2005 crf450 Popping?...

Do you have an aftermarket air screw? Is it still where it should be? Check its position to make sure it hasnt vibrated from where it is supposed to be. Valve Adjustment time? How is it on starting?

Exhaust leak perhaps?

did you take the carb off? check around the intake to see if you have any leaks. Or like 123 said check for exhaust leaks. It shouldn't just "all the sudden do that"

Check the bolts that hold the head pipe to the engine. Make sure they're good and tight. Also, where the header meets the mid-pipe check there for leaks, make sure the clamp (if there is one) is tight. Stock exhausts need a clamp there for sealing. Most aftermarket exhausts don't use a clamp there. Also, be sure the sealing packing is in there if it's a stock system.

Check where the exhaust can meets the mid-pipe for looseness or cracks. Oh yeah, check the header for cracks as well. Check the spark plug and see that it's tight.

Turn the gas off, pull the 17mm plug out of the bottom of the carb and drain any gas/water. Turn the gas on for just a second and let fresh gas drain out then reinstall the plug.

Is the gas old? The fuel screw is in the bottom of the carb up in the front of the float bowl. It's a tiny slotted screw if it's stock. Get a little screw driver and turn that thing in until it stops. Then turn it out about 1 1/2 to 1 3/4 turns. If it's cold out you might have to go slightly farther.

well, yes, it's an air/fuel ratio problem, but there are dozens of different ways that it could be affected. How long has the bike sat? Bad/old gas will cause that along with a dirty carb, an exhaust leak like already mentioned, any vent hoses not hooked up, air filter not sealing right, carb boots loose/not fitting properly/ or torn... exactly what led up to it popping? All of a sudden? Right after you worked on something? The more details, the easier it is to pinpoint the problem.

i didn't read it all but on my 05crf450 i re-shimmed it in April 5 rides later it was popping then wouldn't start again. I totaled my truck last weekend so i haven't been able to dig into it, but it is showing the same characteristics as before when the valves tightened, i have been told a total valve redo is the answer.............

Four stroke? Yes? Problem solved.

lol thanks and lately ive been full of luck, just all bad though. Id have it finished and ready to ride right now but getting to work is a higher priority, but one of my buddies was telling me you can shim it again but it will only last a couple rides, when he finally got his valves all redone he said he rode it for another year without a problem, it was a crf250 though.

Valve issues are possible, did the valves get adjusted any time recently? Valves shimmed too much could do this. The same as an exhaust leak. Is the air screw in the bottom of the carb?

If you do it right, you wont have to do it for a long time. Being in Australia I wouldn't know where to direct you. Here in the states I sent my head to RHC and I am pushing over a year of moto, trails and general play riding on my valves with no issues much less a need for adjustment.

Ferrea valves, hardened seats, no problems
http://www.ferrea.com/Wc5766baa88f4a.htm

Low compression: Have you pulled the tank and inspected the motor? The auto decompression is on the cam, it works on the principal of the cam spinning. I wouldn't think that is the issue. Pull the bike apart and I am sure you will find the issue. Check the carb while you are there. Make sure everything is still where it should be.

From what I see at the track it's normal.

The low compression is because of the valves. When the valves wear and all the backlash is lost the valves hang open releiving the compression just like the decomp is on.

You're getting hosed.

parts should be under $400. I've routinely done that job for others for under $600.