Smokstak Bulletin Board

Two Cycle Motor Oil

I have a little (old) single cylinder 250 cc 2 stroke motorcycle. I had replaced the piston and liner twice before it was even warm enough to ride this spring. After finally getting everything adjusted right, and spending nearly $400. I started using AMSOIL 2 stroke oil at a recommended mix of 100:1. It ran very well all summer until last week. I have all my firewood cut for the season and had a bit of chainsaw gas (40:1) left over so I dumped the remainder (about a quart) into the tank of the motorcycle, which I am guessing had about a gallon of the 100:1 mix gas in it. I had to run an errand on the bike later that day for about 12 miles. On the way back, I noticed the motorcycle steadily slowing down as I'm trying to give it more throttle. All of a sudden at about 30 or 35 mph, the back tire locked up and went into a skid. It took a couple of seconds but I figured out that it might be best if I squeezed the clutch, which I did and coasted to a stop. What happened was that the engine locked up, locking up the back wheel. After I stepped back into the real world, I tried turning the engine over and it was TIGHT and hot as hell! I was about 6 miles from home and I managed to get a ride, brought the truck and little trailer back and hauled the thing home. By that time, the motor had cooled off and was once again loose but wouldn't start. The following morning, it was reluctant but did start up and made some god-awful noises so I shut it right back off. Due to lack of time, I didn't pull the cylinder off but I know the piston and probably the sleeve are shot. Logic says it locked up due to over heating due to lack of oil. I can't understand why adding 40:1 mix with 100:1 mix would cause that? I think there is something very important that I should learn from this and need your help doing so. -- Marty

Marty, first of all a person could get a degree in the science of oil! I didn’t, but I have worked on two strokes for 24+ years and 16 of them were in a dealership on saws, trimmers and mowers. Every factory school goes into detail on what oil to mix and how to mix it. And every school says the same thing about too much and too little oil in the mix! Cut the oil too thin and it runs lean. Add that extra little bit left in the quart thinking if a little is good then more is better and you have another lean mixture. Heavy on oil, but lean on gas. I have seen it both ways when a guy adds Homelite mixed 16:1 to his Husquvarna that runs at 50:1 and it burns up. And then run the 50:1 in the Homelite and it runs the best it ever has, until it locks up! I know some machines are sticklers on mix (Husquvarna being one!) and that’s all the knowledge I have to pass on. – Randy

In high compression high performance outboards, piston damage is common. These engines run at the max most of the time. Compression and timing are as high as they can be for high performance. When people use regular gas because premium is too expensive, they burn a piston. After the engine is rebuilt, they fill the tank with regular gas, double the oil, lowering the octane even more, because oil is like Diesel. The more oil you add, the lower the octane. If a piston is scuffed on the wall or a bearing is burnt, it’s probably a lack of oil. If the piston is burnt from the ring land down, it's probably detonation. High timing can aggravate this. See if the timing is adjustable on your bike and check it, lower it a couple of degrees. Use premium gas. Hope this helps. -- J.B.

J.B. My understanding of octane is that the higher it is, the slower it burns. Again going by logic, (maybe that's my whole problem) I would think that high octane gasoline in a two stroke wouldn't be so good because if indeed it does burn slower, the exhaust has much less time to exit the cylinder than in a four stroke. I would think therefore that the fuel mixture would still be burning as it exits the exhaust port and ignite the incoming fuel. ????? By the way, the timing is not adjustable on this engine and yes they have pushed the performance to the max. It is an extremely radical little motor. – Marty

Premium fuel burns slower, but not nearly as slow to still be burning when the ports are open. High timing ignites the fuel way before top center and it should burn past center to push the piston down. Low octane explodes and tries to push the piston backwards before top center, causing damage. A lean mixture burns real slow, causing backfiring and sneezing in a two cycle because the fuel is still burning when the ports are open. – J.B.

