SmokStak

By Staff
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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.

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