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Do Single Cylinder Road-Trail Bikes Lurch?

Discussion in 'Technical and Troubleshooting Torque' started by matt232, Dec 4, 2007.

  1. G'Day all,

    Executive Summary:

    Question for single cyclinder road trail bikes. Do your bikes "lurch" when travelling at any speed with a neutral throttle? Or do I need to start tracking down a cause for this?

    Long Winded Read:

    As some my have gathered I went and got a 1999 Honda NX650 Dominator, single cylinder 4stroke dual sport beastie. The first weekend I took it up to the Snowies and back again with the Musketeer ride, a 1600+ km thrashing, without finding much wrong with it other than a lack of power trying to get over bigger and more sporty road bikes. This I should be able to sort out with a little practice, I hope.

    However since I've been riding around the city and down freeways, I've noticed that there is a bit of a lurching motion when the bike is at neutral throttle. For someone who prides them self on being a smooth rider this is a little off putting and is taking a bit of confidence away. When I say lurching I mean that it feels like very brief drops in power and a regular return to power (it almost feels kind of like I've got a buckled wheel and its occasionally rubbed on the swing arm but the lurching doesn't seem to change rythym with change of speed). Its not terribly pronounced but it seems to produce a rocking motion in my upper body which perhaps makes me look like I've mentally lost it (this is debatable).

    Admittedly I have previously only ridden 4 cylinder 4 strokes, most recently the 2004 CBR1000RR Fire Blade which you'd expect to be smooth as silk. So I could be a little spoilt and just need to learn the new way.

    In the process to fix this I've had the chain and sprockets replaced which I think has made a slight improvement but its still definitely there. When I returned to point out that the $400 spent hadn't fixed the problem the mechanic took it for a test ride, he came back looked at my funny and said it was the smoothest single he had ridden.

    I just don't see how this could be normal, then again perhaps I just need to get used to it. So do these types of bikes have a slight lurching motion when at neutral throttle at pretty much any speed or do I have a problem I need to track down?

  2. I should also mention that when the bike is accelerating I don't feel the the lurching motion.
  3. My WR400 doesnt lurch at all.
  4. If it's not just huffing and puffing smoothly then you'd have to look at jetting, carby floats or air leaks as a start.

    My TTR occasionally does this as it's starting to run perilously low on fuel but that's the only time.
  5. Pretty common. I'd look at jetting 1st. Well, to be honest, I'd have someone else look at the jetting, I'm rubbish with it.
  6. It's more frequently called "hunting".

    It is frequently apparent on very light maintenance throttle and will disappear as soon as you accelerate a little.

    I doubt that it is apparent at 110kmh, but I could be wrong.

    You might Google the bike and the apparent fault, using "hunting" rather than lurching as the keyword.

    Sometimes it can be due to drive train slop or backlash, or possibly worn cush drive components in a rear wheel.

    A more common cause is carburation which is too rich on the pilot jet. The pilot jet actually works around 1/8th throttle, not just at idle. The idle mixture screw affects the mixture at idle, but not at the 1/8th point where the size of the pilot jet itself is the main fuel metering device.

    As an aside I have a Moto Guzzi 650 with dellorto carbs which have a serious design fault - at one particular throttle opening the amount of fuel drawn through the pilot jet is so much that the bike will "lurch" or kangaroo almost uncontrollably. Opening or closing the throttle just a fraction sees the problem go, but it is easier to change to a higher gear and load the engine a little more to avoid it. Fortunately you should have a keihin carb without this particular problem...

    Another cause of richness is worn needle jet and jet needle combination. This will frequently see 8 stroking at around town speeds. These components can be seriously worn from 20,000km on with a large cylinder such as you have.

    Does the jet needle have slots and a circlip which sets the height of the needle? If so (and japanese carbs have frequently taken away this adjustment recently) it is easy to raise the circlip (which lowers the needle in the slide). This will give you temporarily leaner carburation at 1/4 to 3/4 throttle opening - if it reduces the problem you know you are on the right track. Actually, if the jet needle and needle jets are worn the richness will show up at even lower throttle openings...

    Hope this gives some help if you cannot see a pattern elsewhere on the internet.


    Trevor G

    PS Have you checked the airfilter - is it clean? If badly blocked it will also cause richness...

    Does the bike have an aftermarket exhaust? Is the airfilter standard?
  7. Thanks guys, especially Trevor, a fair bit to check out.

    The bike is sitting on 22000km or there abouts, has reportedly been left sitting in garages in QLD and Vic not doing much for long periods of time and I'm guessing was not prepared for that idle time. The good thrashing heading to the snowies and back seemed to make sure everything was well cleaned out.

    I've not seen the jets, however I have a new tank coming for the bike in the next week or two and will have that replaced so that should be a good chance to check it out.

    The bike was just serviced at 20000km and I assume that the air filters were checked but I'll eyeball it myself. As for aftermarket exhaust, nope stock.

    We'll I'm off to got hunting for "hunting".
  8. searching for "dominator hunting" gives....mixed results. Time for a new keyword me thinks.
  9. My MuZ 660cc Single Skoprion Traveller has not done this.

    Unless you are talking about chain snatching which most singles will do depending on the gear you are in compared to engine revs.
  10. I had a similar problem with the 9R when I rode it without the ram air intakes for the first time. It was an issue with the balancing of air pressure between the airbox and the caruburettor float bowls.

    So... Yeah, it's an air/fuel issue. To start with, it might be worth going a half a can of carby cleaner down the butterfly, just in case there's some goop or gunk in there. Beyond that you might need to get it tweaked by somebody who knows how.
  11. I would try the easiest thing first. As Trev sugggested, try raising the needle in the slide -if it has a circlip, move it toward the bottom of the needle. If not, a small washer will work. See if that makes it better or worse. I suspect it could be running a bit lean. Only a guess though.
  12. Had a GSX750E that had pods fitted. It hunted like all crazy at cruising speeds. Changed it back to the orginal airbox...and all was good.
  13. To be fair, carby overhaul wouldn't have been part of his service schedule and he wouldn't have sat on a freeway long enough.

    Ask him how much, or as Loz said try carby cleaner or buy the kit yourself and a sheet of newspaper and give it a go.

    All the best and let us know how it works out.