Who's up for a CV Carb Discussion?

[SlowOldGuy]

Wow, lot of information here.

Just a few notes.  The slides react to the velocity of the air flowing theough the venturi.  DJ went to the stronger spring to try and prevent the slide from over-reacting.  The slide controls the VELOCITY of the incoming air, not the VOLUME, the throttle plate is what controls the  amount of airflow.

If the slide is allowed to rise too quickly, it overshoots the desired position.  On first thought, you would assume that the higher position would allow the needle to deliver more fuel to the intake.  The real response is just the opposite, i.e. you get the lean stumble that is referred to.  What really happens is when the slide rises too far, too much throat area is exposed and the velocity of the air flow goes down.  Since velocity is what creates the vacuum that pulls fuel from the jet circuits, when this velocity goes down, less fuel is pulled into the intake. So, a bunch of air is let in with an inadequate amount of fuel, resulting in a lean stumble.

The slides are contantly dancing and changing their position.  Drilling 2 vacuum holes is confusing.  Whereas a larger hole "could" increase the slide response time, I've always thought the holes were large enough to not be much of a damper on the slide response.  Drilling 2 holes may actually work against each other.  Any velocity/vacuum difference between the position of the 2 holes may make them interact with each other, rather than helping pull a vacuum above the diaphragm, kind of like a short circuit.

Interesting discussion, though I'm sure Henry's bored by now.

DavidR.

[the fan]

Drilling the hole weakens the vacuum signal above the slide causing it to rise later (or slower if you prefer) a stronger spring would also delay the event, but would cause the slide to close faster, keeping the signal stronger, speeding airflow and aiding fuel atomization. This in turn created more surface area to a given ammount of fuel and promotes a more complete burn. The result of this manipulation is a crisper response when the throttle is reapplied and better drive in these situations.

The key is to understand that altering the timing of the slide, and even to a certain extent raising and lowering the needle, only serves to alter the timing of the event in a CV carb which is often confused with richening or leaning the circuit. It does not. In a CV carb you are altering the timing in which the metered amount of fuel is delivered in relation to the vacuum signal in the carb at that time. This was the hardest thing for me to understand when I started working on street bikes as ost of my experience was with 2 and 4 stroke dirt bikes and ATV's not equipped with CV carbs.

Of course I might have just made it all up......

[SlowOldGuy]

Bill,
CV carbs will run lean if you change anything on the intake side that affects air flow restriction.  This includes air filter type, drilling holes in an airbox, removing the airbox, etc.  Removing any intake flow restriction will slow the velocity of the intake air and cause the slide to seek a lower position which puts the needle is a lower/leaner position for a given throttle setting.

DavidR.

[the fan]

As will enlarging the hole drilled in the slide....

[andyb]

If the slide is allowed to rise too quickly, it overshoots the desired position.  On first thought, you would assume that the higher position would allow the needle to deliver more fuel to the intake.  The real response is just the opposite, i.e. you get the lean stumble that is referred to.  What really happens is when the slide rises too far, too much throat area is exposed and the velocity of the air flow goes down.  Since velocity is what creates the vacuum that pulls fuel from the jet circuits, when this velocity goes down, less fuel is pulled into the intake. So, a bunch of air is let in with an inadequate amount of fuel, resulting in a lean stumble.

Absolutely right.  Might be able to tune around it, but you'd need crazy needles, which would give you other issues.

Just a few notes.  The slides react to the velocity of the air flowing theough the venturi.  DJ went to the stronger spring to try and prevent the slide from over-reacting.  The slide controls the VELOCITY of the incoming air, not the VOLUME, the throttle plate is what controls the  amount of airflow.

Right in a pure sense about the slide, but it reacts to the velocity and limits it.  That lag is why they're CV carbs.  More air demand at the throttle blades means a greater vacuum signal between the butterflies and the slides, sucking air at an increased rate, giving more airspeed, which causes a vacuum at the venturi at the slide.  This pulls the slides up slightly behind the demand, and never allows airspeed to drop too far--assuming the slides don't react too quickly.  In a set of flatsides, if you snap the gas WOT off idle, the airspeed drops to nothing, and the fuel stays in the bowls = stalled. 