Marty, I was just trying to figure out what your problem could be. Out of all the bikes quads and sleds I have raced, if an oil line was plugged or pump was out never did it ever turn over again. My only thought on the matter is your sleeve and piston. I don't know as far as these old motors but my bikes and stuff all have different kinds of cylinders like chrome bore, nickel bore, etc. and different piston types. The only thought that comes to mind is to make sure that one or the other, piston or sleeve are not swelling when it's hot. The only other thing is in my personal opinion is that Amsoil clogs stuff because it's really thick. I have VES on my sleds (Variable Exhaust System) and I got hung out on the trail because it clogged it up! Just a thought! Good Luck – Josh

Too little information given to be absolutely sure (year, gear ratio, etc) but I can tell you this. The older (60’s and 70's) single cylinder 2 cycle motorcycle engines simply do not hold up under a constant high rpm like "on road" use. They were great "off road" when the rpm varied constantly, or in town short trips, but those longer on road treks just seem fry them. I have seen dozens of these single cylinder 2 cycles do just exactly what you described while using the correct fuel/oil mix. I really doubt your adding that small amount was the main factor, likely only a contributor factor, it was the open highway constant high rpm that killed another one. – John

Two stroke engines are picky eaters. Change the oil/fuel mixture, fuel to air mixture, heat range of plug, type of gasoline, type or size of carb., exhaust system, geographical elevation, outside temperature, etc. and you need to re-jet the carb.
Your engine has a main jet that the fuel mixture must pass through to be mixed at the right ratio with the air. Too much oil in the fuel causes the mixture to be thicker, which causes the air to fuel mixture to get way too lean. You thought you were adding more oil by using a thicker mixture. What really happened was that the mixture could not get through the main jet at a sufficient volume to lube the engine. You actually gave it less oil. It is like putting ten-weight versus sixty-weight oil in a drip oiler and still expecting six drops a minute from the same setting. You also starve the engine for fuel which also causes the engine to overheat. Kind of a double whammy.
You will need to adjust the jetting to allow the correct amount of air and fuel to pass. Too big of hole in the main jet = too much fuel = too rich of mixture = cussing, because the engine has no power and keeps fouling plugs. Too small of hole in main jet = too little fuel= too lean of mixture = Seize up and cause skid marks. The solution is to always have the correct main, needle and pilot jet in your go-go bike. The main jet being the most important.
The wrong size pilot jet causes terrible idling and no low end response, again either too rich or too lean. The incorrect setting of the needle valve in the slide causes the engine to not perform well once off idle, up to when it requires the fuel from the main jet. The main jet is most important as the power required from the engine at mid to upper RPM's will need this to be very accurate.
Any two-stroke will run on most fuel to oil ratios, as long as the jetting of the carb is correct. Granted the leaner the oil ratio the less likely the engine will foul a plug but there will be more engine wear.
Anyway, settle on what type of oil you want to use. Second try to use the mfg.'s recommended ratio. If lucky, use their oil also. If all else fails. Find a bunch of jets in a size series close to what is originally in your carb. and then get to work.
Go buy a box of brand new spark plugs of the correct heat range (also keep that consistent). Start by putting in a brand new plug and increase or decrease the size of pilot jet until you have a smooth idling engine. Light smoke and little or no oil discharge. Increase or decrease the size of the jet until you get it right. Change the plug each time after changing the pilot jet.
Next, to set the main jet, put the needle in its middle position and put in a new plug. Start the engine and go out and run it down the road wide open for a couple hundred yards or all the way through the gears. Once wide open at top speed, pull in the clutch and shut the engine off. Come to a stop without power and pull out the plug. If it is black, you are too rich. If it is white, you are too lean. Brown is optimum. Keep trying until you get it right and change the plug after each run.
Once that is accomplished, set the needle to the correct position by adjusting it up or down until the correct mid range response is correct. This will take most of the afternoon, a little patience and a box or two of plugs. How do I know about this? I used to build motorcycle oval dirt track engines. None of the manufacturer’s settings mattered when the only stock item on the bike was the manufacturer’s name. Take care and good luck. --Mark

Excellent post Mark! Good info for all initial setups of 2 cycle engines regardless of application. This method works great for anything from big chainsaws with 5 foot bars to MX bikes! What I might add to it is that sometimes two setups are needed depending on the elevations you work/play at. The elevations in the Pacific Northwest where we commonly play/work varies from sea level to well over 6000ft. Many of us have multiple mixture set-ups in the toolbox ready to go. Beach sand dune riding one day, coastal range mud the next and then the Cascade Range Mountains on the third. Two or possibly three, mixture set-ups, depending on need. -- John


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