# starting a Briggs without the blade??



## luckyvision (May 24, 2007)

There have been a couple of times i have tried to start push mower engines without the blade on & they kick back hard enough to pull the starter rope out of my hand. I allways thought this was because the blade acted as an additional flywheel, but it's come to my attention that a (partialy) sheered flywheel key could be the culpret..

that would explane a missing blade (bent from hitting something) plus i know lawn mower engines can run other things that don't have the rotating mass of a blade, unless those engines have a cast iron flywheel instead.

so am i right or wrong? i can understand how a bad key could throw the timming off, but will some engines also kick back if the blade is absent?


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## geogrubb (Jul 28, 2006)

luckyvision said:


> There have been a couple of times i have tried to start push mower engines without the blade on & they kick back hard enough to pull the starter rope out of my hand. I allways thought this was because the blade acted as an additional flywheel, but it's come to my attention that a (partialy) sheered flywheel key could be the culpret..
> 
> that would explane a missing blade (bent from hitting something) plus i know lawn mower engines can run other things that don't have the rotating mass of a blade, unless those engines have a cast iron flywheel instead.
> 
> so am i right or wrong? i can understand how a bad key could throw the timming off, but will some engines also kick back if the blade is absent?


YES. have a good one. Geo


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## 30yearTech (Sep 14, 2006)

The ignition timing of most small engines is set slightly advanced, meaning the spark plug fires prior to the piston reaching top dead center. Without the extra inertia of a spinning blade or some type of counter balance such as a heavy flywheel, then when the plug fires the engine tries to spin over backwards. 

Lawn mowers can be started without a blade, but you have to crank the engine extremely fast so as to overcome the tendency for the engine to fire over backwards. When a flywheel key gets sheared, the timing gets advanced even more and then the engine will have the kick back even if there is a blade attached.


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## luckyvision (May 24, 2007)

Well then i guess the next question would be if i wanted to use an engine for another application that didn't have the inertia of a blade, would installing an offset key be the way to do it, or would it be better to do something like finding a heavier flywheel?
I don't have a particular application in mind, just expanding my mind..


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## 30yearTech (Sep 14, 2006)

Using an offset key to retard the timing would make the engine easier to start, but once started engine performance would suffer. I would opt for a heavier flywheel or some type of weight on the crankshaft in lieu of a mower blade.


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