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Telemaster Sales UK -> RE: Merco 61 - carbon date it? (5/15/2008 7:39:56 AM)
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quote:
ORIGINAL: Pete Harvey
I've just started tring to get one of my boats going. It has a Merco 61 in it (as do two of my other boats), only it keeps cutting out when i put it in the water. The horizontal screw on the front of the carb might have somthing to do with it (mixture?) , but I do not how best to adjust this. I have posted a link to my site -my personal model website - where I have put a video, where I started it 3 times withi it cutting out. If anyone can give me advice, i would be grateful.
regards
Pete
[image]http://i102.piczo.com/view/2/9/q/a/w/m/9/e/r/e/s/x/img/i344784995_11027_6.jpg[/image]
Hi Pete
Your Merco has an airbleed carb. It's a very good carburettor but of a type not widely used today. PAW diesels and some of the Enya engines still use air bleed carbs but most carbs are twin needle today. The difference lies in the small horizontal screw at the front of the carb. This screw controls the slow running/tick-over speeds by restricting the supply of air to the carburettor at these speeds. If you screw it in it enriches the mixture by reducing the air supply. If you unscrew it, it weakens the mixture by allowing more air in. This is the opposite way round to the slow-running control on a modern twin needle carb where the slow running screw controls the fuel supply.
To get a two stroke engine running, assuming the engine is already run-in, the first step is to adjust the high speed needle valve, the serated brass fitting on the right of your picture. This needle controls the fuel supply at mid to high speeds. If the engine keeps cutting when you open the throttle to its widest position, the mixture is too weeak. In this instance, unscrew the brass needle a quarter of a turn and try again, until the engine runs at maximum speed. If the engine runs unevenly with the throttle wide open, usually accompanied by lots of smoke from the exhaust, it is too rich. In this case, screw in the brass needle until it runs evenly and maximum rpm are reached with the throttle wide open.
Having reached optimum rpm at maximum speed, unscrew the brass needle by two or three clicks to prevent the engine running too weak and potentially causing damage.
The engine should now have reached operating temperature and you can turn your attention to the slow running jet. With the main throttle stick on your transmitter in the fully closed postion but with the trim setting fully open, the engine should tick over. If it is ticking over, try advancing the throttle rapidly and watching what happens. One of the following will occur:
1. The engine will accelerate rapidly to high speed. In that case leave the air bleed screw alone!
2. The engine will accelerate unevenly up to high speed accompanied by lots of smoke, this indicates a rich mixture. In this case, UNSCREW the air bleed needle a quarter turn to allow in more air in then try again. Still too rich? Continue adjusting the needle by small amounts until the situation in 1 is achieved.
3. The engine will cut as soon as the as the throttle is advanced. This indicates a weak mixture. In this case SCREW IN the needle valve by a quarter turn to reduce the air supply and try again. Still too weak? Continue to screw in the needle valve by 1/8 to 1/4 turn until the situation in 1 is reached.
If you can't get the engine to tick over at all, does it cut out straght away when the throttle is chopped. Yes? Too weak. No? Uneven running and smoke? Too rich. Procede as above until it ticks over then whack the throttle open and observe results and take remedial action as in 2 and 3 above.
Still cutting out? Is the glow plug in good condition and of the correct heat range. Try a Firepower FF7 plug and try again.
Still cutting out? What fuel are you using? In my experience, Mercos like to run on 5% nitro and 18%-25% oil. Some claim that the oil should be castor but this is a controversial area.
Still cutting out? Where is the fuel tank in relation to carburettor. The center-line of the tank should be level with the carburettor jet.
Still cutting out? You did allow the engine to reach operating temperature and adjust for optimum highspeed running didn't you otherwise any fiddling is useless.
Still cutting out? Join your local boat club and get some expert advice!
All of the above is based upon my experience of using air cooled Mercos in model aircraft. It should hold good for boats, but when the water jacket fills on a marine engine, the motor will cool and will probably benefit from a slightly weaker idle mixture.
Best of luck
Dave Davis
BMFA Club Level Flying Instructor.
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