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post #1 of 12 Unread 11-08-2019, 01:31 PM Thread Starter
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What causes hunting-searching RPM on Briggs 12.5HP?

So I got my Snapper up and running again after another carburetor and fuel filter change. It's got a 286707 Briggs 12.5HP I/C and originally had the LMT carburetor. It runs pretty decent and you can mow the whole yard without cutting off. Start up pretty easy. However you can still hear while driving around or mowing the engine searching for a steady RPM. It fluctuates up and down every couple seconds. Not enough to cause any issues such as cutting off. It's just enough to know there is still something wrong. I've changed the carburetor a few times even back to the original Briggs LMT. It doesn't matter what carb is on there it still does it. I got the mixture right on, the idle at 1200RPM and high speed idle match the engine at 3000-3100. Did a governor adjustment but it was right on too, I think it was 1750RPM. I rechecked the ignition coil gap, checked the flywheel key. Adjusted the throttle lever control. I'm just wondering what else could cause this issue. I notice there's a guy on youtube that did a video that was having the same problem and as trying to find solutions. Could it be there is a burnt exhaust valve or cam problem? It doesn't smoke though. I know there governor bracket rack and pinion control arm appears to have surface rust but it moves freely. Anything else I could check?

Snapper 3312X6S w/ 12.5HP Briggs engine
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post #2 of 12 Unread 11-08-2019, 01:52 PM
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Re: What causes hunting-searching RPM on Briggs 12.5HP?

Hunting is usually caused by a lean fuel condition. (too much air and not enough fuel) Clean all the fuel filters or replace and make sure their is not a screen in the fuel tank restricted.

It can also be caused by a incorrectly adjusted governor static adjustment or a governor spring weak. Try holding the governor rod at a steady position by hand to see if the engine still hunts or does not idle smooth.

First try removing the breather and using your hand over the intake so as to add some choke. If it smooths out you know it's running lean.
If it smooths out somewhat applying choke and you have tried different carbs you need to check the plastic intake area gaskets and the plastic intake itself closely. (both where the carb mounts to the intake and where the intake mounts to the engine.

Of course it's possible that you have two carbs that needs a good cleaning, but not as likely. You can get a cloned LMT carb for about $15 from flea bay.

Adding some choke at no load and while mowing under load will give you a hint if it's running lean and causing the governor hunting.
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post #3 of 12 Unread 11-08-2019, 02:13 PM
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Re: What causes hunting-searching RPM on Briggs 12.5HP?

Idle circuit is plugged,


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post #4 of 12 Unread 11-08-2019, 05:08 PM
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Re: What causes hunting-searching RPM on Briggs 12.5HP?

Quote:
Originally Posted by grassmech;12371601[B
]Idle circuit is plugged,
[/B]

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A hint of slow carb idle ckt plugged or restricted (or a air leak at the manifold) is the engine will not slow idle smooth or at all.
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post #5 of 12 Unread 11-08-2019, 07:44 PM
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Re: What causes hunting-searching RPM on Briggs 12.5HP?

If that LMT carburetor doesn't have a fuel/air mixture screw, I'd say it's running lean, which new(er) carbs do to help the manufacturer pass EPA laws.

I have a 2012/13 year Craftsman 8.75hp push mower and a '03 Craftsman with 24hp motor that doesn't have a carb mixture screw and they hunt for the right fuel ratio all day long. Using better octane gas helps, but doesn't cure the problem of carbs being built using leaner mixture jets.

Have a 1990 ONAN powered mower with carb mixture screw that I can adjust where engine smokes like a diesel, or lean it out enough that it too will hunt constantly from not getting enough fuel. Of course engine hunts when hitting heavy grass or weeds when mowing, so that's normal. It's telling me the governor is working correctly if it corrects itself quickly and doesn't do hunting, constantly.

There is ways to overcome a lean carb condition by a drill press, steady hand, needle nose vice grip, and some very small millimeter drill bits, but I can't say how, or what to drill out, to allow more gas through the carb and into firing chamber. But, if one has ever replaced a car/truck carburetor with a Holley carb that uses replaceable jets, probably know what I mean when fine tuning a carb to engine.

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post #6 of 12 Unread 11-09-2019, 12:47 PM
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Re: What causes hunting-searching RPM on Briggs 12.5HP?

I've read in a book I have about making your own alcohol fuels,that using a powered drill to ream out carb jets can cause them to explode--your supposed to use the drill bit by hand in a chuck instead..(Alcohol requires bigger jets than gasoline,as it has less BTU's ,so you need more of it to get equal power)..

