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56 Posts
Discussion Starter #1 (Edited)
Okay I'm a little annoyed that I keep losing so to a Stock YFZ that's piped and jetted and getting shit for it..........My quad is not running how it should!

I bought a 41mm FCR Carburetor from a 07 CRF450R.
It starts up first kick or 2 No issue's Hot & Cold, I have to play with the throttle a little to keep it from Stalling while at idle. It don't seem to have the Bottom End I expected along with the Throttle response how it used to, it revs fine no choking out, but When I try to "Crack The Throttle" it will choke out a tad not to much but obviously it should be able to peg the limiter if I crack it.

My friends YFZ450 when you Crack the Throttle, there is there Rev Limiter

Yes my TP Sensor is connected & Correct.

I rode an 05 TRX450R that has same Set up as mine and it has the Response of a YFZ450... Thus, what made me wanna go to the FCR Carb.

I do know my Intake Valves are practically Shot and have ALLOT OF HOURS need to be Replaced but I don't think that would kill my Throttle Response would it? I know that may be what is killing my Bottom end power I'm certain of that... I get Smoked Hard core by both my buddy's YFZ's from dead stop, but Top Speed I can creep past no problem at 84 MPH

Quad Details below

Jetting Mods:
"Elevation Tucson AZ 2400FT Above Sea Level"

Main Jet - 170
Slide Needle - At Mid Section "Out of 5 Clip Settings runs best in mid clip"
Pilot Jet - 48
Pilot Screw - 2 1/2 to 3 Turns Out "R&D Flex Jet"
Main Air jet - 90

Typical Mods:
15T Front Sprocket/Stock 38T Rear Sprocket
Big Gun Rev Box
No Air Box Lid
Uni Air Filter
White Brothers Exhaust "No Super Trap Plates Strait Through Exhaust"

Super Moderator
3,684 Posts
A lot of stuff to cover here; one thing at a time - first the carb (we'll get to other mods later).

Carbs are not magic - they are mixers - they mix fuel with air - what did you expect to gain from changing carbs? Since Honda does not specify venturi size, what is the difference between the venturi of the TRX carb vs the CRF carb? Those dimensions will give a clue as to the performance ghosts you are having.

My first concern would be the throttle position sensor - since Honda does not provide details on TPS resistance for the TRX, how do you know the CRF TPS is set correctly when connected to the TRX CDI box? The desired values would be the same resistance from zero to max throttle settings on the CRF carb as you had on the TRX carb.

Beyond that, the lag you experience on acceleration could be due to too little back pressure on the exhaust or not a strong enough shot from the accelerator pump. You could try installing a smaller "leak" jet in the float bowl or adjust the pump linkage to initiate the fuel shot immediately as the throttle is operated.

A 48 pilot jet is probably too large - the std. pilot jet was a 42 - try going back to it or try a 45 - it is probably the reason you have to play with the throttle to keep it idling - it loads up and needs to be cleaned out by blipping the throttle.

Jetting carbs is fun

56 Posts
Discussion Starter #3 (Edited)
Carbs Aren't Magic? I'm well aware of this....
The Fact is almost Every TRX Guy knows the 07 CRF450R Flat Slide Carbs are a Big upgrade for the 04/05 TRX450R's from the 400ex Style Carburetor.

As for the Throttle Position Sensor, I'm pretty sure I have that hooked up Correctly by doing Allot of Research making sure which wire was 5V In, TPS Signal Back to the ECU and Ground... I set it's Position using the CRF450R TP Sensor Resistance Values at closed Throttle as well.

I will try the 42 Pilot and see if that helps the Idle.

But I did order some brand new Kibble White Valves + Head Gasket Kit and Springs to replace the Intake Valves due to the lack of Clarence they have and I wanna say that may play a big roll for my Power issue.

(I have had the low power issue even before the FCR Carb Upgrade) "Sorry forgot to mention that in the very beginning"
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