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Throttle Response of the 4 stroke

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Jun 27, 2007
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#1
I've been riding 2 strokes for 15 years and looking to make the switch to the 4 stroke.
Currently I'm looking at a 2005 or newer CRF450R. But, I'm concerned about the throttle response of the 4 stroke. You know... the 'bog'!
My 2 stroke throttle response is immediate! So there's my concern

Questions:
do all 4 strokes have this?
Does anyone know if the 2005 or 2006 CRF450 has this 'dreaded bog' issue?

Finally, is it fixable? if so, what mods are needed to get that 'snap' we all are looking for.

Thanks for your responses!
 
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#2
my bike has no bog. just warm it up or down shift. it will bog it it isnt warmed up though so make sure its nice and toasty. it doesnt help if starting in 3rd gear either though

if it still bogs get the carb rejetted
 
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#3
My '04 YZ450F doesn't bog but then it was setup by my dealer who knows what he's doing (I ride with him almost every week). Where does the bog occur and when? On my bike, turn the throttle- it goes. My old KDX200 and KX bogged more.
 
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_JOE_

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#5
My 06 crf 250 is supposed to be the king of bog. A JD jet kit, AP mod and timing set properly, and some pilot jetting work and the thing is instantly responsive. Some places such as Tokyo Mods will do it all for you. Go for it, the linear power delivery is great.
 
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#6
Thanks for the replies. Sounds like proper jetting and a boyseen AP is the answer to reduce the bog.

Has anyone had good results with the Power Now and Power Now Plus carb inserts?
 
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#8
Fine tune the ap circuit=no bog. Anything to make the ap circuit hold more or pump more,bad. JD's needles and secret o-ring on the ap adjuster excellent,diaphragm mod is questionable. My experience with the crf 450 has showed me the most powerful off road bike I have ever ridden,way to fast for closed course competition,but to each his own. And the crf,as usual for honda,the best built! Bog worse than a 2t,only when it had issues!
 

_JOE_

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#9
Yeah, some FCR carbs have really rich accelerator pump circuits. Mine had a 50 leak jet. The leak jet controls how much fuel is allowed to return to the float bowl. I have tried a 70 and 90 so far. I seem to like the 70 best. No bog, but a smooth transition from bottom to mid. I also shaved the rivet on the diaphram. I use the fat o-ring from my JD kit. I have been impressed with the low to mid, but am going to try dropping the main a size in hope of some more revs. :ride: :cool:
 
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#11
Any fcr carb,yes they all have the same issues,07 and newer may be different!Get the jetting,timing and valves in spec and it is an indestructible machine(except for the snapped rods thing). Response is best felt with quality fuel,mtbe based fuels deliver! The 70 is what I settled on for a mid temp baseline,I believe the pilot went a couple smaller and the main one,needle 2nd from top.The popping and bogging gone,that bike can get up,unbelievable! The diaphragm mod,I do not know exactly where its going,increase the ap squirt and to what extent,the burst of raw fuel just clears the slide,its to help the off idle transition,not dump a half a bowl of raw fuel in and out to burn in the header!The ap adjuster,if you watch it,it floats,hesitates and sometimes does not squirt. The o ring mod takes care of that,if you wire it solid the arm bottoms out,that is where I thought grinding the diaphragm came from,but the o ring lets it stretch. Jd's multi taper needles seem the next ticket!
 
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#12
Friesen said:
So any bog can be fixed with say... JD jetting kit for proper jetting, even with the stock AP?
The stock AP works fine if it is adjusted properly. Many people adjust this to squirt too much fuel into the engine too soon. This floods the engine and causes a bog. Adjust the AP so that the slide opens a little bit before the AP squirts fuel. Most of the jets in your carb don't actually start to work until the slide opens a little.

Same problem with many of the aftermarket AP's. More is not always better.

Just my $ .02
 

_JOE_

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#13
As I understand it, the rivet shaving must be done if wiring the arm, and increases the duration of the squirt. With a larger leak jet you shoot a leaner charge. In essence you are using the same amount of fuel over a longer duration. Mine runs really strong and the response and torque down low is imressive. As ol 89er said, the timing is also crutial.
 
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#14
I'm the poster of this question: Update.
I bought a 2006 CRF450R and have a FMF Q2 pipe on it.
the original owner left the jetting stock with the exception of turning the fuel screw out 1/2 turn.

No Bog and throttle response it dead on.
I'm very impressed! and pleased.

Thanks all!!
 

_JOE_

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#15
Great to hear! I'm glad the switch to a thumper was a pleasant one for you.