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Discussion Starter #1
Hey all, I just bought a 2003 250, carb was dirty and it ran poorly (couldn't get up to speed). Cleaned that sucker out about 3 times with chem dip, on the third try it ran like a champ with one exception: It idles very rich. I have to keep the idle mixture screw in all the way, turning it out at all will lower RPMs no matter what. I felt like it needed to go in even more, so I got a carb kit and put a new screw in (which was identical to the old screw) and had the same issue... so I replaced the slow jet just to be safe - still, it runs rich. Hell, if I use -any- choke at idle, the bike dies right away, it just can't handle it. With throttle it's normal, a slight drop in performance. I can start the bike on choke if and only if I have some throttle in there. It won't start with choke if it's idling, the mixture is just way too rich. (I don't need choke whatsoever to start the bike though, but it's not exactly cold here in NC right now)

Yesterday I had a buddy over to practice on the bike and after about an hour of low throttle and lots of idling it would creep up from the 1400ish RPMs at idle to 2... sometimes 3000RPM (at idle!). I'd have to let it sit for a minute to get it to come back down.

My mixture tuning process is by the book: Get the bike warmed up (10 mins of riding), set the idle/air screw to 2 1/4 turns out from lightly seated, tune to 1400 (I have a tach), then move the mixture screw. Engine performance goes up the more I turn it in, and always drops if it's not all the way in. At low and high speed everything is a-ok, this is just at idle. No obvious tears or issues with the carb gaskets, either.

I sprayed starter fluid around all seals to make sure there wasn't a vacuum leak, and changed the air filter too. The spark plugs are new, and while they do build up carbon fast at idle, once the bike is at speed that carbon burns off and they look perfectly tan/white. The plan is to throw fresh plugs on as soon as I fix the idle mixture issue.

Any thoughts?
 

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James - I have a theory, but I have no way of confirming it. Let me start off by saying, I have an AFR gauge on my Rebel. My Rebel runs very well. And except for a downsizing of the main jet, by carb is stock. I wrote a thread about the AFR of a well running Rebel. If you care to peruse it, it is here.

Here are the actual values of a well performing Rebel, back when the carb was 100% stock:
  • Choke (while cold) Idle AFR: Pig rich. 8.0:1 or less.
  • Idle (while fully warmed) AFR: 11.2 to 11.6. The warmer the engine the slightly higher.
  • Cruise AFR: I didn't get a chance to measure these. But they are lean to extremely lean.
  • WOT AFR: In 5th gear, they ranged from 9.0 (rolling on at 55 mph) to 11.5 at 70+ mph.
    • I had originally noted a highest AFR of 10.5, but it was a bit higher yesterday.
So yes, at idle it is rich. And it is supposed to be. My belief as to why Honda set it up that way is because a) it idles well when rich, and b) because it is an air cooled bike. Rich is safe. And being rich provides some amount of fuel that doesn't burn, at least not completely, and tends to cool the combustion chamber. At idle an air cooled engine is going to get hot (since it is not moving). Air cooled engines (like some prop planes and old Porsche engines) are tuned very rich as well. That would be my guess as to why our high performance monster Rebel engines idle so rich.

But here's my theory. I've been reading about several bikes that run similarly as yours. And the owners claim they never need to choke their bike (which I believe). However, I think these particular bikes could be having an issue with their choke plungers, and in essence, the bike runs around slightly choked, even though the choke lever is set in the OFF position.

The issue you mention about the idle slowly creeping up, I think activating the choke actually also every so slightly tips in the throttle too. It doesn't take much. But just a touch of throttle revs it up and bit and goes a long way from preventing the engine from stalling. Some small problem with the plunger position (like maybe the choke cable is not run the correct cable route and is putting just a skosh of tension on the cable and it slowly pulls the plunger up (richening the brew), and just barely opens the throttle (increasing rpm), this also would either not effect top speed performance, but effect it so very little it would be undetectable.

Anyway, that's my theory, I might be wrong.
 

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Discussion Starter #3
This is incredible info, when I get home I'll read that other thread and the rest of this and reply back. Thanks so much for the write-up.
 
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