TMX 35mm CARBS

By Randy Norian

Swapping 35mm carbs onto the RG500 seems like a logical modification.  Most
125 MX bikes come standard with a 35mm carb, and 125 GP bikes use up to a 38mm
carb.  Ultrathin slide carbs on MX bikes generally have terrific throttle
response, so it isn't unrealistic to expect good driveability on the RG when
fitted with 35mm TMX carbs or their Keihin equivalent. 

It's been a few years since I made this modification, so this will just cover
the basics as I remember them.

Just how much power is this mod worth?  I didn't dyno the bike at that time,
way back in those days I was timing 60-120 mph intervals (2 way average) to
sort out my tuning.  Before this mod, I timed 2 way average of 8.49 seconds,
60-120 mph.  I backed this up with a 120.96 mph quarter mile pass.

After the 35mm carbs were installed, 60-120 average dropped to 7.22 seconds,
and quarter mile speed increased to 124.5 mph.  The motor simply made more
power at the same rpms than before. I was able to go faster while shifting at
the same point, or sooner, although there -was- a slight gain in peak rpms
that the motor would turn.

Driveability is excellent.    Throttle response is good, and the bike still
gets about 30 mpg when ridden moderately.  It will accept full throttle as low
as 4500 rpm; if anything, the bike rides better with the big carbs than it did
before. 

35mm carbs have 1.56x the area of 28mm carbs, so this is a huge increase in
inlet area. 

TMX carbs are a primary-type main fuel circuit, as opposed to the stock carbs'
bleed-type circuit.  TMX carbs have a very thin slide and generate a strong
signal at the venturi, so the have excellent throttle response, even when
using a large carb on a small motor.   Early TMXs like mine do not have
removeable needle jets, allowing you to only change the needle to tune. Later
models are fully tuneable. 

35TMXs are physically large.  They are taller and deeper than the *tiny* stock
carbs.

This is not a simple bolt on-- it's not terribly difficult, but there are
obstacles. 

The stock carbs have area of 6.15 cm2.  The stock port size at the inner disc
cover is about 8.2 cm2.  The port has a max width of about 24.3mm and max
height of about 38.4mm.  The stock port is 52 degrees wide.  With a stock
disc, you have 1174 degrees x cm2 of open time-area.

The port is partially open for the first (and last) 52 degrees of the intake
event.The port is fully open for the duration of the disc cutaway, minus  the
width of the port. Example:  The stock disc has about 143 degrees of cutaway.
It is fully open for 143-52 = 91 degrees.  However, the port is half open
during both the opening and closing phases. Averaged out, that gives another
52 degrees of effective full-open time-area. 91 + 52= 143 degrees.   143
degrees x 8.21 cm2 = 1174 degrees x cm2.  The port is full or partially open
for 195 degrees. 

The effect is this: the port is fully open for the length of the disc cutaway.
Total inlet duration is cutaway + port width (in degrees) .  An increase in
effective time area comes from the increase in AREA of the port, not the width
of the port.

The TMX35 carbs have area of 9.62 cm2.  In addition to installing the larger
carbs, I enlarged the passage through the cases to match the carbs.  My
enlarged port is 44.4mm wide at its maximum and 27.8 mm tall, max.   My
enlarged port has a sectional area of 10.2 cm2.   This gives an intake time-
area of 1461 degrees X cm2.  This port is fully open (with a 143 degree
cutaway) for only 86 degrees.  However, it is partially open for the first and
last 57 degrees. Effective full-open duration, 143 degrees.  So, there are 143
+ 57 degrees of full or partially open rotation, for a total of 200 degrees
open duration.  But still only 143 degrees of effective full-open time. 

One more example.  We create a port that is 100 degrees wide.  It is fully
open for 143 - 100 = 43 degrees. the rest of the time it is partially obscured
by the disc.  There are 100/2 + 100/2 degrees of 'half open' port.  Effective
full-open time is still 143 degrees.  Got it?   The increase comes from the
larger port area, not the increased disc duration (which would now be 243
degrees of intake duration).   

Ideally we would like a very tall inlet port,  with small port width (in
degrees). This is accomplished by using the largest diameter disc valve
possible.  Then we could have large intake time-area, with conservative total
disc duration.

