OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Victa Two Stroke Engines Effect of a smaller flywheel to coil air gap?

Hi all, this question probably relates to all small engines with electronic ignition but we'll keep it simple and stick to a powertorque engine for the purposes of this discussion. What effect will a smaller than specified air gap have on spark and ignition? The powertorque specifies a 0.25mm air gap but what would the effect be if that air gap was reduced to say 0.10mm?

This quote from a list of old wives' tales in the Briggs FAQ might help:

"An armature air gap that is too wide will prevent spark. Not true. Well, sort of not true. Briggs & Stratton air gaps cannot be made too wide to prevent spark providing the coil is healthy and the engine is spun over fast enough. A wide air gap, say .030" will ever so slightly retard the ignition timing as the magnetic field takes longer to build within the coil windings."

As I understand it, you may get a spark at a lower cranking speed with a relatively small gap, and the larger the gap the later the ignition timing, but it just isn't a very sensitive adjustment within reason. For the GXV120, Honda specifies a gap of 0.4mm plus or minus 0.2mm.

One reason to avoid ultra-small gaps is that dirt sticking to the outside of the flywheel will cause abrasive wear to the ignition module.

Thanks for that Grumpy, it's a fair call on being abrasive on the module.
The reason why i asked is yesterday i was trying to get a powertorque with a weak spark to fire up. I removed the engine off the deck and noticed a very small air gap. I reset the gap to the specified 0.25mm and still no improvement. Swapped the coil over with a good used one and exactly the same result. At this point i'm thinking "what the..". There was no module to change as this was a later model powertorque.
Today i decided to tackle it again and opted to swap out the flywheel, with great success, it fired up on the first pull.
So 2 things were learnt: 1) A smaller air gap has minimal effect on spark, 2) Flywheels do occasionally stop generating enough magnetic current to produce a decent spark.

I remember when i changed a coil and didnt have the obligatory business card spacer that anything less than the business card spacing resulted in the flywheel magnets sticking to the coil. The flywheel was ever so slightly off centre and would rub the coil, once it was spaced right I got the spark.

bigted, the Briggs test for whether the flywheel magnet still has enough magnetism is to put the flat end of a 10" long flat blade screwdriver on the magnet, then lift the flywheel and see if the magnet securely holds the screwdriver. It sounds as if the one you were using didn't pass it.

Here is the full Briggs FAQ on magneto theory:
http://eu-en.myfaqcenter.com/Answer/?p_faqid=3805

tiger, the clearance is measured to the plain surface of the flywheel, not the magnet part. Also, you need to have equal gaps to the two sides of the magnetic yoke.

Thanks Grumpy, interesting reading, obviously my flywheel would have failed the test and i like the theory behind the physics.

This post is deffinately one for the memory bank. I can't say I've ever encountered this problem before however checking the magnetism is a great idea.
Good question bigted. I'm yet to see something grumpy hasn't got a comprehensive answer for which makes this forum great.

I know not everyone is a fan of power torques but I think they are one of the more reliable engines as long as you mix the fuel right which is a simple process.