Hard Start, Popping, and Black Spark Plug

5 replies to this topic
  • ssbogger77

Posted June 11, 2010 - 03:16 PM


I searched and search for the past day and I'm tired of reading. The bike is a 05 YZ450F.

Starting routine is choke, two twists of throttle and kick.

Bought it 3 months ago and could get it started in 3-5 kicks cold and 5-10 hot. I then checked the valve clearance and shimmed them back to spec. They were out pretty bad. After that, I tried to crank it and it took like 10 kicks cold. Ever since it has been the same, anywhere from 7+ kicks and sometimes I just get tired of kicking and give up. When I do get it cranked it pops/backfires like crazy revving it up. Same thing on decel. It also tends to do it while trying to crank it. It will backfire while I kick it. I know there have been all kinds of threads on this and I am tired of reading them as they aren't specific to my bike.

Engines need spark, fuel and air to run so...

I pulled the plug and grounded it against the engine and I am getting spark. The plug did appear to be black. It didn't have build up on it but it was just black which makes me think it is running rich.

Next, I checked the timing and it is spot on. The cam chain is tight, the dots on the cams line up, and the lobes point out at TDC. While here, I went ahead and checked the clearances and they were all in spec.

Next, I read up on carbs and jetting. I pulled the carb and blew out the jets with air. I didn't blow any air through the carb as I didn't was to mess anything up. I pulled the float cover off the bottom and it appears that the leak jet was not even installed. It was just loose in the float bowl. When I pulled off the cover the leak jet just fell out. I screwed it back in the hole where I think it went, the only one it would fit in on the float cover.

Posted Image

I put it all back together and tried to crank it again. 5 kicks and it fired up. Seems like it may start a little easier but the popping is still there. I mean it's horrible.

My jetting is as follows:

Altitude: Less than 100'
Start: 72
Leak: 40
Pilot: 45
Main: 170

I dont know where the needle clip is set because I couldn't find it/don't know where it is.

Thanks in advance.

  • grayracer513

Posted June 11, 2010 - 07:21 PM


If it's popping while holding the throttle steady or accelerating, and the plug is black and sooty, the problem could be an intermittent electrical open. Look for a loose or dirty ground connection, weak wires at connectors, etc. It could be a faulty stator, too, but this is difficult to verify except by swapping in a known good unit. Sometimes you can connect an analog VOM and tap or wiggle the coils. If the VOM jumps, you may have your culprit.

  • ssbogger77

Posted June 14, 2010 - 08:24 AM


Ok so I'll look for loose connections. What you're saying it that it may be missing spark every now and then causing the fuel to not ignite and thus providing a black plug? Also, would I be correct in assuming that the un-burt fuel would then be pushed out of the cylinder on the way out through the exhaust only to be ignited the next time the spark plug catches a charge which is why I am hearing all of the popping?

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  • grayracer513

Posted June 14, 2010 - 08:50 AM


Yes. Erratic ignition can do all of that.

  • Luigic81

Posted July 01, 2010 - 10:46 AM


this sounds exactly like the problem I'm having with my '06 I just bought, was there ever any resolution to this issue??

  • ssbogger77

Posted July 02, 2010 - 07:55 AM


Not yet. If I figure it out I will post back.

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