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06 crf 450 afterfire/backfire HELP!!

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Hi am am new to this forum

i just bought a used 2006 crf 450 it doesn't have very many hours on it the hour meter says 40 hrs but i came from Arizona USA. i live in Canada BC my altitude is about 720. the problem is when i let go of the throttle the bike it pops and bangs (after-fire)like crazy. When i hold the throttle revving it does the same thing after-fire popping and banging??

I just checked my valve clearance my intake is 0.16mm or 0.06in (to honda specs) but the exhaust valves are at 0.20mm one is tight the other fits nice (out of spec i am in the process of getting the new shims) i have a 168 main jet and the needle clip on 5 slot i have tried changing the clip position and adjusting the pilot jet several times no luck.

Would the exhaust being out of spec clearance be the reason for the after-fire ???

Any help and advice would be nice 👍

Thanks Aaron

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the valves out of spec could definitely do that.

Also check you timing. and if the previous owner cleaned the carb make sure he didnt put the slide thing in backwards (seems to be a common problem with ppl doing this to that carb as it fits both ways). The jetting from az to your elevation would not change the way it runs barely at all so if it was jetted right in az your jetting is close enough where your at now.

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i checked the hot start to make sure its not sticking and just cleaned the carb last night and played with the pilot screw 1/4 turn at a time but made no difference also changed the needle clip several positions, also the spark plug is black so it burning rich. i am going to check for exhaust leaks and shim exhaust valves to spec but need to wait a few days until the shim kit gets here i live in a little town and the nearest bike shop is over 2 hrs away. it after-fired before i took the carb apart as-well

Thanks everyone

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okay so i went out tonight to take the cam assembly off so i could get at the exhaust shims, when i went to get it in top dead center i lined every thing up but i noticed the cam was 180 degrees the wrong way at top dead center, the lobes were facing the front of the bike ??? i didn't think a bike could run that way, i am thinking this could be the problem for the after firing???

thanks aaron

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Rotate engine another half cycle!

Proper top dead center is achieved when the cam lobes are facing toward the rear of the bike (pointing towards air filter) and your timing marks are lined up. I personally use the ignition timing marks on the fly wheel, you can access them under the engine plug on the left case next to the oil fill. I still get it close using the marks accessed thru the right side engine plug then fine tune on the left side of the bike (both ignition and cam marks are easily viewed from one side of the machine that way)

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I would recommend checking the fuel / pilot screw to be sure the o-ring Is In place. If that does not help find out what size pilot jet Is In It. I had the same problem with my 03 when I got It. It started hard, would not Idle, and backfired HORRABLY. Mine had no pilot screw at all!! An aftermarket easy adjust pilot screw Is the shizzle for any FCR!

http://cgi.ebay.com/ebaymotors/Works-Connection-Keihn-FCR-Carb-Fuel-Mixture-Screw_W0QQcmdZViewItemQQhashZitem5ad7a93572QQitemZ390165247346QQptZMotorcyclesQ5fPartsQ5fAccessories

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okay ill take the barb apart and inspect everything more carefully. i like the idea or being able to adjust your pilot screw a lot easier than the stock one, its hard to do :S

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okay here is a update

i reshimed the valves to spec, cleaned the carb, jetted it, reset the timing, installed new header pipe gasket and checked for leaks, and it still backfires!!!! i have not replaced the spark plug yet should i try that next?

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okay here is a update

i reshimed the valves to spec, cleaned the carb, jetted it, reset the timing, installed new header pipe gasket and checked for leaks, and it still backfires!!!! i have not replaced the spark plug yet should i try that next?

Make sure you dont have the vaccum release plate that sits on the slide upside down. The cut away in the center of the slide points down, not up.

Take note of the orientation of part no. 16 in the drawing

14MEB2E1_MEB4E1800G.gif

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thanks for the advise but i looked and its good, i was told it could be my slow jet so i am going to give that a try go to a bigger one like a 45 or 48, it has a 42 at the moment

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so it back fires on deceleration?

If so its a little lean on the pilot, but that can usually be dialed out with the fuel screw. Ive got a 40 pilot in mine, with a Thunder Alley reverse cone megaphone exhaust and it only burbles a little when cold and nothing at all when hot at sea level in SoCal.

A 42 should work.

Im wondering if there isnt something in the pilot circuit.

Have you checked the pilot jet for debris? That thing is small. A single grain of sand could restrict it significantly.

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Make sure hotstart is closing all the way, I totally overlooked mine, and found the cable was rusted half open, doh!

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Make sure hotstart is closing all the way, I totally overlooked mine, and found the cable was rusted half open, doh!

yep checked that it was good 👍

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