Learning curve for a used Tohatsu 30hp buyer

Greetings from Singapore! This is my first post here.


I recently bought a seldom used Tohatsu 30hp motor and the plate says: 2006 model MFS30B 3AC. Mounted to a 14' cathedral hulled tender, it could barely make 7 knots with the wind and current when 3 persons were on board. No amount of changing on the kicker plate could get the boat to plane, even when both passengers are seated on the bow. Other than that, it started ok and ran smoothly.


I took out the fuel filters on both the plastic tank and on the motor, blew them clean, replaced with fresh petrol, removed the shelter to lighten the boat, scrubbed the hull clean, but no joy. Not until I swapped out the 9.9 x 13" prop with a 9.9 x 11" that it began to run better. I attached a pulse reading tachometer and got 5300 rpm at WOT with 3 people on flat seas. I thought I had the problems licked, but then it happened.


While starting up to get home, I got the starter rope wrapped around the tiller handle when the engine came to life. Even though I pulled the kill switch, the rope return spring was displaced and I had to remove that assembly and use the emergency starting rope to crank up.


While getting back, I hit some rough water and noticed splashing water drops getting into the void that was where once the pull start would be covering. I slowed to an idle to wait for the traffic to pass and the water to flatten. I could have left it on idle for half an hour or more. When I eventually throttled up to move, the engine bogs down and dies. After many tries, I managed to get started again, and while it idles nicely, it bogs down and dies when I throttle up in gear.


I eventually managed to get the engine to go up in RPM by gradually opening up the throttle and allowing the motor to catch up in revs, and we made it back to the marina. Back in the shed, I flushed down the engine and next day, fixed the return spring back into the starter housing. Then I found it difficult to fire up.


Pulled the plugs and found all black, with the third plug a bit moist with oil. Replaced with new NGK DCPR-6E. No improvement. Replaced engine oil, oil filter and lower unit oil since my hands are already oily. This time, it starts up promptly.


Yesterday, 3 of us loaded up, and went fishing. The motor ran smoothly and we could come to plane despite the largish ice box and all the gear, until we stopped to jig for half an hour. Since we were close to the pier, I left the motor in idle so that I could quickly get in gear to reposition when we drift too close to the pier legs. After 30minutes or so of idle time, we moved on, and the motor bogged down again when I throttled up. This time, it stayed running, but at a slow RPM.


I checked the temperature of the water stream on the tell tale and it was a normal warm. Rechecked the fuel tank's air vent was open and the fuel hose was securely attached, and they were. So again, by slowly increasing the throttle, I managed to coax the RPMs back up again, and the rest of the day was normal. But I dared not leave it on idle for longer than a minute anymore.


Back at the boat shed, I found the motor now refuse to start up unless I give it a bit of throttle, but when I did, it'd fire up in one pull always. I looked around (in the darkness of twilight) and couldn't find a slow idle adjustment screw on this motor, so I packed up and left.


Now, I'm on my way to Malaysia for a freshwater fishing trip. But my mind is going crazy thinking what could be the cause of the engine behaving that way? Could it be the anti overheating circuitry coming on that the engine will not throttle up after an extended idling time? If so, wouldn't the water stream feel hot to the touch?


And what about the slow idle adjustment? Should I go tighten the throttle cable a bit more?


Any thoughts are welcomed as I'm new to 4-stroke outboards.


And thanks for taking the time to read this as I couldn't sleep on the journey up north. So while I wait for the bus to take us to our destination, I'm typing this with hopes that I can get my mind off the thoughts and fall asleep.

BR,

Lawrence Lee

Lawrence,

The MFS30B is fuel injected, and has an ISC (idle speed control) valve... not an idle stop screw. There is also a vapor separator inside the cowl, as well as a high pressure filter (in addition to the bowl-type filter and tank filter) which should be changed every 2 years or so. The motor should also use a water-separator filter. If left to sit for a month or more, the vapor separator should be drained. If the injectors are slightly crudded up, idle could suffer slightly, and foul plugs (if the injectors are dribbling).

You really should try to get a Factory service manual (copyrighted, so no free dl available), and pay very close attention to the fuel system section.
The owners manual is available free at http://tohatsu.com/tech_info/manuals.html
An overheat would cause the beeper and the led to come on, and the speed governor clears once the motor cools (does not require a restart) so you are likely not overheating.

Idle speed is about 850 RPM when warm, and about 300 more when cold. It is determined by the ECU, which operates the ISC valve.
WOT should be between 5250 and 6250. It will work better near the high end of that range.

