question about CV carb setup

Bike is a 1980 XS650.

Decided to check/adjust/synch the carbs this weekend. The bike has been
running good, just decided to see if I could make it run even better and get
a bit more power. I had read that the easiest way to set the idle mixture
is to take out one plug and ground it against the engine, then start the
bike (which will run on one cylinder ok) and adjust the carb on the running
cylinder to achieve highest smooth idle. As the idle mixture screw on these
carbs is a GAS screw, not an air screw, I figured that the more I turned it
in (clockwise) the higher the bike would idle as the mixture leaned out.

BUT... what I saw was the opposite. I started about 2.0 turns out, then
turned it CW in 1/4 turn increments. The rpms dropped. And dropped. And
dropped. And finally stalled about 0.5 turns out.

So, what I ended up doing was watching the tach and opening the mixture
screw until the RPM maxed out and leaving it just at that point. Did the
same to the other one, then synched the carbs. The bike runs no better and
no worse than before which, I suppose, could be a lot worse.

Am I missing some key concept here? Should I have done it with both
cylinders firing? How can the engine idle slower with a leaner mixture?

As a secondary issue, I am thinking about re-jetting for more top-end power.
I am used to riding a sportbike and like the upper range power when I'm on a
curvy and/or uphill road. This bike is good on acceleration from 0-60 but
absolutely sucks from say 60-90. Very little power. Would larger main jets
likely help me out here or am I wasting my time on an old bike that ain't
going to go any faster?

Thx

Dave S.

 

So, what size pilot jets do you have?

XS650SE's imported to the USA had very small #27.5 idle jets which
might require the idle mixture screws to be opened 3.5 turns.

XS650G's imported to the USA had whopping big #42.5 idle jets that
might require the idle mixture screws to be closed down to 1/4 turn
open or less.

I wouldn't adjust a carburetor unless both cylinder were firing,
because the dead cylinder would still be sucking gasoline and that
would wash the oil film off the cylinder wall and it would also dilute
to oil

The mixture was too weak to provide the minimum amount of power
necessary to
sustain the idle at the desired RPM.

Have you checked the cranking compression? If the cylinders have low
compression, below 150 psi or so, the motorcycle won't achieve its top
speed, no matter what you do to the carbs.

Have you done a plug chop after a high speed run? What did the
porcelin insulator inside the plug look like? If you're running
unleaded gasoline, you should see a narrow ring of fluffy black soot
deep inside the spark plug, where the porcelin meets the steel shell.

If only the nose of the spark plug is white and free of soot, the
mixture is too rich and the engine is running too cold to develop
maximum power.

For more information on rejetting, go to www.factorypro.com and read
the articles on CV carb tuning and mainjet selection for low RPM
engines.

Steer clear of Dynojet's bullshit about making your carburetor's
various circuits operate independantly and avoid the temptation to
dick around with raising and lowering the needles.

Understanding how the idle circuit operates is key to understanding
the CV carburetor.

 

It's an XS650G, Canadian model (speedo in km/h) with the adjustable needles.
I don't know what size the pilot jet is I'm guessing it's completely stock
but haven't disassembled the carbs to date. The stock anti-smog factory
setting was about 2.0 and the best idle was achieved at about 3.0 turns out.

No, I haven't checked it. I have a compression tester but it never occurred
to me that I'd have a compression problem given that the engine's only got
20K miles on it. I'll check, only takes a minute.

Didn't look inside the plug. I have checked them, they're nice tan color,
no soot apparent but as I said I wasn't looking for any small ring of soot.

I've read it, I was hoping that somebody with a similar bike might save me a
whole lot of time and effort trying different sized main jets... there are a
bazillion of these bikes on the road.

I thought I had a pretty good understanding, hence my confusion about the
idle going down and not up as I leaned out the mixture. I'll try it again
with both cylinders running, I think the drag of the operating cylinder just
took too much power to accurately set the other carb.

 

So you probably have the #27.5 idle jets. Back in the days of slide
valve carbs, you would see round idle jets that were about #30, and
round main jets up to #150, because the engineers expected the rider
to use a lot of throttle and didn't have to worry about keeping the
atmosphere clean.

