethanol in all fuel and that turned out to be the root cause of the problems. Long story short, after living with it this way for a couple of years, I replaced the carburetor and making sure only Ethanol free premium fuel is used. It runs like new now for the last few years and I always turn the fuel shut off valve to the carburetor to the off position and run till fuel exhaustion if its going to sit more then 24 hours. Its been trouble free since.