Tohatsu 3.5 HP - Hard To Start / Fuel Economy

[dwedeking2]

I've had my Tohatsu 3.5 HP 4 Stroke (new) for 4 months. Was running great up until about 2 weeks ago, when I noticed a decline in fuel efficiency. By decline in fuel efficiency I mean, I would normally fuel up at the dinghy dock and an tank of gas would get me to the boat and back (1 mile each way) with spare. Now I'm filling up on each leg, as I only make it there and part way back.



Since I had to change the oil this last weekend I figured I'd clean the carb while I had motor up on the mothership, thinking this may be the issue. I've clean the carb (blown out the jets with carb cleaner etc) before without issue. After this weekend it's been hard to start (tore down and put back together a few times on Saturday trying to remedy the issue). From a cold start I now use no choke and full throttle, starts after 4 or 5 pulls. After warmed up, it's horrible. Full throttle and 20+ pulls (just as I'm ready to drop it off the transom to a watery grave it gets scared and starts up).



Once it's running it's smooth as butter (other than low fuel efficiency).



My gut (but I'm not a professional mechanic) says idle jet is dirty but I've cleaned it out a number of times on Saturday and it was clear each time (carb cleaner spraying out the opposite end as a fine mist).



Any ideas?


Thanks in advance

[MarkSF]

The jets on these small carbs on small 4 stroke motors are too small to clean effectively. The best approach is prevention. Do you run the carb dry after use, and do you use a fuel stabiliser?


So what now. You could replace the jets, or replace the carb. While replacing the carb is not cheap, presumably it's cheaper than throwing the outboard in the drink.

[MarkSF]

Having said that, at 4 months old, isn't it under warranty? Why dont you get it serviced at a Tohatsu repair facility? That's what I did with my Suzuki that had similar problems. They replaced the carb.

[Dockhead]

That seems like completely wrong fuel economy to me.


That motor (I have the Mercury version) is supposed to use 1.4 liters per hour AT WIDE OPEN THROTTLE, which more than 40 minutes on the 1 liter built in tank.


Depending on your dinghy, that ought to be getting you miles, even at wide open throttle.


At economical partial throttle, you ought to have endurance of well over an hour, maybe an hour and a half, for a range of perhaps 6 miles or so on an average dinghy.


2 miles seems way off to me -- maybe there is something fundamentally wrong with the carb?


You might want to check whether you're leaving the choke on, or if the choke linkage is such that it's not getting shut off completely.

[dwedeking2]

MarkSF,



The dinghy is run daily (actually every 10 hours as I'm working two jobs) and up until I took the carb apart on Saturday ran well (other than the fuel burn rate change). It started within 3 pulls at full choke with just a little above idle throttle, and restarted in one pull. I do run stabilizer in the gas.



I hadn't run it into the mechanic as I've been working all week and would prefer to fix it myself to a) have it running properly now and b) learn more. I may just order a new set of jets first.



Dockhead,


Yes, when I first got the motor that was my burn rate.



I'm not sure if I'm combating two issues (burn rate being one, and start issue being another) or if they are both signs of the same root problem.

[Shrew]

I've had my Tohatsu 4HP for a couple of years now and find it fairly temperamental to start. Here's what I had luck with.

1) Startron and stable (marine for ethanol) with every tank of gas.

2) Close the fuel petcock and run the carb dry if the motor is going to sit more than overnight.

3) I find that if there are too many people sitting too far aft, the backpressure in the exhaust becomes an issue. I try to sit forward, such as on the seat and start the motor before loading the dinghy.

4) I don't bother with carb rebuilds. Compare the rebuild kit price to a new carb. For $100 every 5 - 6 years, I buy a new carb when it really becomes a problem.

Admittedly, 4 months seems too short to begin developing an ethanol or lacquer/varnish issue

[Shrew]

Also, I find that to start the Tohatsu, I end up slowly pulling the starter until I can feel the copmpression stroke. To compress, the starter rope ends up about 1-2 feet extended. That is where I start the 'Pull' stroke. That tends to help. If i'm not on the compression stroke, or start with the pull rope too far in (which would mean there is little compression), I have a much harder time starting it.

Pull the starter very slow 1-3 times, you will feel it begin to tension about 30-40% out on one of the strokes. That is where I start. I've had good luck with that method.

Just 'pull and pray' doesn't work well.

[Dockhead]

Originally Posted by dwedeking2

. . .I'm not sure if I'm combating two issues (burn rate being one, and start issue being another) or if they are both signs of the same root problem.

It's likely related.


Out of spec fuel consumption rate is a very serious issue which should be solved. Too much fuel burned means its being burned too rich or leaking somewhere it shouldn't, both of which are bad.

