Engine rpm's going up and down when at idle

Discussion in 'Mechanic and Repair' started by ron mexico75, Dec 13, 2012.

  1. ron mexico75

    ron mexico75 LawnSite Silver Member
    Messages: 2,054

    So my Exmark metro with the Kawasaki motor has just started to not sound 100% right. Basing it off of other equipment I own that has done the same thing, Im thinking the carb might have just started to get blocked. When the enigine is just idling on low you can hear that infamous up and down revving sound. Although not real bad it just doesn't have that nice steady hum.

    I poured a good amount of seafoam in the tank. Im using the machine later today to see if that helps. My guess is that it will not so what next? Would taking the bowl off the bottom of the carb help or is that a waste of time?

    Do I need to take the entire carb off and soak it in cleaner over night? If so, do I need to order new gaskets or anything? I've never worked on this mower before so any help would be appreciated.

    At the begining of the season I started using that Star Tron enzyme fuel treatment. Maybe it was too late and I didn't notice the rpm's going up and down very slightly? Is that stuff just a gimmick or because I wasn't using it from when the mower was new I was just a little late to the party?
  2. dutch1

    dutch1 LawnSite Silver Member
    from Jayhawk
    Messages: 2,249

    Apparently you missed the "Mechanic in a Bottle" thread:hammerhead:. There isn't normally a magic elixer for crap in the carb, which I would suspect.

    Likewise there is not one magic solution, but some things you may want to consider:

    Have you tried adjusting the low speed idle needle(s)
    Idle circuit blockage--remove and clean carb
    Loose linkage at idle speed allowing surge--correct
    Governor out of adjustment--adjust
    Worn throttle shaft creating air leak--replace throttle shaft
    Dirty air filter
    Poor plug condition

    Just a few thoughts off the top of my head
  3. 44DCNF

    44DCNF LawnSite Bronze Member
    Messages: 1,827

    Seafoam has helped me in this situation a number of times.

    Try to keep the engine running from full throttle as you push the throttle into the choke position. Choke it from full throttle but only long enought to make it stumble then release choke so it will continue to run. Do this several times to increase velocity of gas through the carb jets. That and the seafoam should work to clear the restricted passage. On smaller engines with primer bulbs you would use the bulb to force additional gas through the carb durign the same move-abruptly choking from full throttle but working to keep it running.

    You may also be in need of adjusting your choke and throttle setting. Synch them per the operators manual if the seafoam does not take care of the erratic idle.
    ntlwncare likes this.
  4. ron mexico75

    ron mexico75 LawnSite Silver Member
    Messages: 2,054

    Ok I appreciate that info. Im not that mechanically inclined so I would have to get bent over and take it into the shop to have any of that stuff checked or completed besides changing the air, fuel filter or plugs.

    I'll try this today. On my exmark I have a seperate control for the choke. The throttle is just low to high. So basically put throttle on high and then pull choke but don't let it cut off. Then push choke back in so its running wide open again? Do that several times?
  5. piston slapper

    piston slapper LawnSite Platinum Member
    Messages: 4,337

    One of my favorite threads.....no doubt...

    The surging may be related to the change between the summer fuel and the winter fuel..
    They formulate the fuel you buy at the pump differently for each season..
    Unfortunately you have very few adjustments on todays carbs to compensate for these changes..
    I have 2 or 3 mowers that are not liking the latest mix...surging when warm...
    I usually just spray some good flammable brake cleaner through the carb at WOT....choking it a few times while I'm at it...to suck through any crud the cleaner loosens up ...
    Its worked everytime so far...Mechanic In A Spraycan...and a lot easier than pulling the carb.
  6. BigFish

    BigFish LawnSite Platinum Member
    Messages: 4,190

    Not to mention the carb does have an idle mixture adj.( dep. on model) with limiter.
    OK, I just read Dutch1's reply........he mentions the idle mixture screw/s.
  7. ron mexico75

    ron mexico75 LawnSite Silver Member
    Messages: 2,054

    Well, I took the fuel tank off, swished around some gas, dumped it in a catch container. I put some rags and paper towels in and moved them around with a piece of wood to collect all the loose debris.

    I changed the fuel filter too. I put in a little gas and a few ounces of that star tron enzyme tretment and let it run at WOT for about 30 minutes. I then moved the throttle down to low for several minutes and it idled fine, no surging. The fuel filter was pretty nasty so maybe it was that or the carb had a little gunk....who knows. Im happy that's all it took and its running fine now.

Share This Page