Yzf400 pops and snaps where do I start

10 replies to this topic
  • Macktruckdriver

Posted September 23, 2008 - 06:26 PM


Ok I have a 99 YZF400 stock with a whitebrothers muffler the one with the baffels like a supertrap. I just adjusted the valves because it pops alot and they were way off not much clearance. That didnt help it runs great if Im crusing but I cant get it to idle yet and if you just grab the throttle it pops and snaps alot almost like a back fire. Im guna check the valve clearances again tomarrow but some direction might help. Thanks.

  • grayracer513

Posted September 23, 2008 - 06:35 PM


First, read:


Adjust the idle mixture:


Verify that there are no air leaks in the hot start plumbing and that the valve itself closes. Check the exhaust.

  • Macktruckdriver

Posted September 24, 2008 - 01:13 PM


Ok so what kind of exh leak am I looking for like one at the head pipe or something. If I just pinch off the line for the hot start between the head and the hot start button that should rule out the button if nothing changes.

  • grayracer513

Posted September 24, 2008 - 02:03 PM


the most common exhaust leaks occur at either end of the header.

  • Macktruckdriver

Posted September 24, 2008 - 02:14 PM


Ok well I checked the hot start and didnt find anything and I put a clamp on the line to the head to eliminate that. I tried adjusting the idle mixture but I still cant get it to Idle at all. If I have the choke out it will fire right up and Idle but high as expected but one I push it in it will die. I kept it running and sprayed eather around the intake boots and intake and id never reved up at all so there shouldnt be an air leak. I dont know what I should do about checking the exh. Thanks so far.

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  • grayracer513

Posted September 24, 2008 - 03:10 PM


An exhaust air leak will not prevent the engine from idling, it will only cause decel backfire. Since it won't idle without the choke, ignore the exhaust for now.

Your problem is more likely a blocked pilot jet, pilot air jet, or other obstruction of dirt or varnish in the carb.

  • Macktruckdriver

Posted September 24, 2008 - 03:23 PM


I guess Ill have to pull the carb again and Ill just get new jets they are cheap. I pulled it because before it would fire right up one kick with the choke on but die after 3 seconds and there was no way to keep it going. I found one of the jets pluged with corrosion since there was probably water in the bowl for a while the screw holding the float on was all rust. I guess Ill have to just clean the carb again and see what happens. i still dont have a manual Im trying to get one. Is there a link to the breakdown of the carb and what jets are what.

  • grayracer513

Posted September 24, 2008 - 04:03 PM


i still dont have a manual Im trying to get one. Is there a link to the breakdown of the carb and what jets are what.

You newbies really should spend some time looking at the Common Threads Sticky.



There should be no reason to replace the jets. If you have access to a set of small number drills (#80 up through about #15), use the largest drill you can push through the jet without removing any metal.

  • Macktruckdriver

Posted September 24, 2008 - 06:08 PM


Well Im not much of a newbie just confused on the dilema. Never had a problem like this. Also this web site is way different than the ones iv been using. I just bought the bike Wed night pulled the oil filter and it was full of metal shavings. Thursday night I had it to the bare case to replace all the bearings and had it back together friday night. I just dont have a manual yet and found this web sight this weekend. Thanks for the help though.

  • grayracer513

Posted September 24, 2008 - 06:23 PM


Metal in the oil filter isn't unusual. The engine oil lubes the trans, and transmissions shed metal. The amount and shape of the chips can be meaningful, but the normal amount causes new owners a certain amount of concern.

  • Macktruckdriver

Posted September 25, 2008 - 01:51 PM


Iv had many four strokes but never tune them I just leave them stock. I know there is uaually a little but not enough metal that all the bearings in the tranny and crank were rough to turn and the entire filter was full of metal like big shavings.

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