That motor, like all 4-stroke outboards, will actually gain crankcase oil, especially at low RPM and in cold weather. They work better with 10w-30 non-synthetic oil, filled below the full mark on the dipstick -- never over.

Thanks for your insight Paul!

pvanv said:

The MFS30B is fuel injected, and has an ISC (idle speed control) valve... not an idle stop screw. There is also a vapor separator inside the cowl, as well as a high pressure filter (in addition to the bowl-type filter and tank filter) which should be changed every 2 years or so. The motor should also use a water-separator filter. If left to sit for a month or more, the vapor separator should be drained. If the injectors are slightly crudded up, idle could suffer slightly, and foul plugs (if the injectors are dribbling).

Click to expand...

I did ask our local Tohatsu distributor for the high pressure filter, but he told us it's not necessary to replace as he hasn't seen one needing replacement in the 10 years he was local distributor for this motor, so it was one of the things I didn't replace when I serviced the motor after buying it over. I'll go order one in and also drain the vapor separator when I get back.

It is our habit to run the carb dry on our old 2-stroke, when flushing down the engine at day's end. So after the first trip out on this motor, the boys did what they usually do, which was to run this motor with the fuel line unplugged till it stopped. They even told me it took a good 15 odd minutes of idling before it ran out of fuel. I told them not to do so as it's not necessary for this motor.

Should this practice continue? I've never heard of people running dry their fuel injected motor before, and my own experience bleeding air lock on fuel injected diesels tells me running a fuel injected engine dry of fuel is bad. Could my problem be due to having run the fuel dry since I coincidentally started having this problem after the fuel was run dry.

pvanv said:

You really should try to get a Factory service manual (copyrighted, so no free dl available), and pay very close attention to the fuel system section.

Click to expand...

I'd like to. My local Tohatsu distributor doesn't carry a copy. Would you ship overseas?

pvanv said:

That motor, like all 4-stroke outboards, will actually gain crankcase oil, especially at low RPM and in cold weather. They work better with 10w-30 non-synthetic oil, filled below the full mark on the dipstick -- never over.

Click to expand...

I do not comprehend this statement. How does the crankcase gain oil? Why is mineral oil preferred for the motor?

My Tohatsu dealer sold me Motul brand 15W-40 Fully Synthetic oil. He assured me it would not react with any oil used by the previous owner, as I was concerned that the previous owner had used a fully synthetic lube. Cool weather here is 28° C. But mostly, it's 32°-40°C in the afternoons.

I did notice that when I replaced the crankcase oil, I measured 1800ml as I had also replaced the oil-filter. However, after turning over the motor and allowing it to drain back down, the dipstick read below the range of marks, so I added another 150ml to bring the level up to middle of all the xxxxx marks.

Only necessary to drain the separator when storing for a month or so. The high pressure fuel feed pump was running the separator down for the 15 minutes. Air should not be an issue, the vapor separator acts like a carburetor, allowing vapor off the top, with pure fuel at the bottom.

I am no longer working for a dealer, so I can't ship parts. Check the dealer locator on the Tohatsu site, or see boats dot net to ship overseas.

Search the internet for 4-strokes "making oil". The crankcases don't get hot enough to vaporize blowby products, so the level actually rises. This is worse with synthetic oils. But if your level is in the middle, you should be OK.

Might just need to flush the fuel system, and maybe the fuel cooler water passages.

pvanv said:

Only necessary to drain the separator when storing for a month or so. The high pressure fuel feed pump was running the separator down for the 15 minutes. Air should not be an issue, the vapor separator acts like a carburetor, allowing vapor off the top, with pure fuel at the bottom.

I am no longer working for a dealer, so I can't ship parts. Check the dealer locator on the Tohatsu site, or see boats dot net to ship overseas.

Search the internet for 4-strokes "making oil". The crankcases don't get hot enough to vaporize blowby products, so the level actually rises. This is worse with synthetic oils. But if your level is in the middle, you should be OK.

Might just need to flush the fuel system, and maybe the fuel cooler water passages.

Click to expand...

Thanks again for the reply.

I just got back from snakehead fishing. Too much catch and release in the lake had made them wise. I'm home with a sore wrist from making over 500 casts and only landing one small one on artificials.

I'll head back to the boat shed once my wrist stop hurting and drain the vapour separator.

Drained the vapour separator last Friday and now she runs smooth and starts on the third pull at worst!

Thanks Paul for your guidance.