Engine surge when not under load

wsmm · Sep 3, 2012

Briggs and Stratton single cylinder 12hp. This unit is on a lawn tractor, when just riding around in any gear and at almost any speed the engine speeds up and down (surging). Once I engage the blades on the deck the engine smooths right out and runs at a steady speed. I've tried just about everything I can think of with no results. Any help or suggestions will be appreciated.
Thanks
Bill

joe csuti · Sep 3, 2012

The most probable cause is plugged/restricted idle passages in carb body due to dirt/gum.

Remove carb from eng,disassemble carb,remove hi/low speed needles,main jet nozzle(if removable)and remove any welch plugs in carb body(if used)soak carb body & all parts in carb cleaner overnight,run a fine wire thru ALL holes/passages in carb body & blow out with compressed air,if carb body has welch plugs & same were removed you'll probably have to buy a rebuild kit for carb,as plugs are usually damaged/destroyed removing them.

You might try a quick & dirty fix 1st before removing/dis-assembling carb,goto a local auto parts store/walmart & get a can of seafoam carb cleaner,add 1-2 oz to gas tank & 3-4 oz to gas container,run several tanks of gas thru eng & see if that cures problem,if not dis-assemble carb,clean & rebuild or replace carb.

Many times seafoam works,sometimes it doesn't.

ShadetreeRet · Sep 3, 2012

First, follow Joe's advice. That should work. However, occasionally, the governor will get out of adjustment and cause one to surge. May want to check this out also.

Anonymous-0 · Sep 3, 2012

Just want to add: If the Briggs engine has a Walbro carburetor, It probably had an idle mix needle on it. It probably has a plastic cover over the screw. Try turning it as far counterclockwise as possible. The black cap will stop it at some point.

dbrojr · Sep 3, 2012

check the carb flange mount bolts,they may be loose and allowing an air leak...happens with my onan twin a lot...makes it surge.

old · Sep 3, 2012

If it has a choke on it and choking it helps then you have a lean running carb and by loading it down with the blades it opens the throttle up wide so has more vacuum to suck more fuel in

dhermesc · Sep 4, 2012

That would be my first attempt also. It worked on my Wards 2000 that surged like that for three years and no other cleaner would do a bit of good. Added about 4X the stated amount and ran it hard for several hours. Started making that part of my opening season breakin for all my small engines after that.

Most likely it is a partially clogged passage in the carb.

If it operates correctly under different types of load (light load, medium load and heavy load) but only has an issue at no load it is most likely not the governor.

greygoat · Sep 8, 2012

Engageing the blade/blades gives a Flywheel
effect, and helps smooth out the engine. I have
the same condition with a Snapper, rear engine
rider..10 years old..it always smooth"s out when
working! (12 Hp B&S engine)

Engine surge when not under load

wsmm

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joe csuti

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ShadetreeRet

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Anonymous-0

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dbrojr

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old

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dhermesc

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greygoat

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