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Surging engine

Skysoldier's Avatar

Join Date: Dec 2004
Posts: 86

05-19-07, 06:29 PM   #1  
Surging engine

Unit is a Murray 4hp, 22" mower. I bought it at closeout from Home Depot, no owner manual.
Suddenly engine wants to change speed continually. It will crank up, run fine for a brief period, then suddenly slows, and begins surging. The engine will begin to accelerate, then rpm's drop, it will stay slow for whatever period, then again accelerate. It contines that cycle, then seems to run fine for a while, and then it start the cycle all over.
It will still cut, just have to go at it a little slower.
It shows some black smoke out of exhaust, but nothing I would call voluminous.
ANy ideas?
Thanks in advance fo help

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Bigwind's Avatar

Join Date: Nov 2004
Posts: 167

05-20-07, 12:35 AM   #2  
FIRST: IS the unit under warranty? If so I suggest you take it back for servicing! A closeout sale should have no impact on the warranty but if it's not covered or is already out of warranty, WHILE YOU HAVE OWNED IT then here's a few thoughts:

Surging is usually caused by a lean condition which is caused by the carburetor! You may need to remove and clean your carburetor (Be sure you don't bend any of the linkage to the governor and if your not familiar with doing this I suggest you draw the linkage or take a picture before you remove it...it will keep you from swearing when it's time to replace it!)

Without knowing what engine you have I can't say exactly where things are located but here is two things I would try first: Locate the screw on the side of the carburetor, it has a spring between the head of the screw and the carburetor (Not the idle screw that serves as a stop on the throttle) and turn it out/left at 1/4 turn increments while the engine is running. If it smooths out great! If not return the screw to it's original position and lets clean the main jet. Again I'm assuming the type carb but I would remove the bowl under the carb and look carefully at the screw / bolt that was holding it on. It may have a hole horizontally through the bolt and quite possibly a hole from the tip down through the bolt and dying into the horizontal passage. Spraying it with carb cleaner will sometimes get the gunk out? If not take a metal tie wrap and remove the paper or plastic covering and use that small wire to push any foreign matter out of those holes. Easy does it you don't want to damage/enlarge the passages)If there is a blockage the engine is usually hard to start but it can sometimes cause a lean condition as well.

If this doesn't work I would remove the carb and rebuild it. This isn't a major job and a carb kit is fairly inexpensive. If it comes to this then I would suggest posting back and someone can walk you through the procedure. Good luck, Ron

cheese's Avatar
Forum Topic Moderator

Join Date: Jul 2001
Posts: 16,573

05-20-07, 01:30 AM   #3  
Bigwind has you on the right track, however, I think this engine might be the classic design, and I would start by cleaning out the foam air filter element if so. The black smoke is what makes me think it's clogged/dirty. If not, proceed with the instructions in the prior post.

"Who is John Galt?" - Ayn Rand (Atlas Shrugged)

God bless!

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