B&S 5hp backfiring

ChrisBFRPKY

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Sep 24, 2020
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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.
 

bertsmobile1

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Nov 29, 2014
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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"
 
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