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Thread: Idling @ 8000 rpm

  1. #1
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    Idling @ 8000 rpm

    Hi, some of you might remember my previous posts about the partial rebuild I've done of my GSF250 Bandit. The good news is that I fired it up the other day and it starts easily (far better than before the rebuild) and ticks over smoothly. The bad news is that it is idling at 8000rpm!

    Some other KBers have very kindly given some advice about this already, but we're still a bit stuck so I thought I'd see if anyone had experienced this issue before.

    The first thing I thought was idle speed screw - I had removed and refitted this and possibly got it wrong. So I adjusted the idle screw so far that it was no longer opening the throttle at all, this had no effect.

    Next I loosened the throttle off completely so that the throttle was completely shut when closed - there is even slack in the cable confirming that the cable isn't stuck somewhere or has been incorrectly routed. No difference.

    Also checked that the slides move correctly and throttle butterflies open/close correctly.

    BTW I have been firing it up then killing it after just a few seconds so can't say what happens if it is allowed to run. Not keen on thrashing the nuts off a cold engine that has just been rebuilt.

    Apparently an air leak can cause a high idle, but I would have thought that firstly, it's unlikely that I would get a leak in more than one carb and secondly, assuming that only one carb was faulty, I wouldn't think that the engine would run at a smooth, high idle. I would have thought it would be a bit rough?

    Now if it is an air leak, trying to find it could prove to be a bit of a bitch - can anyone recommend a good method? I've been advised that an ether spray can be used to find leaks. Sounds good but as mentioned above I don't want to leave the engine running, I need something I can try with the engine off.

    Having said that, am I barking up the wrong tree thinking air leak? Maybe it's something else completely?

  2. #2
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    It's almost impossible totell with them fitted, but are the carb slides in backwards? I know Drew's discussed the possibility but to me, that's the most likely culpret.
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  3. #3
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    Quote Originally Posted by White trash View Post
    It's almost impossible totell with them fitted, but are the carb slides in backwards? I know Drew's discussed the possibility but to me, that's the most likely culpret.
    Hi thanks for the reply. Yeah I did have a chat with Drew earlier and he mentioned the slides, we checked that they were dropping back down into place properly. Bit confused about how they could be in backwards though. Am I right in thinking that the slides are the bits I've highlighted in red on the attached parts diagram? If so as far as I can tell they can only fit in one way? Wouldn't take too long to have a look I guess, just couldn't figure it out from memory of what the carbs look like inside and the parts diag. Cheers.

    Click image for larger version. 

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  4. #4
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    Hi all

    Just a funny story I thought id add, when I was young and dumb ( still might be) I did the carb on a bike and it wouldnt start so I push started it down my drive way and it started right up on full power !! Threw me right off the back and ended with wrecked skin and a bike lying there doing 10,000 rpm, morale of the story for me was to make sure the carb needle was in the hole!! Ahh memories

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    Quote Originally Posted by cbfb View Post
    Am I right in thinking that the slides are the bits I've highlighted in red on the attached parts diagram?

    Click image for larger version. 

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    Yep, them's the slides.

    I've just had another thought, did you put the needles back in the slides in the right order of clips, washers, and spacers? If you had them out of course.

    To start it up and leave it running, drain the coolant and pour in hot water. With the wires off the coil turn it over for a little while to pump the oil to the top and circulate the water a bit. Then fire it up.

    Those motors are screamers so it wont harm it to have it running that high for a couple minutes while you try a few things. Give it a blip on the throttle and see how quickly the revs drop back to 8000.

    Also, with the butterflies totally closed like they are with the idle screw out, it souldn't run at all for a near total lack of air. I think no matter what the issue is, the mounting blocks/rubbers that the carbs clamp into need to come off again and be sealed.

  6. #6
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    Quote Originally Posted by NAR RG500 View Post
    Hi all

    Just a funny story I thought id add, when I was young and dumb ( still might be) I did the carb on a bike and it wouldnt start so I push started it down my drive way and it started right up on full power !! Threw me right off the back and ended with wrecked skin and a bike lying there doing 10,000 rpm, morale of the story for me was to make sure the carb needle was in the hole!! Ahh memories
    Ouch!

    Quote Originally Posted by Drew View Post
    I've just had another thought, did you put the needles back in the slides in the right order of clips, washers, and spacers? If you had them out of course.
    I was pretty methodical when I had them apart, lining the parts up on the bench in the right order and putting them back in reverse, so I would be suprised if I messed it up.

    Quote Originally Posted by Drew View Post
    To start it up and leave it running, drain the coolant and pour in hot water. With the wires off the coil turn it over for a little while to pump the oil to the top and circulate the water a bit. Then fire it up. Those motors are screamers so it wont harm it to have it running that high for a couple minutes while you try a few things. Give it a blip on the throttle and see how quickly the revs drop back to 8000.
    Useful tip! Might have to do that, but your last paragraph is probably what I'll try first...

