Question about RD400 Service Date - Octane Rating and Timing

 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?

 @nf.sympatico.ca:


factory

overheats

I had a '73 RD350, used regular gas and set timing to factory specs.
Never any detonation or pre-ignition problems.

 

You can no longer get street gas that has octane ratings that were
available then.

A lot depends on how the bike is to be used. If you ride the bike hard
or race it on the pump gas that's available these days retarding the
timing might be necessary. If you're not noticing any problems with
the factory settings, you might be okay. I say might, because your
experience level at tuning an engine matters very much. If you know
what detonation sounds like, there'll be plenty of warning, if not
you'll get into trouble without being aware of it. I knew a racer that
was completely insensitive to the sound and when he came out of turn
11 at Sears Point the bike was detonating so hard that it would turn
heads in the pits, but the rider either couldn't hear it, or didn't
understand what the sound meant.

On the RDs I set up for racing, I would install an exhaust gas
temperature gauge. Thermocouple sensors mount right in the exhaust
stream as close as I could get them to the port. The gauge responds
very quickly and dyno and track testing showed that with the engine at
peek rpm in top gear, full throttle, the best temperature was about
1100 degF. You can get this temp with too much ignition advance and
too rich a mixture but the engine will be down on power, so your plugs
have to look right, as well. This happened to me at Daytona tuning a
TZ250 that wasn't pulling the gearing it should and the plugs looked
rich. I backed the timing off a bit and leaned it out and it started
getting the trap speeds it should, in those days, about 150 mph.

                                     Bill Smith

 Back in '77, I roadraced a RD-400, and had really good luck retarding
the timing to about 1.8 or 1.9 mm btdc.  I never had any problems with
it, and it would regularly run 13.90s when drag racing and about 114 mph
or so (after getting fully warm) when roadracing.  There were only a few
other 350-400 cc bikes that could match its speed and accelleration, but
sadly many, many faster riders than I.

Dan


ACH wrote: