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Discussion Starter · #1 ·
Last time I set my carb up, it was in the 40s (degF). Now it's in the 80s. The bike starts and runs fine as it ever did, but it smells a little like it's running rich.

I used to have to retune my '64 Ford any time the weather changed more than 30 degrees on average. I got the impression that was just what you had to do with carbs.

Am I wrong? Should I leave well enough alone?

The plugs had a ton of caked on carbon that I cleaned off with an brass brush, but I'd not describe them as "sooty". After going to all the trouble of cleaning them off, I remembered I had a new set and replaced them.

I just adjusted the valves and now I'm going to clean the air cleaner. If I take apart the carb to clean it, I'll have to adjust it all again anyhow, and since everything is running okay, I don't feel like I need to clean the carb yet.

Thanks y'all!
 

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I've never adjusted the carb other than idle adjustment. 1 Rebel has 49000 miles on it. The other just past 8800. I don't think that you'd need to worry.
 

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I change idle from time to time sitting at stoplight due to weather changes..
summer, winter, autumn, spring..
 

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^^^^That's the same thing that I do. If it's too high or low, you already know that the engine is warm. Best time to do it as it only takes a slight twist to adjust it.
 

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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.
 
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Very well written & easily understood. Thank you for this post.
 

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Discussion Starter · #9 ·
Wow! This is a great post. I started to go down the path of a DIY engine computer for a previous vehicle so this is the right depth of detail for me.
AND you spot on nailed what I was noticing - running rich with warm engine at idle. That’s obviously when you’re going to notice it (you’re sitting there not moving) but it didn’t meet my expectations bc I would expect to be stoich at idle. Your post explains why my assumption was wrong.
From your description, everything is spot on with my bike. And you’ve got the tools to actually measure what’s happening, which often is not the case with these situations.
Thanks again - saving this post for future reference.
It also has me reconsidering my next bike purchase some - I really appreciate the simplicity of an air cooled engine in terms of maintenance and repair, but water cooled engines have the opportunity to better regulated. (Especially if I could find one with EFI - is that a thing for bikes in the mortal price realm?)
 

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I should have also mentioned. I downsized my main jet to a 0.105" (from a 0.108" if memory serves, that's just 1 step different, there are no in-between sizes from 0.108" to 0.105"). I was chasing the elusive last bit of hp. I likely got a skosh more hp (like maybe 0.1 to 0.5 hp increase), but it was not noticeable to the speedometer or buttometer. I have a thread on here somewhere, showing pictures of me removing the exhaust and installing the AFR gauge.

My W.O.T. speed didn't change noticeably. There's so much wind drag on the bike at 75 mph indicated (72 mph GPS verified) that my little (estimated) 0.1 to 0.5 hp increase didn't register a noticeable increase in speed. My AFR at WOT did change, as well as the fuel range / mpg (also at WOT). The AFR went leaner to about 11.8 to 12.0 AFR at WOT. And I did another whole tank with the throttle pinned open on the interstate. I think that range/mpg info was included, maybe in the same thread. Seems like my mpg increased by about 3 mpg, and I gained another 8 miles or so before Reserve had to be tapped.

Overall, it is not worth changing the main jet. Honda got it right. Because, if one runs a more open exhaust, it *may go a bit leaner based on some amount of extra air flow. But from Honda's jetting with the 0.108" main jet, I doubt the modified exhaust AFR would go any leaner than what I achieved by my main jet size. I don't know that factually, it would have to be measured with the AFR gauge. But I seriously doubt any jetting changes would be needed.

Nowadays carbs have advantages over EFI. Nowadays, from what I understand, vehicles are tuned to (try to) run 14.7 AFR under all conditions. This in fact does minimize pollution based emissions. And is why there exists so called, "tuners" out there that can modify a vehicle's target AFR to hit say, 12.8 AFR or so. That AFR (12.5 to 13.0 or so) has been found through lots of dyno time, to yield the highest power output. I think it is less expensive and easier to hit a target AFR with a carburetor than with EFI (both would need an AFR gauge). Well, at least to hit a target AFR at WOT. Hitting a target AFR at other driving conditions is actually more complex. That's another topic.
 

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Man that's crazy you went down a size in the main. Honda sold the nighthawk with a 110 main instead of the 108 they put in the rebels in the ladder years. Some what whiteish spark plugs with the stock 38/108 jets have me bumping up slightly. Id rather run a little richer being air cooled than leaner, which thanks to epa, carbed bikes have seemed to always be sold a bit lean from factory.
 
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