sinkingfast
Seaman Apprentice
- Joined
- Dec 31, 2003
- Messages
- 34
TWO STROKE : HOW TO AND WHY TO INSTALMENT 2<br /><br />This instalment has proven difficult to write due to the need to keep it simple to understand for the not so spanner friendly, yet with enough factual data to make it understandable.<br /><br />The basic principle of the combustion in the cylinder
THE BANG<br /><br />The engine uses a mixture of fuel and air in an amount precisely quantified by the carburettor or injection system. By a premix or an auto-lube, a precise amount of oil is also present. The three elements combine to give a measured Bang with the necessary amount of lubrication for the motor. Unlike a four-stroke engine with its ever-present oil pressure, the lubrication is more critical on the two stroke.<br /><br />In order for us to discuss the workings and failures, we need to go a bit abstract and use our imaginations now to paint a picture.<br /><br />Picture a boat speeding along at 5000 rpm
Got it?<br />OK, STOP TIME. Freeze the picture.<br />Walk over to the boat, ( yeah yeah, save me the jokes about walking on water.)<br />Take the engine cover off the motor.<br />Look at the cylinder head, the top bit where the spark plugs go in.<br />Make the head disappear in your mind.<br />Now you are looking directly down into the cylinders and you see the piston crowns (The top faces of the pistons).<br /><br />What you will see is a round piston crown, the centre of which will have a darkish burnt area where the petrol/air mix has done its work during the bang.<br />Round the outside of that is the area forming the outer ring towards the edge. It will be less marked, almost clean in fact. This area will be wet and shows a shinny layer
the area where the oil lubricant has done its work during the bang. This wetness will extend over the edge of the piston crown and disappear down the sides, over the rings and down the skirt. Thats what is lubricating the upper part of the two stroke engine.<br /><br />You have now a picture of what is happening so we can discuss it further. <br />Oh
. And you can come back to reality again.<br /><br />Ok, we now have a basic idea of what is happening inside the top of our engine. The bang is being made and that bang is making the pistons move, and through other mechanical cycles and devices, it ends up as a turning motion at the prop.<br /><br />Factors affecting the burn<br /><br />Manufacturer has spent loads of time and money refining and balancing the burn and bang to get the best efficiency against reliability.<br /><br />The Spark to ignite the bang is timed to micro precision to make the bang occur at its most efficient moment.<br />The gasoline is metered to micro precision to make the bang efficient.<br />The air input is metered to micro precision to make the bang efficient.<br />The lube is micro controlled to make the band efficient.<br />Mechanical parts all work within a given tolerance for efficiency.<br /><br />MESS WITH ANY OF THESE AND THE EFFICIENCY OF THE BURN ALTERS.<br /><br />It will have a direct effect on the working temperature of the burn.<br />It can increase efficiency
..or ruin an engine. Its that simple.<br /><br />Now with all that in mind, see why we started with instalment one, concentrating on just fuel and lubricant supply. We need to standardise the supply, create a norm to work to. Think what stale weak gasoline, in-effective oil or incorrect settings with the fuel air mixture or oil amount can do. We established a known BASE LINE to work with.<br /><br />We will mention temperature at the piston crowns over and over with two strokes, and I cannot stress how important it is.<br /><br />Ok
..We are going back to the imaginary scenario again.<br /><br />We will reduce the amount of gasoline going to the burning process inside that motor. We are not going to alter anything else, just the gasoline. The driver of the boat will suddenly notice a difference; the boat will feel like a turbo has kicked in. The efficiency of the burn has increased.<br /><br />Remember we can stop time in our scenario. Again we look at the piston crowns and the burnt area in the middle is bigger, it is literally allowing the burn efficiency and temperature to rise and develop more power BUT THE OIL WETTENED AREA IS REDUCED. The driver is happy and so is the engine to a point.<br /><br />OK, we will take it further. We will reduce the gasoline amount a bit more, nothing else just the gasoline. The driver has now had his hat blown off by the power increase and is grinning from ear to ear. Unfortunately the engine isnt. The burn has increased the temperature even more. Now the oil ring area on the crown is getting dangerously near the edge.<br /><br />Lets go again on our imaginary engine. We reduce the gasoline as before and the driver can feel a difference, but this time the effect is not nice. The motor feels different, harsh, the note will change. As we look inside our engine, the oil pattern has disappeared off the piston crown, in fact it has reached past the rings. The rings are not lubricated, the cylinder wall is dry, and our imaginary engine is still doing 5000 rpm.<br /><br />Seizure. Plain and simple. Big damage. Big repair bill.