A recent topic on the Engine Ads SmokStak bulletin board at http://www.smokstak.com/ caught my interest when an antique Maytag oil mixture discussion gave way to some high-tech two-cycle oiling discussion. As ever, various individuals started, commented on and concluded the following bulletin board thread.
I have a little single-cylinder 250 cc two-stroke motorcycle engine. I 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 two-stroke oil at a recommended mix of 100:1. It ran very well all summer - until last week.
I had all my firewood cut for the season and had about a quart of 40:1 chainsaw gas left over, so I dumped it into the motorcycle, which still had about a gallon of the 100:1 mix in it. I had to ride about 12 miles on an errand later that day, and on the way back I noticed the motorcycle steadily slowing down as I tried to give it more throttle. All of a sudden, at about 30 or 35 mph, the back tire locked up and I went into a skid. I squeezed the clutch and coasted to a stop. The engine had locked up, locking up the back wheel. I tried turning the engine over and it was tight and hot as hell.
The following morning I started it up and it made some god-awful noises, so I shut it right back off. I know the piston and the sleeve are probably shot. Logic says it locked up due to over heating due to a 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 I need your help doing so. - Marty
Marty, a person could get a degree in the science of oil. I didn't, but I have worked on two-strokes for 24 plus years, 16 of them in a dealership working 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 (if a little is good, then more is better, right?) and you have another lean mixture. Heavy on oil, but lean on gas.
I have seen it both ways. A guy adds Homelite mixed 16:1 to his Husquvarna that runs at 50:1 and it burns up. Then he runs the 50:1 in the Homelite and it runs the best it ever has, until it locks up! - Randy
Piston damage is common in high compression, high performance outboard engines. These engines run at the max most of the time, with compression and timing as high as they can be for maximum 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 and double the oil, thereby lowering the octane even more. 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 and lower it a couple of degrees. Use premium gas. - J.B.
My understanding of octane is the higher it is, the slower it burns. Going by logic (maybe that's my whole problem), I would think that high octane gas in a two-stroke wouldn't be good, because if it does burn slower the exhaust has 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 would 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 so slowly as to still be burning when the ports are open. High timing ignites the fuel way before top dead center, and it should burn past center to push the piston down. Low octane fuel explodes early and tries to push the piston downwards before it hits top dead center, thereby causing damage. A lean mixture burns slowly, 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. In all the bikes, quads and sleds I have raced, if an oil line was plugged or a pump was out it never turned over again. My only thought on the matter is your sleeve and piston. I don't know about these old motors, but my stuff has different kinds of cylinders (chrome, nickel, etc.) and different piston types. Make sure that one or the other, piston or sleeve, are not swelling when it's hot. - Josh
Too little information given to be absolutely sure (year, gear ratio, etc.) but I can tell you this: The older (1960s and 1970s) single-cylinder, two-cycle motorcycle engines simply do not hold up under constant, high rpms. They were great off-road where rpm varied constantly, or in-town short trips, but longer road treks just seem to fry them. I have seen dozens of single-cylinder, two-cycle engines do 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 and constant high rpm that killed it. - 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 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 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.
You need to adjust the jetting to allow the correct amount of air and fuel to pass. Too big a hole in the main jet equals too much fuel, which equals too rich a mixture, which leads to cussing because the engine has no power and keeps fouling plugs. Too small a hole in the main jet equals too little fuel, which equals too lean a mixture, which leads to a seize-up and skid marks. The solution is to always have the correct main, needle and pilot jet in your bike, the main jet being the most important.
The wrong size pilot jet causes terrible idling and poor low-end response, again because it's running either too rich or too lean. The incorrect setting of the needle valve in the slide causes the engine to perform poorly off idle up to the point when it requires fuel from the main jet. The main jet is the most important for supplying proper power at mid to upper rpms. Any two-stroke will run on most fuel/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.
Settle on what type of oil you want to use and try to use the manufacturer's recommended ratio. Use their oil, also. If all else fails, find a bunch of jets in a series of sizes close to what was originally in your carb and then get to work.
Go buy a box of brand new spark plugs of the correct heat range (keep that consistent). Start by putting in a brand new plug and increase or decrease the size of the pilot jet until you have a smooth idling engine with 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 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 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 two-cycle engines, regardless of application. This method works great for anything from big chainsaws with 5 foot bars to MX bikes! I might add that sometimes two setups are needed depending on the elevation you work/play at. The elevation in the Pacific Northwest where we commonly work and play varies from sea level to well over 6,000 feet. Many of us have multiple mixture setups in the toolbox ready to go. Sand dune riding one day, coastal mountain range mud the next -and then the Cascade Mountains on the third. - John
'The wrong size pilot jet causes terrible idling and poor low-end response.'
SmokStak is an engine conversation bulletin board with over 37,000 messages on file and is part of the Old Engine series of Web sites that started in 1995 as 'Harry's Old Engine.' Harry Matthews is a retired electronic engineer and gas engine collector from Oswego, New York, now residing in Sarasota, Florida.