Cause of throttle surge?

Discussion in 'Lawn Mowing' started by Envy Lawn Service, Sep 30, 2003.

  1. Envy Lawn Service

    Envy Lawn Service LawnSite Fanatic
    Messages: 11,087

    Hey guys, I did a search and didn't find anything. Also maybe this should be in the mechanic forum, but it will get more veiws here.

    I just want to know what causes an engines throttle to surge up and down?

    Running fine, hits thick grass, then vroom-vroom-vroom!!!

    When I stop it will settle out. :confused:
  2. Black Water

    Black Water LawnSite Senior Member
    Messages: 250

    Air filter?
  3. xpnd

    xpnd LawnSite Senior Member
    Messages: 378

    Short of choking the machine in tall grass or an old machine with worn block, surging is symptomatic of improper and varying ratios of the fuel/air mixtures. The four most common I have found are:

    The governor spring is broken or in the incorrect adjustment hole.

    The small spring that takes the play out of the throttle linkage is either broken, missing or came out of one of the holes.

    The throttle shaft itself is worn to the point that it is bouncing around in the carb body and the amount of air being sucked in between the shaft and the carb body is effecting performance.

    The carb is loose on the machine.

    I'm sure there are others but these four have always solved any surging problem I have had.
  4. Envy Lawn Service

    Envy Lawn Service LawnSite Fanatic
    Messages: 11,087


    Air filter is new and clean.

    Governor spring is there and there is only one adjustment hole.

    Small spring I have not checked, but I will.

    Too new to be worn.

    Carb is mounted tightly.

    A better discription may help some. It starts this when conditions are enough to "engage" the governor and put it to work holding RPM's. It seems how extreme it is depends on how much the governor is working if that makes sense. When I go over uncut grass after it has started it's surge, the effects get worse as the load is taken off the engine. I can turn the throttle down some and it will stop. Then I raise the throttle back up and it does not return until it's under load again.

    I'm thinking it's in the springs. Too much or not enough tension maybe.

  5. ducky1

    ducky1 LawnSite Senior Member
    Messages: 252

    Sounds like the wrong tension spring on the gov. If you have a kaw engine they seem to be a little picky about having the right spring.
  6. mh1314

    mh1314 LawnSite Member
    from Texas
    Messages: 133

    What ype mower do you have? My 2 stroke does that when I've gotten water in the gas and the 4 stroke seems to do it when the jet and carb bowl has trash in it.
  7. Rustic Goat

    Rustic Goat LawnSite Bronze Member
    Messages: 1,194

    Other possibilities;
    -Carb float problems
    -Carb problems in general
    -check all hoses & linkages for looseness, leaks, proper settings
    -choke butterfly opening/closing freely
    -kinks/sticking linkage
  8. pinnacle

    pinnacle LawnSite Senior Member
    Messages: 791

    Carb probs here.

    I would clean the carby out.................... (got a carby kit??)

    If no joy.................I would have a good look at fuel flow.
  9. John Gamba

    John Gamba LawnSite Fanatic
    from ct
    Messages: 10,812

    Bring it back to your dealer, Its a defective carb.
  10. Envy Lawn Service

    Envy Lawn Service LawnSite Fanatic
    Messages: 11,087

    John, I think you are right. Defective carb. The reason I say that is because since posting this the engine has also started to backfire when I lower the throttle. I turned the low idle screw up some and that helped.

    Also it is now spitter sputtering and blowing black smoke when I bring it from an idle to full throttle.

    As for the throttle surge, I thought maybe the governor spring had a bit too much tension on it. So I bent the tab just a little to relieve a tad. Now it only surges when mowing in the thick uphill. But it still does it and I can tell I've dropped a few RPM.

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