Tohatsu 50D High Speed Problem

[Seabay]

Tohatsu 1993 50HP, Model M50D3E3, problem.
At the end of last summer the motor begins to miss and run bad above about 3,500 to 4,000 RPM. At wide open throttle the maximum RPM is 4,600. It should be 5,300 RPM. I was using a 50:1 fuel mix in addition to the auto mixing feature when I did the full throttle test. The motor runs smooth at idle and at slow speeds. Compression is 136 PSI, 137 PSI and 137 PSI starting from the top cylinder. I tested each cylinder?s spark by using an inductive tachometer with the motor running on land using a water hose attachment. I ran the motor up until it missed about 3700 and beyond to 4,000 RPM. Spark was good on all cylinders through all speed ranges. I ran the motor disconnecting one spark plug at a time but I could not tell a difference between disconnecting different cylinders.

I always run fresh water through the motor after a run in salt water. Last year was the first year I had to use E10 Gasoline (10% ethanol). I always use mid range octane gasoline. I replaced the head gasket over the winter and found the cylinders to be in good shape. The motor is used on a 1986 Capri Bayliner 1400. The water pump was replaced last year.

Does any one know what the problem is? Any suggestions?

 

[cbmkr]

Re: Tohatsu 50D High Speed Problem

I was a certified tohatsu mechanic in the late 80's early 90's, this sound like a fuel problem on a high speed jet. they can varnish up easily and just a little will make them miss. there is a simple fix to see what is going on. on the side of each carb is a small brass nut about 10mm remove this (fuel will drail out) behind the screw is the high speed jet. Use a new pipe cleaner soaked with starting fluid and slide in through the hole and jet, move back and forth a couple of times and pull out . if there is any color on the pipe cleaner the mani jets are dirty. you must claen them all and add some fuel system claener merc or omc will work.

 

[TOHATSU GURU]

Re: Tohatsu 50D High Speed Problem

That symptom is broad enough to cover just about any problem....But cleaning the carbs is always the first step. No spray cleaner is strong enough to do anything so you will need to take them apart and soak them for at least three hours in carb cleaner. Put them back together and then see what happens. You have a 50% chance that you will have fixed it.

 

[Seabay]

Re: Tohatsu 50D High Speed Problem

I tried the pipe cleaner and starter fluid through the carburetor drain holes and the pipe cleaner comes out clean. I have ordered carburetor gaskets. I intend to remove the carburetors, open them up and try to clean anything associated with the main jets. If I do not see anything varnished up or out of the ordinary I may try soaking the carburetors. The Clymer manual has a caution that says "Do not submerge or soak the carburetor in a caustic carburetor, cleaner or hot tank. Do not expose any plastic parts to any carburetor cleaner." I have rebuilt several automotive carburators with success. I have use Gumout carbureator cleaner for automotive repair but with that warning I guess I should stick to isopropyl alcohol for metal and I tried the pipe cleaner and starter fluid through the carburetor drain holes and the pipe cleaner comes out clean. I have ordered carburetor gaskets. I intend to remove the carburetors, open them up and try to clean anything associated with the main jets. If I do not see anything varnished up or out of the ordinary I may try soaking the carburetors. The Clymer manual has a caution that says "Do not submerge or soak the carburetor in a caustic carburetor, cleaner or hot tank. Do not expose any plastic parts to any carburetor cleaner." I have rebuilt several automotive carburetors with success. I have use Gumout carburetor cleaner for automotive repair but with that warning I guess I should stick to isopropyl alcohol for metal and soapy water for plastic parts as Clymer recommends. Is there a specific aerosol solvent or other solvent safe for this purpose?

With the carburetors off I should be able to see part of the reed valves. Is there anything I can tell about the condition of the reed valves by looking at them inlet manifold assembly?

One thing I did at the end of last summer while in the water was to test the fuel pump. I forced fuel through the system by squeezing the fuel bulb while the high speed miss was occurring. This has no effect on the problem.

I use 6 gallon plastic fuel tanks. At the end of the season I fog the engine, empty the carburetors and clean the fuel filter and oil filters. I notice from the Tohatau parts manual that the oil filter and fuel filter elements are different part numbers. They look the same to me. How can I tell which one is which?

 

[TOHATSU GURU]

Re: Tohatsu 50D High Speed Problem

The Clymer warning is generic and is designed to keep people, who do not have a clue, from leaving the the carbs in cleaner too long. Do yourself a favor and take it into a dealer and let them diagnose and fix it.

 

[Seabay]

Re: Tohatsu 50D High Speed Problem

The dealer I bought the motor from in 1993 is now a dealer for a different brand of outboard motors. When I try to get it fixed in the summer they say they are too busy, bring it in the off season. When brought it in the off season they say it has to go in the water to test that problem and it is too cold to do that. Then they replaced the water pump and did nothing to fix the main problem. I went to the next supposed to be Tohatsu dealer that I have used in the past for my Evinrude kicker motor. He said he works on Nissan outboards only and will not service Tohatsu motors. I went to the third supposed to be Tohatsu dealer he said he only works on low horsepower Tohatsu motors and is not equipped to service a 50HP motor.

So much for Tohatsu dealer service. That is not an option.

I really can not complain about having an outboard motor that ran good for 14 years. A motor with 136PSI compression should be fixable. If I can not get a dealer to repair it I will learn to do it myself. It does not appear to be that difficult to do.

 

[TOHATSU GURU]

Re: Tohatsu 50D High Speed Problem

If you will forward the name of the last two dealers to me I will have them lectured. Also, what brand of engine does the first dealer do now? Obviously they must do a very poor job representing their latest brand if they are not willing to help you, a previous customer.

 

[Seabay]

Re: Tohatsu 50D High Speed Problem

Yee of little faith. I removed the carburetors and found a piece of gasket from the air silencer had broken off and lodged near the high speed jet on the bottom carburetor. I think this is was the cause of the main problem. I was surprised on how clean the carburetors were. I guess the E10 gasoline really cleans everything out. I did remove what looked and felt like a few grains of sand in one carburetor. I was happy to see no signs of eroded rubber parts that means the carburetors survived both MTBE and E10 gasoline without damage. I followed the ?Carburetor Tuning" section of the M40D/M50D Tohatsu Outboard motor Service manual and with the use of an automotive carburetor tester flow meter the motor now runs better than it did when it was new.