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surging after 1km open throttle edit: RESOLUTION

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by slickncghia, Jun 25, 2008.

  1. G'day guys

    the bike is surging/spluttering/about to stall whenever im cruising on the freeways or at WOT for about a k or more.

    dr800 running standard mikuni 33mm sling shot carbies.

    *fuel seems fine (has happened on multiple tanks)
    *needle jets and jet needles look fine
    *main jets are clean (running 1 size up from stock atm to compensate for airbox opened up and zorst)
    *dont think its the fuel filter as fuel is poring out ok from the petcock.
    *has no fuel pump (originally did) gravity fed
    *over flow hose dosent seem to be blocked

    originally we were thinking it must be the float levels. however after several adjustments raising higher and higher (smaller distance) it dosent seem to have made any effect...

    any ideas what it could be? other than the floats all that i can think of is perhaps the gravity fed system possibley cant keep up with the demand at that throttle position for that length and is running out of fuel.

    bike runs fine accellerating and decelerating around at backstreets speeds just surges after a k or so on the freeway

  2. Is there any reason that you removed the fuel pump?

    If it was fitted with a fuel pump as standard, chances are because it needed it.

    Try fixing the fuel pump as a big bore single requires a lot of fuel at large throttle openings.

    Good luck with it :wink:
  3. i didnt remove it. the previous owners did.

    however looking at the flow of fuel it should be plenty gravity fed.

    there was a block in the main vent of one of the carbies. cleared that but the problem still remains ....hmmmm
  4. also its not a lack of airflow getting into the tank, tested it with the fuel car cracked open...

    any ideas guys? id really love to get this running for MR
  5. Surging would suggest an excess of fuel in the system, not a lean situation, Fuel in the intake manifold evaporating/riching the mixture with a shut throttle, causing a surge as the fuel ratio approaches an optimal mixture - try adjusting the mixture to lean it out. Did the last owners fsck with the jet/needle settings? Are you running an aftermarket pipe?

    Try going the other way - lean it out a bit. The mixture may surge the motor as it approaches and passes the stoichiometric ratio - in hard acceleration, apparently the mixture can be as rich as 8:1, let alone 14.7:1, however, if it gets too rich, it will cause fouling, and hence sputtering and stalling.

    Try switching back to stock, then trying that for a bit (I'm assuming a size up means richer)
  6. I was thinking rich rather than lean.

    Get the float level right, then if it's still there I'd be looking at a worn needle jet.

    have you been too:
    Product and technical support
    Motorcycle tuning tech
    Cv Carb tuning
    low RPM engines
  7. maybe surging wasnt the correct work...its more of a cough/splutter

    i will give it a try back at 120's however i think if it was the main jets it would be effected at all times on full throttle not just after a substained period of time.

    however i guess its a possibility that it could be too ritch that it could be fouling the plugs.

    yes the previous owner stuffed around with the jetting heaps and appears to have had even less of an idea than i do. was running 135 originally and was way ritch. there is also another issue with one of the carbies has the afr screw all the way in and seized causing it to run lean on the pilot circuit, however this is not my main concern at the moment and i dont think it would be having any effect on the coughing/spluttering at full throttle.

    any further ideas for me to try tonight?


    *edit* Oh yeah and it has an aftermarket 1 3/4 header and pipe. and i opened the airbox up a fair bit. all things that i would think would lean the afr out, hence require a bigger jet. i might try 1 up and 1 down if i get time tonight
  8. well the float level is back to stock atm.
    ive looked at the jet needle (the needle thats attached to the float) and it looks fine (without getting the microscope out)
    also the needle jet looks to me like a niddle jet should.

    is there anything in particular i should be looking for when it comes to the needle jet?


    *edit* good link btw. havent seen that one in all my net crawling.
  9. Not the needle attached to the float, but the mid-range CV needle.

    Have you experimented with the height of this?

