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Collection, Preservation and Display of Old Lawn Mowers

Ransome Marquis 4a BSA sloper ignition (?) problem

I am new to this forum. A couple of years ago I purchased a Marquis - having had one 20 or more years ago.

Was going excellently, until just recently. Suddenly started cutting out as the throttle was opened. Blocked jet, I thought, and gave it all a good clean out. No difference.

Then I noticed that on spinning it over with the plug out, there are far more sparks than one might expect (should be I per rev I guess), and occasionally none at all. Cleaning and resetting the points made no difference, so I think thinking something - capacitor or ignition coil maybe - might be breaking down.

So, some questions I hope someone might be able to help with:

  • Does the diagnosis sound right?
  • Is there a special puller available for removing the flywheel, or does one use a normal 2 or 3 jawed puller gripping round the outside of the flywheel?
  • I've read that some recommend replacing the entire backplate with an electronic module (Meco from Central Spares?). Is this a good move or should I just change parts on the unit I have?

Hoping for some comments ....

Forums

wristpin Thu, 23/05/2013

Flywheel removal.  Using a legged puller around the rim  is not recommended as there is a risk of distorting the flywheel . In an extreme case (and I have seen it) the flywheel can fracture leaving the hub still on the crank! Most of us in the trade or who frequently work on them will have made or adapted an existing puller to pick up on the three tapped holes in the flywheel. From memory they are Whitworth threads but you should be OK with UNC hardware. If you are of the metric generation I suppose that there is no real harm in tapping them out to say M8 and making the appropriate puller just so long as you remember what you have done! 

Sparks. Have to say that I have never come across excessive sparking but the coil must be reasonably ok to generate them so possibly the condenser id the culprit - cheap enough to replace from Villiers Parts, Villiers Services or Meetens.  The Meco  unit replaces the points and condenser and uses the existing coil - no need to replace the whole stator. Last generation Slopers used a fully electronic ignition and I believe this is available from Villiers Parts but may need a different flywheel - you need to ask.  When I was in the mower business  we used to fit a lot of Meco units with good results and I cannot recollect one ever failing.   With the points removed there is room inside for the Meco unit or you can leave them in place for originality and run a wire out and mount the Meco outside the magneto -  using one of the valve chest fixing screws. If your Sloper is one with a proper on/off ignition switch as opposed to the spring loaded plug shorting device you can pick up the feed to the Meco from there, remembering that you will still have to go into the mag to disconnect the points and condenser or perform some pretty niffty key-hole surgery through the point adjusting window!

From my experience to get a really smooth running and slow idling Sloper the valves need to be sealing reasonably  well and be correctly gapped so it may be as well to deal with them while you are about it.

 

http://www.villiersparts.co.uk/

http://www.villiersservices.co.uk/

http://www.meetens.co.uk/

 

hillsider Thu, 23/05/2013

There is not much to add to the info from wristpin other than have you tried a new spark plug?  The reason for asking is that I once experienced a Honda rotary mower that would idle sweetly all day but any movement of the throttle lever caused a cough and splutter and instant stalling of the engine. After much checking and cleaning of carb etc something made me try another plug this was an instant cure yet there was plenty of spark on the old plug and no obvious damage to the insulator.

nmw01223 Fri, 24/05/2013

Thanks for your detailed reply. I can probably make a puller if I have to. Where does the Meco unit get its trigger from, then?

 

I'll also try hillsider's suggestion of a different plug. Always take the easy route first!

wristpin Fri, 24/05/2013

It's got brains!  No doubt we have an electronics expert or two on the forum who will give chapter and verse but my understanding that it has a transistor as an electronic switch replacing the points and that in turn is triggered by sensing the build up of magnetic flux as the flywheel magnets pass the pole pieces of the stator. As an added bonus it senses engine speed and advances and retards the spark accordingly. So as well as replacing the contact breaker and condenser it also does the job of a moveable cam with fly weights or a manual advance and retard. - if you are old enough to remember that interesting feature to aid hand cranking an engine!

As an interesting aside, don't try fitting one to an Allen Scythe without giving thought to the fact that some 2-stroke Villiers engines used on them have  their max rpm governed by a centrifugal "hit and miss" mechanism controlling the contact breakers - by-pass the points and you have an ungoverned engine! You can still fit one but have to use the existing poits/governor to short it out when critical velocity is reached!

nmw01223 Sat, 25/05/2013

Thanks. What I meant though was it must pick up probably a changing magnetic field from somewhere - probably it has a hall effect switch inside, or possibly some variable reluctance (VR) arrangement. However, in both cases I guess something magnetic has to physically move close to it and away again to provide the varying magnetic field so it can then say "spark now".

I would have thought the contact points cam in the centre of the flywheel would be much too gently changing for this, so which moving 'bit' is used to trigger it (if you know)?

However - the motor's going again, running quite sweetly actually! All I did was clean the plug (which didn't really need it) and fiddle with the on/off switch. I suspect the switch might be a bit dodgey, but then its a standard toggle switch, so any replacement will do providing i get one that will manage the 200-300V or more involved.

 

wristpin Sat, 25/05/2013

As I said - it's magic!

You obviously know more about electronics than me  (or have swatted up on buzz words!!) but there is no physical or "proximity" trigger. You can throw away the points and condenser and if you really want to attach the Meco to a couple of feet of wire connected to the low tension side of the coil and an earth wire from the Meco body to the engine frame and hang it up two feet away from the mower and it will work. Don't ask me how, but it senses the build up of voltage? in the primary and lets go at the appropriate moment sensing the rpm and advancing or retarding the ignition as it goes - clever eh?. 

Have seen a build up of crud around the cutout switch cause a loss of spark, particularly when damp but still not sure why you should have had multiple sparks