The result of all this fooling around is that my large carbs/ ports have a
slight increase in port-open timing compared to stock, but a huge increase in
intake time area.   A small port motor with 159 degree disc cutaway will have
211 degrees of disc-open timing, but only 1305 degrees x cm2 of intake time
area.   My motor can use a stock disc, have only 200 degrees of disc-open
timing, and still have 1461 degrees x cm2 intake time-area.  Sort of like a 4
valve motor vs a 2 valve motor.   I think this is part of the key to the
excellent torque at low rpms that we are getting from my motor (500cc but 66
ft-lbs).

Back to carbs!

I bought all 4 carbs through Sudco Mikuni in California.  This set me back
about 600 bucks, although I hear these carbs are down to 100$ apiece these
days. 

I bought rubber spighot adaptors to attach the carbs to the engine, also
through Sudco Mikuni.

I split the cases and enlarged the ports through the case, to the dimensions
mentioned earlier.

  At the inlet, the openings were enlarged to about the
outer edge of the O-ring recess.  I bolted the spighot adaptors on and used a
little sealant, except for the left rear carb (the alternator carb). On that
cylinder, I installed a 1/4 inch deep wedge-shaped spacer to tilt the carb up
and out a little to clear the alternator case.  I also ground a small divot in
the alternator cover to clear the lower drain plug on that carb.   TMX carbs
have quick change plugs on the float bowls that allow you to change main jets
in just a few minutes, without removing the carbs.

TMX carbs are designed to function normally while tilted, I believe they are
good to at least 10 degrees tilt off-axis.

I  sourced miniature hose barb fittings from a hobby shop; the fittings were
designed for use on fuel lines on RC planes.  Drilled and tapped the fittings
into the carb throats for the oil injection lines, and installed check valves
inline to prevent the oil tank from siphoning into the carbs.

I swapped the stock SIPC (crossover) tube onto the 35mm carbs, with no
noticeable effect.  I think the 35mm carbs are sufficiently large that this
feature is not necessary any longer.

The stock throttle pull is enough to fully open the carbs (just barely),
however there is not enough exposed cable sticking out the ends of the
throttle cables for the slides to close all the way.  You need to cut away
some of the coiled sheathing to allow more of the cable to stick out.  I heat
the cap with a propane torch and then pull the metal endcap away with a pair
of pliers.   Bend the cable over to spread out the coils and cut it with a
pair of snips or something. I used a dremel, but be VERY CAREFUL!!!! if you
slip and nick the cable, you will have little frayed strands sticking out and
grabbing onto things, at best it will be annoying and at worst it can make the
throttle hang up.   I stretch out the now floating length of coil and 'unwind'
it from the cable.  I then smooth  the shortened coiled end and reattach the
metal encap, with a touch of heat. Too much will make the cable cover wad up. 

Now you have 35mm carbs installed, but they were designed to be used one at a
time, on a dirtbike.   A set of four of these will have a truly amazing
throttle pull. You'll have a grip that can crush mortal bones in just a few
short weeks of riding this setup.

I took a set of low-rate springs from a set of CV carbs slides and rewound
them for use in the TMX carbs.  They have plenty of tension to drop the
slides, but allow for a light throttle pull when all 4 are ganged together.  

Jetting:  The TMXs flow lots of fuel with very little signal.  On a disc valve
motor, the intake *pop* really gets the gas moving.  I had to jet the carbs
down severly in order to get them to run on my bike.  They came with jets in
the high 300 range installed,  I had to drop into the 270 range to get them to
work on my bike.  I dropped to a #10 pilot jet, and kept  going leaner on the
needles  until I was 2 needles leaner than what came on the carbs.  I use a
6EN11-58 needle, middle position.  This setup ought to get you running in the
ballpark.   The tiny pilots indicate that we are getting a much stronger
metering signal, though conventional wisdom says that big carbs carburete
poorly.  The TMX carbs are a generation ahead of TMs in this respect, I think.

Intake: I tried to use the stock intake trunks, which had worked very well for
me up to this point, but they are too small for the 35mm carbs and the carbs
go very rich at top rpms.   Removing the trunks was necessary to get the TMX
carbs to run.  I made airboxes on each side of the motor early on, however now
I use 4 individual filters.  I will use a semienclosed, fresh air fed airbox
as soon as I can make one, as power drops 1% with every 10 degree F.  increase
in inlet air temperature.  I suppose you can choose whether to pursue this
free HP or not.

That ought to cover the basics!  Have fun!

Randy Norian