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Hi, I am running a 30 year old Briggs & Stratton 12hp motor in a Westwood mower. For over 1 hour she runs fine, and then she starts hunting and stops. A few minutes later she will start and run but only for another few minutes before repeating the shut down. I have checked the governor, carburettor and fuel supply (new fuel filter) and fitted a new spark plug with no success. The engine oil has been recently changed. Temperature this morning was no higher than 14C. Does anyone have any ideas why this is happening?

Make sure the vent in the fuel cap isn't blocked causing a vacuum in the fuel tank.

Need the model #. Eg: 28A707-1234-E1. Probably on a shroud.
If not fuel starved, like ILENGINE said...
Coil getting hot and failing? Run engine. When quits running - pull plug, ground plug, have buddy pull rope, look for spark.

Your symptoms are consistent with floating debris in the fuel tank getting sucked into the outlet and plugging it.
When the engine stop, the debris floats away till the engine starts running again.
I use an external tank to confirm this .
When the mowr stops, I hook the tank off then go mow, if I run the tank dry then I know it is a fuel supply problem

You can
1) loosen the filler cap to counter a faulty tank vent creating a vacuum lock
2) rip the fuel line off the tank side of the pump and blow back ( loosen the cap first ) up the fuel line, at least 2 big lung fulls .
If it runs another 1/2 no problems then time to drain & clean the tank.

Some mowers and some engines have known problems , like running the fuel line over the head which can boil the fuel.
So engine & mower model numbers are important.

Guys, many thanks for your suggestions. The details off the front of the engine are these:
- Model 281707
- Type 020601
- Code 89101331
which I understand makes it a 1989 motor. Do these details help? The fuel vent on the cap is clear. I've always thought of it as fuel starvation problem as the engine 'hunts' with revs rising and falling before stopping, so think that bertsmobile1 has probably hit the nail on the head; thank you. I have never cleaned the tank out, and I know the mower lay dormant for a good while before I got it 4 years ago, so that's the next job. I'm not sure that this model has a coil does it? I have not seen one, and thought that it was generating the spark from a magneto, but I will look again. Sadly, the mower is not at my house so it might be a day or two before I can get back to it. Thanks guys; further comments appreciated.

A magneto ignition consists of a magnet that passes by ( or rotates within ) a coil and a triggering device .
In the case of a lawnmower the coil has 2 sets of windings just the same as a battery coil so the voltage generated by the magnet in the primary can be stepped up to be sufficient to jump the plug gap under compression pressure.
The trigger to time the spark comes from a hall effect chip that is encased in the magneto housing.
We commonly refer to the coil + chip kust as a coil and not as a "Module" which can cause confusion.

agp47 said:

Hi, I am running a 30 year old Briggs & Stratton 12hp motor in a Westwood mower. For over 1 hour she runs fine, and then she starts hunting and stops. A few minutes later she will start and run but only for another few minutes before repeating the shut down. I have checked the governor, carburettor and fuel supply (new fuel filter) and fitted a new spark plug with no success. The engine oil has been recently changed. Temperature this morning was no higher than 14C. Does anyone have any ideas why this is happening?

Click to expand...

Dies when HOT Won't restart until cooled down? Replace Coil. Use OEM don't be tempted by cheap aftermarket coils they may last a season if lucky.

Thanks for all your replies, but bertsmobile1 wins the prize! Thank you for your advice about floating debris in the tank. Visually all seemed ok when looking into the tank, but after draining most of the fuel, disconnecting the hose to the filter and carb and removing the tank, I found that the outlet pipe was almost completely blocked. I pushed a thin rod up into the tank and the muck that appeared on the end of it had to be seen to be believed. I realised that the problem had been found when the remaining fuel in the tank only came in drips rather than a steady flow before rodding the small plastic outlet. I have used the mower this week for almost 3 hours without a hiccup. Thanks also to biggertv for the coil suggestion, but before fixing the problem it would start again within a couple of minutes of stopping, so thought it was not that. Back to work!

You would not believe the sorts of stuff that I find in fuel tanks . From things you would be very embarassed if your mother found them ( family channel ) right up to 3 garden gloves .
Glad to hear you got it sorted.
I usually drain the tanks, leave them a hour or more to dry in the sun then blow back up the fuel line till all of the crud blows out of the filler spout.

Hi, agp47, I'm glad you got yer mower running due to the advice about junk in the gas. I am working through some issues with my Gravely ZT 50" mower with a Briggs & Stratton 16.5 HP twin-V OHV. Going on 10 years, I can say that it has been mostly trouble-free for that period. Maintenance has been minimal, keep the oil full and change it each season. I "usually" drain the gas tank for off season and run the engine dry to keep carb from gumming. Had to replace the (2) damper controls after about 5 years. They are the thin struts or "shock absorber" looking things that attach to the steering bars. You know they are shot when get rocked forward and back in you seat like a rag doll.

I recently got two symptoms with the engine that remain a mystery and which I fixed just by messing with it till it started. Symptom #1 was no gas to the cylinders, the spark plugs remained "dry" and engine cranked and cranked but wouldn't run. Fuel and filter were clean, fuel pump made a little stream of gas when unhooking its hose to the float and bowl. Finally discovered that for an unknown reason, the wire running to the fuel solenoid (attaches to float bowl) was "dead" (no 12 volts) while cranking. Furthermore, the hot wire to the fuel solenoid showed continuity to ground when I expected it to be an open circuit when all is off. I now suspect that I have a short created by insulation worn off of the solenoid hot wire where it rubs the engine cooling fins somewhere. Moving, jostling the wire magically fixed the symptom, but I am getting occasional misfires when the engine runs, suggesting it may still be grounding intermittently as engine vibrates.

Symptom #2 was no spark to the plugs. I followed a Youtube by "Taryl Fixes All", a funny guy with buck teeth prop, but he's very knowledgeable and helpful. Taryl saved me some bucks, teaching me how to check the magneto coils and the inline diodes in the wires feeding those coils. The diodes (two because of the twin-V) checked out at 0.55 ohms each and measured this in only one direction as they should. The magneto coils (some call ignition modules, or armatures) also checked OK, measuring 4.34 ohms between base and plug boot and 4.43 ohms between kill terminal tab and plug boot. It was as if the kill wire was grounded, say from terminal "M" to ground in the key switch. I am concluding that the issue was with the Start relay which went away when I swapped out the PTO relay and Start relay. They are interchangeable literally "black" boxes about 1 inch cube. I have replaced both handfuls of times. No clue why the swapped relay acted "proper" in its PTO relay role because blades should not have turned if the relay was bad.

Gotta reminisce a bit. Some of you might remember my sad story of when my Troybilt Horse cultivator took the literal plunge and died. I left it running unattended once while trying to test it and it slipped into gear and plummeted down my storm cellar, head banging the exterior door and tiller blades grinding at the concrete steps. The brass worm gear I had just installed was ground into filings. The sounds were horrific and mournful and I felt like the captain going down with his ship.

Well, another more humorous story was the sudden screeching stop of a small tractor-like mower that came from one of those popular stores where you buy stuff out of a box, no support, no dealership. It was one of those cheap yard tractors with undersized engines (and mower deck barely larger than the self push variety) that you run into the ground like many similarly cheap push mowers. After the loud screech there was no power to anything, just a free running motor. Long story short, the bolts that fasten the engine to the chassis had "all" fallen out and the mere tension of the drive belts pulled the engine back till the PTO shaft rubbed the chassis. A kind neighbor gave me fine-threaded machine bolts to replace them and all was back in business when the belts were fitted on the remounted engine pulleys.