I have opened up a few carb jets with my torch tip cleaners before,they are like a tiny round file --normally I'd avoid altering a jet,but some carbs were designed way too lean for emission control to ever let the engine run right..

Some old Briggs engines I had "hunted" up & down because the little holes in the throttle linkage and govenor let the wire link slop around --Briggs put a coiled up spring around the link that hooked to both the throttle and govenor levers to hold them tight and prevent them from being able to move without the link moving with it..if that spring is missing or broken it can cause it to hunt up & down constantly..especially if the throttle shaft bushings have some slop,which lets air sneak past and alters the air/fuel mixture..


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post #7 of 12 Unread 11-09-2019, 01:00 PM
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Re: What causes hunting-searching RPM on Briggs 12.5HP?

Try a new governor spring(s), as Forrest Gump and Tractor-Holic mentioned. I have a Kohler (15.5hp) on my older Craftsman that experienced the same issue and a new spring solve the problem.
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post #8 of 12 Unread 11-09-2019, 01:21 PM
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Re: What causes hunting-searching RPM on Briggs 12.5HP?

Experimenting with a partial choke may confirm a too lean idle condition as Forest Gump mentioned. Increasing the idle speed can help as can enlarging the low speed jet.

Don't overlook the spark plugs as a possibility. It is amazing what changing plugs can do. Also the brand of plug can make a difference.

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post #9 of 12 Unread 11-09-2019, 04:12 PM
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Re: What causes hunting-searching RPM on Briggs 12.5HP?

Carefully check the manifold if it is plastic, as I've found cracks in one of mine. It was on a push mower. Also, be sure that the gaskets are sealing well. These can also be sources of a lean running engine.

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post #10 of 12 Unread 11-11-2019, 09:10 PM Thread Starter
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Re: What causes hunting-searching RPM on Briggs 12.5HP?

I've tried the new governor spring. I've tried different spark plugs too. I did put a new intake gasket on the aluminum elbow part going to the carburetor and a new o-ring in the plastic part of the intake. I'll have to take a look at it closer. I did change the muffler and pipe since it was rusted out. I'm not sure if the exhaust pipe flange is perfectly flat. It might be leaking at the pipe flange. Could an exhaust leak at the flange cause any problem like this? I know the arm on the governor bracket is a little sticky. With the new throttle control I can't get the lever to push against the choke link. I have to do it by hand the last little bit. But then it fires right up on the first start or pull.

Snapper 3312X6S w/ 12.5HP Briggs engine
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post #11 of 12 Unread 11-12-2019, 02:50 PM
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Re: What causes hunting-searching RPM on Briggs 12.5HP?

The exhaust "leak" wouldn't affect the hunting or cause any..

All the govenor parts must move freely with no binding,otherwise it'll be constantly trying to "stabilize" the rpms but cannot ..I'd try to free it up and see if that helps.it might be the whole problem--the throttle lever should operate the choke fully also,perhaps there isn't enough wire showing on the cable,you may need to trim back the spiral housing around the wire to gain enough travel,then adjust it properly..

Have you done a static govenor adjustment ?..

This photo may not be similar to your setup,but I want to show you the spring I mentioned,it is not the one the govenor lever works against,this one slides over the throttle link and hooks to both levers that link goes to,to hold tension against the link & levers.so any slop in the holes the link goes thru will not affect the govenor's operation..
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post #12 of 12 Unread Today, 12:52 PM
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Re: What causes hunting-searching RPM on Briggs 12.5HP?

Years ago, I found replacement of the governor spring did the trick for me. They're so tiny that sometimes they get stretched and weakened.

In the past:
Craftsman, MTD, Snapper, 1961 Ford 801 Powermaster, 1992 LB 4.5HP 580546, 2003 LB 10552 6.5HP Duraforce, IH Cub Cadet 105, 2001 Cub Cadet 2146 14HP Linamar 38" deck, love of my life (Sold 10 Dec 2011), 2007 Cub Cadet 2550 22HP Kohler Vtwin 50"deck (Sold 22 Aug 2018)
In use:
1994 JD LX178 sweet little mower, only 15HP, twin cyl, 38" deck, liquid cooled!
1998 JD 445 22HP Shaft driven wheels, shaft driven 60" deck, diff-lock, hydraulic lift, power steering, fuel & temp gauges, fuel injected, and liquid cooled! This machine has it all!
2005 Nortrak NT 254 25HP Diesel 5' FEL, here's a case of cost-cutting, I only need a tractor 3 or 4 times a year

Going to try to stay Liquid Cooled the rest of my life! Hey, it gets HOT here!
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