Who's up for a CV Carb Discussion?

[fj1289]

Started taking apart 3 sets of carbs today.

Set of 84 FJ1100 carbs - totally stock (still had plugs on the idle mixture screws -- and the worst I have ever seen of any carb for varnish - looks more like tar!)
Set of 90 FJ1200 carbs - unknown jet kit in them -I think Dynojet.  Had different slide springs than stock (seemed to be stiffer) and the slides had a second hole drilled in them about the same size as the stock one.
Set of 89 FJ1200 carbs - carbs I used prior to switching to flatslides.  Had an unknown jet kit in them - I think Dynojet since it had different slide springs (I always thought they were stock springs until I got the 1100 carbs open today).  When I was running these carbs, I rejetted them with Mikuni jets.

Questions/Thoughts:

Was there ever a jet kit for the FJ that specified drilling another hole in the slide?

I assume the stiffer springs in the Dynojet kit is an effort to slow down the slide to overcome any bogging tendancies due to the typical 4 cylinder 4-1 no airbox midrange flat spot?

I assume the shop that installed the kit in the 90 FJ1200 drilled the extra hole in the slides to speed up how quickly the slides come up?  I'd think it would have been easier just to keep the stock springs.

If the slides are modified and come up too quickly, do the carbs become NQCVs (Not Quite Constant Velocity)?

Chris
(thought I was done with CVs!!! ) 

[racerrad8]

I will get it started with my thoughts...

Questions/Thoughts:

Was there ever a jet kit for the FJ that specified drilling another hole in the slide?

Not sure, I have seen one set of second holes drilled. I have also seen probably 10-12 sets with the factory hole enlarged.

I assume the stiffer springs in the Dynojet kit is an effort to slow down the slide to overcome any bogging tendancies due to the typical 4 cylinder 4-1 no airbox midrange flat spot?

The stiffer spring was designed for quick and constant wide open throttle such as drag racing or in my case car racing; the street rider does not get to wide open throttle and hold it for very long. I never understood the premise since the sudden vacuum drop at quick WOT lets the slide stay in a down position since there is not enough vacuum to draw the slide up. I can only presume that the thought was when the intake vacuum surge did begin, after the drop, it would slow the slides upward movement making a more gradual enrichment. Personally, I remove them all and install stock springs back with stock/unaltered slides.

I assume the shop that installed the kit in the 90 FJ1200 drilled the extra hole in the slides to speed up how quickly the slides come up?  I'd think it would have been easier just to keep the stock springs.

The belief of the extra or enlarged hole was to get the slide to raise faster, but in reality I have noted the slides are actually slower reacting. When they have the enlarged hole, they do not reach as quickly. They also seem much more "sluggish" or not as active when doing blip throttle tests.

If the slides are modified and come up too quickly, do the carbs become NQCVs (Not Quite Constant Velocity?)

Not sure, I do not have a flow bench to test...

Chris
(thought I was done with CVs!!! ) 

Glad to see you have come back...

Randy - RPM

[andyb]

Was there ever a jet kit for the FJ that specified drilling another hole in the slide?

Not that I'm aware of.  The FP kit didn't, DJ didn't, and V&H didn't either.  May have been someone experimentally tuning, you know, drill holes in parts to save weight, right?   Enlarging the stock holes is included as part of the directions with both the DJ 1985 and 1984 kits (which have different part numbers...oddly...).  It's not in the instructions on the 1200 models.  Interestingly the instructions specify drilling different hole sizes between the years.

I assume the stiffer springs in the Dynojet kit is an effort to slow down the slide to overcome any bogging tendancies due to the typical 4 cylinder 4-1 no airbox midrange flat spot?

The different springs were not part of the DJ 1984 kit, but are included in the others.  According to their instructions:

The DynoJet slide springs may offer better drivability in this area [0-1/8 throttle] but may slow down your accelleration.

Makes me guess that they're trying to soften the transition between the pilot circuts and the needle, but the stiffer springs prevent the slides from rising as quickly, limiting throttle response on large inputs (i.e., closed to WOT).  They'll also smooth any stumble out to a degree, kinda like making the carbs "more" CV than they are.

I assume the shop that installed the kit in the 90 FJ1200 drilled the extra hole in the slides to speed up how quickly the slides come up?  I'd think it would have been easier just to keep the stock springs.

The belief of the extra or enlarged hole was to get the slide to raise faster, but in reality I have noted the slides are actually slower reacting. When they have the enlarged hole, they do not reach as quickly. They also seem much more "sluggish" or not as active when doing blip throttle tests.