Modern CV carbs take direct control of the main jet out of the rider's
hands and they now use a much larger idle jet to give the engine good
off-idle and midrange performance without ever having the carb go onto
the main jet.

The size of the orifice is based upon a #100 jet having a hole that's
1.00 mm in diameter, so a #42.5 pilot jet has a hole with an area 2.4
times that of the #27.5 jet.

And, remember that the screw-adjustable idle mixture outlet is only
one of four idle mixture ports in the carb throat. When the idle jet
is large, more mixture will flow through the three transition ports
controlled by the throttle butterfly and you have to turn the mixture
screw all the way closed.

Besure the engine is warmed up and open the throttle all the way so it
can get air to compress. The compression pressure should rise evenly,
over about five compression strokes.

If you're running unleaded gasoline, a tan insulator indicates you're
burning a little oil. A wet ring around the threaded end of the plug
indicates you're burning a lot of oil, as does shiny black carbon
burned onto the ground electrode right where it's welded to the outer
shell.

#135 main jets are plenty large, if you're not running an open exhaust
system. Remember what I said about the main jet size being based upon
1.00 mm. If you go from a #135 to a #150 jet, the bigger jet has 1.24
times the area of the smaller jet, and you probably don't need
anywhere near that much orifice area if you're just trying to optimize
the performance of an engine.

Well, tuners have understood for a long time that there was more power
to be extracted from an engine by leaning up the mixture, but you will
always hear amateur shade tree tuners talking about throwing a jet of
larger jets into their carbs to extract some more horsepower that the
engineers at the factory stupidly overlooked.

The actual reason that racers put larger jets into their carbs is to
keep the engine cooler by reducing the temperature that the fuel/air
mixture burns at. If you're not going to be running at full throttle
on some long race track somewhere, chances are you don't need bigger
main jets, you just need to work on the idle mixture circuits by
optimizing the idle screw settings.

If you have to adjust the idle mixture screw more than about 3.5 turns
open with the small idle jets you have installed, you can put some
#30's or #32.5's in there.

But you might just be chasing a rule of thumb by doing that. How much
effort and expense do you want to invest in chasing a rule of thumb?

?I'll try it again

That's what I was trying to say in the previous post.

 

According to the manual, I've got #132.5 main jet, #85 air jet, #135
throttle valve, #42.5 pilot jet, and #30 starter jet. Don't really
understand the difference between pilot jet and starter jet, unless one of
them has to do with the choke (okay, okay, "enricher circuit").

Well every year it gets closer and closer to an open exhaust system as rust
eats more holes in the mufflers. I tried several retail muffler repair
"systems" and found that they're all junk. Don't really want to sink $250
into mufflers to go on a $250 motorcycle.

Last year I tuned a Honda CB750 for a friend who couldn't get it to idle
smoothly... the previous owner had drilled some "extra" holes into the
airbox: bypassing the air filter. Duh. Those Honda engineers really
dropped the ball on that one!

Yeah, I know what you mean. I just don't think there's much more
performance to be had out of this old dog. I'll check the compression just
fer fun; other than that it's likely as good as it's going to get. I
shouldn't be going that fast on this bike anyway... the handling and braking
are not confidence-inspiring after riding my VFR.

BTW, you said tan=oil burning. This bike doesn't burn appreciable amounts
of oil (although it does seem to drip it in my garage from the clutch
pushrod seal and main crank seal). I was always told that you want your
plugs with "a layer of light tan ash" on the porcelain... white or grey
meant too lean/hot, black soot too rich.

 

Yes, the starter jet feeds gasoline straight out of the float bowl to
the bypass passage when you "choke" the engine.

Drilling large holes into an airbox ruins it as a resonant chamber.
When I remove the 3-inch long rubber snorkel from the airbox on my
GS1100, it started lean surging violently at 6000 RPM, which was 100
mph in 3rd gear. I reinstalled the
snorkel and the surging was gone. I removed the airbox cover entirely
and the engine didn't surge. But the engine really hated having to
suck air through the hole where the snorkel had been when i removed
it.

