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Thread: Engine Fluttering Sound

  1. #11
    Senior Member Jt105's Avatar
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    The valve check interval is 16k miles, so that service is due.
    Overheating can be caused by low coolant level, stuck thermostat or bad water pump.
    There has been a couple of cases where the timing chain jumped a tooth due to a bad cam chain tensioner. If that happened then the engine wouldn’t run quite right.
    By checking the valves, you can verify the timing, set the valve clearances and flush the coolant. That hits all the main mechanical reasons for odd engine performance.

    JT

  2. #12
    Super Moderator Engine Fluttering Sound 670cc's Avatar
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    Well, you can answer the valve question by simply doing a valve check/adjustment, so that’s easy.

    I’m really puzzled about the overheating. Besides the cooling system causes that Jt105 mentioned, another thought I had is that engines burning lean mixtures run hotter. The lean burn would also explain the fluttering. But could possibly it run lean enough to overheat? That, I don’t know. What you could do is ride the bike awhile with the engine in the fluttering behavior, then stop, and pull the spark plugs and read their color. Are they stark white, like a lean burn? Are they both the same? They is a separate fuel injector for each cylinder.

    Have you taken note if the gas mileage has changed since this problem arose?

    Back on the idea about poor fuel delivery, it might also be worth the effort to measure the fuel rail pressure.
    Last edited by 670cc; 22nd November 2018 at 11:39.
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  3. #13
    Senior Member Krampus's Avatar
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    Quote Originally Posted by 1wiseguy2 View Post
    Does it do it when you roll on the throttle quickly or just any acceleration under 3000? What if you are above 3000 rpm and roll on the throttle hard/quickly?
    So what would this above/below 3000 RPM condition be indicative of then?

  4. #14
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    There's a cooling fan behind the radiator. If the coolant goes above a certain temperature, the fan should start running and keep the engine from overheating. Does the fan start running?

  5. #15
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    Quote Originally Posted by Krampus View Post
    So what would this above/below 3000 RPM condition be indicative of then?
    In carbs it may mean the primary jet is clogged. In Fuel Injection I am not sure. If it was a filter then it would definitely bog down above 3000 as it would need more gas than at a lower rpm. Injectors just change the stroke length of the injector to change the amount of fuel injected. At least on diesels. What did your plugs look like when you pulled them out? Did they look white like you were burning lean? Check your valves as you are over the interval. Does it do it before it warms up to operating temperature? Injection does put more fuel in when cold. Is it colder where you are right now? Does it take some time to warm all the way up? If not then it could be an electrical device that needs to get hot before it acts up or when the engine gets warm the fuel system lowers the rich fuel mixture when hot and a weak fuel pump then reveals itself? Ask your dealer mechanic if they will put a fuel pressure gauge on it and let you go for a ride until it starts doing it. If that is ok then I would be suspicious of a coil and those can be tested too.

    Sent from my SM-N960U using Tapatalk
    Last edited by 1wiseguy2; 23rd November 2018 at 05:13.

  6. #16
    Commuter Extraordinaire Engine Fluttering Sound
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    Quote Originally Posted by b_rubenstein View Post
    There's a cooling fan behind the radiator. If the coolant goes above a certain temperature, the fan should start running and keep the engine from overheating. Does the fan start running?
    The fan is working. I checked that after it overheated the first time.

  7. #17
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    Quote Originally Posted by 1wiseguy2 View Post
    In carbs it may mean the primary jet is clogged. In Fuel Injection I am not sure. If it was a filter then it would definitely bog down above 3000 as it would need more gas than at a lower rpm. Injectors just change the stroke length of the injector to change the amount of fuel injected. At least on diesels. I would pull the plugs to see if burning lean as suggested earlier. Then adjust your valves while you are there behind the radiator. Very interesting that it has to warm up and be operating at temperature. Injection does put more fuel in when cold. Is it colder where you are right now? Does it take 30 miles for it to warm all the way up? If not then it could be an electrical device that needs to get hot before it acts up or when the engine gets warm the fuel system lowers the rich fuel mixture when hot and a weak fuel pump then reveals itself. Pull the plugs. Ask your dealer mechanic if they will put a fuel pressure gauge on it and let you go for a ride until it starts doing it. If that is ok then I would be suspicious of a coil and those can be tested too.

    Sent from my SM-N960U using Tapatalk
    I think I still have the plugs I replaced and both plugs looked identical. What I mean is, one did not look like it had the side affects of a lean running system when the other looked normal. To me, nothing looked out of the ordinary on the original plugs. The symptoms occur immediately. Doesn't matter if the bike is hot or cold. I think the fluttering issues is still present above 3000 rpm but it is much less noticeable and acceleration is better.

    There has been mention of loose grounds and such. I plan to look at those types of things today along with air filter since I'll be tearing into things.

  8. #18
    Senior Member Griff's Avatar
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    My somewhat amateur thinking would also be pointing to a coil problem as is mentioned above. The misfire under load/acceleration has me thinking in that direction. However I have also seen poor running on my former KTM because of a clogged fuel injector. I have also previously seen fuel injector problems on here and that might turn up with a search.

    I would doubt if it is a valve clearance problem and my thinking there is that the bike would not tickover properly due to poor compression becoming more of a factor at such low revs.

    One other aspect to look at is tank venting.
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  9. #19
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    When it starts doing it pop the tank lid off and see if it continues. That will tell you if it is a venting issue.

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  10. #20
    Senior Member Krampus's Avatar
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    Quote Originally Posted by 1wiseguy2 View Post
    In carbs it may mean the primary jet is clogged. In Fuel Injection I am not sure. If it was a filter then it would definitely bog down above 3000 as it would need more gas than at a lower rpm. Injectors just change the stroke length of the injector to change the amount of fuel injected. At least on diesels.
    Quote Originally Posted by Krampus View Post
    So what would this above/below 3000 RPM condition be indicative of then?

    Well, the NC is a FI engine. The injectors are electrically controlled; control unit determines how long they stay open with input from the O2 sensor. Never seen reference to stroke length on an injector for a gasoline engine.

    As to this RPM versus fuel requirement thing... RPM has so little to do with it. It is a function of throttle position. 3000RPM on a NC is much a sweet spot for fuel consumption; you use more accelerating hard up to 3000 RPM.

    You're not on the main jet of a carb until 3/4 WOT+. Attaching two diagrams to help those who missed out on the joys of carb tuning understand.

    Engine Fluttering Sound-img_how_to_remove_the_throttle_cable_from_the_carburetor_5917_600-jpgEngine Fluttering Sound-jet2-jpg
    Last edited by Krampus; 24th November 2018 at 16:45.

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