Drilling the hole weakens the vacuum signal above the slide causing it to rise later (or slower if you prefer) a stronger spring would also delay the event, but would cause the slide to close faster, keeping the signal stronger, speeding airflow and aiding fuel atomization. This in turn created more surface area to a given ammount of fuel and promotes a more complete burn. The result of this manipulation is a crisper response when the throttle is reapplied and better drive in these situations.

Second half makes sense.  First half loses me.  The vacuum signal above the slide can be considered a pressure (well, lack of pressure) as compared to what the airbox is seeing.  The larger the hole in the slide, the faster air can flow from above the slide into the venturi, raising the slide more quickly.  Explain more maybe?

In a CV carb you are altering the timing in which the metered amount of fuel is delivered in relation to the vacuum signal in the carb at that time.

Okay...erm.. You're controlling the amount of fuel is metered in to the air stream, by controlling the velocity of the air and by controlling the orfice size at the needle jet.  That's richening/leaning a circuit to me...?  You're doing it by playing with air and fuel rather than just changing the fuel metering.

CV carbs will run lean if you change anything on the intake side that reduces air flow restriction.

Fixed that for ya, but we knew what you meant  

If you have an airbox, it's always going to be under a slight vacuum.  Pods are less restrictive, and will be under a smaller amount of vacuum, because their lack of restriction allows air to rush in and fill the vacuum more freely.  The carb takes air in for the bottom side of the diaphrams from the airbox area.  So it actually should make the carbs more sensitive at the slides because the counterpressure is greater?  Probably cancelled out by the lower vacuum later on?  Dunno, lots to think about.

[turbocamino]

this has been some great info to digest...especially for me being that i have an 89 with a dynojet stage 3,with stiffer slide springs, and what i feel are some pretty good flowing slipons.  I also think the needles are to high as bike seems fat thru the mid-range . ok...... so..i just bought a 4/1 kerker...i known (read) that i may loose some giddy-up thru the mid-range with a 4/1. so...i'm thinking :scratch_one-s_head:i would drop the needles 1 or 2 grooves and now having learned velocity's role, maybe i should put softer springs back as well?  slowing fuel and air.leaning it further than just the needle change    ..yes no maybe?

[Harvy]

Well, just to keep the thread going......

I used the DJ E4117.001 kit....... is for European Models only........ mine was a Jap Domestic grey import into Aus.......essentially the same as the Euro models with the restricted inlet manifolds.

I have installed the kit as instructed:

1. Remove vac slide from carbs. Remove stock needles & spacers noting order of assembly. Use slide drill (#19) provided to enlarge slide lift holes. Do not drill needle holes!
2. Install Dynojet needles on groove no2, using all stock spacers. Install the Dynojet washers above the e-clip.
3. Remove the main jets and replace with DJ112 for use with aftermarket exhausts.
4. Locate fuel mixture screw and turn clockwise until seated, then back out 1 1/2 turns.


After installation I adjusted needle to 2 1/2 position with a shim.

Ran fine thru the rev range with this setup.

Since this install the bike has run 50000 kms and I have replaced the OEM 28mm manifolds with the rest-of-world 32mm units.

Thus now has me thinking.......with the change in inlet manifold diameter, there is an inherent change in vacuum and velocity so its no wonder she is running a bit ragged at the moment up in the Kookaloo zone.

I am now thinking of getting new DJ112 mains, DJ needles and FP emulsion tubes......and re-tune from there. OR should I just chuck the DJ kit already installed and just get FP kit for US models (given I now have the bigger throats) and FP tubes.
I'll tear em down and give em a good clean while I'm at it.

COMMENTS ANYONE?


Harvy

[Harvy]

Also.... a note on drilling the slides...... the #19 bit takes very little out of the existing hole. I can't really see that it is going to be detrimental to the FP kit to leave the slides as is (drilled) if I go that route.

