Wont start when hot/bogging down and stalling when hot
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  1. #1
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    Wont start when hot/bogging down and stalling when hot

    Hi all,

    New to riding and ATV's so if I don't understand something your saying I apologize.
    I am essentially using this as a tracker for the things I do to try to fix it. feel free to leave any input.

    as far as my bike goes, I only just started taking things off of it like plastics and everything so I really don't know what kind of condition anything is in. today will be the first day I start tearing into it. I have ordered oil and filters, a new air filter, new spark plugs, and some other things


    Anyway I just bought a 2009 450 XC and when the bike is cold it starts just fine and runs well. I rode it around the house yesterday and after it warmed up it would no longer idle. a little more riding after that and it started stalling out. it sputters and lurched back to the driveway and died again. after that it wouldn't start at all. I haven't tried it since then.

    Today I am going to try to take the carb off to clean it out really well and to check the jets. it has an hmf full exhaust on it so im going to see if they also put the recommended jets in or not. ill see if I can find the factory settings for the air fuel mixture screw and set it to the before reinstalling the carb.
    now ive done some googling and the "valves" could also be a cause for this, or at least one of the easier things to fix. im assuming that means the Exhaust and intake valves? ill have to look up some videos on how to go about checking those.

    That's it for now I suppose. Advice or tips are appreciated

  2. #2
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    Very common question. Before you do anything you’re gonna want to adjust valves and pull the carb and clean it out well, and check for an intake boot tear, VERY common for these. You won’t notice a crack unless you physically pull the boot between the carb and head off and inspect thoroughly. Also start with a nice clean air filter. When you get it warmed up and it stalls, I bet if you pull the hot start it’ll start right up. Just from what you’re saying I bet if you go down one or more on your pilot and re-adjust fuel screw you’ll be golden. People jet these bikes half warm, when you start a cold bike you need more fuel, that’s what the choke is for! Then you end up super rich when the bike is all warmed up! I wouldn’t suggest stock jetting. I don’t have experience with these bone stock, but everyone tells me how bad the stock jetting is. You could also have a restriction somewhere in the intake, or maybe you just simply left the choke on haha.
    The choke is the black knob, the hot start is the red one above it, little hard to get to. Good luck!

  3. #3
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  5. #4
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    Hi wild and free,
    Thank you for your reply. I didn't mean I would go to stock Jets. I meant I would check to see if the previous owner left it stock or went to the recommended Jets by hmf. If they are still stock I'll rejet it. Thanks for the video!

  6. #5
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    Also, that video is for the dirt bike. Will it be roughly the same for the ATV?

  7. #6
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    It is the exact same procedure, make sure you get the locking bolts on the tappets snug but not too snug, you don’t want to stretch them, but you don’t want them falling off and wrecking things, I really recommend a torque wrench for them. You can use HMF recommendations just make sure you use genuine Keihin jets. KTM use the same FCR flatslide carb as a lot of other quad and bikes, here’s an in-depth write up on the internals- https://www.yfzcentral.com/forum/32-.../topics/129065

  8. #7
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    Fantastic. And when you say the fuel screw do you.mean the fuel and air mixture screw? Do you know what the stock setting for that is?

    I'll see how far I can get with it tonight and keep this updated

  9. #8
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    Yes but it is not a fuel and air mixture screw, Calling it that may make it confusing, four strokes have a fuel screw, that’s all it controls. Two strokes have air screws. Not saying that to be a smart a$$, just Making sure you fully understand. It’s at the bottom of the carb on the side facing toward the front of the quad, there is no number of turns sweet spot for this jet.

  10. #9
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    General rule of thumb on a fuel screw is to use it as a fine adjustment for throttle response and/or if you get lean popping on deceleration. If you're less than 1 turn out from fully seated you should go to a larger pilot jet. If at 3 turns or more then go to a smaller pilot jet. Not sure how well the HMF header seals to the flange on the head, but the stock head pipes would bottom out on the flange welds before seating internally. This made them notorious for sucking air and popping on deceleration. Over jetting on the pilot/fuel screw circuit would mask the problem, but a better solution was to get the header sealed up.

    If jetting is confirmed, air boot is ok, and valves are in spec another place to move to would be the pickup coil for the ignition. There's been more than one person here that's had a problem with that when their machine gets hot. Keep a spare plug handy and pop the plug boot off and check for spark with the spare plug grounded to the motor if the problem persists. That'll at least point you in the direction of an electrical problem if there is one.
    Mine:
    2017 Husky FX 450
    2009 505 SX
    2008 525 XC-modded for mx (currently 450 cylinder)
    Dad's:
    2009 450 SX
    2008 540 XC

  11. #10
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    Okay so I took the carb apart and cleaned it. They did not change the Jets because it's a 42 and 165. Hmf recommends 45 and 175 so I'll order some of those. The fuel screw was at 1.5 out

