Motorhead Memo: Intelajet redux

By Kip Woodring

The intention this month was to show a step-by-step installation of a Thunder Products Intelajet circuit on a Harley CV carb. We’ll save that. For now, let’s review some reasons you’d want to install one!

There are several components that need to be optimal for a true high-performance carbureted Harley street engine. But the “ins and outs” can be tricky for the best of builds.

When the exhaust valve opens a positive pulse (A) travels to the collector/megaphone/muffler, and the vacuum created should also help draw a fresh charge into the chamber. Meanwhile, part of the energy is reflected back as a negative pulse (B) which should not spoil the fresh charge. This happens in multiple waves… moving so fast that most Harleys are tuned to the fourth wave. Since the pipe “draws and stuffs” both the spent gases and the fresh charge… even a “tuned” exhaust usually only works efficiently in a band approximately 1500 rpm wide and some even make things worse everywhere else in the rev range. All of which affects carburetion.
When the exhaust valve opens a positive pulse (A) travels to the collector/megaphone/muffler, and the vacuum created should also help draw a fresh charge into the chamber. Meanwhile, part of the energy is reflected back as a negative pulse (B) which should not spoil the fresh charge. This happens in multiple waves… moving so fast that most Harleys are tuned to the fourth wave. Since the pipe “draws and stuffs” both the spent gases and the fresh charge… even a “tuned” exhaust usually only works efficiently in a band approximately 1500 rpm wide and some even make things worse everywhere else in the rev range. All of which affects carburetion.

Once you’ve done your homework regarding heads and cams… they’re installed and set. Intake manifolds are what they are… usually not working that much to help, but you definitely don’t want one that works against you either. Because, again, once installed, intakes are usually a set-and-forget proposition. That leaves exhaust, carb and air cleaner which are out there where you can mess with ’em after they’re installed… a process loosely referred to as “tuning.” Here’s where things get interesting.

As far as air cleaners are concerned, one would do well to study the design of the Buell Helmholtz type. Ugly as sin but efficient and quiet. Strip the thing’s housing and you see the nucleus (and inspiration) for the elbow-style cleaners so popular today. There’s something to be said for a “stack” with a bend in it, because it adds to the inlet tract length… and often helps air straighten out and fly right… into the carb, at least. Just be sure you’re not creating the same issues that plague the intake manifold. (If you’re not sure what those are, you’ll get some clues in the photo captions hereabouts.) To avoid dealing with it at all, simply use one of the many more traditional types… Screamin’ Eagle, Ness Big Sucker and other variations of the same theme. As long as there’s roughly twice the diameter of the carb bore between the outside edge of the filter and the inlet of the carb, it should be fine. Not hard to get right and easy to change or mod if you don’t.

When Harley first offered Screamin’ Eagle intake manifolds, they were for butterfly carbs. Intake velocity was so slow, there was a spacer about a half-inch thick included in the kit… to give the incoming charge a little extra room (and time) to get moving in the right direction. Since then, manifold designs have slightly improved (a gentler bend on the short side mostly) and factory carbs have increased velocity, especially SE Mikunis! Today these spacers are obsolete… or are they just forgotten? One wonders if any real improvement in Harley’s badly-shaped manifold might still come from adding a spacer.
When Harley first offered Screamin’ Eagle intake manifolds, they were for butterfly carbs. Intake velocity was so slow, there was a spacer about a half-inch thick included in the kit… to give the incoming charge a little extra room (and time) to get moving in the right direction. Since then, manifold designs have slightly improved (a gentler bend on the short side mostly) and factory carbs have increased velocity, especially SE Mikunis! Today these spacers are obsolete… or are they just forgotten? One wonders if any real improvement in Harley’s badly-shaped manifold might still come from adding a spacer.

Exhaust systems are so plentiful and misunderstood, most folks go with looks and sound. That’s too bad, because the right choice can make the build come alive and the wrong one… kill it. The problem, in a nutshell, is resonance, reverberation, fluid dynamics and sonic waves if you prefer… and the fact that whatever design you choose, you’re stuck with. For a component that affects the results you get from your build as much as this can… you’d think there’d be some form of adjustability on almost all exhausts in the name of tuning. As it is, there are only a couple that come to mind—the venerable SuperTrapp disc system and Patriot with its “lollipop-on-a-wire” setup. Adjustability is really needed in the header pipes… but if you want to add lollipops to mufflers or head pipes that came without ’em… Google it. You’ll see several ways to do it and there’s a YouTube video or two that shows how, as well. Who knows, they might help!

These are still available and useful, if you are serious about DIY re-jetting for conditions of temperature and/or altitude. I grew up in the Rocky Mountains and this little slide rule was practically essential for jetting a bike from the Mile-High City to above timberline. It’s a solid reminder of how little it takes to upset your “perfect” carburetion and the frequent work of changing jets to keep it right for the conditions at hand. Even then, it’s only a rough guide to a fix.
These are still available and useful, if you are serious about DIY re-jetting for conditions of temperature and/or altitude. I grew up in the Rocky Mountains and this little slide rule was practically essential for jetting a bike from the Mile-High City to above timberline. It’s a solid reminder of how little it takes to upset your “perfect” carburetion and the frequent work of changing jets to keep it right for the conditions at hand. Even then, it’s only a rough guide to a fix.

Guidelines for head pipes usually start (and stop) with “equal length” and stepped diameters or so-called “anti-reversion” cones. Two-into-one advice, long tube on the back cylinder for torque, merge collector of between 2 ¼” and 2 ½” in exit diameter with megaphone muffler containing corresponding large core baffle. In my opinion, this still bespeaks the wild west situation that exists for exhausts, where just about the only way to get it right for your specific build is “buy it and try it”… no guarantee and no specifics. It comes down to word of mouth from those who are happy with their choice or blind faith in advertising where every company builds the “best” performance pipe. Most of the time you can get in the ballpark, but perfection remains elusive. (Look at some crude illustrations close by, with brief explanations of why.)

