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What would cause a Briggs & Stratton 5hp to backfire on occasion? Any ideas?

Do you have any idea how many different models of 5hp engines Briggs has built? If you really want us to help you, you must help us. Post the model, type and code numbers for the engine. Yes, we need all three, plus what type of equipment is the engine on.

Rivets said:

Do you have any idea how many different models of 5hp engines Briggs has built? If you really want us to help you, you must help us. Post the model, type and code numbers for the engine. Yes, we need all three, plus what type of equipment is the engine on.

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Sorry, it's my dad's. It's on a Craftsman rototiller, approximately 1990's or 2000's. That's all the info I have.

Is it running when it backfires? Or does the backfire occur when the engine is shut down? Or is it not starting and you have a backfire when cranking the engine?

When running, it varies in ram's with the occasional backfire. Is there a way to check the timing?

Rogero said:

When running, it varies in ram's with the occasional backfire. Is there a way to check the timing?

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If it is running like that, try to partially close the choke and see if it runs better.

Rogero said:

When running, it varies in ram's with the occasional backfire. Is there a way to check the timing?

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If this is something like a model 130202, sounds like it could be a little lean on the fuel. Check the small stuff first before going all in on what may be a minor issue. I'd turn the fuel adjustment screw counterclockwise about 1/4 of a turn and see how it runs there. Make sure the air cleaner is on it too.

If no improvement, you can move on to checking into a few items, like the flywheel timing key, air gap setting on the coil, spark plug, carb diaphragm, valve lash, and if the valves are seating properly etc.

Usually when one starts to lean out like that, it means there's likely something clogging a carburetor passage or the diaphragm is getting a little stiff with age. A new diaphragm and a few shots of carb spray would be a good idea regardless. That and a quick lapping job on the valves is basic maintenance on the old flat heads.

Rogero said:

What would cause a Briggs & Stratton 5hp to backfire on occasion? Any ideas?

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Through carb or muffler?

Rogero said:

Is there a way to check the timing?

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Yes with an old inductive timing light firing at the magnets. Just look a the flywheel key, key slot and flywheel. Should be lined up perfectly.

Slomo, please tell me how you use a timing light pointed at the magnets To check timing. How do you start an engine with the shroud off. Do you use an external battery source? Finally, if the timing is off and the key is good, plus air gap is correct, how do you adjust the timing. Asking because at my old age I want to start using my timing light, which I haven’t used on a small engine for over 40 years.

Other than checking the flywheel key and air gap on the coil, there's not much else needed to check the timing for a stock Briggs 5HP engine.

Slomo is correct about being able to use a timing light on a Briggs but it may not be as easy or straightforward as you'd think. I used a timing light to set the max ignition timing on every stock, modified and open class Kart engine I ever built. It's completely unnecessary to do this for something like a stock tiller or lawnmower engine though. If you're interested in doing it for racing purposes here's how:

1. Connect timing light induction lead to spark plug wire and you'll need a 12 volt battery from somewhere to power the timing light.
2. Install a degree wheel and a pointer then find top dead center.
3. Remove spark plug and turn the engine crankshaft the correct direction of normal engine operation with a drill (or an electric run in motor).
4. Note reading on degree wheel, this is where your timing is. (You can also use this method with a tach if you want to aim for a specific timing at a specific engine RPM)

Adjust timing by removing the flywheel key completely, lap the flywheel to the crankshaft and reinstall it without the key. You can move the flywheel forward or back on the shaft to get the exact timing you want and tighten it down. There are also fancy little doodad coil brackets you can buy that adjust timing by moving the coil but I never used those normally unless I was experimenting with some weird coil combination.

FWIW
Moving the coil closer to the flywheel advances the timing and produces a bigger spark
Moving it further away retards the timing and produces a weaker spark
You can fine tune most mower engines like this if you want to
Not much scope for advancing but a lot for retarding
We used to fit a DC amplifier ignition system to old British MX bikes
The unit was designed for model aircraft engines from memory it was called Runtronics
A mob in the USA made a mounting bracket for the pick up that moved both in & out and left right so you could adjust both the timing & power .
This of course was a kettering system , not a magneto and threw cluster sparks powered by some NiCds
We used to get about 17 run hours out of 5 D size cells
I have fitted one oft these to a racing mower engine and boy did it make a difference
However after winning national titles he got disqualified for "illegal modification"