Tohatsu 4hp 4stroke won't idle

The motor ran great for about a year, then i let it sit with ethanol for about 3 months... When i tried to start it then next time, my friend noticed gas coming out of the powerhead. I put it away and let it sit for another couple months before I finally got around to fixing it...

When i finally started working on it, the first thing i did was take off the carb and clean it. I also changed all fuel lines. Upon starting the engine I experienced the following symptoms:
- it would start with the choke, but would quickly stall unless the choke was removed and generous throtle applied
- it would run but only with generous throttle
- it would not idle or run well at low throttle
- after running for awhile, backing off the throttle at all would stall it
- after stalling once or twice it will refuse to start at all until several hours of time off

I thought that the issue was still the carb so I cleaned it again and removed the brass plug over the main jet needle. I played with this setting as well as the idle set screw. After this... the same exact symptoms!

I assumed that the issue was still the carb, so I bought a new one (this time a 6hp model since that seemed like a good idea!), replaced the spark plug, replaced the fuel filter and unfortunately, I still get the exact same symptoms!

After about 15 minutes of running and stalling and refusing to start, I checked the spark plug and saw a small bit of white residue on one side of the plug. This might not be a good diagnostic however, since I used engine starting fluid to start the motor and the old plug was soaked in gas when i removed it, presumably due to the motor laying on it's side during transport two days prior, though i'm also not sure about that.

If anyone has any advice I am all ears and well beyond my wits end! Thanks,
Fred

I'm guessing your model is MFS4A2, or possibly a 4B, right? Your original symptoms sound like you had varnishing in the carb. That can usually be cleaned only with a 4-hour bath in real carb dip (after a complete disassembly). There is no brass plug over any sort of main needle. You probably drilled out the EPA-mandated plug over the idle pilot screw. That screw should not normally require adjustment, and will only affect idle at any rate. Changing the idle stop screw would only be needed after you have the rest of the bugs worked out. Why did you change the fuel lines? Why did you change the fuel filter?

Now then, since you installed a new carb (and yes, the carb for a 6A will work fine), and still have similar symptoms, you may have either gotten a bad carb, or something else is your problem.

Do not use ether-type starting fluid, unless you want to crack a ring. The plug will not get soaked in gas from laying the motor on the wrong side... however, you could fill the cylinder with oil, which could cause liquid lock.

What is your oil level? It should be between the 2 hash marks on the dipstick. If you are over full, you could get these symptoms.

At this point, I would recommend you take the motor in to a dealer, as they will have the expertise (and factory service manual) to diagnose it.

You are correct, I have the 4B model and the brass plug was removed from the idle jet not the main jet.

For further info... I have used ether type starting fluid throughout this process, so how likely is it that i damaged the engine? Also, the plug was covered with OIL when i changed it... however, after running for about 10 min and checking it it was clean. also the oil level seems to be a bit high, although I have not added any oil since i got it. I will change the oil this afternoon.

one further peice of information: when starting up, each pull of the starter cable prior to the engine starting made a slight gurgling noise... although this was only noticeable when the plug was covered in oil, so it may have been a result of oil in the cylinder from laying on its side during transport.

I find it highly unlikely that I would have the exact same problem with a varnished carb, a cleaned carb and a brand-new carb if the problem was merely carb related. Are there any other common reasons for these symptoms?

I feel like the engine wont idle, but when i decrease the throttle it is flooding or something.

Thanks for your help!

are my symptoms in any way indicative of a bad carb gasket? The gaskets all look great to the ***** eye and don't appear to be brittle or cracked at all so I doubt if they are the problem, however, It really seems like a fuel related issue... another idea: could the fuel pump be a problem? I took it apart and it looks alright. Also the carb bowl fills with fuel after a couple pulls on the starter so it seems to be working.
Fred

Sounds like the motor was shut down and laid up incorrectly. Hence the gurgling, oily plug, etc. Once you get the correct oil level, and a clean plug, and a good carb, you can see how it goes. If you did not do complete carb disassembly (jet, nozzle, et al), followed by a 4-hour soak in real carb dip, you did not adequately clean the carb. Sadly, if your fuel has ethanol, it might very well have some water content in that fuel... and leaving that junk sitting in the carb will corrode it. That cannot be cleaned by ordinary means, so if that is indeed the case, you need a new carb.

The 2 carb gaskets and the thermal plastic spacer that goes between them seldom get damaged.

If you are running from the internal tank (and the tank is at least 3/4 full), the fuel pump is not needed, as the gas level is above the carb height, so no, it is doubtful that you have a fuel pump issue.

At this point, maybe you should consider getting the motor to a dealer so they can diagnose it.

As soon as I get home I am going to change the oil and try again... I have one more idea as well:

When all this began, I lost the small round metal disc with holes in it that covers the intake of the throttle body. I am getting a replacement today. Is it possible that this disc limits the airflow enough to affect the carburation? It seems possible, since the carb/jets are so small and the metering is so precise. I asked the dealer this question in the beginning and he assured me that the disc did nothing other than act as a filter, but if it also is used to restrict airflow then this may be the real problem...

Thanks for the help. If this does not fix it then I will take it to a dealer. I will post when I know more.
Fred

The air box is in fact a backfire arrestor. If might cause some slight enrichment of the motor... but since I have never operated a 4/5/6 with that removed, I can't say for sure... But I'm sure it's not too significant.

Full Success!!

The motor is now running perfect... I can NOT stall it at any throttle setting. It purrs at idle and roars under throttle. What I did...

Replaced oil. Replaced ALL fuel lines AGAIN. THOROUGHLY cleaned the fuel petcock (as well as the all other connectors in the fuel line). Cleaned gas tank and rinsed with fresh gas....

Once all the old gas was removed (and any part that had ever touched the old gas was either replaced or cleaned) the new carb had no trouble doing it's job. Also of note: I have the 4hp B-model engine, and I replaced the carb with the 6hp A-model carb. This worked for me and I have heard that the EPA emissions restrictions were lighter on the A-model making it less finicky (and uonly slightly less efficient). It was a bolt on replacement and no adjustments were necessary.

Thank you so much Paul for your help. This site is awesome and I can't wait to get out there and fish!