Shop Talk | Breath Deeply

Tech Tip

Over the past few issues, I’ve received more questions than usual concerning air filter replacement; specifically whether or not replacing the OEM air filter with something from the aftermarket will necessitate a modification to the fuel system.

Just so we’re all on the same page, what we’re talking about here are aftermarket drop-in replacements; the writers are not modifying the air box, exhaust or intake tract in any way, they’re simply replacing their stock unit. Generally, the aftermarket filters are advertised as being less restrictive, or “high flow” so the concern is that installing one will create a lean condition, requiring an adjustment to the fuel map.

With some qualifications, the short answer is no—installing a direct replacement aftermarket air filter shouldn’t require you to re-jet or re-map, which, at the risk of sounding like a smartass, is why they’re called “direct replacement.”

That said, every engine is different, and while 99 percent of them will run just fine on a replacement filter there is always that 1 percent that have to be difficult. So yes, under some circumstances, installing a less restrictive air filter could conceivably push an engine that’s already running too lean over the edge. However, in that case I’d suggest the engine already had fuel issues that should have been dealt with before the new filter exacerbated them, and I’ll also point out that in my experience the problem is usually something minor like an exhaust burp when decelerating.

On some older, carburetor-equipped engines, you may—and let me stress, may—have to richen the pilot screw setting by a 1⁄4 to 1⁄2 turn to prevent a hiccup, but re-jetting? Not likely.

The bottom line here: When it comes to replacing that worn-out stock filter, an aftermarket direct replacement unit has a lot going for it, and should be considered a fit-it and forget-it procedure.