[twatwaffle]

So I threw a 2005 450R silencer (just the muffler, not header) on my 450X. The bike turns over right away, seems to run well once warmed up, but there's lots of backfiring/popping on decel and some while holding the throttle at constant speed (although I'm riding on pretty rough terrain and that could be due to decel as a result of less than steady throttle application).

The airbox is open up top with a Twinair filter. Jetting, from what I last remember, is the NCVS needle, 165 (?) main, 48 pilot, Flexjet fuel screw (turned something crazy like 3.5 turns out...but it's the sweet spot I found after putting the pipe on). Merge spring too.

I would assume that I'm running a touch on the lean side. Is this something I can correct with the fuel screw or should the pilot be changed as well?

[JimDirt]

What is your elevation ??

If its backfiring then you are lean , the R muffler (same thing i have on my 06X) makes the bike leaner , as well as the airbox mod , so you need to richen it , , BUT , with a #48 pilot , i would say your rich enough on bottom , , but the fuel screw at anything above 3 means your lean , so its possible you have a clog somewhere in the carb , or a loose connection at the muffler/header connection , ...... or , your intake valves are tight .....

A 45 pilot should work about anywhere , though i had a experience where i had to run really rich at 500 ft elevation , to get either of my bikes to run , i had to run a 180/48 with 2 1/2 turns out , but anywhere else i have ridden , from 2,000 ft to 8,000 ft , i use 162/45 , 1 1/2

I would double check the mid pipe connection , or the head connection , , make sure nothing is clogged inside the carb , and check your valves , make sure the fuel screw O-Ring is good

[twatwaffle]

Hey Jim; thanks for the reply.

I'm at about 1500 feet and ride between 40-90 degrees.

I just took the carb apart. I've got a #55 leak jet, 165 main, #70 'starter' jet and a #45 pilot. I've torqued down the header a bit more and had put it together last week with a new copper gasket, as well as tightened the midpipe clamp, both of which were a touch loose.

Based on what I've read, those jets should all work for my location and elevation. Should a #48 pilot maybe help to get that fuel screw closer to that 1-2 turn area? Other than that, everything should be good. Valves are brand new and in spec.

[JimDirt]

Yea , your about right where you need to be on jetting , so your fuel screw should be in the 1-1/4 to 2-3/4 range

Also have you cleaned (with a actual Jet Cleaning Tool) or replaced the pilot jet with a new one to confirm the pilot jet is not clogged or partially clogged ?? , as it can still be blown thru and still have a partial clog and be enough to cause a lean condition

I am assuming you rode it (on the stand tuning/testing will still get some popping and is not a accurate test) after tightening the connections , and it still is decell popping ??

Did you try adjusting the fuel screw to a 1-1/2 base setting , and trying to ride it ?? , did the bike start easily ? , did it pop like crazy ?? , this will tell if the header and mid pipe tightening did any improvements , i am also assuming you have confirmed the Hot Start is working as it should , since if its stuck in the Open position , it will also cause a lean condition , have you confirmed the O-Ring on the fuel screw is intact and not dried out ?? , also you need to confirm the Floating Slide Plate seal is in good condition , and that the clamps on your intake and air cleaner boots are tight and there are no cracks in the boots , all these can cause air leaks as well and lean the mixture

[bdaztoy]

I am having the exact same problem with my recently purchased 2006 CRF 450 X. I currently have a 170 main, 45 pilot. I live in Phoenix, Arizona. I performed the CRF's only air box modification. The bike starts hard hot or cold, you must pump it, pull the choke and when it finally runs it pops and bangs on decel. The fuel screw is out 3 turns. I covered up the air box side holes with duct tape and put a rag over the cutout on the top of the air box and its a whole lot better. The carb has been cleaned twice and most recently all 3 pieces split and cleaned thoroughly. Will a 48 pilot fix the problem or do I need to regroup?