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Subject: Carb Vent Tubes From: papazit "at" juno.com (Chris L Johnson) Date: Mon, 3 Nov 1997 11:07:08 -0500
On Mon, 03 Nov 1997 09:15:18 -0800 Dick Barrrett
Dick:
The newer Dell'Ortos have only two vent ports or tubes. Tekei (TY
Yamaha), Mikuni, and older Dell'Ortos, like your Roo, have 3 in general,
with an odd 4th port on the Fantic-supplied Dell'Ortos. I'm not sure if
the newer or older Keihins have 2 or 3.
Traditionally, the top two ports on the carb are simply vents for either
side of the float bowl. They allow free gravity flow of fuel into the
float chamber. They also dump excess fuel. These are high on the carb
and won't dump fuel unless the bike is leaned way over. The bottom port,
usually on the float bowl, is a high float level overflow. The port is
connected to a tube in the float bowl chamber that is about the level of
the top of the bowl, higher than the normal operating level. If the
float needle valve dumps too much fuel, this port will dump it to the
ground, maintaining near the proper float level. If excess fule is in
the chamber, the bike will run rich until the motor brings the level back
to nominal, with aid of the overlow vent.
For 12 years I ran all my vent hoses up high to keep fuel from being
excessively dumped. I learned this trick during my cross country racing
days. I could go farther without refueling. While this gave me
significantly better fuel economy, especially with 3-port carbs, it would
occasionally result in richness when the bike was jostled or put at an
extreme angle. While not a problem on a go-fast bike, it caused problems
on my TYs and my Fantics...though rarely. I had to remember to always
shut off the petcock before transporting the bike, or when zipping down
mountains engine off. Otherwise, the engine would flood with fuel. One
time, I forgot on a long transport, and had so much fuel in the carnk the
engine went solid during the first kick start! Had to turn the bike
upside down, remove the plug, and pump out the fuel.
I can attribute hoses run high to a lean condition on my `96 Section.
The trapped fuel in the lines looped up and over the carb would on rare
occasions prevent replenishing of float bowl fuel and thus lean the bike
out. The `96 carb has only 2 hoses, up high on the carb.
Your question has brought up the issue of the role of only 2 vent hoses
on the newer Dell'Ortos. Is one a vent and the other a float level dump?
One would think they are only 2 vents in the conventional sense, as they
are both high on the body of the carb. Next time I have one apart, I'll
confirm or deny this. I do know the newer carbs are less prone to fuel
waste, possibly by better float needle valves and no high float level
dump vent. I think simpler is better in this case.
Now for that oddity: the 4th port located on near the float bowl (lower
than the 2 other vents on the carb body under the fuel inlet anf filter)
on older Fantics with PHBH26 carbs (large body carbs). The 305 up to at
least the `93 Roo had these as OEM carbs. The role of this port is a
mystery to me. It goes to the needle jet area at the upper nozzle (NOT
to the float bowl), and the normally open port at the bottom carb inlet
edge that goes directly to the nozzle is blocked by a brass plug. This
port is a bypass for the mixing area of the nozzle, so low pressures due
to the venturi effect at the juction of the nozzle and the slide do not
affect air supplied to the upper nozzle. What they did with this nasty
bit of creeping elegance (an engineering term for over complexity) is to
supply this air not from the beginning of the carb inlet, but from the
air box. Why? Beats me. All I know is I have had to unplug this hose
from the air box on some Fantics to get rid of inconsistent behavior.
Some riders also mistakenly plug a vent hose into the air box and get
weird effects from pressure drops in the float chamber. If it's working
OK leave it alone. You can suck a minor amount of dirt into the nozzle
if this hose is not getting its supply from the clan side of the air box
as intended. If not I would recommend throwing on a VM26 round slide
Mikuni, which works super on this bike. My `93 Roo was a basket case
until I put on a Mikuni. Couldn't get that Dell'Orto to run right no
matter what!
Specs for VM26 Mikuni for most Fantic 250s and even other bikes are:
Main: 120 to 140 (depending on the bike; start at 130)
Some bikes will require minor tweaks. The flat slide Mikuni and 28mm
Keihin are excellent choices as well. If you replace the carb on the
older Fantics, don't forget to plug that open hole for the former weirdo
vent.
For all carbs my recommendation is to remove any old and hard hose,
replacing them with high quality clear urethane hose of the largest
diameter the barbs will handle. Run all hoses straight down from all
vents (with that one exception above), to just behind the skid plate so
fuel doesn't run all over the back of the motor. Do not use vinyl hose
from the hardware store. It will get hard quickly. One rider has
cautioned against longer air hoses for deep stream riding, but I've never
had a problem. You can run no hoses at all. The only harm this does is
make a mess, and increase fire potential, which is generally very low
anyway.
Chris (CJ) Johnson
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