The DJ 1990 instructions actually don't tell you to drill the holes out.  Possible that someone used the wrong kit or followed the wrong directions.  Also possible that it just was some idiot.  Randy is correct here, as the larger the hole in the slide, the less pressure differential between the vacuum lifting the slide and the airflow below, so it'll react slower.  Probably useful if you've got an engine that's slightly overcarbed. 

If the slides are modified and come up too quickly, do the carbs become NQCVs (Not Quite Constant Velocity?)

I'd imagine that they become very horrible flatsides at that point, that are pig rich through much of the lower half of the revs, then hit 6krpm and take off.  As the slides rise up, they're balancing the vacuum difference and the orfice size.  If they rise too fast, you'll have a slightly rich midrange.  If they rise too slowly, you end up with soggy throttle response through the midrange.  Probably best to err on the side of caution and go to the stock springs and stock bleed hole size.

[fj1289]

Glad to see you have come back...

Thanks Randy, but all things being equal, I wish I still had my FCRs!

Andy,  good info an all the DJ kits.  I also forgot about the V&H kits - they had a good thing going (assuming they were delivering quality products) pipes, carb kits, air filters, ignition advancers (other bikes got ignition boxes - never heard of one for the FJ though?)

The rest of this stuff makes my head hurt!    


Some VERY BASICS on CV's for my own clarification...

Obviously, the butterfly valve is the actual "throttle" - unlike a flatslide carb (I'm assuming Mikuni RS or Keihin FCR) where the slide is the throttle. 

Slide position controls fueling.

So...it seems to me the real trick is getting the "fuel control" to match the "engine control".  I think I have a pretty good handle on the fueling circuits (fuel and air) and their interactions in a carb.  I'm trying to get a better handle on the slide control. 

Here's a clip from a writeup I found online (http://www.drpiston.com/Cvcarbs.html)

Now we'll try to figure out how CV carbs work.

When the butterfly valve is closed, very little air is moving in the carb bore.  (The engine is getting some air and fuel through the pilot circuit, which we'll describe later.) With little to no air flowing, the air in the carb bore and the air in the closed chamber above the diaphragm are at close to the atmospheric pressure of the outside air. 

Open the butterfly, and several things happen.

1. Air now speeds through and venturi effect (depression) at the point of the slide (variable venturi) is created.

2. The depression at the venturi is transmitted up through the holes in the slide to the closed chamber above the diaphragm.  This lowers the density of the air in that chamber.

3. The open air below the diaphragm now wants to rush into that chamber to equalize the pressure, but it can't because there is no passage.

4. So it does the next best thing and tries to push its way in through the underside of the diaphragm. 

5. The diaphragm can't let the air in, but it is flexible so gives way it is pushed up by the outside air pressure.

6. As it goes up, it pulls the slide with it, and the slide pulls the tapered fuel needle up in the fuel hole.

7. More air flows, more fuel is pushed into the air stream, and the engine accelerates or runs at higher revs.

But how does this improve things over the simpler slide carb?

When the throttle is cranked on the slide carb, the slide is pulled up immediately by the throttle cable, expanding the variable venturi suddenly, and causing the lean stumble described above.

When the CV butterfly is opened, the slide does not immediately jump up to a much more open position.  It raises gradually as the increasing engine revs provide the needed depression (at the venturi), which is then transmitted to the chamber above the diaphragm.  As the slide rises, the increasing depression also encourages more fuel to enter the carb bore and combine with the greater air supply now available. And the higher the slide goes, the more fuel the tapered needle permits to flow. In other words the genius of the CV carb is that the fuel from fuel hole can now "keep up" with the increasing air available--maintaining the mixture at proper ratios during the accelereation process.

In summary, the CV carb provides quick enough acceleration (no lean stumbles to slow things down) which is also smooth.  And overall we get a "kinder, gentler" carb which gives us less twitchy responses as we make small throttle adjustments.

I'm good with all that, but it seems there are a lot of exceptions to the theory when put into practice!  Also, I think there is a wide range of what people would consider a "twitchy" carb vice a "responsive" one. 

I think I'm starting to get it...!
A larger hole in the slide SHOULD let the slide move quicker - more like a Mikuni RS or Keihin FCR.
BUT, the big difference is when the slide on a CV carb comes up too fast and you get the airflow "stumble" in the carb, the CV slide is coming back DOWN because you've lost the "vacuum" that WAS holding the slide up.  And, I'd imagine where the larger slide hole allows the slide to rise faster, it will also DROP faster. 

