CV carburettors: They're different...

[forchetto]

There are lots of tuning and modification articles dealing with the normal slide carburettor, but few telling it like it is with CV carbs.
Some time ago I found this write up in a newsgroup that explains things very clearly. It certainly opened my eyes when dealing with these instruments.

At the end I include a sectioned view of a CV carb showing the all-important transfer or transition ports.

Here goes:

Here is the secret of the CV carburetor: up to 3/4 throttle, the engine
is getting most of its mixture from the pilot jets, not the jet
needle/needle jet combined orifice.

Compared to the pilot jets in an older slide valve carburetor, the CV
carb has a huge orifice. Whereas a slide valve carb might have a #17
pilot jet ( the hole is 0.17 mm in diameter ), a CV carb might have a
#30 to #45 pilot jet.

We all learned to calculate the area of a circle in junior high school.
The #45 pilot jet has SEVEN times the area as the #17 pilot jet.

So the CV carb's pilot jet flows most of the fuel into the carb when
the throttle isn't wide open and the slide isn't lifted far enough to
pull the jet needle out of the needle jet.

You can really enhance your off idle throttle response and improve the
midrange power by finding the pilot mixture screw and tweaking it about
half to one full turn counterclockwise.

The pilot mixture screw is probably hidden under a blind plug, forward
of the diaphragm cap on top of the carb, or underneath the carb,
forward of the float bowl.

If you cannot see a slot screw on the side or top of the carb or underneath it, the EPA plugs are still intact. You have to pilot drill the plug, screw a
small sheetmetal screw into the pilot hole and then pull the screw and
plug out with a pair of pliers.

Now you can adjust the pilot mixture screw by turning it counterclockwise a bit.
Note that CV carb idle mixture screws work the opposite way. Clockwise is leaner,counterclockwise is richer.

Riders get into trouble when they expect the idle RPM to increase the
more they turn the screw counterclockwise. When they've gone too far,
the exhaust sound is dull and thudding and maybe the the idle RPM is
too low.

So they adjust the master idle knob to increase the idle RPM. Then,
when they twist the throttle grip open and closed, the engine RPM hangs
up at 5000 RPM and they wonder what the heck is going on?

The amateur tuner doesn't know about the three transition ports right
by the throttle butterfly's bottom edge. The transition ports are
getting fuel from the pilot jet too, but they aren't controlled by the
pilot mixture screw, they are controlled by the bottom edge of the
butterfly.

The amateur tuner tweaks the pilot screws CW and CCW and can't figure
out which way they should be turned because he doesn't understand that
the transition ports are getting fuel that BYPASSES the pilot screws...

Pilot fuel is sucked out of the float bowl through the idle jet. It
goes through a branched passage to a pattern of transition ports that
are covered by the edge of the throttle butterfly when it's closed. Air
from the fixed air jet is added to the same passage. Fuel/air mixing
occurs in the passage. As the throttle butterfly is opened *slightly*
the vacuum in the carb's bore can suck fuel/air mixture out of the
transition ports and the engine speed can increase smoothly. There is
another branch of the same passage that has an outlet about 1 inch
downstream of the transtion ports. Vacuum is weaker there. The pilot
screw adjusts fuel/air flowing out of that port...

When the idle RPM hangs up at 4000, 5000,even 6000 RPM, that tells the
experienced tuner that the pilot mixture screw is open too far. So, he
has to turn it back the other way to get the engine to idle smoothly at
the smallest amount of throttle opening and the least number of turns
out of the pilot screw.

You can do so much with pilot jets and adjusting the pilot mixture
screw, The strategy for adjusting the pilot mixture on a CV carb is to get the
engine to run smoothly at the specified RPM with the smallest amount of
throttle opening possible. The amateur tuner fools around with the
pilot mixture screws, not understanding that the engine is drawing fuel
air mixture from the transition ports that may be slightly uncovered by
the throttle butterfly, according to the setting of the master idle
knob and the synchronizing screws...

Now, one of the tests for pilot mixture setting is that you open the
throttle quickly, and close it. The engine speed should increase, but
it should return to the original setting. If it hangs up, the pilot
mixture is too rich, because the engine is ALSO getting fuel from the
pilot outlet port AND the transition ports.

> What about main jet size?

Let me educate you on the Mikuni/Keihin jet sizing system. It's just
like the pilot jet sizing system.

The orifice size is based upon a #100 mainjet having a hole that's
exactly 1.000 millimeters in diameter.

The area of this orifice increases as to pi times the square of the
radius, so you can see that going from a #100 main jet to a #120 main
jet will result in an orifice with an area that's 44% larger and will
flow far too much fuel whenever the engine has enough vacuum to suck
fuel out of the float bowls.

The engine will be drowned by excess fuel if you go up too many jet
sizes at once.

Going from a #100 mainjet to a #110 mainjet is not just one size jump,
it's FOUR sizes larger, #102, #105, #107, then #110.

Even the #110 mainjet has 21% more area than the #100 main jet. What
would make an intelligent rider think that his new exhaust pipe by
itself was going to raise his horsepower by 21%, requiring 21% more
fuel?

