Nerf Herder

Hi, I have an old merco 35 glow engine in a 1 metre balsa and ply racing hull using a 45x direct drive prop and flexible joint. I get plenty of water through the cooling head, the hoses I use are far bigger than the nipple on the engine, and the engine is fairly open to the air. the problem i have is the engine overheats and stalls after about 5 mins use[&o] . I am using straight fuel, no nitro with 20% oil and have the needle set so i get plenty of smoke from the exhaust and good throttle response. Any idea why this is happening anyone???

Mr Cajun Gator

Richen it more, if it dies after 5 min. of running, break the engine down and check the bearings.

Nerf Herder

Ok will give it a go, do you think the prop size i am using is about right it seems to rev ok but is not particularly quick? I assume i have enough water flow, I have an 8mm diameter scoop behind the prop (home made from brass tube) what do u reckon??

Mr Cajun Gator

Yes if the prop is to big for any nitro engine, it will have lower RPM's and cause it to load up and die after a few mins. of running. Also something to think about, after the engine has been running for a few mins. it will heat up, as the engine heats, it becomes leaner. Have you tried running a few with any nitro content? A weak glow plug can also cause the engine to die. When the engine kills, is it a sudden stop, then richen it, if it is a slow stop then lean it.
Hint: when setting the needle move it very little at a time, listen and watch what the boat is doing. This will take some time, but it is well worth what you learn from your engine. Also make a note of the weater conditions, temp and humidity. This will teach you exactly what your engine wants.
Oh yea one more thing to check, Are you refilling the tank every time you run? Is the fuel level in the same place when it dies? If so, the pick up lines inside of the tank may have a pin hole or crack. Also cut off about 1/4" of the ends of the fuel lines, just to make sure that they have a good fit and are sealing.

Nerf Herder

Yes it runs great for a few mins then looses power, Ive been running it richer each time which seems to be helping so I suspect i just need to keep getting richer. I've had quite a lot of experience with plane engines but not boats so I was unsure about prop size but I think I've probably got that about right now, I was mistaken in my original text I have actually settled with a 40x prop. The fuel tank is a very good point, I will look at that. It's my own design hull and I had initial problems with the fuel tank level, I now have it set so when it is full the level is the same as the needle valve which prevents it flooding while starting. I believe the problem is due to excess heat but I will check the tank. I did try some 10% nitro, easier to start but other than that no difference, so I went back to the straight stuff hoping it would run cooler.

One more thing, it's a bit of a long shot, does anyone know if there is a pull start out there anywhere that could be used with this engine?

Thanks for your help[8D]

Mr Cajun Gator

If you have any smaller props, could you try one just to see if the engine still dies after a few mins?

Nerf Herder

I tried a 45s, much lower pitch same result