    If not, leave the main one above stock (sounds about right) and play with different needle heights. also inspect the brass hole it slides in and out of. It should be very round.
  10. i had a look at the "mid range" (there are too many needles all named simmilarly) needles the other day. they were set 1 clip up (ritcher) again by the previous.

    i didnt however inspect the hole that it slides in and out of in any great detail other than that they werent blocked.

    however if this was out (which it may well be) i wouldnt think this would really be effecting it enough to be causing surging/coughing at freeway speeds...would it?

    it runs fine around the streets its just freeway speeds that has an issue. spluttering ect when i return to normal street speeds after a short while it returns to normal functioning. (to me indicating that either the empty float bowls have had a chance to refill on closed throttle or that the fouled plugs have had a chance to burn themselves back to a clean state)

    sry im doing my best to decribe the symptoms here
  11. Depends on the bike. If you've got the throttle most of the way open and are pushing it, then it will be on the main jet, otherwise the needle and needle jet are very much in play.
  12. Float bowl pressure not equalised with airbox?

    Sorry if I keep on suggesting it. :)
  13. OK just re-read the title. Is it OK up to 1km on constant throttle then goes bad?
  14. yeah i suppose its probably cruising on around the border of neddle and main jet (around 3/4 ish)

    however the problem also occurs at just WOT after the same amount of distance. which is all main jet territory.

    i guess ill have to go do a plug chop when its surging to tell once and for all if its fouling or if its not getting enough fuel.

    when i check the plugs at home (after riding a 5k back from the testing ground) the plug is black around the ring and light brown around the pin and side electrode. indicating that at some stage it was running way too rich...however when i first got the bike it was jetted WAY to ritch and wasnt running properly (135s) so im not sure if it just hasnt had a chance to burn that off or not....but i think it would after a week of testing.

    i still have a funny feeling its float/needle jet related. i guess a another plug chop is the next logical step.

    any other ideas that i can try tonight please let me know

  15. If it's fine for a while at that throttle position, but gets worse, I would suspect fuel starvation.

    check/replace vacuum lines to the tap and clean and check the tap. Also make sure your tank vent is clean.
  16. what could be causing this loz? the overflow hose isnt blocked. ive tried running with the petrol tank open (ish)

    any other areas that could be causing this?

    yeah, i honestly cant be certain whether it would stall/surge at say 60km/h on constant throttle because it hasnt happened yet (possibly cause im always accell and decell) however it only seems to present itself on the freeway or WOT
  17. Is there a tube, or pair of tubes, leaving the carb float bowl and leading to an open end somewhere near the front of the bike?

    The fact that you've got a custom airbox on it can play merry hell with the air-fuel mix. Pressure waves, incorrect pressures, tube lengths, all that sort of thing. The pressure needs to be constantly equalised between the airbox and the carb float bowls, or you get symptoms very close to what you're describing.

    Air intake systems are tuned very precisely to optimise the air/fuel mix at different speeds as more or less air is getting pushed through the airbox. Before I ran the float bowl pressurisation tubes up into the airbox, the bike would run fine at low speeds, but then surge and cut at freeway speeds when the airbox wasn't getting enough pressure due to the front rams being removed.

    The fact that your problem only happens at freeway speeds would indicate to me that it's an airflow issue affecting the air/fuel mix.
  18. cheers

    theyres no raim air style system on the bike originally or now

    theyres basically just a big triangle filter in the airbox that sucks in air from the side. the only mod i have done to it is opened up the hole a little more. to let a bit more air in.

    the airbox is on the side of the bike.

    ive attached a pic to explain. but yeah it should be sucking a fair bit more air in the side, however because its on the side i dont know if it would be getting turbulence issues or not....i guess i could try masking the airbox back up to its original state and going for a ride...might be worth a try

    any thoughts?

    edit* theres no tubes leaving the carb float bowls (there is just the overflow tube/vent thingy)
  19. and lol the actual cutting is about as rough as that ms paint pencil depiction

    reckon that will have any effect?
  20. My Spada does thin after coming down from high revs.

    ie cruising at 100+ and coming back to town speeds. Only ever does it when the outside temp is cold though,

    warm day everything is fine. 2:00 am on th calder passing woodend and it shows its head when pulling back into town.

    i got the carbies clead, checked reset, balanced etc but no improvement....only lasts far a few minutes and all is good again