Constant vacuum carburetors rarely run on the main jet. You could even
probably get away with running the stock main jet or one that's just
maybe two sizes larger than the original jets.


> Will a needle shim be enough?. Should definitely be in the right direction.


Nope. That's an urban myth amongst amateur tuners that don't undertand
how CV carbs work. CV carbs have short fat needle without much taper,
and whacking open the throttle just doesn't lift the needle out of the
hole enough to matter. The grooves on jet needles with clips help
control when the tapered part of the needle begins to have effect, but
you'll get more off-idle acceleration effect out of turning the pilot
mixture screw just exactly the right amount.

You run at full throttle so rarely on the street or the highway, it
just doesn't make sense to mess around with main jets and needle
shimming. It makes a lot more sense to learn about the pilot jets and
how to set them correctly.

You may ask, "But why do all the tuning manuals say to start tuning by
finding the correct sized main jet?"

It's not about ultimate power, as shown on a dynamometer. You
can get a higher horsepower reading out of an engine that's just warm
enough to carburate cleanly than you'll get out of a hot engine.

Heat builds up during a long race and it builds up faster on a high
speed course like Daytona.

Racers who really *race* need a main jet that's big enough to pass
extra fuel so the engine won't overheat. The racers are willing to
waste fuel to keep the engine cool.

But the typical rider who buys a Dynojet or other kit thinks that he's buying "instant power" and an instant solution to the EPA mandated lean burn jetting from the factory.

Sorry, it doesn't work that way. The rider who succeeds in getting what
he wants out of his engine is the rider who understands the technology
of that engine and why that particular technology was chosen for it.

[rich_e]

Well, now a know a little about CV carbs. My ATV has one. It'll start but will not run past an idle. If you touch the throttle it dies. It'll only run for a second or two, at an idle, before it stalls. It acts like it has run out of gas, but there's gas in the float. Strange. The fuel flow from the tank is good (it also has a vacuum operated petcock), you can see the fuel squirt out if you disconnect the fuel line and crank the engine. I guess I'll pull the carb completely apart. So far I've only pulled off the float bowl and confirmed that the float wasn't stuck and that the needle and seat were clean. There must be some dirt in there somewhere.

Thanks for taking the time to post this info.

[forchetto]

Even though they're meant to be very efficient and clever, I find them a total PITA...they're notoriously difficult to tune and any little problem that would be nearly ignored on slide type instruments will upset them. A small vacuum leak at the manifold, messing about with their carefully designed airboxes, punctures in their diaphragms.
It looks as if you might have a blocked passage in the pilot circuit probably through little usage allowing gum and varnish from dried-up gas to accumulate. You may have to perform a thorough clean of the idle air jet and its drillings, as well as the pilot jet ones.

[rich_e]

Forchetto:

I just notice that your location is Spain! I lived in Zaragoza (1973 - 1977). My father was in the U. S. Air Force.

Spain is a wonderful country. I hope to return some day for a visit. I had a couple of 50cc motorbikes, a Derby and a Puch. Fun machines.

[dr1445]

sorry to take issue with your view on adjusting cv carbs. the idle circuit is good for idle to 1/4 throttle, after that the fuel metering duties are past on to the needle, then the main jet. it is true that they all must work in harmony. adding a shim to raise the needle has major effects even with the stock needle. for instance a 1 mm shim added to a suzuki dr650 will add 2.5 hp if the airbox snorkel is removed. since a stock dr only has 30hp i would consider that a large change. thank you 1mm shim. see the thumper talk forum and the suzuki dr, there are even dyno charts. if you go over to the suzuki savage forum you will find folks cure the backfire on the ls650 with stock jetting by raising the stock needle and also get a performance boost. if you mess with the needle you also loose a around 2to4 mpg by raising it. just passing on what greater minds than mine have found and documented.

[forchetto]

Originally Posted by dr1445

sorry to take issue with your view on adjusting cv carbs.

Not really my view as I got that write-up from a newsgroup.
I did ignore it when tuning my PGO 250 buggy with a Kymco engine and Mikuni CV carb. Did the jets, needle raised, spring cut off and it worked great...
In fact I'll probably be ignoring it again and trying the needle thing on my Qingqi QM200...

Have searched for article about that hp increase for a DR650 and found it here:
http://www.thumpertalk.com/forum/arc.../t-566238.html

There are also the dyno charts here:
http://www.procycle.us/dr/dyno1.htm
and here http://www.procycle.us/images/dr_dyno.jpg

[VinceDrake]

Thanks for the post, I enjoyed it. I really wonder what (if anything) the pilot and transfer passages are doing at 3/4 throttle and up... Hmmm.

On a semi related note:

Years ago I got to watch a Mercedes D-Jetronic Fuel Injection System operating in a glass box, with a blower attached. Pretty Cool, could see Coldstart Injector working, etc. I wonder if anyone has a vid of a 'glass' (Plexi?) CV Carb. I'd really be curious to see what passages are doing what at WFO, etc...

Thinking about it, maybe I should try to make one.... Hmmm.

--Vince