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Symptoms of running lean?
|Author||Topic: Symptoms of running lean?|
posted 06-13-2002 11:16 PM ET (US)
There is a lengthy VRO discussion here and it mentions that lean conditions will cause a 2 stroke to eventually self destruct. Is there any way to tell when an outboard is running lean. Or, better yet, is there any noticeable symptom of when a motor is starting to run lean?
posted 06-14-2002 11:15 AM ET (US)
Usually when people talk about motors running lean, they are referring to the fuel/air ratio. Signs that the fuel/air ratio is too lean are things like; backfires at high throttle settings, hesitation when the throttle is opened, or the motor actually slowing down the more you open the throttle.
There will be no symptoms (while the motor is running) that you have too little oil in the fuel until the motor grinds to a stop. It is possible to get an indication from looking at sparkplugs though.
posted 06-17-2002 10:15 AM ET (US)
Often the first symptom of lean running will be a melted piston and seized motor. It does not take long either. In my case, about 20 seconds after a carb jet plugged at 3000 rpm and I noticed a power drop off until the motor seized with a broken piston. You might be able to get some indication of an early warning by pulling the plugs and comparing them. If one has an electrode that lookes eroded, or small bubbles of what appears to be metal deposited on the ceraminc insulator, then that is your lean cylinder.
posted 06-17-2002 10:51 AM ET (US)
My port motor is "sneezing" at idle.
I was told that it is running "lean."
Is this true and a possible symptom of running lean?
posted 06-17-2002 09:57 PM ET (US)
Sneezing might be cause by lean running, dirty plugs, or too much oil in the gas. Can you adjust the idle jets for your carb?
posted 06-18-2002 09:54 AM ET (US)
They are a couple of '89 120 Johnson VRO's.
I'm assuming the OMC manual would have an adjustment screw location for me if it exists, I hope. I have new plugs in her and the VRO is operational. Someone also recommended cleaning and rebuilding the carbs, if that was never done. Last resort I guess.
posted 06-18-2002 01:46 PM ET (US)
Rebuilding carbs is cheap.....maybe $5070 a carb if you remove them. Then you need to have them synced and linked. they do not have idle adjustment screws per say and you adjust via the butterflies.
posted 06-18-2002 11:19 PM ET (US)
I was afraid that the answer would be what it appears to be here. There usually "ain't" no symptoms until the motor self destructs. I guess this is all the more reason to run Stabil in the gas because it prevents gum buildup in the fuel system. I would imagine that lean conditions occur when a jet gets partially blocked.
$5070 Nick? That must the be the rebuild service which uses the 14K gold jets. (Couldn't resist...)
posted 06-19-2002 02:48 PM ET (US)
Whalerron, to try and answer your question, in a 2-stroke motor, there are three
components that make up the fuel mixture recipe, air, fuel and oil. On my engine which is not pre-mix (mixing oil into gas tank), there is an oil pump that pumps oil into a check valve where it mixes with fuel coming from the fuel pump and then this mix goes into the carbs. My oil pump has a linkage connected to the rest of the throttle links. This part of the motor I do not dare touch and only check to make sure it is within shop manual specs. As is, it seems that at closed throttle, the marks show that the pump may be pumping a
little more oil than needed, but again, I will not mess with this.
At idle speed, this fuel/oil mix enters the carbs, when it is sucked into the engine
by vacuum created by the pistons as they finish the power stroke. Now, the actual thing being sucked in is air, through the carb barrel, and as the air rushes in, it in turn passes over a "hole" (ventury, for lack of a better word) through which the fuel and oil are sucked into the air and on into the block and then into the cylinders. As you accelerate, the jets take over and supply the fuel/oil mixture. Nissan tech support explained to me that only the jets supply this mix after approx. 2k rpm, others have suggested that it works the other way around in that at idle the jets shut down and at speed both supply fuel, but either way, this is not important other than that you can not control your fuel/oil to air ratio at higher rpm without modifying the jets. You can only modify your fuel to oil ratio by making adjustments to the oil pump.
Now, as far as I understand, here are the dangers: If your oil pump is not
supplying enough oil for the rpm the engine is running at, the engine will overheat from
friction caused by lack of lubrication. Then it will seize. If there is too much oil, it will foul the plugs faster, may cause excessive carbon build up...
