Need some carb help...

8 replies to this topic

Posted March 19, 2009 - 12:40 AM


I've got a '99 WR400 that is sumo'd. Awesome bike with all new parts. Big Gun pipe and carb is supposed to be jetted for it. I've had the bike since December and have put 900 miles on it since then. The other day, I changed the oil & filter, replaced the antifreeze with Engine Ice and all was well. I rode the bike to work that night (29 miles) and everything performed flawlessly. The next morning on my way home, I got about 3-4 miles from work and the idle got stuck high. It was almost as if the throttle was not returning to rest. After checking it, I saw the throttle had full range of motion. I had to limp it home with a high idle for about 23-24 miles, dulling my shiny ceramic header coating :banghead: Anyway, I've tried adjusting the mixture screw out, but still have a high idle. It has performed perfectly. This is an "all of the sudden" problem. Please help me guys:banghead: :)

  • Birdy426

Posted March 19, 2009 - 01:38 AM


Pull the carb and check the slide for cracks. The Gen 1 FCR carbs on the 400s have a history of cracking, and ultimately failing, ingesting a bunch of metal into the intake. If it's cracked, it would cause an air leak, which would give you a high idle (esp if your mixture at idle is a bit rich)


Posted March 19, 2009 - 01:51 AM


Thanks, I'll pull it when I get home this morning. I'm of the belief that the carb is from a later model bike. It has a stand-alone hotstart knob (attached at the frame, not the carb) as well as TPS. I don't know for sure, but it seems as what was passed on to me when I got it.

  • Birdy426

Posted March 19, 2009 - 04:45 AM


If the hot start is on the frame, it's a Gen 1 carb. The Gen 2 carbs have it integrated into the carb body.

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  • Birdy426

Posted March 19, 2009 - 07:00 AM


You might also check the hot start vacuum lines for leaks...disconnect it where it ataches to the head and put a cap over the nipple. If your idle drops to normal, you have a leak in the octapus somewhere...

  • ncampion

Posted March 19, 2009 - 07:20 AM


My money is on the slide crack. Replaced three of them on my 400 before going to an Edelbrock. Problem solved.


Posted March 19, 2009 - 04:50 PM


Thanks for the tips guys. I didn't get to pull it today, but I will tomorrow. I'm scared that it's going to be the slide as well. I'll try the trick with the hot-start first. Thanks again, and I'll post up my results.

  • Birdy426

Posted March 19, 2009 - 06:40 PM


Don't be too can quite often find a Gen 2 carb on e-bay for less than the cost of a new slide, and you will get better idle and off idle perfomance. If you snag a YZ carb, you also lose the Air Cut Valve from the WR carb, which makes it a bunch easier to jet. If you end up buying just a slide, there used to be a guy here on TT who sold billet aftermarket slides that eliminated the problem, but I don't really remember any of the details...


Posted March 20, 2009 - 12:59 AM


Thanks again, I tried the hot-start trick, but the idle was still high. I guess I'll check into late-model carbs if the slide is cracked. I won't know until today when I get home. I really appreciate the speedy response!


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