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EVO Carbs

Started by wingingit2, April 13, 2019, 05:58:58 PM

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JW113

I don't know why I'm doing this. Bored or something?

wingingit. You seem to have selected the perfect handle.

The All Balls kit is not a magic performance kit, it looks to be a very complete stock rebuild kit. Good choice. The only thing lacking is a replacement slider diaphragm, which tends to get gas soaked soft and rip over time. It also does not supply a new slider.

The 6 Sigma kit is a different story. Just so you know, I can read very well. The question is, can you? This is copied from the 6 Sigma Jet web page, just for sake of discussion.

Jet Kit Includes

    Main Jets
    Pilot Jets (if required, not all kits will require the pilots)
    Needle Adjusters
    Nylon Spacers
    Drill Bits for:
    Slide Hole Modification (for CV carbs)
    Slide Spring Modification (for CV carbs)


I don't recall anybody recommending drilling out the slide hole, or using a lighter than stock spring. In fact, quite the opposite. Let me tell you a little bit about that, though. And please, feel free to fire back more insults so I feel good about wasting my time trying to share info with somebody that doesn't really want it.

CV carbs are by no means perfect for every application. They have some very nice features, but throttle response is not one of them. At the throttle, a CV feels like a real dog compared to an S&S or Mikuni. That's because the slider function adds a time delay as to have fast the power comes on, particularly right off idle. In most cases, it is a minor delay, but a delay none the less. Some people absolutely hate that, and choose to kick the CV's superior fuel metering characteristics to the curb in favor of throttle response.

Hence, the "magic carb kits". There are a handful or more of these available, and they all do pretty much the same thing: modify the slider function to make it open much faster, and with much less air mass through the carb. In short, these kits are trying to turn a CV into an S&S Super E, or rather a close approximation thereof. You might as well remove the spring and JB Weld the CV in uppermost position, and then play with the main and pilot jet until the bike will run. And there you go, instant throttle response.

What the performance kits do, though, is ruin the CV's ability to carefully manage the fuel metering transition between idle, low speed, and full throttle. Fixed jet carbs like S&S tend to have some pilot to main transitional gaps, and the typical compromise is to jet rich to compensate. Jetting rich or lean, though, means flat spots somewhere in the throttle range. And jetting rich is what kills fuel consumption. I've not heard of too many cases where after installing a CV magic carb kit, the owner did complain about why his bike is only getting 25-30 mpg. A typical stock, stage 1, or stage 2, 80 cube EVO without fairing or windshield should expect right close to 50mpg at steady highway speed.

The other big advantage that the CV brings to the party is it's ability to compensate for altitude changes. The CV's slider (and thus jet needle) operates from air mass passing through the carb venturi. As the mass of the air changes (lower mass with higher altitude), the slide adjusts accordingly and leans out the fuel delivery. Fixed jets carbs, when jetted at sea level, run increasingly rich as they go up in altitude. Or in your case, if jetted appropriately at 4500 feet, will get leaner at lower altitudes.

As Hillside pointed out, on the dyno, it makes almost no difference what carb you run, as long as the carb is sized appropriately to the size and HP potential of the engine. CVs tend to nose over right around 100HP, as that is when their air flow capability flattens out. For 99% of EVO powered motorcycles, a CV has more than enough air flow capability.

So enough of all that. I think you should follow your own plan and don't listen to any one of us. Just wing it, see what happens. I'm sure you'll learn something in the process. The internet, mixed with it's truth and lies, is fun to read and all, but experience is the best teacher. And you seem like a guy that would rather learn from their own experience rather than somebody else's.
:beer:

-JW
2004 FLHRS   1977 FLH Shovelhead  1992 FLSTC
1945 Indian Chief   1978 XL Bobber

Breeze

JW; Most of the things in your post are what I (a butt dyno, home mech.) have found to be correct about the cv. I put a 42 mm Mikuni  on my Evo that dyno'd about 85 hp with a CV, it dyno'd almost the same with the Mik. (enough yrs. ago that I don't remember exact figures or have sheets). The only real difference I noticed was the engine responded to throttle blips quicker  :baby:, a very consistent rattle from the slide at all steady rpm's and a noticeable lightness in my back pocket. My mpg was only slightly less with the Mik   Good explanation by you IMO.
I'm starting to believe my body is gonna outlast my mind.