<br /><br />Just by altering one we ended up with a mess. What about the effects of reducing or increasing the air supply, making the spark occur at different times?????? Each one will effect the burn temperature efficiency, sometimes good but usually bad. See why we need baselines now?<br /><br />Maintenance to reduce the risks<br /><br />Take a while to look at your motor and if you have the manual read it and refer to it.,,,,,, it will help you identify parts and systems.<br /><br />Carbs. On the side of the carburettor, some manufacturers fit mixture screws to increase or decrease the gasoline amount. These usual are only relative to tick over or low speed jets. They can have small plastic collards on to limit the movement. These are ideally shaped to get caught in and moved by cloths used to wipe down the motor. The effect of these being wrong is small but we are discussing ALL factors. I recommend you note the position of these on a notepad, for future reference. As an example I do by:<br />TOP 1 right side 11oclock position. And so on.<br /><br />If there is doubt that the screw has moved, note the position. Turn the limiting collar, counting the movement or rotation of the collar anticlockwise until it stops. Be gentle with that movement. Feel it dont force it.<br />Return it to its original position. Do the same in the clockwise direction until it stops and note the movement needed. The basic approximation for this setting is that it should be in the middle of its range. Example<br />Full anticlockwise 9 oclock<br />Full clockwise 4 oclock<br />Basic setting around the 12 to 12.30 position.<br /><br />If you are not sure, leave it alone. But note the positions anyway.<br /><br />While we are on the subject of carbs, an average V6 has 6 different sized main jets, the ones to do the fast speed mixture balance. Each one is precisely matched to the temperature needs of each cylinder. Imagine Joe Spanners taking the carbs apart to clean them and mixing up the jets. The engine will run like a bag of spanners and sooner, rather than later, fail. <br /><br />WHEN IN DOUBT, CHECK THE MANUAL<br /><br />Hopefully by now you are starting to think differently about that metal thing on the back of the boat. Lets see if we can go a little further.<br /><br />Get someone to move the hand lever on the remote control. Watch the levers on the carburettor sides open, close, open, close, open, close. See all the moving levers?<br />Do they catch on anything?<br />Do they move smoothly? A bit of WD40 goes well here.<br /><br />Oh, and that top lever. The one that seems to do nothing more that rotate something at the bottom of the starter ring
<br />Well, that is the lever setting the ignition timing for the burn, synchronised to the throttle movement and has instant and immediate reaction to the burn temperature. I wonder though how many of you never realised its importance before. Check its movement and gave it a spray with WD40. It should move smoothly, effortlessly and without obstructions. If it stuck in one position, it would either be in advance: and make the motor a bugger to start, or retarded: making the temperature of the burn go skywards and
.. Sound Familiar?<br /><br />Are the nuts tight at the base of the carbs where they bolt on to the motor? <br />Loose nuts = bad seal. <br />Bad seal = incorrect vacuum to draw fuel/air<br />Incorrect burn
.. See where I am going?<br /><br />The Sparks The spark plugs themselves are rated to a particular burn temperature range. Use the ones specified by your manufacturer. That is why he recommends them. <br />Clean plugs mean efficient burn. An efficient burn means clean plugs.<br /><br />Here we have a personal thing. I believe from experience, that cleaning plugs can ruin them if you dont do it right. I prefer to remove a plug, spray into the burn area something like carb cleaner, genclean or brake cleaner. I give it a shake and a blow with air. If you blow on it with your mouth, close your eyes, that cleaner stings,<br />No wire brushes, no sandpaper, nothing. Get you home cleaning is OK but if you have had to do that, get new ones and keep the old ones as spare Get you homers.( I know someone will tell me that they have been cleaning their plug with a broken brick since 1741 but come on, this is not what we are about here.)<br /><br />Now we will read the plug. The cleaner should have removed any residual oil hiding in there. Look at the plug, not at the top but right deep inside. Use a bright light and look at the base where the porcelain centre meets the metal at the base. That surprised you didnt it. The bottom is where the most accurate reading of burn are taken from. Ideally you should see a brownish, haze colour on it.<br /><br />Black is OK. A bit rich on the mixture perhaps but dead safe<br />Brown is ideal good balance.<br />Greyish is OK A bit lean but OK<br />White is a bit hot.<br />White with lumps HMMMM running a bit too hot. Think about the fuel mixture amount and refer to a manual or a dealer.<br />Oh
. and if all you can see is a metal threaded connector, you are holding the plug upside down 
