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Hello people new to the site have a little problem with my 450x, the bike is standard besides a slip on DRD muffler and some foam air filter everything else stock as far as I know. valves are spot on at .006 and .011 I ride between sea level and 700mtr so about 2100ft the main jet is 145 and pilot is a 45 the air box is un touched standard and air temps change from 5 deg c to 45 deg c so the only problem I am having with the bike is popping at light throttle cruise speed and deceleration any help would be nice.

Welcome to the site !!

Use this as a Jetting Guide (below) , also make sure the Fuel Screw is adjusted within spec ..... it should be between 1-1/4 to 2-3/4 turns out from lightly seated .....If the bike sat for any length of time (over 2 months , and you used gas with Ethanol in it , then either remove and clean (they make a needle tool to clean the jets) or replace the Pilot Jet ......

As far as jetting itself (which should cure your issue if the fuel screw is adjusted as mentioned) ... The chart below is where you should be ... The factory jets them lean to pass emissions .... not to make them run good ..... so go by this and you should be good .....

Your Pilot Jet is good , just worry about the Main Jet , also check the Needle position in the Slide , it should be on the 3rd from top position (stock is 2nd) ..... Also make sure you do not have any air leaks , like the Header where it connects to the head , and where the mid-pipe joins the muffler , as that can cause the popping as well ...

Jetting Guide for Main Jet:(pilot 45) CRF450R/X

Sea level....165
2000 ft.......162
4000 ft.......160
6000 ft.......158
8000 ft.......155


Temperature/Elevation:
One main jet size (up/down) for every 2,000 feet or 25 degrees in temp.

Thanks Jim so using the guide looks like I should be at least 165 on the main and leave the pilot as is, everything else looks to be fine no running hot etc and pilot wasn't blocked might just need the bit more fuel due to muffler change Im thinking ? Is the 145 main a stock jet ?

I believe the stock Main is/was 142 , as it is for the earlier years (05-09) they may have went to a 145 in 10 , but I think it varies by what state is was originally sold (elevation dictated the stock jetting somewhat), and they basically only changed radiator shrouds (in 08) along with gas tank size went from 2.3 gal.for 05-07 to 1.9 in 08 on (dumb for a off-road bike) and smog stuff for late 06 on years ........ (my 06 did not have any smog pump crap and has the 2.3 tank).....

Hey Jim cheers for all your input Im 48years of age and only got my bike licence 6 months ago ride plenty of bikes off road over the years but this one first road rego for me.

It should do you good (the X) I have had mine since brand new ... the only time it ever failed me , was my own fault , I let dirt get past the air filter and it took out the Intake valves real quick ... I replaced them with Stainless Intakes and have not even had to adjust them since , that was back in 06 , other than that ... zero issues ....... These bikes are durable and you should have a blast riding it ....

Hey Jim thank you for all the help so far so I have another question for ya what screw on the carb is for setting it up ? I see two red one on bottom and flat blade screwdriver one mid way up on left cheers bud. Im picking up 165 main tomorrow after work.

The red one with the numbers is the air / fuel screw. That one you have is an aftermarket addition and makes it easy to make your adjustments. Which changes is altitude and air temperature sometimes requires just a slight adjustment to stay running perfect.
The larger slotted one on the side is the idle speed adjustment. The small slotted one on the bottom with the rubber tube is the float bowl drain.

That's a really nice looking bike you have there.

Thanks for your reply Neil, that air fuel screw was set at 3 3/4 turns out so I have picked up a 78 pilot and only went with the 155 main as up top I think everything was fine it was only playing up at light throttle and cruise so will see how it goes after fitting tonight.

Sounds good - Let us know how it goes. We all love to her back on people results.
If the A/F screw was that far out to run close to right it certainly needs to step up one size on the pilot jet.

Thanks for your reply Neil, that air fuel screw was set at 3 3/4 turns out so I have picked up a 78 pilot and only went with the 155 main as up top I think everything was fine it was only playing up at light throttle and cruise so will see how it goes after fitting tonight.

#78 ??? , did you replace the Starter Jet ??? (short round fat slotted jet) or the Leak Jet ??? (small slotted jet that goes in the float bowl) , the Pilot "should" be around #42 or #45 (long skinny slotted jet with holes on the side of the thin tube) .... The Fuel screw should be between 1-1/4 to 2-3/4 turns out from lightly seated as I mentioned above , if it needs to be 3+ turns out , then the Pilot jet is either clogged or too small , or you have a internal blockage in the carb ..... Also , do not forget to put the bottom plug back in the float bowl that is missing in the picture

This is what the Pilot Jet should look like:

Sorry Jim that was a typo, just fitted 48 pilot and 155 main about 10 mins ago but having a couple beers tonight in the shed so test it on way to work tomorrow
And yeah from everything I had a read of so far Neil pointed to pilot too small cheers guys

Ok feed back the bike is running like a dream and a lot more responsive and snappy and only pops on heavy decal Cheers for the help guys

Excellent. You might be able to get rid of that slight decel pop by a 1/4 turn out on the red A/F screw. Slight adjustment with air temp and density or altitude is not uncommon either.

Good job.

Ok bit more feed back after riding a couple more days , fuel screw out 2.5 turns atm and after pushing up a few hills the pop is still there about 1/3 throttle, I have not looked at needle clip position yet. trying things in stages