So we need to have a major conversation about carbs.

User /r/Cerpin-Taxt wrote me the following on a thread on the Virago 750:Do you seriously ever post a comment that isn't shilling for mikuni?You've been here for years and the only advice I ever see you give is telling people to buy mikuni carbs and mikuni carb adaptor kits to fit them to bikes they aren't made for, no matter the issue people are having. I've even seen you telling people to fit mikuni twin carbs to a 4 cylinder engine despite the fact this will only hurt performance.What's your deal?So let's talk about it.The carbs found on the vast majority of bikes we'd call cafe starting points are going to be CV carbs. They generally look something like this:https://i.ytimg.com/vi/9TMBGn0DH24/maxresdefault.jpg...or this:https://i.ytimg.com/vi/UEvg_RgIp4s/hqdefault.jpgThe first image is of a type of CV ("Constant Velocity") carb usually seen on Hondas starting around 1979-ish. The second pic is of the more common type, in this case from a Yamaha XS650. The "dome" up top is lower. We're going to focus on that type (but all the CVs work the same way) so you can start to see why these carbs stink to hell and gone.Previous image, you're seeing a couple of butterfly valves. You're looking into the backs of the carbs, so "from the engine's point of view". Let's look at what they look like as airflow comes IN:https://www.picclickimg.com/d/l400/pict/272334036732_/XS650-Yamaha-XS-650-1976-77-Mikuni-BS38-Cleaned.jpgBefore we go further, these are Mikuni BS38 carbs. As you look into the intake side, you see that there's a big round vertical slide in there. The main needle is on the bottom of that round slide. As it rises the needle comes up with it, and being tapered allows more fuel flow the higher the slide rises. Great.So, obviously, when you whack the throttle open you're opening that slide up, right?WRONGO! And that's the problem.Your throttle grip on the right is wired to the butterfly valves on the rear end of the carb, which allows more fuel flow, yes, but there's a delay between the opening of the butterfly valves and the rise of the slide.So what causes the slide to rise?Well go look at that pic of the business end opening again. There's three extra air openings around the intake "mouth" of the carb - in this case one big one, like a "curved oval" at the top center and two smaller ones at the lower left and lower right. Let's get y'all a better pic:http://www.roofis27.com/motorcycle/13_02_11/010.JPGAs more air comes in, airflow into those extra ports forces the slide up. Incoming airflow starts to move based on the butterfly valve.So why is this a problem?Well let's start with the very name of the carb: BS38. No, I'm not making a bullshit joke. "38" is the bore size at the main venturi (restriction) point inside the carb. IF you replace this with a "smoothbore" carb like the Mikuni VM series, the proper replacement would be a VM34 (unless you've modded the engine, then go with VM36).But there's other issues. There's TWO major things in the way of airflow: the butterfly valve at the rear and the main slide towards the center. On the VM series there's only one thing blocking airflow: the main slide.In the CV carb there's a fundamental delay between whacking the throttle open (your direct control over the butterfly valve) and the bike actually moving (the slide going up and letting more gas and air in).As bad or worse, the butterfly valve is in the way of the primary airflow path AFTER the fuel/air mixture at the venturi (the carb's tightest internal point where air speeds up and gets mixed with fuel). The butterfly valve at the last point of airflow in the carb ends up disrupting airflow into the motor.Finally, the airflow into the carb has to be finely tuned to enter those smaller airways to raise the slide. That air has to come in straight and clean. Hence the airbox concept - that's what it does, smooths and evens the incoming air stream. That's why CV carbs and pod filters never run really 100% right.So what do we do instead?We want carbs with as little in the way as possible of the airflow, and carbs where we DIRECTLY control the main air flow modifier by mechanical linkage to the right wrist.There's several carbs that fit that bill. One of the most common, cheapest and easily tuned is the Mikuni CV series but it's not the only one.The Mikuni VM carbs have a roundslide like a CV, but you raise it by directly yanking on a wire going right into the top of the slide. And there's no butterfly valve at all, not for the choke and not for the main intake. For a choke it just dumps a bit more gas in for starting, which is simple but works fine. It has three stage jetting: a dial-adjustable idle, replaceable pilot (midrange) and replaceable main (top end power) but adjustments to the needle assembly are possible.https://www.dimecitycycles.com/mikuni-vm34-complete-carburetor-kit-fits-yamaha-virago-750-920.htmlMost variants don't have an accelerator pump but it doesn't seem to be necessary.What else is out there?On Harleys, the longest-living aftermarket carb (in terms of age of the design) is the S&S "Super" series, "E" (medium) and "G" (huge). It's basically 1/4 of a classic Holley 4-barrel carb for a musclecar turned sideways. No slide - it uses a butterfly valve instead.https://www.jpcycles.com/product/401-022/s-s-cycle-super-e-carburetor-only?mrkgcl=444&mrkgadid=3308569184&utm_source=google&utm_medium=cpc&utm_term=490342421466_product_type_motorcycles_product_type_air_intake_and_fuel_systems_produc&utm_campaign=Google+Shopping++Manufacturer+Intent+-+Air+Intake+and+Fuel+System&product_id=401-022&utm_content=pla&adpos=1o1&creative=278867793335&device=c&matchtype=&network=g&gclid=Cj0KCQjwjMfoBRDDARIsAMUjNZqmABJqK85jMyyrvWprVt9vwha03JaiK9KsfsooCVGEL6G10k0FuTwaAjQ4EALw_wcBI tried it on a '96 Buell S1 Lightning. Worked well, but the fuel efficiency was disappointing. Not great for a daily driver. For a chrome show bike you drive on weekends, 35mpg might be fine. For me, nope!So the next try for that same bike was a Mikuni HS42:https://www.ebay.com/itm/Mikuni-HSR42-Easy-Kit-Carb-Carburetor-90-06-Harley-Softail-Dyna-Touring-42-18/331687596692?