    Quote Originally Posted by Drew View Post
    Also, with the butterflies totally closed like they are with the idle screw out, it souldn't run at all for a near total lack of air. I think no matter what the issue is, the mounting blocks/rubbers that the carbs clamp into need to come off again and be sealed.
    I think you're right, there should be bugger all air getting in there with the slides down and the throttles fully shut. I will pull the carbs off and remount them. Did you mention using gasket sealant or something on them yesterday?

    Also can I ask a slightly embarassing question - I am pretty sure about this but sometimes the dumbest things can turn out to be the problem... On the picture below, the hoses I have highlighted don't appear to go anywhere, I assume that's OK?

    Click image for larger version. 

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  7. #7
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    Based on their position, I would have said those are your incoming fuel hoses, from the tank tap....or some sort of rebreather from the rocker cover.
    Do you realise how many holes there could be if people would just take the time to take the dirt out of them?

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    Quote Originally Posted by MSTRS View Post
    Based on their position, I would have said those are your incoming fuel hoses, from the tank tap....or some sort of rebreather from the rocker cover.
    Vaccum hoses ... ???
    When life throws you a curve ... Lean into it ...

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    Usually only one of those. And a slightly smaller gauge than the fuel hoses.
    Do you realise how many holes there could be if people would just take the time to take the dirt out of them?

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    Quote Originally Posted by MSTRS View Post
    Based on their position, I would have said those are your incoming fuel hoses, from the tank tap....or some sort of rebreather from the rocker cover.
    Maybe I've got them confused on the diagram... but the fuel and vacuum hoses are quite distinctive so I'm confident they're correct. And the breathers coming off the rocker cover are much bigger and short so they can only fit in one place.

    These hoses are quite long (a perfect length for sitting at the front of the subframe right where the airbox oil drain hose sits) and show no sign of stretching at the end and no sign of hose clips, so I doubt they've been connected to anything. Also I made notes of where everything went when I removed the carbs and these weren't mentioned, so I'm assuming they're some sort of air vent hose that just stays open? I ruled out overflow because the overflow pipes clearly come off the bottom of the float chamber. Unfortunately I've only seen a limited number of carbs and can't remember seeing these before.

  11. #11
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    Quote Originally Posted by FJRider View Post
    Vaccum hoses ... ???
    Quote Originally Posted by MSTRS View Post
    Usually only one of those. And a slightly smaller gauge than the fuel hoses.
    Yep there is a vacuum hose and a fuel hose (the fuel hose as you say being larger diameter). But it's obvious where these come from and go to. Also the diameter of these hoses match the fuel tap connectors. The mystery hoses are too wide to go on the vacuum hose connector.

  12. #12
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    Best be id-ing what they do then.
    Carbs only need fuel and airflow. The fuel is supplied to the bowl/s to feed the various jets inside, and the air comes in through the throat/s attached to the airfilter box.
    Do you realise how many holes there could be if people would just take the time to take the dirt out of them?

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    Quote Originally Posted by MSTRS View Post
    Best be id-ing what they do then.
    Carbs only need fuel and airflow. The fuel is supplied to the bowl/s to feed the various jets inside, and the air comes in through the throat/s attached to the airfilter box.
    Yeah I agree, trying to do just that. Of course things are complicated a bit by rebreathers and fuel overflow but nah I can't figure what the hell these random hoses are for. Will do more Googling...

  14. #14
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    Quote Originally Posted by cbfb View Post
    Yeah I agree, trying to do just that. Of course things are complicated a bit by rebreathers and fuel overflow but nah I can't figure what the hell these random hoses are for. Will do more Googling...
    I've got two breather hoses similar off the carbs on the GSX. They go nowhere. Never have, never will. Tried blanking them off as I thought they should be capped but the bike ran worse. I can only assume their something to do with the CV carbs.
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    Quote Originally Posted by White trash View Post
    I've got two breather hoses similar off the carbs on the GSX. They go nowhere. Never have, never will. Tried blanking them off as I thought they should be capped but the bike ran worse. I can only assume their something to do with the CV carbs.
    Thanks you just answered my next question! Reading up on the interweb I have discovered that these buggers seem to be air vent hoses. A good description of their purpose that I found:

    That is what makes carbs work. That hose provides the atmospheric pressure reference signal to the fuel in the float chamber. The difference in pressure between atmospheric and the lower pressure of the air flowing thru the carb moves the fuel from the float chamber to the air stream where it accelerates, bursts into a fine spray, mixes with air and vaporizes so it can burn in the combustion chamber. For an experiment, put your finger over the end of the hose and see what happens. Nothing bad, just try it.
    So I was gonna ask if anyone had tried blocking these off, expecting the engine to run badly or idle - which you've just confirmed. Apparently it's a problem with dirt bikes going through deep water.

    So looks like that was a bit of a red herring, but at least we've ruled it out.

    Going back to thinking about the inlet manifold now....?

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