)<br /><br />At this point dont bother peering down the plug holes trying to read the oil ring pattern. This can only be done after the very precise set of conditions of a plug chop have been done and will give you the wrong reading.<br /><br />The gap for the spark plug is stated by the engine manufacturer. Again the wrong gap will effect the burn efficiency. Setting it to the standard is simple but do not lever against the centre electrode to alter it as it can damage easily. Use a proper tool.<br /><br />The leads to the plugs should have no splits in them, if so change them.<br />Have you seen any rubber collars on the H.T. leads that go to the plugs? They are there to keep the leads away from hot surfaces of the head and from chaffing on edges. Have they moved? Slide them so the leads fall naturally and away from surfaces that could cause damage.<br /><br />On the head, manufacturers sometimes put a detonation monitor. Does yours have one? Check the manual. Is the wire to it connected correctly and tightly?<br />This device monitors the bang. Remember our boat driver, minus his hat? We said he heard the engine note and feeling change just before if seized. Well that little device is listening to your motor in case you dont and will tell you by changing the ignition towards a safer setting and in some circumstances, stop it altogether.<br /><br />Filters That inline fuel filter
you know, the one you havent replaced in years, but just swilled out to save money. Well if it is allowing debris past, the debris goes into the carbs, passes through the jets, gets stuck blocking a jet and reducing the gasoline passing through it
Yes you are thinking correctly, The burn, the temperature, the oil pattern, the trip to the bank.<br /><br />Small rubber pipes that supply the vacuum? Pinch them gently with a pair of smooth grips. If they spring back, good. If they stay flattened or deformed, they are weak and plasticised. Change them.<br /><br />I can go on forever about the motor but its impossible to think for you.<br />Look at your motor, the electrical connections that might alter the spark, the loose or corroded pipe clamps, that get you home wire repair that you never got round to do properly because it still works.<br /><br />I can go further into detail if you require although I am deliberately leaving out certain aspects of the data because it is really not a requirement for the average guy. We can cover plug chops to allow us to assess the burn/oil pattern but as I said earlier, only after certain routines have been done prior to the reading.<br /><br />Is that level OK for you???<br /><br />What do you want next???<br /><br />OK GUYS
.. OVER TO YOU. <br />
)<br /><br />At this point dont bother peering down the plug holes trying to read the oil ring pattern. This can only be done after the very precise set of conditions of a plug chop have been done and will give you the wrong reading.<br /><br />The gap for the spark plug is stated by the engine manufacturer. Again the wrong gap will effect the burn efficiency. Setting it to the standard is simple but do not lever against the centre electrode to alter it as it can damage easily. Use a proper tool.<br /><br />The leads to the plugs should have no splits in them, if so change them.<br />Have you seen any rubber collars on the H.T. leads that go to the plugs? They are there to keep the leads away from hot surfaces of the head and from chaffing on edges. Have they moved? Slide them so the leads fall naturally and away from surfaces that could cause damage.<br /><br />On the head, manufacturers sometimes put a detonation monitor. Does yours have one? Check the manual. Is the wire to it connected correctly and tightly?<br />This device monitors the bang. Remember our boat driver, minus his hat? We said he heard the engine note and feeling change just before if seized. Well that little device is listening to your motor in case you dont and will tell you by changing the ignition towards a safer setting and in some circumstances, stop it altogether.<br /><br />Filters That inline fuel filter
you know, the one you havent replaced in years, but just swilled out to save money. Well if it is allowing debris past, the debris goes into the carbs, passes through the jets, gets stuck blocking a jet and reducing the gasoline passing through it
Yes you are thinking correctly, The burn, the temperature, the oil pattern, the trip to the bank.<br /><br />Small rubber pipes that supply the vacuum? Pinch them gently with a pair of smooth grips. If they spring back, good. If they stay flattened or deformed, they are weak and plasticised. Change them.<br /><br />I can go on forever about the motor but its impossible to think for you.<br />Look at your motor, the electrical connections that might alter the spark, the loose or corroded pipe clamps, that get you home wire repair that you never got round to do properly because it still works.<br /><br />I can go further into detail if you require although I am deliberately leaving out certain aspects of the data because it is really not a requirement for the average guy. We can cover plug chops to allow us to assess the burn/oil pattern but as I said earlier, only after certain routines have been done prior to the reading.<br /><br />Is that level OK for you???<br /><br />What do you want next???<br /><br />OK GUYS
.. OVER TO YOU. <br />