_trkparms=aid%3D222007%26algo%3DSIM.MBE%26ao%3D2%26asc%3D20160323102634%26meid%3Dd1d63c7767e8451887f83158e6faa2ac%26pid%3D100623%26rk%3D3%26rkt%3D6%26mehot%3Dag%26sd%3D172123104243%26itm%3D331687596692&_trksid=p2047675.c100623.m-1That pic shows it set wide open, and as you can see there's almost nothing in the way of airflow going on! This is also what a VM series looks like wide open. The HS42 is a "flatslide" instead of a "roundslide" like the VM, and in theory has more potential. It's also a "pumper" (accelerator pump) which Harleys seem to need.The BEST carbs for parallel motors would be the Keihin CR or FCR series smoothbores. They're a lot like a VM but the slide control is more accurate and you can lock a bank of them together so that you wack them all the same amount in a row. The FCR is the same but includes an accellerator pump. These cost a lot more than VMs though. Here's a set of CRs for an EX500 for over $700:http://power-barn.com/kawasaki-ex500-keihin-cr-special-racing-carburetor-kit/A VM34 kit for something like a Virago 750 or XS650 will be less than $400 and be at least 90% as good, in my experience. You only need that extra 10% from a Keihin set if you're doing a full track bike.The age of the CV carbs (whatever brand or size) really started in 1979 as emission standards tightened. CV carbs almost never pass more gas than needed. They do so sluggishly, but they're fuel and emissions efficient. The Japanese makers (and others) used them to avoid going to catalytic converters for decades, until fuel injection got sorted out in factory offerings.Before that we see basically primitive versions of the Keihin CR series as factory setups in bikes like the SOHC Honda 4-bangers:https://diggit-assets.s3.amazonaws.com/listing/161630/image/6288777996195920483-1024x768.jpgThis pic is screwed up because it's upside down but try and cope :). You're seeing the intake side (right behind the air filter) and again we see butterfly valve chokes. Those aren't too big a problem because it's the airflow after the fuel/air mix point (venturi) that matters most.You seldom see these from the factory on 1970s twins, but on 4-banger motors from Honda and Kawasaki (KZ900, etc.) they were pretty common.Be aware that some of these are a real bitch to tune because access to the idle screw was blocked. The most notorious was the last of these carb sets found on the 1980 CB650 which I tried to tune and just...utterly failed.https://www.motopsyco.com/2013/10/25/how-to-adjust-valve-lash-1980-honda-cb650-sohc/You'd think those carbs would rock. On paper they should. In practice they're utter turds and worse, that particular bike is rare and there's no aftermarket support at all.Now. Let's talk good news. Since the days when I was dealing with weird cases like this (and it's been a while, I'm 53 years old) something has come along to help:https://www.summitracing.com/parts/inn-3918?seid=srese1&gclid=Cj0KCQjwjMfoBRDDARIsAMUjNZpfuiqsw5TuwDdPp4akjJPzrcKvpkxusN5_oaI13IuyoyAMvPV-9mkaAuUNEALw_wcBThat is a "wideband fuel air ratio gauge" - it tells you the exact fuel/air ratio at every RPM as you ride. It comes with a Bosch sensor that you screw into the exhaust - it comes with a fitting you weld into your exhaust system somewhere.The only problem is, the cable between the sensor and the gauge is too long as it's meant for a car and you can't cut and splice it shorter without ruining the calibration. So permanent mounting is difficult. But I don't think it matters - this is mostly needed while tuning the carbs.With a critter like this, I think it's possible for a newbie to develop a new jetting recipe for a carb/bike combination that's never been done before. The noob in question will need pro advice on what size carb to start with and a guess from a pro as to what jets will be close enough for it to start and run (if badly) but from there, the gauge will tell you where to go next on jet sizes.What happens if a carb is too big?I've made this mistake before, late 1980s. The first bike I ever learned to tune and my first cafe build (or at least that's what we'd call it now) was a Yamaha XS650. I had a choice between VM34 carbs and VM36, and I went with the latter because MOAR! but yeah, not my best move because I didn't have enough cam and valve work to flow enough air to match.As a result, whacking the throttle wide open too fast bogged the motor. Once I was wide open the power was solid as hell (and WAY better than stock!) so I learned to ride the throttle up and control the motor's increase in speed myself. Which actually didn't suck once the learning curve was done :).I should have run a pair of VM34.Either way, the difference in throttle response and total top end power over stock CV carbs was just incredible and remained awesome on every bike I ever modded later, from lower power rides all the way to that Buell.IF ANYBODY KNOWS OF A SOURCE FOR FUEL INJECTION MODS FOR VINTAGE/CAFE TYPE BIKES, DROP A LINE IN HERE!!! I've been looking. I can find "development kits" and some parts, but nothing like a turnkey solution. via /r/CafeRacers https://www.reddit.com/r/CafeRacers/comments/c5lboc/so_we_need_to_have_a_major_conversation_about/?utm_source=ifttt