Villiers MK12 Issue

I had a play with my Villiers MK12/compressor today, it started after a few pulls and was running at around 1200 RPM according to my £20 Chinese ebay tacho. After about 10 minutes it gradually slowed and stopped, I took the plug out (new NGK equivalent of original) which was dry and had mid-grey deposits on. I checked for a spark, it was there but I would say it was weak. I refitted the plug and the engine started first pull but would only run for around 15 seconds before dying again, it did this a couple of times.

I am thinking that either the coil or condenser is failing when hot and the spark is too weak under compression, any thoughts?

Thanks in advance.

mattblack wrote: ....
I am thinking that either the coil or condenser is failing when hot and the spark is too weak under compression, any thoughts?

Thanks in advance.

I would tend to agree.  BTW Clinton service manual suggests gapping a plug to 150 thou' to check spark in free air.

That's interesting, thanks.

I have a couple of spark testers, but I don't know if they would still show a spark with a failing coil. The other thing I am now thinking is a reoccurrence of the problem I had with the blocked fuel tap, maybe a partial blockage which is allowing just enough fuel through to refill the float chamber while the engine is stopped.

More tinkering required!

If the coil/condensor are failing due to heat it should run for another 10 minutes from cold?

When it would only run for about 15 seconds it was still hot.

I might have a go at making a spark tester with an adjustable gap, in the meantime I will check the easiest (and free!) option of the fuel supply.

Just a thought.
Have your multimeter handy, run the engine until it packs in, then quickly whip off the HT lead and check continuity/resistance

Matt, I've just two Magneto's rewound, from cold the engine would run then after a while stop, I would change for a cold plug and it would run for a while and stop, the next rally it wouldn't work at all so sent both made for service and repair and both were open circuit and needing rewinding.

Maryalice

Maryalice, Don't forget these are simple Villiers coils.

Matt, I would also test the condensor - but it means removing the breaker box to get to it.

Fingers crossed it is the condensor as it's a much cheaper replacement.

https://villiersservices.co.uk/index.php?main_page=product_info&cPath=8_18_51&products_id=24

They maybe simpler but all magnetos have a LT side and a HT side so they still work the same way, only difference is a villers mag has spread all the bits out and not contained in a small box

I am off work next week so I might have some time to have another look at it, I will have to post any reading I get as electrickery is not my strong point.

maryalice wrote:They maybe simpler but all magnetos have a LT side and a HT side so they still work the same way, only difference is a villers mag has spread all the bits out and not contained in a small box

Yes, I know how they work. My point was that I, personally, wouldn't dream about having a Villiers coil re-wound

" electrickery"....  you must have watched Catweazle.

My magneto man does them without any issues. All coils are wound the same. You didn't state your point which I wasnt aware off, all the coil winders I know of use coil winding machines to make the rewind easier and tidier, I would never rewind a coil myself as it's not easy by hand

maryalice wrote:My magneto man does them without any issues.....

Any chance you can share his contact details?

I've got a week off so had a play today. Not good news, looks like it's a 'spares' engine...

While I had the cowling off I decided to take the head off as there was oil appearing at the joint, I found a lot of oily carbon in the combustion chamber and exhaust port and a semi-circle of wet carbon on valve side of the piston crown and the same on the head, bearing in mind it had only run for probably a couple of hours at most since being decoked. I found the valves were very loose in the guides so can only think a lot of oil is finding it's way past. I took the exhaust valve out, the part number shows it to a standard size (valves with oversize stems were available, but of course the guides have to be reamed out with the proper Villiers tool)

I decided to continue checking the coil anyway, checking in situ gave zero resistance on the primary (with points open) and 2.75 K ohms on the secondary (Which I believe is too low?) :




However, when I removed the coil and points housing ect and retested I got the same result for the primary but open circuit for the secondary

I checked the condenser and it was open circuit, which is correct I think?

Either way, it looks like the coil is dead and it is hardly worth spending on a new one with the valve guide issue as well.