Tohatsu M40D2 randomly bogs down

[Greggo1]

Hi All,
I have a 14 foot Aluminium V hull tiller steer open dinghy. My 2001 M40D2 starts reasonably easily on about the 4-5th pull. After about 10 minutes on 3/4 throttle it can instantly, but not always, lose revs down to about third throttle at best. Winding the throttle on harder only results in a bit more vibration. It is not coughing or spluttering like it's missing. It actually continues to rev reasonably smoothly, it just will not rev out. If I stop it will idle perfectly. While under way in this phase, it can occassionally snap back into life instantly, taking me by surprise and nearly landing me on the floor. What is stranger is if I turn it off for 5 minutes and restart it will run fine on the cruise speed for 30-60 seconds again and then resume it's decent back to a boggy one third throttle. Fuel is fine and no water is evident in the under cowell filter.
Any ideas?

 

[TOHATSU GURU]

Re: Tohatsu M40D2 randomly bogs down

You have at least one cylinder that is dieing on you. Take it into a dealer and have them check the electrical system and fuel system. You should also find out why it takes so many pulls to start your engine. A correctly tuned engine should start on the first pull or at least by the second.

 

[deejaycee_2000]

Re: Tohatsu M40D2 randomly bogs down

sounds like a reed valve acting up .......

 

[chevyjet]

Re: Tohatsu M40D2 randomly bogs down

I would suspect one of the coils or the wires to them, most of the time a failing coil shows up with the engine hot.

 

[Greggo1]

Re: Tohatsu M40D2 randomly bogs down

Thanks for the replies lads (and lasses I guess, to keep it all PC).
After putting new plugs in a couple of weeks ago, I finally got back out on the water this morning and did the "wait for problem/cowl off/take a plug lead off one at a time" thing, and narrowed it down to the bottom pot. I also noticed, and I'm not sure if this method of diagnosis is recomended in any workshop manual, that I received an equal amount of tingling shock off the lead to the faulty bottom pot as I did the other 2 leads! A human hand is not a multi meter but at least I know it had some spark!
So anyway, going forward I need to address the possibility of poor fuel supply or dodgy reed valve (my initial theory) or dodgy lead/coil on the bottom pot, which spurs my questions-
1. Can you buy only one lead/coil or must they be bought as a set (x3)?
and
2. How many hours labour would/should a mechanic charge for replacing 3 reed valves(I guess if one is gone I may as well do the other 2)?

 

[TOHATSU GURU]

Re: Tohatsu M40D2 randomly bogs down

!) Individually purchased.
2) Depends on what you are asking the dealer to do:

If you take the engine in and say "change the reed valves on all three cylinders".....!.5 hours.

If you take it in and say "whats wrong and fix it" and it turns out that it is the bottom reed set.....Anywhere from 2 to 4 hours depending on where they start their test procedures.

You do not need to change the other reeds if one cylinder has a crack or chip....And I wouldn't assume it is a reed yet. Reed failure is not that common. You are more likely to have a bottom carburetor problem.

 

[Troutboy NZ]

Re: Tohatsu M40D2 randomly bogs down

I had exactly the same problem on my M40D2, clean and retighten the GROUND bolts that secure the coil packs to the exhaust cover and see if that fixes it. My engine was only 20 hours old when it started doing this - tighten and 100 hours later no problems. can't hurt to run something like Yamaha ring-free through the tank either to clear up any potential carb deposits.