A bit of background info. We have a Tohatsu M8B two-stroke outboard on our 10-foot RIB dingy. The engine is about 4 years old and has seen a fair amount of use in the charter business before we acquired it (along with the mother ship) in August 2010. It has run well for us until this latest episode.
The problems probably first began when we were in Pte-a-Pitre Guadeloupe. Austin and I had made a run to shore for something when all of a sudden the engine stopped. We had had a leaking fuel hose a couple days earlier so we were familiar with the kind of quick but not sudden stopping that happens when the engine isn't getting enough fuel. The first thing I checked was that the air vent on the gas tank was open and it wasn't!! That's an easy solution, open the vent and off we go. However, only a few meters farther the engine died again! We were close enough to the dinghy dock at this point that we broke out the paddles, got to shore and tied off so we could take a more thorough look under the hood.
With the engine cover off, we discover that the fuel line has ruptured upstream of the fuel filter. I'm guessing that the pressure built up when the tank valve was closed and ruptured the old, and slightly decayed, fuel line. Again, not much of a problem. There just happened to be a chandlery a few steps away and they were happy to give us the required 5cm length of fuel hose. No problem and off we go.
A few days later, but only a few minutes of engine run time as we were travelling up the island and not using the dinghy much, we were heading into town (Deshaies Guadeloupe - 10 points for the correct pronunciation) when, at medium RPMs, we stalled again. Uh-Oh. It still sounded like a fuel starvation issue. Keeping the revs really high allowed us to get to the dock and back to the boat again. However, each time the revs dropped the engine stalled.
Lucky for me Deshaies has a great bay-wide wifi connection (HotHotHotSpot) so I was able to research the symptoms on some forums and download a Tohatsu parts manual with an exploded diagram of the carb. It really seemed like my problem was a clogged low speed jet so I set about dismantling the carb and spraying everything down with carb cleaner. My first "attempt" was a bit pathetic. All I did was remove the air inlet and spray cleaner into the carb. That didn't work. For my second attempt I took the carb right off the engine, separated the bowl from the main carb body, pulled the jets out and soaked the whole thing in some 3M spray-on carb cleaner for about 5 hours. Testing this would have to wait as we left Guadeloupe for St. Maarten at about 3am.
Once we arrived in St. Maarten I put everything back together and gave it a try. The engine still stalled at high RPMs. One other thing I noticed along the way was that I needed to have the choke fully out to start even when the engine was warmed up. This plus the stalling at low speeds really suggested a carb problem and likely something to do with the low speed jet (as mentioned in some forum archive). That said, because I had cleaned everything I also suspected that the diaphragms in the fuel pump may have been deformed and were not pulling fuel through very well unless the engine was really working hard (this was not the case as I later found out).
I bummed a ride from a cruising friend (thanks, Julian, from Oumâ) to the local chandlery who are also agents for Tohatsu to see if replacing the fuel filter, hose, pump diaphragms and jets would solve the problem. While there I spoke to the Tohatsu tech and he confirmed that it's most likely a carb problem and gave me another diagnostic check to try. This is what he suggested:
Let the engine RPMs drop but just before it dies, pull the choke out. If the engine comes back for a few seconds it's a carb issue. If not, it's something else. The theory is that pulling out the choke causes more fuel to be pulled in. Kind of like "forcing" the fuel through clogged jets or passages.
And that's exactly what happened when I tried it. Cool!
So, back to the boat with a bag full of parts and some new ideas. Since I had time on my hands I decided to replace one thing at a time, test and see what the real problem was. By mid-afternoon, after a couple breaks for lunch and beer - I'm in the Caribbean after all - I had replaced the fuel filter, fuel lines and the diaphragms for the fuel pump and still, the engine stalled at high RPMs. The only thing left was to replace the jets. However, before doing that I had one last look at them, in the daylight this time, and what do I see? A small bit of something in the middle of the low speed jet! I broke out the can of carb cleaner and a can of compressed air and whoosh! it was gone. I'm pretty quick at putting my carb back on now but I think I set a record this time.
One pull, choke all the way out, and the engine started. Running it high, OK, running it low and .... it still worked! Turning it off and starting it with the choke back in and ... it still worked! Problem solved. I ended up not needing to replace the jets. I probably didn't need to replace any of the other parts either but now they are new and the engine does sound better as a result. And I have a pile of spares, which means now I'll never need them.
My theory as to the cause is this: First, we had been having some slight difficulty starting the engine for a little while. Nothing huge but to start on a first pull we needed to have the choke out every time. This suggests that there was a slight blockage to begin with. Then, when I left the tank air vent closed and starved the fuel and created a vacuum in the fuel supply system I think that the little bit causing the blockage got sucked farther in and nearly completely blocked the low-speed jet. I'm not an expert so maybe it was just something that happens from time to time but this is my theory.
I hope this helps someone someday and feel free to post suggestions or questions.