That tuning "secret" is now 30 years out of date, now that tetraethyl
lead is no longer used to raise octane.

How wide should the narrow band of soot inside your spark plug be?
Keep leaning the idle mixture until you start seeing small black
specks that look like crains of pepper on the insulator nose. That is
carbon that is flaking off the top of your piston, and watching for
this evidence of lean mixture preignition works fine on old engine
with carboned up pistons.

You'll never see the specks on a newly-built hot rodded engine though.
But you might notice the strange groaning sound that an exhaust system
makes when the engine is jetted too lean and the crackling sounds out
exit of the pipe.

Older 2-valve per cylinder engine designs gave the burning gasses more
time to
transfer heat to the crown of the piston and the ignition had to be
advanced to over 50 degrees BTDC in some inefficent designs which
didn't swirl and tumble the fuel air mixture and vaporize it.

Modern 4-valve per cylinder engines reduce the total number of
crankshaft degrees that the engine uses to light the mixture and
extract most of the usable power from the expanding gasses by about
30%.

So the later designs don't need to waste so much fuel to cool the
combustion process and the engines can be jetted leaner.

 

Okay, so I'm looking for white porcelain with a band of black soot near the
bottom of the insulator in the groove between the insulator and the external
metal case.

Are we still talking WOT plug chop? Or low-speed cracked-throttle chop?
How much do the idle jets contribute to the wide-open performance? I guess
that'd be about 9.3% based on areas of the jets. More than I thought it
would be before I did the calculation.

The black specs, are they a good thing or do I want to richen it say 1/4
turn after I start seeing them?

This is starting to sound more like art than science. I adjust the idle
mixture for full-throttle performance (!) and hope it will idle at all with
the throttle closed? I'm sure it will, I was very surprised that I could
basically go from 0.5 to at least 5 or 6 turns and the bike still ran. Then
you hear about people talk about things like richening the idle for better
"off the throttle response". I guess the key is to just mess with it until
you get tired or find that nirvana which is optimum jetting and mixture
adjustments.

Thanks for the help.

Dave S.

 

Nothing. The vacuum downstream of the wide open butterfy is too weak
to suck mixture out of the idle circuits.

When you read performance tuning instructions for carburetors, the
authors alway make it appear that the idle mixture ports only operate
from
closed throttle to 1/8th throttle and then the throttle slide cutaway
supposedly starts helping acceleration, and then in their fantasy
world the carb supposedly runs on the needle until the throttle is 50%
open, then they clain the carb runs on the main jet.

And all the above is a steaming pile of capital-C Crap. The various
fuel/air circuits of a carburetor overlap, the idle circuit provides
less and less fuel/air mixture as the throttle opens further and
further and the slide raises more and more.

An increasing amount of fuel/air mixture passes between the jet needle
and the needle jet tube that it goes up and down in, but this amount
of fuel/air mixture is less than the amount of fuel/air mix that could
pass through the main jet, once the needle has raised high enough so
the difference in area between the jet needle and the needle jet hole
is larger than the orifice hole in the main jet.

At some point, around 50% of the possible distance the slide can
lift?the jet needle/needle jet combination orifice begins to pass more
fuel than can pass through the idle jet.

This is called the "cross over point", where the engine is getting
more fuel through the JN/NJ than it is getting through the idle jet.

Although your idle jet can only pass about 10% of the amount of fuel
that the main jet can pass at full throttle, your carburetor is still
running on the idle jet 75% of the time, until you open the throttle
so far it lifts the slide all the way.

GOK what the slide position is at any given time in a CV carb, but
once it has lifted the slide all the way you're theoretically on the
main jet if the carb is jetted anywhere close to where it should be.

When I see the specs, I richen the mixture slightly.

It is an art. ?

No, no. Remember the pattern of three transition ports below the
throttle butterfly? If you open the idle mixture screw, you are only
adjusting the fuel air mixture through ONE port, about 3/4ths of an
inch downstream of the transition ports.

