Bog on Throttle Hit...525xc HELP!!!
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  1. #1
    Senior Member KTMATVHQ Member
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    Bog on Throttle Hit...525xc HELP!!!

    OK guys,

    I've done alot of reading, and I've made all the carb mods that should help this problem: When I'm riding, and I need to pop over a log or a rut, I blip the throttle from near zero throttle and it falls flat on its face! If the revs are up, or I'm on the gas a bit, it's fine. Seems to be a 0-1/4 throttle circuit issue... at lower revs, when you're basically coasting between whoops. It's really dangerous... It runs GREAT otherwise, but this has me stumped... I've adjusted and confirmed a nice even dual-squirt from the AP that is not hitting the front of the slide(hard to see the inside slide wall however) and the jets are all new. My quad, mods and tuning are:

    Stock bore & stock head 2008 525xc, 15 hrs on the quad total since new.
    KTMBill pipe
    Open airbox
    806 cam
    carb bored to 41mm (Dave Hopkins)
    emulsion nozzle and dual AP squirter from Thumper
    O-ring mod on AP linkage
    45 leak jet
    HRC Needle on clip #3 from top
    160 main
    45 pilot, 1.5 turns out (yes, it'll barely start cold with no choke)

    I've been riding at lower elevations, 0-3000 ft, 70-90F weather. I just can't figure out if I'm getting too little fuel(such as a clogged passage in the carb that I cant see), or if it's flooding... I tried a 40 pilot with no luck. Any help is appreciated, I'm stumped!
    -Glen

  2. #2
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    make sure your accelerator pump is functioning properly (i assume you did the "o-ring" mod already) but after you check that go up in size with your pilot jet. if it bogs or stumbles like you say i would assume its a lack of fuel when you stab the throttle. increase it by quite a bit just to see if it makes it better or way worse. if it makes it better then you know your headed in the right direction and keep testing till you find the perfect size pilot weather it be slightly decreasing or increasing from what you just tested with. i would just buy a size 50 and throw it in there after you take it for a spin with the 45 and see if there is instant/noticeable improvements. as for your fuel air screw i would just leave it in place. remember Only Change One Thing At A Time when testing so you know exactly whats making the positive/negative difference (example; dont change a pilot jet and also adjust the fuel/air screw)
    2008 KTM 525xc motoworks full, SX a-arms & axle swap, DWT wheels, Razr mx tires, Rath racing flat blacks all the way around, Rox Speed fx fully set up handle bars w/ risers and barpad

  3. #3
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    also if it just started doing this but never did before one could assume its something electrical going on instead
    2008 KTM 525xc motoworks full, SX a-arms & axle swap, DWT wheels, Razr mx tires, Rath racing flat blacks all the way around, Rox Speed fx fully set up handle bars w/ risers and barpad

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  5. #4
    Senior Member KTMATVHQ Guru
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    My 525 setup is very similar to yours. Riding at 3000' with 806 cam, stock head, 13:1 piston, Akro pipe, open air box, and 41mm bore with o-ring mod on ap linkage. I also had my carb done by Fast64ranchero on here so the ap nozzle is setup per his spec. This is how it is jetted:
    168 Main Jet
    45 Pilot jet
    55 leak jet
    HRC needle on 3rd clip down

    Biggest difference I see between them that would affect low-end is the leak jet size. Maybe yours is appropriate for the dual ap nozzle? If I understand correctly, a larger leak jet will let more fuel bleed back into the float bowl when the throttle is hit fast. If the dual ap nozzle and my modded one have similar flow restriction, then my setup should be a little leaner on that circuit.

    I developed a bog with mine at one point that would show up on fast hits into steep jump faces and jump landings when I would get on the gas at the same time as impact. Not a huge deal on landings, but not fun on takeoff. Fastranchero suggested checking the o-ring and the ap nozzle for obstructions. O-ring was fine so I ended up just tearing the carb down for a good cleaning and it fixed it. Wasn't visibly dirty inside, but there must have been some small piece of gunk stuck somewhere in a passage. Made sure I blew backward through the ap nozzle. Never did see anything come out while cleaning.