Here’s a photo of a “lab rat” CV with two Intelajets installed, one in the body of the carb, the other in the adaptor bolted to the intake. You wouldn’t (shouldn’t!) want both (at least not like this) but it gives you a sense of the installation options.
Here’s a photo of a “lab rat” CV with two Intelajets installed, one in the body of the carb, the other in the adaptor bolted to the intake. You wouldn’t (shouldn’t!) want both (at least not like this) but it gives you a sense of the installation options.

Last but not least—carburetors—the very definition of close but no cigar about 90 percent of the time. Ironically, not that many riders seem to mind. Maybe that’s because it’s so difficult to predict (let alone master) the intricate interplay between all the components of a build, particularly the influence of an exhaust. Maybe it’s because even if you get lucky under “controlled conditions” it can all go haywire under different circumstances. After all, the motorcycle won’t even operate without the air/fuel mixture a carburetor provides but the known compromises in that simple fact are responsible for the outcome. It is what it is. Or is it?

Once installed in whatever manner, you drop three main jet sizes, mount the controller and its air filter (smallest in the world, I think) and you can toss jetting slide rules and the angst of re-jetting… away. Intelajet/Dial-A-Jet takes over and jetting issues become pretty much non-existent.
Once installed in whatever manner, you drop three main jet sizes, mount the controller and its air filter (smallest in the world, I think) and you can toss jetting slide rules and the angst of re-jetting… away. Intelajet/Dial-A-Jet takes over and jetting issues become pretty much non-existent.

Carburetors are the only component in a build, designed… intended… to be adjusted. In effect, the single item best suited for tuning the entire package of parts. Evident in the myriad of slides, springs, jets and needles available for each area of carburetor function, it’s almost overkill. In each area of fuel/air supply, from idle to wide open throttle, there’s functional overlap. There’s also the potential for gaps and imprecision. The Harley Keihin CV carb, for instance, is about as close to correctly set up as a carb is likely to be, when it’s attached to bone-stock, emissions-controlled engines. We perceive that to mean, OK at idle, lean from 1/4 throttle to near 3/4, and on the money wide open, where emissions aren’t measured and don’t much matter. Runs fine, then there comes the pipes and the air cleaner and whatever else you choose and… re-tuning! Which almost invariably is done incorrectly. If not because of the tendency to jet rich for no good reason, or any of the other predictable lapses in judgment or verification… then it’s liable to be because of an un-tunable exhaust. Forces you to tune the crap outta the carb, just to compensate. Let alone deal with the breathing characteristics of those heads, cam(s) and manifold!

Gorillas in the mist

Given its impact and importance, the prospect of improving the state of tune of any given build (involving a carburetor) is not to be ignored! Particularly, if the prospect includes an “on demand” function that deals with those pesky variables and deficiencies without requiring any tedious “help” from… ah… you. Last month’s column introduced a couple of products that go about doing what’s needed with a whole different concept. A circuit that supplies a pre-atomized air/fuel mixture… a light mist… in tandem with the circuits already on the carb. This air/fuel mist moves almost as fast as thin air, much faster than fuel, and corrects the lean “bog” that occurs when the throttle gets yanked open. A quick study of so-called back pressure in the exhaust indicates this circuit is also capable of smoothing out pipe pulses that interfere with normal carburetion… which they do! Ditto the turbulence in the intake manifold. In essence, this circuit will fill and smooth the air/fuel curve, which makes more torque, which in turn makes more horsepower throughout the engine’s rev range. Factor in extra-rapid throttle response and you have the veritable 800-pound gorilla… thanks to the mist.

Here’s a “double” done on purpose, correctly and for good reason… on a Mikuni HSR45. Intended for use on a monster motor. Note the install is done in the carb body…
Here’s a “double” done on purpose, correctly and for good reason… on a Mikuni HSR45. Intended for use on a monster motor. Note the install is done in the carb body…

To paraphrase some of the company’s technical info:

A load must be placed on the engine for the circuit to function. You can’t just rev your engine up and watch fuel go up the lines. Without loading the engine, there is no need for extra fuel, so the circuit does not deliver fuel.

Three things trigger the circuit: 1) engine pressure drop (like a VOES switch), 2) acoustic/sonic signals which either amplify or dampen the reversionary pulse waves in the intake tract (same physics as exhaust), and 3) air flow volume and velocity.

The circuit works from just above idle all the way to full throttle, feeding atomized fuel/air mist in linear proportion to engine load. Various speeds and engine loads require rapid changes in your fuel/air curve. The circuit automatically monitors your engine’s needs then instantly alters the air/fuel curve to match. Which means being jetted properly, whether you are pulling a heavy engine load or just cruising.

…as opposed to this adaptor type installation on another 45… completed and in action!
…as opposed to this adaptor type installation on another 45… completed and in action!

A word about “being jetted properly:” Since the circuit is operating in conjunction with circuits already in the carb, a reduction of main jet size is required because you have two tandem circuits now. And… because you can jet for Denver (5,000 feet) or even higher, yet not run lean at sea level. As this is written “the circuit” comes in two forms for Harley carbs. The Dial-A-Jet once drilled, tapped and screwed into the carb has five settings—not adjustable “on the fly.” The Intelajet is more finely adjustable, as well as offering two basic mounting options… directly in the carb body… or installed via a bolt-on adaptor sandwiched between the carb and the air cleaner.

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