OK, time to go finish tearing down the carbs and have another look at the differences between the FP kit I have and the DJ kits in the carbs.  It seems like the FP kits richen the pilot circuit to deal with a quicker moving slide (by keeping the original spring) and the DJ kits slow the slide (with the heavy spring) and don't richen the pilot circuit. 

When do I get to go for a ride again!

[andyb]

2. The depression at the venturi is transmitted up through the holes in the slide to the closed chamber above the diaphragm.  This lowers the density of the air in that chamber.

It's comparing the air density (vacuum, pressure) between the venturi under the slide and near the filter end of the carb where the top of the diaphram is fed by air.

As the engine starts using more air, the air velocity goes up, lowering the pressure.   The lack of pressure bleeds air from above the diaphram  out through that drilled hole, and the pressure lifts the slide.

If there is no hole at all in the slide, it never comes up.  It's actually held down a little, from the vacuum in the main air passageway trying to suck it into the motor. 

If there's a tiny hole in the slide, it comes up slowly, as it takes time to bleed the air pressure from above the diaphram.  if it's too slow, this acts like a secondary throttle blade to the motor, restricting air flow, and restricting fuel by holding the needle "in".

If there's a large hole in the slide, it comes up quickly. The fuel curve follows the demand of the motor and everyone's happy.  Yay.

If there's too large of a hole in the slide, it comes up even more quickly.  This will let it rise and give the motor all the air it wants, but not enough vacuum/airspeed to keep the carbs working.  The motor gets a sudden lean spike, and then as the revs rise the engine rapidly goes rich as the main jet is being used before the motor needs it all.  Basically turns it into a flatsides problem, like snapping the gas open at idle.

So I guess I was mistaken earlier. Have to think that someone was trying to increase throttle response by drilling them out.  Seems to me that if you improved airflow within the engine you'll improve the throttle response by allowing the motor to gain revs quicker and cause the vacuum to spike up faster.   

Dunno, guess this is what the pros get paid to figure out   If you've got the DJ needles, you could turn the carbs to those specs pretty easily, or you could go with the stanard needles shimmed.  You could also go to a FP kit, which I've heard good things about... though someplace I read the FP kits were including mains as big as 132!

[Harvy]

Andy...... my '91 has DJ stage 3....... drilled slides....... needles on 2 1/2 position from memory......... 4-1 exhaust and dual pods....oh and full diam inlet manifolds, not the old 28mm OEMs for Jap domestic model. Of late, top end has felt very mushy. AT the end of my last ride, I gave her a wot run on the stand.........flames shooting out the exhaust......So its now running very rich at the top end.
I believe this has been a gradual process of getting richer.......and am concluding that its the emulsion tubes that have probably gone oval.....the bike is now at 92000km..
Do you know if the FP emulsion tubes will work with the DJ kit?
Haven't seen anyone comment on this possibility..... and maybe I should just replace the tubes with OEMs?

Harvy

[andyb]

Harvy, either the instructions were changed or someone followed the wrong ones then   Doesn't matter, so long as it runs okay for ya.

Definitely sounds like emulsion tubes there.  Because of the mileage I'd wonder also if the camchain has gotten a little rubbery and allowed the cam timing to move away from spec also.

The FP emulsion tubes should work with the DJ needles.  You could also ask FP if they'd be willing to sell you needles... it may be worth looking at the entire jet kit though.  If the emulsion tubes are that worn, I'd really wonder about the condition the needles are in by now?

[Harvy]

Harvy, either the instructions were changed or someone followed the wrong ones then   Doesn't matter, so long as it runs okay for ya.

Definitely sounds like emulsion tubes there.  Because of the mileage I'd wonder also if the camchain has gotten a little rubbery and allowed the cam timing to move away from spec also.

The FP emulsion tubes should work with the DJ needles.  You could also ask FP if they'd be willing to sell you needles... it may be worth looking at the entire jet kit though.  If the emulsion tubes are that worn, I'd really wonder about the condition the needles are in by now?

   I did the install myself so if the instructions were wrong, then I must have bought the wrong kit...... any way, not to worry as it ran fine for around 50,000 km with the kit in......I've still got the instructions which say to run the number drill thru the hole to make it just a tad bigger.

