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2008 CRF450R stutters on 0-1/4 steady throttle

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Hi everyone,

I just bought a used 2008 CRF450R a couple of weeks ago. The bike runs great, starts on first kick every time, pulls very strong and the engine sounds like it's in great condition.

One issue I noticed is that the bike 'stutters' while riding with steady 0-1/4 of throttle, i.e it's not smooth. Idle and all other throttle positions are fine, smooth while steady and very strong WOT.
I assume that there are some adjustments needed on the carb, haven't touched it so far. But it would be great if someone could point me in the right direction.
From the service manual I see the following recommended settings for my environment (Altitude: 700-1000m, Temperature: 18'C-28'C): PS: 1-1/2, SJ: 42, JN Clip: 2nd, JN: NJYR, MJ: 172.

What would be the first thing to check? Or does someone think that it's something other than the carb?

Many thanks.

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7 hours ago, wioffroader said:

Check your jet needle for wear. Just found mine was worn and was possibly letting extra fuel past. Here's a pic. IMG_4304.jpg

Ive seen needle wear before, but that's pretty significant. You may want to trying something different with your air cleaner too.

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On 8/22/2017 at 1:24 PM, eazy230 said:

prob fuel screw or pilot jet. this may help

https://thumpertalk.com/forum/showthread.php?t=467595

 

 

or437546

Id like to point out the overlap in the circuits. The pilot is really the only system to run completely out of its range and be ineffective. The throttle valve, needle jet and Needle itself make huge impacts, but the diagram doesn't show the main making as big of a difference as it really does. Especially as you affect the needle position. And it really shows you why that thing needs to be clean. It'll also show you why your metering surfaces and orifices need to remain unmolested. Dont jam wire through a jet, scratch at the needle etc. One must be gentle with the parts!

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Put a 7 sizes bigger main jet in your carburetor and only use 0-1/4 throttle and you will never know. Main does not  matter at this opening. It would however matter if it was plugged solid not free flowing..

Quote

 

 

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2 hours ago, eazy230 said:

Put a 7 sizes bigger main jet in your carburetor and only use 0-1/4 throttle and you will never know. Main does not  matter at this opening. It would however matter if it was plugged solid not free flowing..

 

You would notice a difference at 0-1/4 throttle with a giant main jet. The poor tune would become worse and worse, but it'd be noticeable at lower throttle settings. Which is exactly why you tune the main jet first, then the needle. Because if you try to tune the needle, with a main that's too big(or small) , if you happen to get it right, when you finally get the main dialed, you'll be back dialing the needle again.

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I challenge you to remove your main jet and start your bike, it will idle perfectly.  Your needle taper is the restriction until it clears main.  Therefor you can start and idle, even blip throttle to close to a third without a main jet in your carb.  Ask me how I know, mine came loose and dropped into carb bowl while riding, rode fine around parking lot get on track, ran very poorly. Took me almost 3 hours to figure out what happened.  This was on a 2001 CR250. Now a main jet that is too small becomes  the restriction way to soon therefor would have an effect at lower than 1/2 to wot.

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Hi everyone,

I just bought a used 2008 CRF450R a couple of weeks ago. The bike runs great, starts on first kick every time, pulls very strong and the engine sounds like it's in great condition.

One issue I noticed is that the bike 'stutters' while riding with steady 0-1/4 of throttle, i.e it's not smooth. Idle and all other throttle positions are fine, smooth while steady and very strong WOT.

I assume that there are some adjustments needed on the carb, haven't touched it so far. But it would be great if someone could point me in the right direction.

From the service manual I see the following recommended settings for my environment (Altitude: 700-1000m, Temperature: 18'C-28'C): PS: 1-1/2, SJ: 42, JN Clip: 2nd, JN: NJYR, MJ: 172.

 

What would be the first thing to check? Or does someone think that it's something other than the carb?

Many thanks.

 

In the Keihin FCR-MX, the needle clip position ( or length) primarily affects 1/4 throttle position. The quick solution would be to lower the needle 1 clip. Because the bike is nearly 10 years old you should probably replace the needle jet as well. The needle jet, or emulsion tube, is the long, cylindrical tube that the main jet threads into and the needle rides inside of.

 

The pilot jet is for idle speed only.

Sent from my XT1650 using ThumperTalk mobile app

 

 

 

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Thanks a lot for all your suggestions guys. I re-checked all adjustments in the carb and reset everything to service settings. The clip was one down and the pilot screw was opened 2 turns... so the bike was running little rich.
2nd clip now and 1-1/2 on the pilot, runs perfectly smooth.

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16 hours ago, angel.koseski said:

Thanks a lot for all your suggestions guys. I re-checked all adjustments in the carb and reset everything to service settings. The clip was one down and the pilot screw was opened 2 turns... so the bike was running little rich.
2nd clip now and 1-1/2 on the pilot, runs perfectly smooth.

Glad you reported back with your fix.  A lot of people fail to do this and it does not help those who have same problem later and do a search for answers.

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+1 ^

Bugs me to death when someone doesn't come back and finish the thread! haha! .. 

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