Back to Stock for the GB350S?

Honda GB350S - Exhausts and filters? To be or Not to be? My original question for AI was: Do you think that by replacing my stock exhaust on my Honda GB350s with a free breathing exhaust plus changing the paper filter for a reusable one that is free breathing will this configuration take the ECU automatic compensation over its threshold and lean the mixture too much? The answer was: That is a classic "tuning rabbit hole" question. You’re essentially trying to turn your GB350S into a more efficient air pump! The short answer was: Yes, it is very likely you will push the stock ECU past its comfort zone. Modern bikes like the GB350S (especially being Euro 5 compliant) are already tuned to run quite lean from the factory to meet strict emissions standards. When you "open up" both the intake and the exhaust, you change the volumetric efficiency of the engine. How the ECU Handles Changes The stock ECU uses a Closed-Loop system during idle and steady cruising. It relies on the oxygen sensor to make small adjustments to the fueling based on exhaust gases. The Threshold: The stock ECU typically has a "correction factor" (often around 10-15%). If the new airflow requires a 20% increase in fuel to maintain the ideal air-fuel ratio, the ECU will hit its limit and trigger a "lean" condition. Open-Loop Issues: Under heavy acceleration (Wide Open Throttle), the ECU ignores the O2 sensor and relies on pre-set "maps." Since those maps assume a restrictive stock exhaust, the bike will almost certainly run lean at the exact moment it needs fuel the most. Risks of Running Too Lean If the mixture stays too lean because the ECU can't compensate, you’ll likely notice: 1. Increased Heat: Lean mixtures burn much hotter, which can stress the air-cooled engine of the GB350S. 2. Popping/Backfiring: Excessive "decel pop" when you let off the throttle. 3. Flat Spots: A noticeable loss of torque in the mid-range. 4. Engine Longevity: Long-term risk of glowing exhaust headers or, in extreme cases, damaged valves. Recommendation: The "Golden Rule" of Tuning If you only changed the muffler (slip-on), the ECU could likely handle it. By doing both the high-flow filter and a free-breathing exhaust, you are significantly changing the pressure balance. To do it right, you should consider a Fuel Controller (like a Power Commander or a BoosterPlug) or an ECU Flash. This "tricks" or overrides the stock mapping to provide the extra fuel your new hardware is demanding. So what concerns me more about this is the statement: Open-Loop Issues: Under heavy acceleration (Wide Open Throttle), the ECU ignores the O2 sensor and relies on pre-set "maps." Since those maps assume a restrictive stock exhaust, the bike will almost certainly run lean at the exact moment it needs fuel the most. Since the beloved GB is a low power bike the tendency on A roads and larger is to OPEN the bike up to cruise and maybe on overtakes which can take longer. With my various tests using Black Widow exhausts and finally the little IXIL exhaust plus DNA air filter I found the following: 1. With the DNA air filter only I perceived a slightly more responsive engine at the bottom end but less so with the mid or top end. With the addition of the exhaust (baffled) or DB Killer present, I noticed the same, improved engine response and bottom end ranges but at this point I noticed a little surging at low speed. This started to make me suspicious about the engine map. At the point of removing the baffle I noticed a significant drop off in the top end, lowering the top speed as well. It was this that then made me consider to reflect on some earlier comments from some of my viewers and subscribers, some very experienced in engineering and engines in general, and consider the very possibility that this long stroke ‘lazy’ engine really was at it’s maximum tune out of the box! So the definitive answer after some testing for me anyway is: If you want to change the look of the GB350S and the sound and lose a little weight, then fitting an aftermarket exhaust should not harm the engine and will only very slightly lean the mixture at open wide throttle settings only, otherwise the ECU will mostly compensate. If on the other hand you want a performance boost, then you need to fit an aftermarket fuel adjustment system or get the ECU remapped to match your setup. In this video I demonstrate these findings. Thanks for watching.