Hmmmmm...last time I had the valve cover off the chain was not exhibiting excess slack.......but you never know......I'll check valve timing. Yes, I think you are right, a full FP kit and emulsion tubes are on the horizon I think.

Cheers
Harvy

[andyb]

The chain doesn't set the tension, the tensioner sets the tension.

If the chain and/or front guide have worn, it can change where the timing ends up.

May as well freshen a bunch of things while you're in there instead of just the emulsion tubes, it's enough work to pull carbs that it's not that big a deal to replace orings and other wear items while you've got it all apart, right?

[fj1289]

Harvy, a good cleaning every now and then does the carbs wonders too.  Also, like Andy was saying, as the cam chain stretches over time, your cam timing retards slightly (each link in the chain gets minutely longer - effectively rolls the cams back slightly).  Might as well degree the cams while you're at it...and then you could add some headwork...and how about a bigbore...and of course then you'll want some larger carbs...and then...what were we talking about fixing in the first place?!   

I've got a Factory Pro jet kit for the 89.  Will report how it compares (as best as I recall) the the dynojet/Mikuni combo I had before. 

Did the Dynojet kits ever include different Pilot Air Jets?  The 89 carbs have 144's w/o the Mikuni "mark" on them -- stock is 155. 

Chris

[fj1289]

If there's too large of a hole in the slide, it comes up even more quickly.  This will let it rise and give the motor all the air it wants, but not enough vacuum/airspeed to keep the carbs working.  The motor gets a sudden lean spike, and then as the revs rise the engine rapidly goes rich as the main jet is being used before the motor needs it all.  Basically turns it into a flatsides problem, like snapping the gas open at idle.

Andy,  I thought that too at first.  Then I tried to figure out why Randy said it makes them move slower.  With the flatslides, you are spot on, but I don't think it is possible for the CV carbs to behave that way.  I think if the slide hole is too big or the spring too weak, you can make the slide rise too quickly to sustain the airflow just like the start of the flatslide scenario, but then I think the difference occurs between how the two carbs act.  With the flatslides the slide stays at the top of the carb because you are holding it at WOT.  With the CV, once the airflow is disrupted/stagnated, there is nothing to hold the slides up, so down they come until the airflow stabilizes again and there's enough airflow/vacuum to start them back up again. 

IF CV's do behave that way, I wonder how much the slide in a CV carb "hunts" for the right postion?  In other words, if you do drill a slide too large, will it "overshoot" and rise too much, lose some airflow, "fall" a bit too much, rise again, etc, each time getting closer to being spot on?  If so, how fast is all that happening?  Then, if we make the hole bigger still, will it "overshoot" more and take longer to dampen out? 

Another train of thought - Does the weight of the motorcycle/car effect the "best" settings for slide movement?  I'd think that the lighter weight bike could accelerate quicker, so the engine RPMs would pick up quicker, so the slide would need to move quicker.   

[andyb]

Andy,  I thought that too at first.  Then I tried to figure out why Randy said it makes them move slower.  With the flatslides, you are spot on, but I don't think it is possible for the CV carbs to behave that way.  I think if the slide hole is too big or the spring too weak, you can make the slide rise too quickly to sustain the airflow just like the start of the flatslide scenario, but then I think the difference occurs between how the two carbs act.  With the flatslides the slide stays at the top of the carb because you are holding it at WOT.  With the CV, once the airflow is disrupted/stagnated, there is nothing to hold the slides up, so down they come until the airflow stabilizes again and there's enough airflow/vacuum to start them back up again.

IF CV's do behave that way, I wonder how much the slide in a CV carb "hunts" for the right postion?  In other words, if you do drill a slide too large, will it "overshoot" and rise too much, lose some airflow, "fall" a bit too much, rise again, etc, each time getting closer to being spot on?  If so, how fast is all that happening?  Then, if we make the hole bigger still, will it "overshoot" more and take longer to dampen out?

It would seem to me that they're going to overshoot to some degree anyhow.  It's more in this case about inertia though.  Remember also that the slides don't see a consistent change in vacuum/airspeed, they see the pulsing of the cylinders, because each carb only sees the airflow of one cylinder.  So they bounce or flutter, rapidly.  Changing the springs won't provide damping, though adding weight to the slide could, via greater inertia.  When I say they would overshoot the optimum balance, that's depending on two things, the speed of the slide's movement and the mass of the slide assembly.  In theory if you got the slide to react more quickly, it'd be moving faster when things balanced out and it would continue moving a short distance.

