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Suddenly running rough

Discussion in 'Technical and Troubleshooting Torque' started by Jastel, Sep 10, 2015.

  1. This morning I fired the bike up to head into work and when I got to the top of my driveway (lucky) it ran out of petrol. No worries just switched to reserve and headed to the closest servo.... an independent. The ride there was problem free and I filled the tank with 95.... I always run 95.
    Anyways after I switched the fuel valve and got on my way I immediately noticed it running rough at low revs, kinda like it was running on less than 4 cylinders, this continued for the 20mins trip into work and when I arrived and switched the bike off I noticed a strong scent of petrol but I could see no leak. I switched the valve off and went about my day.
    Seven hours later I fired the bike back up for the return journey and noticed it was running fine........ for about 5 mins....... then it started running rough again same as before. Runs like its missing from a standing start but smooth about 3000rpm onwards. Again once I arrived home I inspected for any leaks but could see none.... both with engine running and not.

    Does anyone have any ideas on possible causes?

    It had a new fuel valve installed about 500km ago, the spark plugs are about 750km old.
    I put some fuel treatment in it this evening, will see what happens in the morning.

    The bike in question is a 1996 CBR900RR.... carbureted, manual choke etc.
  2. May have got some dodgy fuel, probably has a filter in the tank may be blocked, could even have a blocked jet in one of the carbies. Fuel treatment is as good a place to start as any
  3. It's not a servo I've been to before, I usually only go to Caltex but the circumstances this morning determined this.
  4. Since the only thing you changed recently was the fuel, my guess would be that. Might be an idea to see what happens when you fill her up next and see if the problem persists.

    If it's pretty urgent I'd drain the tank and go get some new fuel ASAP.
    • Like Like x 1
  5. Yeah I'm thinking of doing this right now actually, do a run to the servo with a jerry can. I want to ride to work tomorrow and I'd hate to cause any serious damage.
  6. Be careful there's no shit in the jerry can
  7. My guess is a fouled plug. I know they are relatively new, but the smell of petrol - with no leak. To me would indicate a plug isn't firing as often or strong as it should. At higher revs the compression increases making the mix easier to ignite which may be why above 3000RPm it's smooths out. The cause of the fouled plug of course could have been bad fuel.

    Do you often have to switch to reserve? The reason I ask is if you don't often run it on reserve often, the bottom of the tank may have had a build up of contaminants which are normally avoided when run on the "on" position ( as the "on" valve is higher than the reserve valve) contaminants usually stick around the bottom of the tank because fuel is quite light.

    • Agree Agree x 1
  8. Yeah I get you, I'll whip the plugs out and have a look this weekend.

    I've only run 3 tanks through the bike so far, each time I've run it down to reserve then re-filled it. The bike had been sitting for 3 years before I got it and I've cleaned the tank out twice now.... each time the fuel came out cleaner.

    I just whipped the tank off and emptied the fuel out, looks pretty clean coming out. I've kept some in a jar. The fuel valve was spotless and the filter has a little rubbish on it. I'll post pics shortly.
    • Like Like x 1
  9. Coincidence you write that. I filled up mine today with 7/11 ron95 and prior to that 2 days ago. I noticed today like the bake was running quite rough as well. Not at all times but there seem patches where it is just not smooth but for me it is just slightly discerning right now. My full up day tomorrow night again. Will try another servo.
  10. I just thought it was odd that it ran fine for 5 mins after it had sat for the day.
  11. If it hasn't run for years there could be rubbish throughout the fuel system that is starting to flake off and move, carbies may need a clean out
    • Agree Agree x 1
  12. I just put 10L of 95RON from Caltex in it. Ran perfectly after a few seconds, will see how it goes tomorrow.
  13. I've never had a problem with mobil's 95.
  14. Ok so I started it this morning, perfect, got to the top of the driveway and it crapped itself again. This time running worse than yesterday, won't even idle.
    Will inspect the plugs this weekend.
  15. Ok so I got the plugs out and seems 3 & 4 are having a bit of an issue.

  16. If the plugs are just the cheapies I'd replace the lot, they're probably cheaper than a tank of fuel...LOL. they all seem a bit dark. Also it might be the angle but "2" seems like a smaller gap. Hopefully she's right for the weekend. Fuel in the Jar looks OK.
  17. Yeah must be the angle, the gaps are good.
    I'm heading on holidays for a couple of weeks so I might clean out the carbys upon my return anyway.
  18. Well I cannot for the life of me remove the carbys.... I've loosened all the insulator band screws but they just won't separate from the head...... there's movement but that's it.
  19. You are getting crap under one of the float needle. sometimes you can be lucky and it will clear if you get on it a bit.

    Otherwise it's a carby bottom end clean at least.

    Put some new filters in the lines too.