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Engine speed goes up and down when on less than full throttle.


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#1 Mustard Tiger ONLINE  

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Posted June 19, 2017 - 02:40 PM

Its a lawnboy gt18, so kohler magnum 18.  It was working fine and then last time I went to mow any time I dropped it below full throttle it starts revving up and down.  By the time I get to half throttle it ends up stalling out.  At full throttle it seems fine though.  The air filter is clean, but I am pretty much just randomly guessing when I figured to check that in the first place.  What should I be looking at?



#2 speedbird ONLINE  

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Posted June 19, 2017 - 04:26 PM

Try adjusting the high speed needle screw. I would start by turning out slowly (With the engine running of course) and then back in and see if the engine runs any better. It's worked for me sometimes.


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#3 mtoney OFFLINE  

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Posted June 19, 2017 - 05:25 PM

Engine surging is the signal that the carb is running lean.  So the engine is trying to find more fuel, so it surges.  Some Mag 18's have 2 adjustable jets, some have 1 and some have non.  If you cannot clean up the surge with the adjustment screws, one is high speed, one is the idle circuit if it has 2 adjustable jets.   You can look up the base line settings online in the Kohler rebuild/service manual for the engine.  You gently screw in the jet till it bottoms out, then back out the number of turns in the book.  Then you start the engine and fine tune from there.  Also check the throttle butterfly shaft for to much play as it will allow "false air" get into the mixture stream and cause a lean condition.   Sometimes a complete tear down, soaking in a can of carb stripper and a full rebuild kit is the only way to clean up the surge.  Make sure you replace the fuel filter as well and make sure the fuel pump(if it has one) is working as well.  Also make sure you gas cap is venting as if the vent hole gets plugged up, it can cause the engine to pull a vaccuum on the tank and cause all kinds of problems.    Mike


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#4 secondtry OFFLINE  

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Posted June 19, 2017 - 05:29 PM

 Sounds like a dirty low speed jet. Doesn't want to run on low speed jet so it opens throttle with throttle open it runs fine, rpm comes up and the governor closes throttle to where it doesn't fire the rpm drops the governor open the throttle again rinse and repeat. First thing you have to do is get it to idle. Some times just screwing in the idle jet while counting the turns and then returning to the original setting will do it, usually need to clean the carburetor.   Don  


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#5 secondtry OFFLINE  

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Posted June 19, 2017 - 05:31 PM

Engine surging is the signal that the carb is running lean.  So the engine is trying to find more fuel, so it surges.  Some Mag 18's have 2 adjustable jets, some have 1 and some have non.  If you cannot clean up the surge with the adjustment screws, one is high speed, one is the idle circuit if it has 2 adjustable jets.   You can look up the base line settings online in the Kohler rebuild/service manual for the engine.  You gently screw in the jet till it bottoms out, then back out the number of turns in the book.  Then you start the engine and fine tune from there.  Also check the throttle butterfly shaft for to much play as it will allow "false air" get into the mixture stream and cause a lean condition.   Sometimes a complete tear down, soaking in a can of carb stripper and a full rebuild kit is the only way to clean up the surge.  Make sure you replace the fuel filter as well and make sure the fuel pump(if it has one) is working as well.  Also make sure you gas cap is venting as if the vent hole gets plugged up, it can cause the engine to pull a vaccuum on the tank and cause all kinds of problems.    Mike

Yea what he said Don



#6 boyscout862 ONLINE  

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Posted June 19, 2017 - 05:41 PM

It is most likely a bit of dirt in the carb. The above mixture adjustments may solve it. Check the fuel tank cap vent. If it gets plugged it will cause problems. Check that the carb is tight to the manafold. Check that some crap hasn't gotten stuck in the governor linkage. The Kohler Manual should have a good troubleshooting guide in it. Good Luck, Rick
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#7 ol' stonebreaker OFFLINE  

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Posted June 19, 2017 - 08:37 PM

  Had an 8 HP Briggs doing that several yrs ago and I noticed it was also spitting gas out the carb throat. Figured out the intake valve wasn't quite closing. Fed it a heavy dose of MM oil at full throttle for about 30 sec and the problem went away.

                                          Mike


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#8 buckhvnter3 OFFLINE  

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Posted June 19, 2017 - 09:25 PM

Yea what he said Don


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