    Check that your ap diaphragm is not starting to degrade and come apart. With the low amount of hours you have I wouldn't expect that to be an issue, but it's worth a look. Be sure to keep track of the two small o-rings that are under the cap with the diaphragm.

    Another thing that I have seen cause this problem with the FCR carb is if the needle is set way too rich. Whack the throttle WO from idling/coasting rpm and it will dang near kill the motor from over fueling. I don't think this should be an issue with yours judging by the settings though.
    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

  6. #5
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    Thanks for the suggestions guys! JBeach, the electrical idea is interesting, although if it was say the TPS crapping out in this range it would sputter at slow speed throttle motions I would think. Quad97, interesting setup, and wow your main is RICH for that elevation, IMO. Have you ever tried going down to a 162 or even a 165? I do not have a high comp head/piston(yet) but assuming you're on 110 or better fuel, you might yield crisper power at a lesser main. Just a thought.

    So, I found one problem: The dual AP squirter is squirting downward, hitting the boot area, not making it to the head! So, when I whack the throttle, it shoots for about 1 second, and pools into the boot, not at the valves. I've contacted Thumper and sent them a video, will see what they say. If all else fails, I'll try a JD or Athena kit (for another $80) and see if theirs squirts higher. This is likely my problem, either way I have to recheck my AP timing to not hit the slide, and ensure the dual squirts dont hit the needle shroud in the carb(common problem with aiming these little buggers correctly).

    Will advise when I hear more!

  7. #6
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    yeah its crazy to think electrical could do something like that. i had an 04 YFZ before with the headlights removed and it ran awesome until like a year later it would sputter and like cut out when i was really high in a gear (made hitting the freestyle ramp more sketchy) and it was actually a problem with the current flow. ended up needing to add a resistor to make it run perfect. problem came up out of nowhere tho and was very noticeable in a certain rpm range
    2008 KTM 525xc motoworks full, SX a-arms & axle swap, DWT wheels, Razr mx tires, Rath racing flat blacks all the way around, Rox Speed fx fully set up handle bars w/ risers and barpad

  8. #7
    Senior Member KTMATVHQ Guru
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    Glad you got it figured out. That jetting is how he had it setup when I got it back from having the leak jet and ap nozzle mod done to it. Seems to run fine the way it is. I had a 160 in it from running at St. Anthony (5000') and he asked if it was running fine because that seemed lean to him for my 3000' elevation. I've noticed that I tend to run one main jet size larger than what others have reported for my riding style on the MX track. If I drop down one step it usually runs fine but runs hotter than if I give it that little extra bit of fuel. Throttle response and midrange pull are tuned in fine with the needle and fuel screw. Maybe a deeper fuel bowl would benefit me and allow the main jet to drop one step. Might have to try it sometime. I know humidity plays a part in jetting too, and we have really dry air here.
    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

  9. #8
    Senior Member KTMATVHQ Member
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    Thanks, me too. Talked to Thomas at Thumper today, and he saw my video, agreeing something is arai with the jet. I've got a new one in the mail, hope it's different than this one!

  10. #9
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    OK, update: Got a new AP dual squirt jet from Thumper(thank you for your support Thomas!) and installed it. Took 6 attempts to get it aligned correctly with respect to the cylinder valve intakes, what a pain!!! Now, the BOG: STILL THERE. I've tuned the AP to barely not hit the3 face of the slide, and it still has the bog. The fuel paths are higher now than before, but still hit the cyl head before the valves, splattering before the valves instead of squirting into the valve seat area...

    I think I've got the dreaded issue based on the design of this little guy, it's just not working. This thread talks about it as well:
    http://www.ktmatvhq.com/forum/ktmavh...ap-noozle.html

    Anyone have a stock nozzle they want to sell? I cannot find mine for the life of me. I really want to try the .018" diameter drilled out stocker now, this just isn't working. Ugh...

