Stuart Turner Marine Engines.
Stuart Turner 2-stroke marine engines are a much maligned breed. Log-on to any boating forum to ask a question, or seek advice on how to get the best out of them, and some smart-arse will recommend you deep six it or use it as an anchor. These are the same people who struggle with bugs in their diesel, dirty fuel injectors and vibration.
The reason Stuart Turner, and other 2-stroke inboard engines, get such a bad press is down to a lack of understanding of how they worked in the past and how they can be made to work so much better now.
In the past, nobody bothered much with '2-stroke oil'. The owner at that time would follow the manual and mix engine oil, probably SAE30 as used in the gearbox, with leaded petrol. Being a cautious sort of chap he probably erred on the rich side of the recommended 50:1 mixture, thinking it would do a better job of lubricating his engine.
When he came to run his engine that's when things started to go wrong. Being seawater cooled Stuart Turner engines are overcooled at low rpm. This means the combustion chamber is not quite hot enough to efficiently burn the fuel/oil mix and you end up with that familiar black sooty gunge on the spark plug.
The original Stuart Turner thermostat seems to be unavailable (not that I've ever spoken with anyone who's even seen one) so I'm considering fitting a cooling water bypass which will allow me to recirculate some cooling water at tickover.
Then there was the problem with leaded fuel. The recommended gap for the spark plug is only 16 to 18 thou. Lead gets deposited on the electrodes and easily shorts out the plug. Result? The engine suddenly stops.
Modern, low ash, clean burning, 2-stroke oils at the correct 50:1 mix are far less likely to foul your plug. Modern unleaded fuel is perfect for 2-stroke engines as there is no possibility of lead depositing on the plug and shorting it out. Whatever you do, DO NOT, pour any of those lead substitute concoctions into your fuel!
If your engine starts easily enough when cold, but won't start when hot that is indicative of a weak spark. Possible causes are:
- Plug gap too wide. (weak spark)
- Points gap too wide.(weak spark and affects timing)
- Condenser (capacitor) faulty. (weak spark and affects timing)
- Faulty HT cable. (weak spark)
- You've fitted a suppression cap. (remove it until neighbours complain)
- Magneto fault. (see magneto section)