There is no mention of changing the springs in my DJ kit, nor are any supplied.

I've not seen the US kits...... do they include the drill and/or springs?

I really don't want to have to replace slides as well. If it comes to that, I'll just buy a new set of re-jetted carbs from Randy!

Harvy

[the fan]

Harvy,

Here is a pdf of the USA FJ1200 dynojet contents and instructions. Compare this to yours to see any difference. I don't see any mention of drilling the slides.

[Harvy]

Is there a link hidden in there somewhere Bill?

Harvy

[Marsh White]

Is there a link hidden in there somewhere Bill?

Harvy

Harvy,

Click on the actual post to see it.  Attachments, Signature lines, etc. are not shown in the "Most Recent Posts" view.

[Harvy]

thanks Marsh

[andyb]

1. Remove vac slide from carbs. Remove stock needles & spacers noting order of assembly. Use slide drill (#19) provided to enlarge slide lift holes. Do not drill needle holes!
2. Install Dynojet needles on groove no2, using all stock spacers. Install the Dynojet washers above the e-clip.
3. Remove the main jets and replace with DJ112 for use with aftermarket exhausts.
4. Locate fuel mixture screw and turn clockwise until seated, then back out 1 1/2 turns.

After installation I adjusted needle to 2 1/2 position with a shim.

Ran fine thru the rev range with this setup.

Since this install the bike has run 50000 kms and I have replaced the OEM 28mm manifolds with the rest-of-world 32mm units.

Thus now has me thinking.......with the change in inlet manifold diameter, there is an inherent change in vacuum and velocity so its no wonder she is running a bit ragged at the moment up in the Kookaloo zone.

I am now thinking of getting new DJ112 mains, DJ needles and FP emulsion tubes......and re-tune from there. OR should I just chuck the DJ kit already installed and just get FP kit for US models (given I now have the bigger throats) and FP tubes.
I'll tear em down and give em a good clean while I'm at it.

If you use the unrestricted manifolds, you're lowering the average velocity.  Isn't a problem, they flow more air.

What else is done to it?  Pipe?  Pods?  Where's the soggy spots?  Read the plugs?

If you are close, and the needles aren't in bad shape, then you just need a couple parts rather than the whole kit, and can save some cash.

[andyb]

Also.... a note on drilling the slides...... the #19 bit takes very little out of the existing hole. I can't really see that it is going to be detrimental to the FP kit to leave the slides as is (drilled) if I go that route.

Yes, it takes just a sliver of material off the slide.  It's a very sensitive adjustment!

The US kits include the drills and the springs, but it depends on the year. (All jet sizes are DJ jets, not mikuni sizes)

The 84 kit includes a #28 drill, 110, 114, 120, 124 jets, and suggests you start with the mixture screws 5 turns out (!), and does NOT include springs.

The 85 kit includes a #19 drill, 110, 114, 120,124 jets, puts the screws 3.5 turns out, and does NOT include springs.

The 86-93 kit has no drill, uses jets 108, 112, 114, 118, runs mix scerws 3 turns out, and does include springs.

You could see if they'd sell you a set of springs, I'd imagine.  Dynojet PN DSP044. 

If your slides are drilled already and you don't want them to be, you can plug the holes with a little jbweld or solder them closed and drill new holes next to the old ones.

[Harvy]

Thanks for all the input on my little problem guys.
OK, my first task is to rip em off and strip them down for a good clean.
I'll replace bowl needles, seats and o-rings. Put in new OEM emulsion tubes and DJ112 main jets and DNT723 needles.

That will have it back to where it was 50,000 kms ago.......except it will have the larger manifolds.

I'll use that as my base and should have no problems tuning for the larger diameter from there.
Yes there is an engine re-build in the future.......but she does not use oil or have any issues which require a pulldown.
I have UNI dual pods, DJ stage 3 kit as discussed, real world manifolds, no engine mods at all that I know of, 4-1 headers, Micron can.
OEM ignition. Have not checked cam timing yet (hey....its the middle of summer...I want to ride it, not look at it... ).

Harvy