Yamaha RD400 1981 Road Test
How would you fancy an RD400 that could
blow machines with twice that capacity into the weeds?
The idea appealed to reader Brian Dodson so he invested
two years' work and a lot of money achieving just
It's not an idle boast, we speed trapped his bike
at MIRA where it reached 125.69mph in fairly neutral
wind conditions. We were impressed! We often hear
from readers who claim that their bog-stock xyz goes
20mph faster than our road test version, but when
you get down to the timing lights the bullshit stops.
To get the kind of speeds we were recording you need
just one thing — power, and plenty of it! In
fifth gear the engine would sing on to 10,000rpm and
a genuine 121.54mph. In fact the power spread was
one of the best things about this conversion, the
other good point was the stability.
Even flat out the bike was rock steady and Brian
achieved this with remarkably little in the way of
modifications. The bike is fitted with taper roller
head bearings which Brian says made a big difference.
There is an air conversion on the front, drilling
and tapping the original fork nuts to accept a GS1000
balancing kit. Pressure is set to 8 to 10 psi. At
the rear Koni dampers control the stock swinging arm
while damping at the front is helped by 30 weight
Bel-Ray at 210cc per leg.
The engine tune is a story of gradual development
over a couple of years, and Brian made a few mistakes
along the way. Opening up the reed stops to 12mm lift
helped the power, but the bottom reed broke up after
several thousand miles. Fitting two six thou reeds
cured this although Brian has now gone over to Boyesen
Racing reeds after reading the report in MCN.
The cylinder heads have been lightly skimmed but
Brian was still running with head gaskets so the squish
could probably come closer yet. The barrels were ported
by Stan Stevens Motorcycles at Sevenoaks in Kent and
although we have no details on the port dimensions
we have been assured that the holes are "big"!
To match up with the big ports, Brian bought a pair
of 34mm Amal Mk11 carbs. These were fitted to the
standard intake rubbers which were cut out to match
the new carb bore — the same mod as our own
shoot-out Yamaha RD250. The power range on the new
port timings and the Amal carbs was very good but
the addition of power jets, from Amal, helped things
even further. Brian fine tuned the power jets by extending
the feed nozzle with brass tube obtained from a radio
shop. This brings the jet in earlier as the throttle
opens and it really smoothed out the pick-up.
I checked with the Amal technical department on this
point and they said it was perfectly acceptable to
tune in the power jet in this way. Their standard
jet length is a compromise for all the applications
which the carb can be adapted to, with wild port timings
on a two stroke, bringing the power jet in earlier
helped smooth out the power delivery.
On the exhaust side Brian chose Allspeed Formula
111 expansion chambers. The centre stand cannot be
used with these pipes but they do tuck underneath
the engine, much like the LC model, and increase cornering
The plain section of the front pipe with the spring
fitting made it a piece of cake to shorten the system.
Brian took one inch off the down pipe and modified
the back fitting bracket. This gave even better ground
clearance and let the motor rev out higher.
In this very high state of tune the engine is remarkably
flexible and easy to drive, as Brian says: "It
is more like a tuned four stroke on the road with
usable power from quite low engine speeds."
To cope with the extra power the clutch had to be
beefed up. This is a common problem with mildly tuned
RD400 engines so Brian had quite a problem! His answer
was to visit no less than 40 bike shops searching
for stronger clutch springs. Finally he discovered
that Honda 125 inner VALVE springs could be fitted
with just a little machining of the clutch pressure
plate. Initially Brian had six Honda springs but found
the pressure just too much. He now has three, with
three standard springs and has had no trouble at all
from the unit.
Going faster is one thing, stopping is another.