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Wont hold idle GPX250

Discussion in 'Maintenance and Servicing' started by Operator, Aug 7, 2012.

  1. Okay, I have had another post about this bike. While I am still solving one other confusing issue, I though I would try to solve this. They seem to bear little effect on each other.

    Bike hate starting on choke, just dies, maybe being flooded. Will always start first time without choke, engine was from a bike that had been left a few years so *sometimes* needs start ya' bastard to get running. Once running it's fine. The problem is that it wont hold a stable idle speed. I set it to the factory 1400rpm and it will suddenly rev to 4000 or so and just sit there, I screw the idle back again to 1400 and it will run, then stumble slowly and die. I have cleaned the carbies and there is no sign of oil coming up to the air box (the cause of this issue on my last bike). Any idea's? Is it a balancing issue or idle screw issue or one/both of these?

  2. I would replace the air filter, sounds like the current one has probably been on the bike for a fair while.
    Sounds like you may have an air leak somewhere - are both sides of the carburetors clamped down nice and secure? Gaskets in good shape? No nicks in the boots?
  3. I'll definitely get onto those ideas. The current filter was bought by the previous owner without ever really being run in the bike but I will def check it's condition, re-oil if needed etc. Also will give the boots and gaskets a good check, thanks.
  4. Neglected air filter would cause it to run rich which would mean you probably wouldn't need choke to start. What do the plugs look like? Black and sooty?
  5. Hey I have the same bike, im still getting it goin. Im assuming i might come across this problem, or similar.... how did you go? Also, what year is your gpx out of interest?
  6. It's an '88 model, only 30k on the clock but it has been neglected for about 4 years, left with the clutch cover on loosely attached etc. I have solved every other problem it has thrown at me now, except this over reving issue. I cannot find any air leak visually, no loose screws etc. Sprayed some ether around the carbies, the boots and the intake manifold and there is no sign of the revs altering in the slightest.

    Will pull the plugs out tomorrow morning, been trying to put it back together today and make sure everything is road worthy to see how well it rides in the next few days. I recall pulling the plugs out 3 days ago and they were black and sooty (from memory) and they *were* new before I started it the first time.

    Anyone with a thought as to other causes? Starts, but 50% of the time needs an ether spray. Once running will usually find idle at 1400rpm, touch the throttle just a little she stumbles. Touch it as though you were taking off, will run up to any revs you want; just wont come back down very well. Sits at about 4000-5000rpm after you move the throttle. adjust it back to 1400, same story again. Do it once more and when it is adjusted back it just dies...
  7. Check the cable for the choke, it may be slack which could cause the choke to, well, choke the engine when you think its off?
    as for the start ya bastard, have you checked valve clearances? compression etc?
  8. haven't checked the compression or valves, hoping it doesn't come to that. Doubt it is compression, the bike runs really well and both cylinders fire well and the exhausts (none baffled) both have good strong pulses with each piston firing. I think it is something a little more subtle I am missing. I am going to check my fuel system. I was thinking there may be dirt in the fuel tank or fuel lines or the carbies may have gotten clogged jets or something.
  9. If you need ether to start sounds lean although this is not consistent with sooty plugs
    Are you getting fuel? Blocked fule filter? Stuffed fuel pump?

    If plugs are sooty sounds more like rich. Mixture? SAirfilter blocked? Stuffed carby jets?
  10. Mine does that atm with my temporary fuel tank, because im flooding it with fuel and it it over revs for a while. So my suggestion is check the fuel system all the way from the petcock on the tank through the carburettor to hte engine
  11. The plugs were probably sooty from the rich mixture being created by the ether. Yep, fuel system was my next stop. I realised a low fuel flow would have the same symptoms as an air leak. I will definitely drain the tank, get fresh fuel. Remove and clean the petcock and see if that helps. If it still wont work I'll take the carbies back out and clean them again. This said I cleaned them back at day 1 so unless my fuel was disasterously dirty =S

    Will hopefully get the time tonight, otherwise it will wait till Sunday. Will keep posting the results. (I am running out of possible causes so lets hope the fuel system solves it).
  12. Would oil (any amount) getting into my cylinders affect the revs?.... I know that may seem a stupid question, but I have never encountered anything like this before. I had a mates dad take a look at it and he noticed when reving the bike up by the throttle (not when it ran away on its own so much) would result in blue-ish smoke. This I do know means I have an oil leak, probably rings, into the combustion chamber.
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