2009 YZ450F stumbles and spits under acceleration

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10/24/2018 9:50 AM

So, I just got my carb back from the dealer and the did a full rebuild and clean on the carb. The reason I brought it in to the dealer was because the bike was running weird there was no smooth acceleration and It was spitting under acceleration only when you roll into the throttle. I just got it back and its doing the same thing but worse. There is not smooth acceleration its either all or nothing.

If you slowly roll the throttle back it begins to feel like it has no power and just pops like crazy but goes no where, but if I snap the throttle it goes like a normal bike.

The bike has a full dr.d exhaust and twin air, air filter but I got the dealer to rebuild the carb on stock jetting. I have played around with the fuel mixture screw and it does not make a difference. I have tried everything from .5 turns to 3 and it still does it.

Anyone have any ideas? I have been riding it in Ontario Canada and its like 4 degrees Celsius here but I have had my bike out on a frozen lake so I don't think weather makes a difference.

Thanks in advance.

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10/24/2018 9:55 AM

Possibly an air leak? Jetting not correct?

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2017 RMZ450
2005 YZ250-sold :,(
1998 YZ250
2005 KX250F

80% of the time it works every time
IG @hammerfamily_4 & @2HRacing
Thanks to : Factory Effex, N2Dirt, Acerbis, DT1, Fasthouse, Matix, FMF, ASV, 100% & Mika Metals

10/24/2018 10:54 AM
Edited Date/Time: 10/24/2018 11:03 AM

The cold temperature may play an effect here colder air= more air... question if you keep it choked does it react better? if so I’d say you have a lean pilot circuit... maybe the reason it reacts correctly to a quicker throttle snap is it’s transfering off the pilot circuit so quickly you don’t notice the stumble/hesitation... always start simple first... order I would run through diagnostics... if it runs correctly on choke that normally indicates a lean pilot circuit so either too small of pilot jet or dirty pilot circuit/ possible air leak/ start there... if that doesn’t cure it look into your accelerator pump timing/diaphragm, also check the squirt nozzle for the accelerator pump as I have seen these gum up before and cause all sorts of headaches ...check the vacuum plate seal on the slide..( just because it was at a dealer doesn’t mean they looked at everything)

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10/24/2018 11:05 AM

It is running very lean.

Paw Paw

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10/24/2018 12:53 PM

Ok so I just was trouble shooting the bike for ever and some one suggested to run it with the choke out. it runs way better with the choke pulled out. It only sputters a little bit when slow acceleration but it runs way better with the choke pulled out.

Im not quite sure in the direction i should go from here. obviously like paw paw said it running lean becuas pulling the choke out richens the A/R ratio. it is stock jetting in the bike as of now but fuel is definitaly my issue.

Thanks again

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10/24/2018 2:43 PM

Remember that the choke on your carb is like a separate carb, so this indicates you are very lean with the choke turned off. It could be an air leak, plugged jet or bad fuel screw o ring or some other small item. Start by looking for an air leak, then inspect the jets and o ring. The fact that it runs well when accelerated quickly tell me the accelerator pump is good. Now the pump timing can be off a bit and cause the stumble. Check it as well.

Paw Paw

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10/24/2018 4:16 PM

Ok so I did some looking around and im pretty sure its not an air leak, it is thigh around the air box and tight around the rubber boot going to the top end.

could it still be a plugged jet if the carb was completely rebuilt? I mean like unless the guy who rebuilt the carb put the new jets in dirty I don't thing that a dirty jet would be the problem.

im pretty new to the whole carb thing so how would i go about checking the pump timing?

thanks

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10/24/2018 4:29 PM
Edited Date/Time: 10/24/2018 4:32 PM

Toppman 16 wrote:

Ok so I did some looking around and im pretty sure its not an air leak, it is thigh around the air box and tight around the rubber boot going to the top end.

could it still be a plugged jet if the carb was completely rebuilt? I mean like unless the guy who rebuilt the carb put the new jets in dirty I don't thing that a dirty jet would be the problem.

im pretty new to the whole carb thing so how would i go about checking the pump timing?

thanks

Just because the boots are tight doesn’t mean they won’t leak air/ take some carb cleaner/ starting fluid or wd-40 and spray it around the connection points with the bike running... if the idle picks up when you spray it then you have an air leak at that point... (if this all started after the bike sat for a descent amount of time with fuel in it I would suspect a plugged passage or dirty jet... the mid carb on these carbs are full of wormhole style passages and sometimes dirt will sit in the corner of them causing a fuel restriction)... if you want to check accelerator pump timing ithere are plenty of videos out there explaining how to do so...

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10/25/2018 10:19 AM

Alright so I have spent 2 days trying to figure out why the bike sputters with the choke out. Obvisouly it is running lean like paw paw said. I have checked for air leaks, nothing. I sprayed wd-40 around and idle has not increased everything looks to be sealed. I timed the accelerator pump and it was not far off of perfect when I started looking at it.

I think my next step is to start taking the carb off and a part once again. but im not sure ill find anything.


I was looking at the old jets that came off the bike and the carb rebuild kit came with OEM jets.
Would it be possible that the bike is running lean becuase of the new twin air filter i put on, my full dr.d with stock jetting? the twin air filter is definatly thinner than my old one that i took off. The stock jets that came out of the bike are as follows:


Main is a 160, pilot is a 45


Anyone have any other suggestions before i take it to the dealer to have them look at it?

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10/25/2018 10:50 AM

Toppman 16 wrote:

Alright so I have spent 2 days trying to figure out why the bike sputters with the choke out. Obvisouly it is running lean like paw paw said. I have checked for air leaks, nothing. I sprayed wd-40 around and idle has not increased everything looks to be sealed. I timed the accelerator pump and it was not far off of perfect when I started looking at it.

I think my next step is to start taking the carb off and a part once again. but im not sure ill find anything.


I was looking at the old jets that came off the bike and the carb rebuild kit came with OEM jets.
Would it be possible that the bike is running lean becuase of the new twin air filter i put on, my full dr.d with stock jetting? the twin air filter is definatly thinner than my old one that i took off. The stock jets that came out of the bike are as follows:


Main is a 160, pilot is a 45


Anyone have any other suggestions before i take it to the dealer to have them look at it?

Call DRD and ask what they recommend for jetting.

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2017 RMZ450
2005 YZ250-sold :,(
1998 YZ250
2005 KX250F

80% of the time it works every time
IG @hammerfamily_4 & @2HRacing
Thanks to : Factory Effex, N2Dirt, Acerbis, DT1, Fasthouse, Matix, FMF, ASV, 100% & Mika Metals

10/25/2018 10:52 AM

DRD jetting for the exhaust I bought says you don't need to rejet

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