  11. #10
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    ever get a stock drilled nozzle in? I rebuilt my top end a couple weeks ago and went to an 806 cam, 12.5 CP. i run 41mm bored carb with 165 main with HRC needle, tried 42 and 45 pilots. have leak jet setup and drilled out stock nozzle. since putting the 806 cam in the low end response sucks compared to stock cam. wondering if the miss you're noticing is because the 806 robs some bottom end? I know i'm seriously debating on going back to a stock cam if I can't find a setup just a bit better on response. mid to top flat out rips but thats not the most useful in the woods.


    before I rebuilt I ran a OBEKR needle 3rd clip and 175 main. jetting was flawless with not miss anywhere but after rebuild had the miss. thought maybe the new cam would need jetting tweaks. swapped the jetting from my backup quad (listed above) and saw slight improvement but still not what i want. with the OBEKR needle and 175 main in the backup quad it runs flawless.

    anyone else run into this?
    Last edited by polarisrider150; 07-21-2014 at 02:09 AM.
    Racing KTM 525xc's in WNYOA & GNCC

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  12. #11
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    PROBLEM SOLVED for now!!! So, I got a micro drill-bit kit from Amazon("CML Micro Bit kit"), and used the nearest bit size to .019" as recommended by FastRanchero's XC thread for the AP squirt bore-out(I had a .020" dia bit). In doing this, I first used a sharpened punch to peen-closed the lower AP hole in the dual AP nozzle. I figured it would be easier to simply go backwards in evolution at this time, redrilling to a known-good diameter and re-aiming best I could by hand the single hole angle. I ended up with a single healthy stream of fuel pointed at the left valve, and the throttle response is WAAAAAY better!!!! It'll bog if you stab the throttle too quick, but you have to really be lightning fast, and it's really much much better than it was! For now, ITS FIXED!!!! It really was a lean condition caused by fuel not getting from the dual AP into the valve area, but falling short due to bad aim of the holes as-drilled in the brass nozzle. FYI, this was with a 45 leak jet to maximize pressure & duration, and it seems to work fine, about a 1 sec duration. I'll post some pics next of the operations. Wahoo!

  13. #12
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    PolarisRider,

    That jetting works pretty well for my mods, but I've found the 45 Pilot, 1.5 turns out, and a 162 main with the HRC needle on 3rd clip from top(and I have the emulsion tube from Thumper too), is crispest around 0-1000 ft elevation and 80-100F temps....I do use the 165 when it's colder.

  14. #13
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    Update: higher altitude use above 3000 ft incurs a bit of bog due to overly rich .02" hole in ap nozzle. Im still waiting to hear back from thumper on a failed second ap nozzle, but my advice is to drill it at the next drill size down of .018". This should be sufficient at most altitudes. Plus the hand-drilling makes for a slightly larger hole than a cnc drill operation would when these are made at a machine shop.

  15. #14
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    The carburetor was sent back to us for evaluation. We found the valve body gasket was swollen and pushing out of it's mounting channel. There was less resistance pushing the fuel past the valve body o-ring vs. pushing it out the AP dual spray nozzle so the squirt was weak. By drilling out the dual spray nozzle you made the resistance less than at the o-ring leak so the fuel would squirt better. After checking the carb and replacing the gasket we see a nice strong dual spray that last approx 1.2-1.5 seconds. We did change the oring on the linkage and wired it. The wire puts more pressure on the AP compared to an oring. The wire will not break over time like the orings do and it give a positive connection to the linkage where the oring will flex like a rubber band an will not give as positive a squire as the wired linkage.
    Last edited by ThumperRacing; 09-25-2014 at 08:21 AM.

  16. #15
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    I am speechless...wow. I never would have guessed the inner gasket had an issue!!! Thomas/Travis, thank you so so much for your help, I never would have figured that out!!!! My only logical conclusion was a bad batch of nozzles. Please let me know what I owe you, and my head and cylinder are headed your way for some more excellent service from Thumper Racing.
    2008 525xc; exhaust & skid by KTMBill, suspension by RocketRonRacing, Thumper Racing 540 12.5:1 kit + trail porting +1 KW valves, breather, carb emulsion tube, Mino hi-flow water pump, 806 cam, 41mm carb w ranchero mods by Dave Hopkins, GDC intake mods, Rox Risers, Precision Stablizer, SX stem bearing, PRM arm skids, and the wisdom on KTMATVHQ! Jetting above 25C: below 5000 ft: 162main, 42pilot, 1-1/2 turns out. Above 5000 ft: 155main, 42pilot, 1/2 turn out. Below 25C-up one on main.


 

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