I applied an AFR gauge to my Rebel (250). After having done so on my old Plymouth Scamp with a carbureted old school V8. Originally I added the AFR gauge to improve the starting, but evolved to tune the performance across the spectrum. And boy, what awesome education I received. I was having fits getting the car to start, thinking I was flooding it. Turns out, the AFR gauge let me know that indeed, it was just the opposite. After that, the car starts easy / as-expected knowing what's actually going on. And, based off historic and new drag strip data, I have increased the power by ~65 hp. And the car drives around town almost as if it is running EFI.
I say that because, when it comes to carbs and the tuning of said, I consider myself an expert. I put the AFR gauge on the Rebel. The Rebel is tuned EXTREMELY WELL from Honda, right out of the box. There is actually no need to change a thing. If you think you have a bad tune in your carb, and you're going to pull it off and clean it, I recommend you either verify the components in the carb are the original sizes, or buy a rebuild kit (assuming they have such, they did for an ATV I own), and replace the components back to factory sizes.
You can't just say, "I think my carb is running rich." Because, the fact is, it does run rich. But it runs lean and stoich too. A properly jetted carb suffers AFR variation across all rpm and all throttle positions. It suffers this, but it is not a bad thing. A properly tuned carb runs rich when the engine *should run rich, and it runs lean when it is convenient to do so. And it runs stoich when it is not running rich or lean, ha. One thing to be aware of, my 2016 Rebel 250, with completely stock carb and an AFR gauge, RAN RICH AS FORT KNOX, when cranking cold and using the choke. 14.7:1 is stoich. Less than 14.7 is "rich." Cranking a cold Rebel 250 with the choke runs the AFR to less than 8.0:1. So yes, when the O.P. cranked his bike and said it smells rich ... well yes, it was rich. As it should be at that instance. And you should be thankful it did, because it means it was performing how it should.
Lastly - Here's a basic guide to how your bike is running, if carbureted correctly.
- Just cranked cold, including the choke: Rich as Fort Knox.
- Just cranked warm, with or without the choke: Rich
- Idling while warm: Rich
- Light throttle cruising at steady speed: Lean
- Nominal acceleration: varies between slightly rich - to - stoich - to - slighly lean
- Wide Open Throttle: Rich.
Honda selected a heckuva capable carburetor, and then set it up to run ideally in all conditions, in my opinion. I wish my Edelbrock on my Scamp was as capable. In most conditions, the Rebel engine should be running rich. Why? Primarily because it is an air-cooled engine. Generally an air cooled engine only has issues ... when idling when the engine is warm to hot, in hot weather conditions. With no air flow cooling the engine, it builds heat. So, at idle, in the worst case condition (the engine is very hot), you want the idle to be rich. Because the unburnt fuel has cooling qualities to those engine parts exposed to the extreme heat of freshly ignited fuel. So, air-cooled engines are typically tuned to run richer across the spectrum of all conditions than those of liquid cooled carbureted engines. I ran W.O.T. for about 2 hours steady on the Rebel running the interstate. If I remember numbers right, stock carb setting ran at about 11.0:1 to 11.5:1. That's way rich of course. But max power is generated in the 12.5 to 13.0 AFR zone, but an air-cooled engine (arguably) needs it to be richer than that to combat overheating while running at WOT. It s for these reasons I say the Rebel 250 is carbureted EXTREMELY WELL, right out of the box.