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2005 CRF Pops/Backfires on start after rebuild

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The title pretty much says it all. I just finished putting a new 2007 head and a new stock piston on my bike with all new stock parts. The bike runs great, starts first kick most of the time, and pulls throughout the rpm range as good as ever. The only problem is when it is hot, it pops or backfires 2 out of 5 times on the kick stroke even though it usually cranks when it does this. This is what I have done and the bike set up-

New stuff

  • New 07 head with all oem parts
  • New 05 piston and rings
  • New oem top end gaskets
  • New timing chain

Bike set up is all stock engine wise other than a 450X head pipe and a 35 oz. (heaviest available) Steahly flywheel. I had the flywheel on the bike before the top end and it did not do this although I installed it in the heat of summer. The jets are stock 168 main, 42 pilot.

Things I have checked

  • Hot start sticking- No
  • Pilot or main clogged- No
  • Head pipe leaking No
  • Loose carb joints No
  • Ait filter is clean new oem
  • Timing marks align perfectly(although I had to transfer the marks to to the Steahly using the stock flywheel as a template). I double checked the alignment with the timing mark on opposite side of the engine.
  • Intakes are tight .006 and loose .006
  • Exhausts are .011 each
  • Decompressor is at .014
  • Fuel screw is set at 2.5 turns out - it reaches fastest idle at this setting and does not improve any more if I take it out further

I did have the idle set high to prevent stalling so I lowered it. This seemed to help, but I can't really take it any lower. The fuel screw adjustment does not seem to eliminate the pop/backfire. I'd appreciate your help because I am stumped. I am tempted to live with it because the bike cranks easy and runs great.I just can't understand why it does this on the cranking kick and then runs fine.

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The title pretty much says it all. I just finished putting a new 2007 head and a new stock piston on my bike with all new stock parts. The bike runs great, starts first kick most of the time, and pulls throughout the rpm range as good as ever. The only problem is when it is hot, it pops or backfires 2 out of 5 times on the kick stroke even though it usually cranks when it does this. This is what I have done and the bike set up-

New stuff

  • New 07 head with all oem parts
  • New 05 piston and rings
  • New oem top end gaskets
  • New timing chain

Bike set up is all stock engine wise other than a 450X head pipe and a 35 oz. (heaviest available) Steahly flywheel. I had the flywheel on the bike before the top end and it did not do this although I installed it in the heat of summer. The jets are stock 168 main, 42 pilot.

Things I have checked

  • Hot start sticking- No
  • Pilot or main clogged- No
  • Head pipe leaking No
  • Loose carb joints No
  • Ait filter is clean new oem
  • Timing marks align perfectly(although I had to transfer the marks to to the Steahly using the stock flywheel as a template). I double checked the alignment with the timing mark on opposite side of the engine.
  • Intakes are tight .006 and loose .006
  • Exhausts are .011 each
  • Decompressor is at .014
  • Fuel screw is set at 2.5 turns out - it reaches fastest idle at this setting and does not improve any more if I take it out further

I did have the idle set high to prevent stalling so I lowered it. This seemed to help, but I can't really take it any lower. The fuel screw adjustment does not seem to eliminate the pop/backfire. I'd appreciate your help because I am stumped. I am tempted to live with it because the bike cranks easy and runs great.I just can't understand why it does this on the cranking kick and then runs fine.

Could you have installed the vacuum plate on the carb slide upside down? That will cause the bike to backfire/pop and hard to idle. It also usually makes it hard to start too. It doesn't sound bad enough to be the problem but it is easy to confirm.

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Could you have installed the vacuum plate on the carb slide upside down? That will cause the bike to backfire/pop and hard to idle. It also usually makes it hard to start too. It doesn't sound bad enough to be the problem but it is easy to confirm.

I did not have the carb apart during the rebuild and it was fine before the top end job. Thanks for the idea though. I appreciate the help. :bonk:

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have you tried a larger 45 pilot?

also double check the cam timing. easy to get off 1 tooth.

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The pilot might be clear, but that doesnt mean the passage is also clear.

Id have you pull the carb and give it a good douching with carb spray and compressed air through all the holes.

Take all the jets out, the ones in the bowl and the ones in the mouth.

And double check the vacuum plate. If that doesnt get it, try the OEM flywheel. Weirder things have happened.

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OK, I changed the flywheel back to stock- no change.

Verified the timing with oem flywheel marks - right on

New spark plug- no change

New exhaust pipe crush ring- no change

I did find some success with turning the idle down and resetting the fuel screw. It only afterfires through the muffler on cranking when it is really hot now, but it usually starts first kick :bonk: . I think my high idle setting was contributing, but the afterfire is still there though not as bad even with the low idle and reset fuel screw (now at about 2 turns out). I used Eddie S. instructions from the jetting forum. The crazy thing is this bike runs awesome other than the hot cranking afterfire. The next step will be replacing the pilot, main, and jet holder with new stuff and cleaning the internals of the carb with spray cleaner and compressed air. The old jets that are in there look open, but I am trying to eliminate things here. Is there any chance I will hurt the internal gaskets with "Gumout" spray cleaner? I also have a 45 pilot coming as well to try-just to see and I will check the vacuum plate while I am in there. Thanks for your help.

Edited by FOUR STOKED

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sounds like maybe you lost or installed the washer spring and Oring on the fuel screw.

I would pull and verify: screw, spring, washer, then O ring

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also why did you do the top end? maybe you need to do the crank also. Pull the left cover, inspect the pickup coil. There is 2 soldered points there. as the crank wears, small metal slivers will become metalisized there and stick between the points creating an electrical bridge between them.

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I want to thank everyone for their suggestions and help on the board. I finally seemed to work it out although I am not sure what finally fixed it. In addition to the all the other things I looked at, these were the last three things I did:

1- replace the pilot, main jet, and needle jet holder

2- repack the muffler

3- drain all the gas out of the tank and carb and replace with some new gas.

The backfiring stopped and I was able to run the morning class GNCC at the General in Georgia again. Had a good chance at the holeshot this year as the old girl cranked first kick and pulled good. Went into the woods top five but the finish was another story :thumbsup: .

As a side note, the right crank bearing started chattering pretty good since then and even though I never found any metal around the flywheel, it makes me wonder if that might have contributed to the intermittent pop and it came out changing parts around? JJ might have been onto something there. Seems I have gotten my moneys worth out of the bike I bought way back in October of 2004. Time flys when you are having fun. Anyways, I've got the cases split now for a bottom end overhaul and hopefully will be back in the saddle soon. Thanks again,

FS

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