06 YZ450 backfire and poor idle help

Trying to fix my buddies 06 450. Start but dies at idle and when held at high idle or revved it back fires, blows flames out of pipe then stalls. Starts up again but won't run properly. Searched back firing and did all possible fixes namely ; increase pilot to a 48 from 42, balance of jetting stock (at sea level), checked valves and valve timing, no air leaks, carb cleaned, slide seal replaced. I am thinking bent/stuck valve that is not showing up on a valve adjustment forsome reason. New plug, unplugged TPS and checked resistance ( in spec at closed and 3.79 at WOT! Should be 4-6) pick up a leak down tester tomorrow to see if that's the problem. All stock. Am I. Missing anything? Thanks

bent or stuck valve should show up though. but sounds like youve checked pretty much everything.

same symptoms before/after the pilot change? whyd you change the pilot to begin with?

Thanks. Changed the pilot after reading a lot of threads about jetting. Doing leak down test tomorrow.

You could be barking up the wrong tree. The pilot jet thing to relieve backfiring is generally pointed at popping through the exhaust on deceleration, not the symptoms you describe. A 48 pilot is normally larger than necessary for an '06, which is usually happier with a 45 except in colder climates at lower elevations.

It sounds more to me like an ignition problem, and I think you should do a thorough check of the stator, coil, and your wiring. When checking the stator, rap on the case cover with a hammer handle or something to see if the resistance values change because of it.

flames out the muffler? wouldn't that indicate a valve issue? timing issue?

No, not really. What i indicates depends on when it happens. In this case, it won't idle, and pops and fires out the pipe while holding over an idle. That could be an extreme carb issue, such as a blocked pilot passage, missing fuel screw O-ring, broken or inverted slide plate, etc., but it's also a classic symptom of erratic ignition, like an intermittent open circuit in the stator or coil, TPS, etc., that leads to an interruption of the spark. When the system starts functioning again on the next stroke after it drops a spark, the exhaust will be loaded up with the previous unburned charge and pop off.

Valves that are too tight can cause a lean condition by allowing compression/combustion pressures to dilute the intake stream, but usually, this shows up along with extreme difficulty in cold starting that gets more normal as the engine heats up.

Just did a leak down test and sounds like intake is leaking. Once head off I'll confirm. Thanks for the help.

(I didnt have time to read the other posts im purely replying from your question) I had the same problem with my 2009 yz450f. To start it was fine but when it was running it ran like shit. To begin if your below 3000 ft set the carb to stock jetting. No factory sends their bikes out running like shit. Second since you checked the valves and made sure they are right make sure your woodruff key on your flywheel is not striped. On my bike the TDC marks on the flywheel would not show up when the cams were lined up. This ment my flywheel which holds magnetic information, was incorrectly sending ignition signals to my ECM (electronic control module) The bike blew flames was bogging down and dieing and made my life miserable. Anyways a woodruff key which holds the flywheel in place correctly cost 3 dollars and may be the answer to your quarrel.

I ran thru something similiar and eventually chased it down to plugged pilot. What baffled me as I had the carb off and had cleaned it and specifically cleaned the pilot several times, so was convinced that was not it.

I failed to drain the tank and get rid of the offending gas. When I looked inside with a flashlight it looked ugly in there. Had to pull tank and clean out the gunk. I also shit canned the fuel lines too.

Good luck and report what you find.

e 10 or 10% ethanol gas could lead to a degrading fuel line, thus plugging the pilot.....In any case clean carb,clean out gas tank, new fuel line and install 50 pilot.

e 10 or 10% ethanol gas could lead to a degrading fuel line, thus plugging the pilot.....In any case clean carb,clean out gas tank, new fuel line and install 50 pilot.

Yes! and for the future use an ethanol treating fuel stabilizer, you wont ever have that problem again, unless of coarse you let the bike sit without running for like 3 years

Well it's been a while but just wanted to confirm I found the problem, Greyracer was right it was electrical. The spark plug cap/coil had a fine crack in it and was arcing In the head causing mis firing. Thanks for all the help.

Good catch...thanks for the follow up

I am having a similar problem with my 07, did all the regular stuff, checked valves and timing and resistances in electrical and everything seems fine. cleaned the carb and unplugged pilot jet and returned it to stock jetting(had a 175 main) and needle clip was in wrong position 5th instead of 4th iirc. Bike still backfires in all areas accel and decel. My mechanic who is not familliar with auto decompress think it has something to do with it but I took the cam to yamaha and the tech there said it was working as it should. the exhaust leaks a bit out of the front of the can(DR.D) going to repack it today and make sure it seals well. any ideas if I have missed anything? the tech at yamaha said it could be the coil over plug grounding out on the head but my guy says its def no that im lost, any help would be muchly appreciated thanks.

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