I should preface what follows by saying electronic fuel injection is here to stay, because it is a superior air/fuel delivery system. It manages to keep emissions down and performance up… delivering fuel economy benefits into the bargain. Took me a while to get around to being a fan, rather than just an ignorant victim of this technology. I’m ashamed to admit it, but EFI has something of the same effect on me as electronic ignition had on folks a generation earlier. It’s something not easily grasped at first, that drags you kicking and screaming into a brave new world… technologically.
I’m not saying any system as complex, expensive and “techy” is perfect… far from it, in fact. Part of my problem with EFI came from my “simplest is best” philosophy. I’m not alone. But, watching the progress of EFI sophistication, over decades, brings an even more deeply held philosophy into play. Namely, the human animal and his tools (computers included) have a knack for taking a technology from “iffy proposition” to “way to go”… inevitably. Witness the fact, today, there is a Screamin’ Eagle throttle body, injectors and ECM combination matched to M-8 SE CNC heads that defies logic… yet works supremely well. I mean, you’ve read how precise and effective M-8 EFI is (and XL EFI as well). Yet, when Big Twin performance is upped by sticking a huge (64mm) TB on heads with relatively dinky ports, the only way to make sense of it is to appreciate that pinpoint precision of electronically-controlled amounts of fuel, delivered into a vast, enclosed atmospheric area and properly atomized in a nanosecond, offers up a perfect mixture to a port at peak vacuum. Astounding!
Carbs can’t do it… never have and never will. They can’t because they operate on principles of fluid dynamics that guys like Isaac Newton and Daniel Bernoulli understood over 200 years ago! Carburetors haven’t been around quite that long, but they’ve had a pretty good run over a century or so. More germane to what comes next, though, is the fact they are still with us and still working. Getting back to the notion of the human animal and his tools, there’s been plenty of refinement along the way. Getting back to simplest is best… carbs still are, in many ways. In short carburetors are not obsolete because they have advantages of their own. But, the drawbacks are equally well known. So, for the millions of motorcycles out there still equipped with ‘em, the trick is to minimize those drawbacks. Enter the “Intelajet”!
Motorcycle carburetors are dumb, compared with EFI and most automobile carbs at least. There are three fundamental types of Harley carb… “butterfly” (Linkert and S&S), slide (Mikuni, mostly) and Constant Velocity (CV Keihin, mostly). All work about equally when the throttle is wide open. Ditto while idling. It’s that pesky in between that’s bothersome. By nature of design, butterfly carbs are not real good at ¼–3/4 throttle behavior. Slide carbs are much more responsive at that game. CV carbs, trying to be all things to all people, usually afford the best behavior, but do it a trifle slowly.
The basic mechanisms used between idle and WFO are additional “circuits” of orifices and metering devices, intended to manage those fluid dynamics which aren’t always willing to cooperate. It’s partly the nature of fluids (air and fuel), but mostly the dynamics involved. Air and fuel are supposed to be mixed at something like 12–13 parts air to one part fuel for best power. Yeah, good luck with that. Since one fluid (air) moves a hell of a lot faster than the other (guess) and they don’t naturally mix well in the first place, getting anywhere close is a neat trick.
Old-school carbs with only idle and full throttle circuits were usually kept small to maximize vacuum to minimize midrange “transition” problems. When S&S stepped into the performance arena with the old Super B, going WFO got great, but mid-range behavior was barely acceptable. With the advent of the VM Mikuni, two circuits became three (or four), for idle, 1/4 throttle, 1/4 to 3/4 and 3/4 to WFO helped along by dumping the butterfly in favor of a cylindrical slide to improve air intake velocity. Suddenly, iron Sportsters, Pans and Shovels had a new best friend! The two carb “camps,” two-circuit butterfly versus more circuits and a slide, duked it out for decades. Then Keihin got clever and added both a butterfly and a slide, plus more circuits, one of which, via a rubber diaphragm, actually controlled the slide behavior. This is about as slick as it has ever gotten with Harley carbs. Sorry.
Stuck in the middle
One of the first aftermarket upgrades to come on the scene was (surprise!) a device to add a third (fuel) circuit to the reigning WFO king… the S&S. Set the way-back machine… way back… remember the ThunderJet? It’s still around, making midrange manageable, proof of concept, wouldn’t you say? Thing is, it was more than a nice little improvement, close to a full-on necessity, when increasingly larger size carbs were employed on the street. Somewhere along the way Mikuni made their round slide flat, refined the circuits and offered bigger bores to compete… and it does! The HS40 and mostly HSR42/45/48 have been a particularly keen competitor for S&S ever since. The Keihin CV with all its circuits and robot slide completed the performance triad when it got bigger… a whopping 44mm (1.733 inches if you score that way). There is where the story stood until the demise of carbs on Harleys.
Takeaways from this stroll down memory lane? First, carbs got more circuits over time, second, as motors got bigger so did carbs, up to a point. Make that a limit! Problem is unlike the 64mm TB there are definitely restrictions in the physics of an over-large carb. Plus, many a Milwaukee muscle machine is either tuned incorrectly in some, if not all, of its circuits. Hell, we’ve all known for years now, even factory jetting left a lot to be desired, whether a stock motorcycle or a performance-enhanced one. Band-aids and fixes abound, have for a long, long time-trick needles, jet kits, spray bars (needle jets) and more. Some work well, others not so much and all of ’em a little too dependent on a particular performance combo to give their best. Still, better than any DIY alternative for most, since damn few of us are tuners worthy of the name.
There are ways to avoid it, but too much carb aside (for now), the real issue is circuit tuning and the fact that even if correct for dyno shootouts or cruising the ’hood… it all gets real sloppy when little realities like temperature and altitude are factored in. (Actually, they usually aren’t!) That’s because, unlike EFI where a computer does it for you as you ride, carbs must be disassembled, re-jetted and reassembled every time to get tuned right for prevailing conditions. Even “close” involves having to stop, climb off, twiddle some adjusters, get back on and hope the bike runs decently clear to top of the climb and/or on a really cold (or hot) day. Yup, carbs, horseshoes and hand grenades… close is all you need… right? Well, what if you could get a hell of lot closer a hell of lot easier?
To oversimplify a tad… that ThunderJet add-a-circuit solution has evolved nicely over the years, to the point Mikuni used a device much like it once or twice. The biggest and best stroke of genius was to add (wait for it)… an air bleed. The first iteration, called a Dial-A-Jet, had five settings to offer… and it was good… very good. Still used to death today by off-roaders and snowmobile freaks. Can you imagine why? The next iteration, the one I’m trying to get to here, is the one with the totally adjustable air bleed… the Intelajet. This thing is slicker than goat grunt, as my old friend Moose used to say… because it works!
- Notably improves throttle response
- Compensates for altitude and temperature
- Allows very precise tuning (especially when an air/fuel meter is installed) without taking the carb apart
- Easily adjustable on the fly (is/as required)… not by an ECM… but by the rider
- Way cheaper than an EFI conversion and nearly as good!
You should know… this thing doesn’t do a thing for peak power. Never the intention. What it does do is help power under the curve (average overall power) as well as allowing you to simply turn a thumb wheel to keep it that way under real-world circumstances. My friend Paul has one on his hot Sportster and doesn’t on his near-stock 1200S. One ride on that sharp 1200S and his beast back-to-back is all I needed to make up my mind this is the best thing to happen to carburetors since, well, circuits! Intelajat works on all types of carbs, but you CV fans in particular need to know more and we’ll get there next time! If you can’t wait, I suggest you call Lonn Peterson at thunderproducts.com, 320.597.2700.