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  #1  
Old 01-22-2011, 09:20 PM
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Default Bike runs rough in low RPM's

Okay here's the deal: I bought a 1996 CBR f3. It runs well at idle. When you give it gas, it responds decent, but it stutters in low RPMs. Once it gets past 6k or so, it clears up and sounds and runs fantastic. There is a shorty two brothers exhaust and a k&n filter on it. No jet kit. Is that where the problem lies? Carbs haven't been sync'd in who knows how long but from what I have read, if you put a filter or exhaust on, you should also put on a jet kit because it's like making a pb&j without the pb. Its just not right.
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  #2  
Old 01-23-2011, 12:03 PM
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There is a shorty two brothers exhaust and a k&n filter on it. No jet kit. Is that where the problem lies?
I would say thats the very good place to start. Not only can an aftermarket slip-on require a rejet, but you have a k&N filter, plus the aftermarket slip is a shorty. Sometimes, if you only have a slip on, you can get away without having too many issues. but the fact its cut short, will create problems.

About the only thing you can do is jet for your situation, which just installing a stage one or stage two jetkit will probably not solve your issue. Your issue is kind of unique to your bike. You more than likely have a lean condition down low, and maybe an ideal condition up top. You may also still be lean up top though.

Have you pulled your sparkplugs to look at them?

If you don't have any mechanical knowledge with carbs, I would leave it to a professional. Or, go to a full length slip on. that way, you can get a dynojet kit that will fit your bike better, then only a small amount of tweaking will be required instead of the headache your going to have with that set-up.

I know you probably see a lot of shorty exhaust out there, but most of those will be on fuel injected bikes. Its much easier to fix the lean and rich spots with a fuel injected bike than it is with a carb. You can do it with a carbed bike, but its going to take a long time and someone who knows those carbs really well. And those people are getting harder to find.

As far as the sync goes. Carbs rarely go out of sync. at least out of sync to the point of needing adjustment. But, who knows what has been done to the bike before you aquired it. Out of sync carbs will run rough down low and clear up at the higher rpms. They will also have a very erratic idle. Also, if you rev the bike, the rpms will generally come back to around 3-4000 rpms quickly. but then drop very slowly back to idle from there.
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Old 01-23-2011, 01:15 PM
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Originally Posted by justasquid View Post
I would say thats the very good place to start. Not only can an aftermarket slip-on require a rejet, but you have a k&N filter, plus the aftermarket slip is a shorty. Sometimes, if you only have a slip on, you can get away without having too many issues. but the fact its cut short, will create problems.

About the only thing you can do is jet for your situation, which just installing a stage one or stage two jetkit will probably not solve your issue. Your issue is kind of unique to your bike. You more than likely have a lean condition down low, and maybe an ideal condition up top. You may also still be lean up top though.

Have you pulled your sparkplugs to look at them?

If you don't have any mechanical knowledge with carbs, I would leave it to a professional. Or, go to a full length slip on. that way, you can get a dynojet kit that will fit your bike better, then only a small amount of tweaking will be required instead of the headache your going to have with that set-up.

I know you probably see a lot of shorty exhaust out there, but most of those will be on fuel injected bikes. Its much easier to fix the lean and rich spots with a fuel injected bike than it is with a carb. You can do it with a carbed bike, but its going to take a long time and someone who knows those carbs really well. And those people are getting harder to find.

As far as the sync goes. Carbs rarely go out of sync. at least out of sync to the point of needing adjustment. But, who knows what has been done to the bike before you aquired it. Out of sync carbs will run rough down low and clear up at the higher rpms. They will also have a very erratic idle. Also, if you rev the bike, the rpms will generally come back to around 3-4000 rpms quickly. but then drop very slowly back to idle from there.

First, thanks for the awesome post. I learned a lot in the past...2 minutes. Awesome! looks like I'll be getting a full length slip on for sure..

Question: would it be possible to have the bike run better if I put a regular air filter on in place of the K&n along with the full length slip on? I mean, to avoid the hassle of a jet kit. I'm not interested in power with this bike, I just want it to be smooth.

I pulled the plugs the other day and they were pretty much all grey. They looked good. And I have had the carbs off the bike and taken apart, and they look pristine.

Maybe I'll use this *misfortune* to fab up an undertail exhaust for this bad boy
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Old 02-10-2011, 10:44 PM
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From my experience with carbs, if you don't want to play with jetting then leaving the rest of the system as close to stock as possible is ideal. Definitely stock filter and as was said earlier a full size slip on but retain the rest of the exhaust system (headers, mid pipe etc.). Carbs are more sensitive in changes regarding available air flow and exhaust back pressure than fuel injected bikes. Sometimes carbs are jetted a little rich from the factory and you can find a slight advantage from replacing the filter OR the exhaust. I currently have a 2000 CBR 929 with a k&n filter and Jardine slip on but stock headers and mid pipe, htev still intact. I'm not using a power commander and it runs rather well, however I have been contemplating going back to a stock filter. I had a 1978 CB 750 super sport with stock airbox and filter, stock jetting, no carb mods, stock exhaust. Ran great even 30 years later.
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Old 02-10-2011, 11:48 PM
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They say the k&n filters are more restrictive than the stock filters for the f3. Maybe try a different filter like a uni filter. Also cb750's don't like any airbox mods or air filter "pods" because of vaccum
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Old 02-10-2011, 11:48 PM
 
 
 
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bike, carb, cbr929, fuel, injected, lean, low, motorcycle, revved, rough, rpm, run, runs, rus, slip


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