The transition ports are NOT controled by the idle mixture screw in
any way. Where the amateur mechanic gets into trouble is when he turns
the idle mixture screw too far counterclockwise and the idle mixture
coming out the single port becomes too rich.

The exhaust note becomes dull and thudding, the exhaust smells too
rich, the engine idles too slowly and maybe stalls.

So the amateur mechanic turns the idle speed knob up to get the idle
speed back to spec. This opens the butterfly slightly, uncovering the
first transition port.

When he blips the throttle, checking for throttle response, he is
surprised by the engine's refusal to idle down again. The amateur
mechanic is utterly lost at this point, because he doesn't even know
the transition ports are there to help the motorcycle accelerate away
from a stop.

The transition ports are NOT supposed to be uncovered at ordinary idle
RPM when the tranmission is in neutral. They pass too much fuel for
the amount of power needed to idle the engine.

The idle screws weren't adjusting anything at all after 4 turns open.
?

Opening the idle mixture screws 1/8th to 1/4 turn past the EPA setting
will definitely help the engine start and warm up quicker and
accelerate better at small throttle openings.

No, the key is to understand how the carburetor works. Mark Salvisberg
has a wonderful site at www.factorypro.com, but Dynojet has a site
that is full of capital-C Crap with their claims that their kits will
seperate the interacting fuel/air mixture circuits into "discrete"
circuits like some old slide valve mixer of yesteryear.

Dynojet claims that carburetors need to be tuned for "dynamic"
performance (when the engine RPM is changing rapidly) as well as for
"static performance" (when you are trying to cruise or corner at a
steady speed).

I bought exactly ONE Dynojet kit, and it didn't help my GSXR750 one
pit, it actually hurt the performance during steady speed cornering. I
called up the tech rep and asked him what was going on, and he told me
that I should be able to accelerate rapidly through the terrible flat
spot in the mid range like I was street racing or drag racing.

I told him I couldn't do that, I had to go through certain corners in
certain gears without having to upshift and downshift in mid turn.

 

Other posters have covered the CV carbs pretty well.
Low-speed and mid-range should benefit from some tweaking, but unless
you add a LOT more breathing capacity, larger main jets won't help.
The XS-650s used very modest cam lift and timing, with small
valves. It made for good mid-range, and really good reliability. It
was totally inadequate for "sportbike" performance.
A friend had a BSA 650 Lightning while I had an XS-650. I
could stay even with him to about 70 or so; then the Yamaha stopped
pulling, and the Beezer would walk away.
Someone else I knew explored the top end of his XS, and turned
it into a Britbike killer, but it cost a head, valves, exhaust system,
cam and pistons, and many portraits of Ben Franklin.

 

Coo, he's morphed again.

 

This is a fair assessment. It was Yamaha's first four-stroke, after all,
and they were playing it safe. The first prototype engines only produced
something silly like 20bhp. They had a lot to learn.

Oddly, despite being two-stroke builders, Yamaha's engineers got a lot
of experience working on the Toyota 2000 sports car. Specifically, they
developed the cylinder head. The valve springs in the early 650 (maybe
the later ones too) had Toyota part numbers, and the pistons were
modified Toyota ones.

 

There's a web page (650 central maybe) which has got instructions to convert
the XS650 from a 360-degree to an 83-degree offsite firing angle. I thought
about doing it, retooling the cam, new ignition, at that point you even
start thinking about a retooled head with larger valves, etc. But the cost
quickly put that idea to rest. the exhaust is, I think, only 1-1/4"
headers, and I've also seen people who have upsized the pipes and intake air
flow to squeeze some more power out.

Dave S.

 

And just re-throwing the crank wouldn't improve power on its own -
just make it smoother. As you say, you'd need to carry out all the
tuning work after doing the crank, timing and cam changes.

XS650s are very easy to tweak anyway, and the engine is incredibly
strong, so you've no need to worry about blowing the thing up.

 

Ah, but if you start messing with larger valves and higher lift cams some of
your bullet-proof-ness gets lost.

 

Believe me, an XS650 has got enough of that Magic Ingredient to spare.