Yamaha RD400 service manual could be dangerously obsolete

I found this from http://www.motocarrera.com/advice.htm It looks like a
good site for Two-Stroke Hints/Tips/parts but I never heard of the
following before or seen on anyother site. I know about octane and
pinging/knocking but I never knew that the technical info in the factory
service manual "could be dangerously obsolete because of today's
gasoline. On a Yamaha RD350 or RD400, timing should be set at 1.8mm
btdc, instead of the printed specs of the day. Don't find out the hard
way with a melt-down".

"All fuels have different burn or flash rates which is referred to as
octane rating. The higher the octane, the slower it will burn.
Two-strokes are very sensitive to proper timing. If your timing is set
just slightly too advanced for the fuel you are using, you will
experience detonation, or knock. It is destructive for a two-stroke to
detonate or ping for more than a few seconds before the piston overheats
and starts to melt. Since today's fuel is lower octane than the good
stuff available back when these bikes were new, timing must be set
slightly less advanced than the manual recommends. For Yamaha RD's we
recommend 1.8mm btdc instead of the '70's spec. 2.0-2.1 btdc that the
manuals have printed."

My questions are why do you have to use a spec that's not in the limits
of the factory manual (did the octane rating go down since the mid/late
1970's)?

Wouldn't it be easier to just use a higher octane gas or gas additive?
Is the highest rated octane gas rated the same or higher as gas used in
the 1970's?

Would using "1.8mm btdc" do any harm or damage instead of the factory
settings (2.0-2.1 btdc)?

Does this mean all two-strokes made before the 1980's have to use
non-standard timing settings? How do you know a correct setting for
bikes other then Yamaha RD's?

 

Back before all bikes came with fixed ignition timing, it was
commonplace to adjust timing in the tuning process. As I recall from my
experiences tuning RD350s, retarding the timing .1 or .2 mm often made
the engine run better. The original specs for the RD350 were 2.2mm but
after a couple months there was a service bulletin that said to retard
the timing to 2.0 mm.

Two stroke engines are relatively low compression and RD’s ran great on
regular back when. Running them on premium now would give better octane
than needed for a stock engine. Retarding the timing may lower power if
it’s not needed or increase power if it is needed but it won’t hurt to
try like advancing it to far would.

Try reading the “two stroke tuner’s handbook” if you can find a copy.

Tony

 

You're riding a machine that employs *fixed timing*, as I recall. There
are other factors besides the fixed timing that are going to affect the
rate of flame propagation inside your combustion chamber, like the
spark plug heat range and the part throttle carburetor jetting.

Another poster was asking about how he could build a knock detector
recently. He wanted to tune his road racer to perfection by minimizing
the number of detonations or "knocks" that occurred during throttle
roll off for a corner.

The problem with both 2-stroke and 4-stroke engines is that you can't
jet them for full power, full load and let the excess fuel keep the
flame temperature down, you have to
roll off the throttle and you hear tinkling sounds like loose valves.
This starts after the spark plug heats up during the 2-stroke part
throttle yakkity-yak.

The tinkling is actually pinging and it is trying to erode a hole
through your piston. I have burned holes through pistons before and
seen pistons turn into molten slag.

But it's hard to visualize this fixed timing you're talking about when
you describe it in terms of millimeters before top dead center.

What the heck is it in degrees BTDC?

The piston is rising very slowly when it gets as close to TDC as 2.0
millimeters. A lot of degrees of crankshaft rotation go by as the
piston is near mid stroke, but the piston doesn't move much vertically
at the top of the stroke.

I think there is very little difference in degrees of piston movement
when you are worring about a difference of only 0.2 millimeters.

Here's an old formula from dirt bike days that I found in one of my
notebooks which seems to translate millimeters BTDC to crank angle.

Theta= cos^ -1 [ 1 - (2 X millimeters BTDC /stroke)]

 

They have tampered with the volatilitity of gasoline, the idea is to have
less air pollution during fueling.

Ted

 

You have no idea how much that made me grin. Thanks )