They will auto set themselves to a degree, but it's not relevant like you're thinking.  If the slide is at 50% vs 60% on average at a given amount of airflow, what matters is the needle's position and taper at the orfice, not the actual slide position.  Changing the bleed hole size in the slide is going to change that position which will then alter the jetting because it puts the needle in a different position.  So overall it seems to me that it doesn't matter much if you've drilled them or not, it matters that you've matched the needle to the speed that the slide moves.

A lighter bike accellerating quicker should make a tiny difference in the fueling, but if it's only depending on the rate of rpm gain at the engine, then lower gears are going to fuel differently than higher gears will (meaning first vs fourth, not final gearing).  Also if you're really pushing the motor to get you down the road, then it shouldn't much matter as you're WOT and probably north of 6k rpm, so the slides are going to stay topped out and you're using the mains instead.  In a racing situation it shouldn't make a lot of difference what you do to the needles nor the slides, because you're going to launch around 5k or so and go up from there.  May be able to tune into or out of a slight lean/rich spot just off the line, but it's faster to just change your launch rpm appropriately to the amount of power you're needing, as the jetting will be wrong for only a very short duration until the mains take over.

[andyb]

I've got a Factory Pro jet kit for the 89.  Will report how it compares (as best as I recall) the the dynojet/Mikuni combo I had before. 

Did the Dynojet kits ever include different Pilot Air Jets?  The 89 carbs have 144's w/o the Mikuni "mark" on them -- stock is 155. 

According to the http://www.dynojet.com/pdf/4127.pdf information from dynojet on their 1200 kits, a DJ144 air jet was included and was optional, for enrichening the bottom end.

And stop scaring the poor guy, man.  Just needs his carbs done, that's NO reason to bore things up.  Everyone knows that when you work on the induction side the major improvement is turbocharging, not overboring...

[racerrad8]

Andy,  I thought that too at first.  Then I tried to figure out why Randy said it makes them move slower. 

Well...in my application of a car that basically runs in the range of closed to WOT, I see them reacting slower...

I am sure we are talking about the same thing, but the slide is raised by vacuum; not pressure. While vacuum is a pressure differential it works by vacuum.

The slide is raised by the loss of pressure inside of the slide diaphragm as the intake charge flows past the hole in the bottome of the slide. The vacuum then raises the slide. Based on my experience and common operating condition, the larger hole makes the slide lazy.

My only reasoning for this is the same air speed across a smaller hole creates a high velocity of vacuum (suction) compared to the larger or dual hole arrangement. Maybe I'm way off base, but just passing along my thoughts and experience's.

For an experiment take your vacuum and try it with a large and small hoses. The vacuum will create maximum vacuum quicker with the small hose. The vacuum has to draw a larger surface with the larger hole. Yes it pulls more into the hole when it has reached maximum vacuum but, it does take longer to get there.

Randy - RPM

[andyb]

Yer formatting is strange, Randy~

Can argue it either way.  The pressure differential is what moves it, vacuum on one side and atmospheric pressure on the other.  In a turbo setup if you set the carbs up properly, neither side sees vacuum; the pressure difference (relative vacuum) does the work.

The vacuum experiement makes sense as you describe it, but the other portion is that the bigger hose has more volume in it, and the vacuum motor is pulling a set amount of air out of the system, so it'll take longer to remove that much air.  You'd have to do some math and cut the bigger hose shorter to get an accurate comparison.

This makes me wonder if several smaller holes would act differently than one larger hole.

The volume of air that has to be removed from the inside of the slide and above the diaphram doesn't change (unless you smash the carb tops in with a hammer).  The amount of air coming in from the other side of the diaphram doesn't change unless you do some serious drilling.  So the only ways to change this system is to change the length and/or rate of the springs, change the weight of the slide assembly, or change the orfice size in the end of the slide. 

Hell Chris, you've got a set of slides with different holes, try em all and let us know!  They only take a couple of minute to swap over, you could do that between rounds    They shouldn't make really any difference at the dragstrip though, they'd make the biggest difference going from smallish throttle positions at lower revs to healthy throttle positions as the motor is trying to spool through the midrange.  FactoryPro uses a brake dyno, and does most of their testing during steady state load tests, which wouldn't change this.  Dynojet uses inertia dynos and in theory would be able to better match this effect up for clean fueling.  But remember that in the later DJ kits and in the FP kits, no slide drilling is required.  I don't know if this is to prevent people from complaining that their bike runs bad after installing a kit because they drilled it wrong, or if it's just not necessary.