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Old 11-14-2010, 10:36 AM
  #27  
Speedy-Gonzales
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Join Date: Mar 2004
Location: Bryan, OH
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Default RE: Best way to mount a hall sensor

This Webra was originally set up from the manufacturer for glow but never ran. They always came with the option to convert to ignition but the original ignition units (sensor) mounted on the back of the engine and were a piece of crap. The adaptor in the head is for a glowplug but I just take it out and drill and tap for 10mm plug. I have also added a Walbro WT-540 carb. A superior replacement for the original Delorto carb.

You can insert the trigger magnet into the original prop hub but the wall thickness from OD to ID is inadequate in my opinion and the addition of the adjuastable ring makes for fine adjustments once everything is in place. The magnets I use are 1/8" x 1/8" dia. noedymium and I order them from McMaster-Carr. Very strong little buggers and nickle coated to keep them from crumbling when pressing them in place.

I machine two rings. ( See pics below ) One that has tapered slots to fit over the webs on the crankcase for mounting the sensor and then the timing ring that fits over the prop hub. They are both 1.75" in diameter and this sets the gap between the magnet and sensor automatically once everything is mounted. This gap is very important to get consistent timing and spark. Some guys remove the crankcase webbing which I think is a big "no-no". The webs are there for strength and should not be removed or altered. The sensor mounting ring also permits drilling the sensor mounting holes in the ring and not the actual crankcase which I think is another "no-no".

Ring shown on left is the timing ring that fits over the prop hub with hole for magnet. Ring on the right is the sensor mounting ring that fits over the front crankcase housing with tapered slots to clear strengthening webbing. Both rings are secured with ( err...uhmm ) JB Weld.

You may say that this is a bunch of unnecessary work but I am a machinist by trade and I like to do it right the first time. Do whatever pleases your own fancy. I like my way the best. Not really that much more work involved when you have the machinery to work with. It takes me 45 minutes to do ALL the machine work for both rings and the sparklplug.

Here is the link to the magnets.
http://www.mcmaster.com/#neodymium-i...agnets/=9prxj4
Click on the picture showing the square magnets titled " Ultra-High-Pull Neodymium-Iron-Boron Magnets" Towards the lower part of the NEXT page that comes up look under "Disc Magnets Nickel Plated" #5862K61


I have not done this yet but I am going to run a Webra Bully on methanol with ignition to see how it performs.