[Shrew]

Originally Posted by dwedeking2

From a cold start I now use no choke and full throttle, starts after 4 or 5 pulls. After warmed up, it's horrible. Full throttle and 20+ pulls (just as I'm ready to drop it off the transom to a watery grave it gets scared and starts up).

This would tell me that the choke is wide open, and therefore you are opening the throttle to ensure the fuel and air ratio is correct. Use the choke and you shouldn't need to open the throttle.

This leads me to another step I missed (above). You will need to pull the choke and advance the throttle to 25% as well. Apologies for missing that. I marked the TRUE start position on the throttle which is beyond the one from the factory.

[CSY Man]

Got the same motor, (Mercury) found it hard to start but brand new, only got 3 trips, or 1:20 hours run time. Using only no-corn fuel, use stabilizer and run the carb dry in a fresh water bucket after use.
Best I have done is 3 pulls when cold, but more like 10-12 pulls is the norm.
From this thread it looks like it won’t improve.

Got the extra 3 years for a total of 6 years warranty, should be time for lots of new carburetors or whatever it takes to make it start easy..

[a64pilot]

It’s not a dirty, or varnished up carb, or almost certainly not anyway as any of that leans a motor out.
My bet is you have the float too high, likely bent it a little when the carb was apart. That will make one go rich.

The only other even possible thing that comes to mind is if the carb has a fuel enriching circuit as opposed to an actual choke, if it doesn’t then you could have buggered up the enriching circuit.

Years ago I bought an old mid 80’s 25 hp Mariner, which is of course a Yamaha.
Ran like crap except full throttle, couldn’t figure it out, then after finally getting a parts diagram I noticed the spring and little ball was completely missing from the enriching circuit, so it was on all the time. Got the parts and the little bugger ran perfect.

Oh, the float could have fuel in it too if it’s a hollow one, shake it to see, a solid one you have to weigh, of course you have to know the correct weight to start with.
Fuel in a float makes the level incorrect obviously

[Bill O]

Nothing personal here, but since the now hard starting issue happened after you took it apart, it begs the simple question, did you put it back together correctly or set something incorrectly?
The reason I ask is based on a Tohatsu 6 hp 4 stroke that had similar issues. The guy I got it from claimed to be an engineer, but reassembled the carb and spacer plate incorrectly. He had a heck of a time trying to start it etc and basically threw it down. After disassembling it myself I noticed it was incorrectly put back together (w/the help of the parts diagram). Once correctly reassembled, it started up easily.
On the 3.5 carb make certain there are 2 seals on the "left" side and no seals on the "right".

If you still can't get it to work, I'd go w/MarkSF's advice and take it in under warranty. It's part of what you paid for when purchasing a new motor.


Bill O.

[Mike OReilly]

I’ve got the same engine: 3.5 Tohatsu. No real wisdom to add since I suck as a mechanic, but thought I’d add my experiences. So far I’ve had no significant problems with the engine. I’ve never cleaned the carb or had it fail to start. Mine is going on seven or eight years old now.

I haven’t accurately measured fuel consumption. I notice it varies greatly depending the the throttle setting. I can go for very long time on one litre of gas at low to mid throttle. But if I push it to the higher throttle range my fuel consumption increases dramatically.

For cold starting I use full choke and place the throttle about 25% open. This is above the official “start” position. It usually starts with 2 to 4 pulls. I find that consistent-speed pulls over the full range works best. My partner has difficulty with cold starts. I think it’s because she has a harder time maintaining consistent speed over her pulls.

Shrew, your description of hitting the compression stroke is insightful. I wonder if that’s what I’m connecting with my consistent-speed pull?

Warm starts are usually very easy. No choke, one easy pull.

I always run the carb dry if we’re not planning to use the OB within the next, say, 48 hrs.

[dwedeking2]

Bill,



Nothing personal. Originally on Saturday that was my thought as well, that I put something together wrong. I took it all back apart and put it back together paying particular attention to what I was doing.



I've taken this carb apart and fixed a plugged main jet issue before.



Was hoping the combination of hard to start and fuel burn rate would cause someone to go "oh, yea look at A & B". I'll take it into the marina and have their mechanic look at it.



Mike,

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.

[bwiencek]

First with consumption like that and assuming that it's a 4-stroke engine - check the engine oil to see if it's thinned out from fuel contamination (it will be thin and smell like raw gas). If you're dumping a ton of fuel in the combustion chamber that goes unburnt then it usually washes down the rings which can cause slightly low compression and reduces the oil lubrication qualities. Change the oil and see if that helps at all.



If it's a 4-stroke - try adjusting the valves - it sounds like you've got plenty of hours on the engine so it's probably time for a post-break-in adjustment. Valves that are too tight can bleed off some compression causing it to use more fuel and cause hard starting when hot (and things expand at different rates). I see the valves on the little engines tight more than loose as the valve receeds into the seat & head and/or the valve stretches (usually not a problem on a quality engine part)