wfolarry

I'll add one more thing:
When I flow tested the carbs on the flowbench they were mounted to the heads. There was a big difference between how fast that slide would open on a ported head compared to a stock head. Get the bike on a dyno & look into the carb on a WOT pull. You'll be surprised at what you see.

capn

Us simple folk prefer S&S. Maybe less mileage but runs great on any HD.

JW113

WFO Larry: if you are after high HP numbers on a dyno, CV is maybe not the ideal choice. Proboly 3 or 4 more HP with a non-CV carb. So, I agree. You're also talking stage 3, which I was saying stage 2 (ordinary street bikes) the CV gets the job done. Stage 3, well, maybe consider something else. Like a CV51.
:SM:

Capn: if you're not chainging altitute a lot, and don't care about fuel consumption, yes the S&S carbs work well. Being a Califorina wacko, spewing as few carbon atoms into the atmosphere as possilbe is important.
:SM:

OK just rattling your cage! Smootchies all around!
:hyst:

-JW
2004 FLHRS   1977 FLH Shovelhead  1992 FLSTC
1945 Indian Chief   1978 XL Bobber

wfolarry

What I was referring to was how headwork [among other things] had an impact on how fast the slide would open on a CV carb.
I remember seeing stock bikes that couldn't pull that slide all the way open. So did others obviously that's why they came up with all the gimmicks to get it open faster.

JW113

The slide is not a throttle. It's a mechanism to measure how much mass of air is flowing into the motor. In a somewhat similar way the older style flapper valve mass airflow sensors work on 80s fuel injection systems. So it makes perfect sense that with head work, more air will be getting into the cylinders and thus the slide moves more. The balance of spring force, diaphragm area, and vacuum port diameter is calibrated to raise the needle the right amount to maintain a somewhat constant air fuel ratio. The combination of needle and main jet port is essentialy a "variable jet". Letting the slide slam open without a corresponding amount of air is simply over-jetting the air/fuel mixture, and will hurt power, not help.

-JW
2004 FLHRS   1977 FLH Shovelhead  1992 FLSTC
1945 Indian Chief   1978 XL Bobber

JamesButler

Quote from: wfolarry on April 23, 2019, 03:07:29 AM
What I was referring to was how headwork [among other things] had an impact on how fast the slide would open on a CV carb.
I remember seeing stock bikes that couldn't pull that slide all the way open. So did others obviously that's why they came up with all the gimmicks to get it open faster.

Another "gimmick" to improve throttle response for the CV-40 -and one of the least expensive ones- was to swap out the stock CV-40 spring with the Screamin' Eagle spring CV-44, which was a lighter tension spring.

Burnout

Whatever you do DO NOT CUT A CV SPRING!!!!!
For you non-science majors that makes the spring stiffer.....
Cutting the spring ruins it.

As detailed above:
Modifying a CV for throttle response is futile and counter productive.
You need a different carb to get the blown fuel throttle response.

A CV is a great all around carb, snappy throttle response is not in it's repertoire.

I prefer the non-butterfly type carbs (for most responsive) as they do not take the 20% flow reduction caused by the blade and shaft in the air stream. But with that you take some drivability loss.

A Mikuni slide also needs a quick throttle to match the feel of a large butterfly carb.
A butterfly carb has a large inrush at small throttle openings. So don't mistake that for throttle response, it is all feel.
A slide carb airflow curve is the opposite sloping up to linear, where as a butterfly carb airflow jumps up sharply at first then tapers off.
Remember a butterfly carb has to be 20% larger to have the same airflow as a slide,
the real downer is the losses in the intake tract because the air speeds up and slows down as it goes through the carb.
This is a no no for performance, you want to smoothly and progressively accelerate the air to slam it past the valve.
Any slowing down is a loss that cannot be regained.
They don't call me Ironhead Rick just because I'm "hard headed"

JW113

I think brother Burnout has nailed it.