The mixture screws do the following: if you turn them clockwise until they are
fully closed, you have cut off the supply of fuel and oil to the carb at idle. The more you open them, the richer the mixture becomes, the more you close them, the leaner the mixture becomes. Now, as I was told, if the fuel to oil mixture is correct, but you have set the (pilot/mixture) screws too lean then the engine will most likely also overheat, some call it scortching or melting. Basically, there is too much air for the amount of fuel and this creates too hot of a combustion inside the cylinders, now at higher rpm, this depends on the jets as well. Other mechanics have told me that realistically, if the idle mixture is set too lean, it will start dieing on you at idle.
Modifying the jets is way beyond anything that I dare say I know much about
other than that usually the guys that do this and run into this problem are guys that start to make major modifications so as to see how much horsepower they can squeeze out of their engine. They do this as part of their quest for the ultimate outboard. Take a 50 hp, slap it on a boat the size of a surfboard, tweak the engine out so it produces 70+ hp (or so they hope), and then race it till is goes out in a literal blaze of glory.
I assume you just want to make sure that your engine will not become ruined
prematurely. So, what do you need to do, first, make sure that your oil pump is set to the right setting as per the instructions of OMC in regards to their VRO system. Then, set the mixture also according to the instructions of OMC for your particular engine.
I have been in search of an answer to the same question for a while now, and
though the above is my understanding of mixture, it may help you understand itís
fundamentals. Every mechanic I have talked to seems to have a different opinion as to
exactly what is correct.
posted 06-19-2002 04:07 PM ET (US)
Good job Tubby:)
Again that is for air/fuel&oil mixture. Idle is seperate....and that was $50-70:)
posted 06-19-2002 05:36 PM ET (US)
$50-$70...yup, sounds reasonable.
Ok, as is with a car, an outboard that is running lean is gonna run hot. Is there enough increase in powerhead temperature that a temp gauge would indicate the rise in temperature? I am wondering if a temp gauge would be worth having for this reason.
posted 06-19-2002 07:02 PM ET (US)
Believe it or not, on a 2 stroke, the mixture also contributes to cooling the piston. Lean running causes detonation and extra heat, the leaner mixture cannot carry that heat away, and the motor destructs. But, at idle, you never really generate enough heat for this to be a problem. I agree, the motor will die at idle if too lean. I have never seen an OMC V-4 (or carbed Merc V6) that did not sneeze occasionally at idle. I still think that looking for damaged or eroded spark plugs is the best early indicator of this condition. If your motor has excessive sneezing out the carb, you may have worn reed valves. They sit between the carbs and crankcase. I have had to replace them on old motors as they can fatigue and break.
posted 06-19-2002 08:38 PM ET (US)
A standard temperature gauge will not notify you of a lean condition on a outboard motor(a piston will probably burn first). The temp sensors are on or in the cylinder head water jacket area and you do not get a combustion chamber temperature reading.
The only type of temp gauge that indicates a lean condition on a 2-cycle engine is an exhaust gas temp gauge. There is a sensor for each cylinder in the exhaust manifolds. I have never seen this type of setup for a water cooled outboard motor.
posted 06-20-2002 02:32 PM ET (US)
Whalerron, again, even under normal conditions and settings, the temp. inside the cylinder is going to be much hotter than the temp. a gauge (temp gauge) will show since the gauge is showing water temp as Dive 1 said. If your water intake is blocked or water pump (impeller) gives out, it will show the temp rising because there is no collant. If your cylinders are getting to hot due to the mixture being too lean, then the gauge will only reflect this after the fact.
Aluminum melts at approx. 1200 F, since at say 3000 rpm a constant amount of water is flowing through the engine (assuming thermostat is open) regardless of internal temp. then there is a maximum to the amount of heat the water can absorb while it passes through the engine. If the fuel mixture is too lean it will burn hotter and the internal cyl. temp. will gradually reach a point high enough to melt aluminum (cylinders and cylinder walls). I think what BSMOTRIL is reffering to is that since in a 2-stroke, the fuel entering the cylinder will have oil in it, and since the oil does not contribute to the combustability of gas, and as the EPA likes to make very clear, 2-strokes are notorious for not completely burning all the fuel, I would imagine that the unburned fuel does absorb some of the heat before it exits the cylinder. Too rich, and it will do a good job of keeping things cool, but it will also foul up a lot of stuff, also cause the engine to run rougher at idle. Again, I am not sure how much effect the pilot/mixture screw has at higher RPM's since at that point the jets supply most if not all of the fuel. Check an OMC shop manual for the proper setting. I bought an aftermarket shop manual from Clymer that had a typo in it, it specified that the pilot screws are supposed to be .25-1.75 open, it is supposed to be 1.25-1.75 open plus 1/8th for cold starting.
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