Common timing symptoms

15 years ago

Just wondering what they are. Say if you get the crankshaft and camshaft gears misaligned in a briggs ohv vtwin. Backfiring, I know. Could you also have an engine that is hard to start, lacks power, runs roughly but doesn't backfire?

I have a thread going on my mower issue, just trying to understand the symptoms of bad timing a little better. I understand that if the timing is off, the spark isn't happening when it should, but what are the symptoms besides backfiring?

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Comments (3)

mownie
15 years ago
Two kinds of "mis-timing".
Ignition mistiming can result in dramatic explosions out the exhaust if the spark occurs too late.
If the spark occurs too early, there may be "popping back" through the intake manifold, or the piston may simply stop ascending (if in the cranking mode of operation) and stall.

Valve/cam mistiming can result in some of the same symptoms but with the added risk of a valve being struck by a piston (OHV engines only). Some engines are at risk of the piston striking an open valve if the camshaft and crankshaft are mistimed (or become mistimed in operation). These engines are known as "valve interference engines".
I don't think one tooth of mistiming will cause a piston to strike a valve in a Briggs engine.

Whether the ignition timing is synchronous to the crankshaft rotation, or on the other hand.....synchronous to the camshaft rotation.........will make a difference in how both types of mistiming will affect the behavior of the engine.
Spark timing is always expressed in terms of "degrees Before Top Dead Center" (BTDC) or "degrees After Top Dead Center (ATDC). The COMPRESSION stroke is ostensibly the cycle the piston is in at this point, though ATDC is actually the beginning of the POWER stroke.

On Briggs, ignition timing is synchronous to the crankshaft and can only be affected by movement of the flywheel in relation to the crankshaft (or vice versa). A missing or defective flywheel key is the usual culprit if the spark is out of time.
If a Briggs camshaft is "one tooth off", you might have an engine that will run........but just barely. And note I said "might".
cedarghost
Original Author
15 years ago
I didn't think this post made it up, so I reposted it. Can an admin delete the new one?
Thanks for the reply. I was just curious, because I was always told if it's out of time it will misfire, or not start at all. But that was dealing with cars.
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mownie
15 years ago
Whether it "backfires" or not really depends on the specific "point of being out of time" that the ignition is firing in its out of time condition.
There are 720 degrees of rotation of the crankshaft in one complete 4 stroke cycle. Some points in those 720 degrees will result in a loud bang out the exhaust. Some points will result in a loud "cough back" through the intake and carb.
Some points result in an engine that just will not run.
Being out of time is kinda like a roulette wheel.....the right number wins, the wrong number bombs.