Are you going to ride this bike around town 10% of the time, and take it drag racing 90% of the time?
Or are you going ride around town 90% of the time, and drag race 10% of the time?
Or, more importantly, are you going to ride around town 100% of the time, and pretend that someday you're actually going to go drag racing with your bike maybe once every year or ten?

Answer those questions honestly to yourself, then you'll know if a Keihin CV is right for you, or not.

In terms of day to day drive-ability, I can't think of any carburetor in the world that outshines a Keihin CV.

-JW
2004 FLHRS   1977 FLH Shovelhead  1992 FLSTC
1945 Indian Chief   1978 XL Bobber

Deye76

CV's are great if you like to tinker. For most riders a S&S E will suffice very well. Set it and forget it.
East Tenn.<br /> 2020 Lowrider S Touring, 2014 CVO RK,  1992 FXRP

JamesButler

Quote from: Deye76 on May 02, 2019, 06:12:29 AM
CV's are great if you like to tinker. For most riders a S&S E will suffice very well. Set it and forget it.

I agree and disagree with you, if that's possible. With the CV, you initially set or tinker with the pilot and main jets, air-fuel mixture screw and idle screw to fit your specific engine build, but then you can pretty much forget about it.  However with the S&S, when going from sea level to 4,000 ft elevation, re-jetting is required for it to run optimally, if you plan on staying awhile, whereas the CV self adjusts for elevation changes.  No need for re-jetting which makes it a great and versatile touring carb...

I have several FXR's.  Run the Super E on two of them, run the stock CV-40 on my bone stock '92, run the re-jetted (tinkered) CV on a couple others, and a Mikuni on an '84 FXRDG.  Like them all!  :hug:


koko3052

Quote from: JamesButler on May 02, 2019, 08:00:02 AM
Quote from: Deye76 on May 02, 2019, 06:12:29 AM
CV's are great if you like to tinker. For most riders a S&S E will suffice very well. Set it and forget it.

I agree and disagree with you, if that's possible. With the CV, you initially set or tinker with the pilot and main jets, air-fuel mixture screw and idle screw to fit your specific engine build, but then you can pretty much forget about it.  However with the S&S, when going from sea level to 4,000 ft elevation, re-jetting is required for it to run optimally, if you plan on staying awhile, whereas the CV self adjusts for elevation changes.  No need for re-jetting which makes it a great and versatile touring carb...

I have several FXR's.  Run the Super E on two of them, run the stock CV-40 on my bone stock '92, run the re-jetted (tinkered) CV on a couple others, and a Mikuni on an '84 FXRDG.  Like them all!  :hug:

:up:

thumper 823

I also find the CV a bulletproof reliable carb.
I run it to Sturgis and back and everywhere.
I have a very moded engine and I would guess that more power could be had from an HSR.
But for long touring, it is steady as a rock.
Also I have an AFR gauge.

Fine print- The carb is moded with a drilled hole and lighter spring
At low RPM it runs a little fat compared to what FI would be .
Idle is about 13.5
As the bike hits 80 MPH  and faster the AFR unit steadily shows fatter and fatter which is fine with me .
It is over 10 static CR
I get about 35 MPG,  two up, loaded HVY,  @ about 75 to 80 MPH.
D Troop 3/5, - C/16 ,162AHC, Mekong delta.
Rising from the Ashes  UHIH

Deye76

I've ran S&S B's and E's and G's unmodifie  for many years. Been out west in the Rockies, and through Yellowstone and Glacier, and never once rejetted. Bikes ran fine. I don't try and win a drag race while climbing Pikes Peak. Yes CV's are reliable, but everyone I had, after a while would spit and pop, and require tinkering.  I remember looking at thousands of bikes parked in Sturgis, Milwaukee, etc. with the majority sporting S&S carbs. If you like a CV run it, if you want reliable simplicity S&S will be fine. Not trying argue which is better.
East Tenn.<br /> 2020 Lowrider S Touring, 2014 CVO RK,  1992 FXRP