Briggs and Stratton RPM Surging up & down

LawnmowerBillyMay 8, 2012

I have a 25hp. OHV v Twin engine in my riding mower. It has had a surging problem since I purchased it and it is getting worse. I have done some reading on it and Briggs says it is a bit normal and that it should have gone away after several tanks of fuel or full throttle operation; it's 5 years later and I can't stand it any more.

Does anyone know if this is a fixable problem and where should I start looking, there are no vacuum leaks and carb cleaner as an additive does nothing. Is a carb overhaul the place to start or governor adjustments, there seems to be a lot of this surging problem on the net but no definitive answer. HELP!

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exmar

This may be "apples and oranges," but here's some info.

I have a 7.5 HP Briggs on a rear tine tiller, from it's second season on, it surged and hunted badly. Played with governor settings, there are no carb adjustments, nothing helped. Finally with info obtained here, I partially disassembled the "non serviceable" carb and sprayed it very well with carb cleaner, did that a couple of times. Totally drained the tank and refilled with new gas which had Stabil added, also some carb and fuel injector cleaner. Couple of tanks later the problem had essentially disappeared. This pretty much proved to me that my issue was caused by the "mystery fuel" available at the pumps today. Now I'm rabid about fuel conditioners. Stabil or equivalent goes into the empty can before it goes in the back of my truck to make a fuel run. I've actually bought a shut off valve for that tiller so I can run it dry after use, just haven't got around to installing it yet.

I'd look into what parts you get in the carb rebuild kit, you may have to replace the carb, as the issue is probably in the jets. I know Sears now automatically replaces those carbs as there isn't a lot you can do to them.

Just my two cents,

Good luck,

Ev

    Bookmark   May 8, 2012 at 9:06AM
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romore_gw

Hold the throttle shaft steady with your finger, if it smooths out and runs steady the governor is too sensitive. If it stumbles and fights you have a carburetor issue or intake air leak.
Spray carb cleaner around the base of the carb and the manifold gaskets on the head while the engine is running, an air leak should show up.

    Bookmark   May 8, 2012 at 9:46AM
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grass1950

My surging problem was cured by changing the fuel filter.

    Bookmark   May 8, 2012 at 11:29AM
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baymee(LehighValleyPA)

Regarding carburation in general, usually this is a fuel starvation issue. In almost all of these cases, I've soaked the carbs in NAPA carb cleaner and they end up working great. Disassembly is necessary, along with an air gun, and removal of welch plugs. Sometimes you can do it without removing all the welch plugs.

    Bookmark   May 8, 2012 at 4:03PM
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tomplum

Lot of ifs. Model and spec?

    Bookmark   May 8, 2012 at 10:28PM
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exmar

Kudos to romore, I forgot to mention that I did that. The engine didn't settle down as it normally would when you hold the throttle.

Ev

    Bookmark   May 9, 2012 at 10:46AM
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wertach zone 7-B SC

"Spray carb cleaner around the base of the carb and the manifold gaskets on the head while the engine is running, an air leak should show up.

Isn't that kind of dangerous? I have never saw nonflammable carb cleaner. I use a spray bottle with water and a little soap to do that.

    Bookmark   May 10, 2012 at 1:36PM
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mownie(7)

***"I use a spray bottle with water and a little soap to do that."***
Please tell us what kind of results you are anticipating by spraying a non flammable liquid onto a seam suspected of having a VACUUM leak.

    Bookmark   May 11, 2012 at 12:38AM
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wertach zone 7-B SC

"Please tell us what kind of results you are anticipating by spraying a non flammable liquid onto a seam suspected of having a VACUUM leak."

The soapy water will temporarily stop the leak.

    Bookmark   May 13, 2012 at 12:08PM
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gewf631(Chicago NW Burb)

Soapy water for expected PRESSURE leaks - makes bubbles

Never heard of using that for a vacuum leak. Have always used spray carb cleaner - far easier to hear the sudden increase in RPM's, than a slightly better running engine when the leak is plugged temporarily.

    Bookmark   May 14, 2012 at 9:10AM
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mownie(7)

***"far easier to hear the sudden increase in RPM's, than a slightly better running engine when the leak is plugged temporarily."***
Hear Hear!

    Bookmark   May 14, 2012 at 10:06AM
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wertach zone 7-B SC

I guess you have never had your eyebrows burned off? I have, by someone unexpectedly spraying carb cleaner on a hot motor! Sorry but I believe in safety first!

What if your advice caused the OP to get hurt or worse? Would you feel bad? Advising someone to do something that dangerous is not good.

    Bookmark   May 14, 2012 at 4:33PM
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bill_kapaun

How much alcohol is in your fuel "up there"?
IF it's more than 10%, you may need a bigger Main Jet(s) in the carb.

Engine Model & type#'s are handy to know so one can see exactly which carb you have etc.

    Bookmark   May 14, 2012 at 5:07PM
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KubotaMaster(7A-Arkansas)

Must be using exxon gas that crap has a bunch of water in it. I filled 10 1G jugs and got 2G of water out of them.

    Bookmark   May 20, 2012 at 7:00AM
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KubotaMaster(7A-Arkansas)

Must be using exxon gas that crap has a bunch of water in it. I filled 10 1G jugs and got 2G of water out of them.

    Bookmark   May 20, 2012 at 7:19AM
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bill_kapaun

It's actually difficult to find water in the gas when freshly purchased. The alcohol absorbs it.

Just sounds like some kind of rant from a BRAND NEW POSTER!

    Bookmark   May 20, 2012 at 1:33PM
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