Hi
Just thought I would show you what happened to my brand new engine (SC60) after trying to run it in at home on a water pump.
This motor was brand new, thought I would run it in by circulating the water around the head with a fuel pump to save me time over the lake, the pump was run on 7.2 volts so there was a fair old gush comming out of the side of the boat and back into the water pick up bucket. As you have probably read was having a lot of problems starting it and keeping it going, then I noticed the compression had gone off, when the engine was stripped down I found that the piston had semi sized and destroyed the fit. I believe this was caused by the piston and liner not being able to reach there opperating temperature due the amount of water cooling the engine, this meant that the piston and liner could not expand at the same rate so I guess that the piston expanded more than the liner and seized resulting in the loss of compression. Lucky Piston and liner for the SC only £30. Boat is now running well, with only a trickle of water comming out of the outlet pipe but it runs cool for 10 Min's no problem, so lesson learned there then, looking to get a better carb for it as I think this has a lot to do with the bad idling and poor starting. Oh well nice afternoon, I'm out flying.
Atb
Steve.
Hi Steve / Biscuit,
As the others said, well done for sticking with it.
I think you surmised correctly as regards the overcooling (normally the brass sleeve will expand more than the aluminium piston, so that the fit loosens up somewhat while it is running - to achieve the correct fit when warm and run-in). I believe there is another important factor though - which is related to your running problems. If the engine revs hard when the throttle is closed, it is running in a very lean condition, and hence deprived of oil. If you add this to the fact that it was a new engine - therefore tight, and made worse by the overcooling, it's not surprising the piston and sleeve suffered.
The lean condition in itself will not do any good though, so the carburation does really need to be sorted. I broke a rod on a brand new SC .61 on shore - in the time it took to burn the fuel stored in the crankcase.
The SC carbs can tend to leak air around the main needle - which can usually be helped with a suitably sized length of silicone tube - all the way from the carb body to the outboard end of the needle screw. This will also grip the spring, and help to stop the main needle unscrewing. If it revs when the throttle is closed, with the low speed needle is screwed well out, it almost has to be an air leak or a blockage - unless you are using pipe pressure on the fuel tank. If you use pressure, sometimes you need to go very lean on the main needle to compensate for the pressurised fuel - leaving it short of fuel when you close the throttle. From what I have seen, the SC carb's don't like pipe pressure.
Agreeing with the others again, there are much better carbs out there than the SC, and some good options have already been suggested.
Good Luck and Happy Boating:
Ian