Interesting video and comments...The added foam "jetting" change seems two-fold, first a "choking" of air flow through the restriction, and secondly, according to the YouTube presenter, a possible offset of what he calls "reversion" air flow.
The first factor is much like the claims of aftermarket air filter companies that boast "less restriction", only in this case it's the opposite. I've changed back to a stock air filter and even added the missing OE anti-backfire screen, an additional air restrictor if you will, to the Honda XR650R motorcycle. While "uncorking" these Big Red Pig models involves removal of the stock air box restrictors (done by the PO, none in place), actual filter air volume is a measurement of CFM flow and the filter's surface area.
I'm banking on Honda having enough sense to provide adequate air volume through the stock filter and the vital OE anti-backfire screen. (Many remove the screen for more flow. Envision the bike on its side with fuel flooding out into the air box. This is a major fire hazard with the anti-backfire restrictor removed!) I only re-jetted to what is actually stock on all non-U.S. market Honda XR650R cycles with this same carburetor, compression ratio, etc. This was the conservative approach, and the HotCams Stage 1 camshaft should be okay with that jetting, too. Stock jetting for the non-U.S. market is 175, I went for 172 at our altitude and mountainous, high desert riding environments. I'll see whether the stock air filter creates any kind of enrichment factor.
Note: RPM plays a role in the main jetting needs. I'm not "racing" this motorcycle nor redlining the rpm constantly. In fact, I need to conserve fuel at my remote riding venues and make sure this engine stays intact and reliable.
What I'm hinting about is that filter surface area (or a foam restrictor like this video describes) would simply control air flow volume. So does the choke or a dirty air filter for that matter. The engine's air requirements are based strictly on CFM requirements for the cylinder displacement, camshaft valve opening duration and lift, and the valve timing "events" as the video mentions. The OE carburetor, at least theoretically, should have the right CFM flow for the engine. My XR650R uses a 40mm Keihin, no CFM rating to share here.
CFM flow of a carburetor is based on the engine speed as well. Jack Clifford, an inline six devotee, and I discussed Jeep 4.2L six-cylinder requirements years ago, and he threw out a carburetor formula: 1 CFM for each cubic inch of displacement to run the engine at 4,000 rpm. More rpm, more CFM flow required. Many sport bike motorcycle engines are barely on the throttle at 4,000 rpm.
So, assuming the engine has the right carburetor, and an unrestricted air filter of the correct surface area to match the CFM needs for the carburetor and engine, there's still another factor: the air box flow. This is a science, and latent horsepower is often found in a redesign of the air box, affecting the air flow and velocity to the carburetor or throttle body intake. The filter, in combination with air box flow dynamics and air delivery changes, can dramatically impact performance. There is an entire automotive aftermarket devoted to these performance improvements. OE air boxes are often misshaped to meet fit and engine bay requirements, and so forth, restricting the performance of the engine.
The presenter mentions "reversion flow", and as he describes it, that would be a function of camshaft design/lobe profiles, valve timing and engine speed, manifold vacuum and the induction system design. He may be right on with the reversion taking place, and that could be a factor of air box placement limitations or even air box tuning to deliver a certain kind of performance at a given engine rpm. This tuning could be performance and/or marketing driven.
The "foam jetting" is a restriction of air flow volume at the filter. If the engine is getting enough air (CFM flow) with or without the restrictor foam, jetting should not be impacted. Although the blockage alters the engine's ability to access air, that does not necessarily mean the engine is not getting enough air. Choking the engine with the choke valve is entirely different: The choke valve restricts the incoming air and also raises the vacuum/fuel draw from the carburetor idle and pilot circuits (enriching the mix further)—while at the same time restricting incoming air (CFM) volume.
Not sure whether the bike in the video is carbureted, but I'm assuming that's possible by the mention of "jetting". If the cycle had EFI with an oxygen sensor regulating the A/F ratio, regardless of intake air flow restrictions, the A/F ratio issue would be moot. A Jeep engine with MPI will adjust A/F even for a dirty air filter. A/F must remain constant to meet emission requirements, and the engine and vehicle simply will not perform well with the lessened air volume: Injector fuel flow gets lowered to match the available air. That's the beauty of EFI/MPI versus a carburetor, the EFI adjusts for altitude changes, available fuel and air, manifold vacuum and barometric pressure.
By contrast, the alteration of air intake flow, air velocity or air volume into a motorcycle engine with a carburetor can have a distinct effect on jetting. The video implies that the air box in question actually flows more air than useful. If the engine runs better and does not show signs of excessive enrichment (blubbering, fouled plugs, blackened exhaust soot and such), then it's getting improved flow or simply running richer and obviously better than stock.
Forman, keep in mind that later EPA requirements for on- and even off-road vehicles, including motorcycles, demand the leanest A/F mixes tolerable. Restricting the air box intake with foam, as illustrated in the video, might be just enough to enrich the mix slightly while still allowing enough CFM flow to meet the engine's requirements. This could be the source of happiness for the engine...You'd have to do a four-gas exhaust analysis, with the motorcycle on a dynamometer under various loads, to see what's really going on here.