  12. #11
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    Air filter is incredibly dirty so I may try to wash it. I have a new one ordered but could be some time before I get it
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  13. #12
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    So im all done for today. Carb is cleaned. Waiting for the new Jets to come in. Unfortunately there is cracking in the boot. It's on the bottom side right near the mouth. The cracks go through to the two small holes that connect into the big hole. If that makes sense. A new one is about 45 bucks. I might try to do a redneck fix on it though

  14. #13
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    Fix it the right way. The $45 is going to be a lot cheaper than having to rebuild the top end because it seized.
    08 525xc ≡ 12.5:1 570 bbk, +1 valves, Crower Cam, Houser Bounce Nerfs, Razr 2 rears, Razr fronts, Pro Armor Dominator Bumper, Scotts Steering Damper, Mino Water Pump, FCI intake, KTMBill Skid Plate, FastRanchero 41mm Carb

  15. #14
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    Yeah I suppose that makes sense. I'll order 1 now

  16. #15
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    Alright new cover is ordered. Should be here in a week or so. I'll try to check the valves today after work.

    So the jet needle has that c clip on it. It's set at the 4th notch from the top. Will that still be okay or will I need to change that?

  17. #16
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    Hey quad,

    When I rode it before it did pop on deceleration. Could this be because the jets were wrong? Should I also look into sealing my header better?

    Anyway I cleaned the carb and then plugged up the intake hole and sprayed the bike down a little. there was a decent amount of dirt in the radiator fins so I spayed that off till the water ran clean. I haven't checked as to whether the fan works but ill do that when it all back together

  18. #17
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    Alright so for the fuel screw I suppose ill leave it at 1.5 turns out, get everything back together and get it running. after its running I should first correct any idle issues using the idle knob correct? then after that I can adjust the fuel screw to reduce exhaust popping, or if it seems sluggish on throttle response yes?

  19. #18
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    Make sure you ordered the better boot, if you order another junk stocker it’ll just tear again. https://www.ktmatvhq.com/forum/xc-en...-upgrades.html
    If you try to correct the idle while your fuel screw is off you’re gonna be chasing it all day, set the idle to spec last.
    There are different ways people go about setting their screws, it all achieves the same goal which is to get the cleanest running idle and the best throttle response/starting possible off/at idle. I’ve always done this way: bring engine to a slightly slow idle, fully warmed up. Turn fuel screw in until it almost stalls but not quite (leaning), then turn it out slowly until the engine reaches the highest idle you can get(richening). But stop RIGHT as it hits the peak RPM, if you keep going you’re gonna be too rich. After that, you can tweak if you like to try to get better throttle response, but if you do it right then that should be the spot with the best response. Leave the needle for now, that’s the “mid range” jet, comes into play from about 1/4 to about 3/4 throttle. Worry about the idle/pilot first, then move on to the needle if need be. Be patient and use the information you have, jetting has a learning curve that can be frustrating.

  20. #19
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    hey thanks for the reply,

    So based off that link we were talking about separate things. My boot is perfectly fine(the smaller connector that goes from the engine to the carb). its the actual whole big lid that's messed up. the one that goes from the airbox to the back side of the carb. the are right next to where the clamp goes is torn, so it was able to suck extra air into the carb as I was riding.

    and okay that makes sense. so youre saying start the bike up and ride it a bit till its warm, then use the idle knob to bring the engine to a slow idle, then use the fuel screw to raise the idle to right when it gets the highest idle. Then, after that if I want a faster idle I can use the idle knob to increase or decrease it?

    And okay I wont mess with the needle and ill leave it at the 4th notch. ill only be switching the jets to a 175 and a 45. ill go ahead and wash the current filter and I believe I have filter oil at my house for it for at least until I get the new one.

    Like I said ill be checking the valves later today and just use that one video you posted. I also read through that other post about the carbs in general. the only thing that really caught my attention on that was the accelerator pump and the aftermarket one that guy put on his. I don't have the same one on mine but it is definitely aftermarket. it is a shiny blue color. I didn't see a brand but I wasn't looking very hard before.

  21. #20
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    A shiny blue color? Huh... never seen one of those either. The adjustable jet is a must in my opinion. You can make them automatically responsive no matter how hard you stab the throttle with one and a few other little mods. I don’t know what brand you have on there though. I don’t like the Boyesen as much but it works, the full R&D power bowl is my weapon of choice. Your accelerator pump only come into play on throttle stabs. So once again something to worry about after this idle is fixed. Sorry I didn’t read through you’re whole post, as soon as I read “ripped boot” I automatically assume it’s the one that is notorious for ripping. As for the idle, yes a slightly slow idle is perfect in my opinion. It makes it perfect to notice when you’re mixing at the right amount IN MY OPINION, some people set them high to do it, some people set it right at manual spec, that part isn’t overly important. You can then raise the idle to normal and do it again and make sure you’re right where you need to be. Throttle response should be maximized at that point. It’s quite annoying to get to that screw, might have to rotate carb and re-tighten clamps a little to be able to get to it. Remember that if you have to turn it too far in or out to get the idle to change, or it doesn’t change at all, you have to change the pilot.


 
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