Hello all,

I'm seeking a little advice on my m20 running condition. It has 2 issues really, first of all it is inconsistent and mostly difficult to start. This can be strange. Sometimes it starts first or second time and other times it takes a load of kicking and often best to use a squirt of easy start. When running it seems to be particularly rough at lower engine speeds and idling; when idling is possible, it seems to miss several beats in 10s. I initially thought carburettor as I've read that idle jet pathways in jetblock are prone to blockage. I took carb to bits; including removing jetblock jets etc and had the lot in an ultrasonic acetone bath for half hour. The acetone was pretty dirty which was not surprising after previous rusty tank issues, although i did have fuel filter inline. However that's all sorted after my tank electrolysis/resin work which was a great success; possibly worthy of a separate post in itself. After putting all back together it's not much better.

The plug looks a decent colour with plenty of brown so mixture not too bad. My instinct then sends me to ignition. It obviously has a spark but how can one tell is it good enough and therefore suspect the magneto? Holding plug on head gives a spark but I can't be sure its large enough. Points are clean and correctly gapped, as is sparkplug. There is no arcing on points which makes me think condenser ok.

Are there any sure fire ways to tell if ignition/magneto are ok; preferably when all together?

Cheers Neil

email (option): Nlhclarke@gmail.com

Are you still running the original 75 year old carburetor which most likely has a worn out slide and body? If you are the slide rattling around will never produce a good idle. You can have the carb body bored and the side sleeved or get a new carb.

Usually if the magneto is starting to fail, the bike will be much harder to start with the engine hot if the condenser or windings are going bad. With only a 4.9 compression ratio, the mag does not have to work very hard to make a spark.

My M20 was exactly the same, so I bit the bullet about 4 years ago and bought a new carb, it did improve things a little but not much, when I removed the points carrier, it's like an old rotor arm thing, the end of the armature shaft is shaped like a letter U and that fits nicely into a corresponding female shaped U on the points carrier. Well half of my U was missing and someone had cunningly shaped a piece of metal fitted it into half of the female shape and offered it up to the shaft, well the result was on tickover one minute it was stable the next minute it was trying to stall, it was like setting your tick over on full advance and expecting it to then tickover on retard, it was all over the place. It was always a bugger to start from hot as well, it got so bad I would keep petrol at home and then I wouldn't have to stop when out riding, if I stopped for 30-40 minites it would start fine again. So over the winter I had my magneto rebuilt complete with new armature shaft, what a difference, she'll usually start first prod hot or cold tick over on full retard is a reassuring low thud thud thud snick it into 1st no grinding of teeth because the revs are so low. In short its a pleasure to ride, and I know that when I reach my destination its going to start to get me home

email (option): Alwyndart@gmail.com

Apart from a specific magneto fault have you checked the ignition timing setting?...It won't tick over well if too advanced...Also (and conversely) it will be reluctant to start if you are retarding the ignition too much when starting...Start with as much advance as you can...In other words only retard it if it 'kicks back' at you....As an initial strategyy though I'd check the timing setting and get it as accurately set as you can...

I'm not 'teaching granny to suck eggs' I hope but I wrote a detailed piece on ignition timing for the 'Technical section' which might be worth a look...Ian

email (option): ian@wright52.plus.com

Hello chaps, many thanks for the replies. i can update on a few of these.

Bruce, on the carb comments, yes the carb is original but the throttle slide looks reasonably tight in the bore and doesn't rattle about unduly in my opinion. In idle /low engine speed the throttle slide is stationary in the bore (as viewed from the intake). I think on reflection the bike can be harder to start when hot, like after a 30 min ride or greater. On a constant high speed (40 mph !) it runs very well and does not misfire at all/or very slightly if I slow down.

Ian, I have not checked the timing as yet but am aware of your very informative document in the technical section. I have tended to think the timing cant be too bad as it has previously idled very well and not misfired. I'm not sure if this can drift/change as would require the generator pinion to slip on its shaft which seems unlikely after all these years? Unless there are other mechanisms for timing changes? In any case I will have a go at checking this.

Alwyn, your experience in performance seem very similar to mine although it would be be amazing to find a similar bodge to the points mounting plate as you have experienced. I'll have a go at checking this in next day or so. However, today I checked something easier and potentially related. I unscrewed the end of the HT lead as it enters the magneto and the observations begged questions. I'm not sure if the way/where they cables separated is normal, or the condition is bad. I attach a couple of pics to see if this looks normal.





Also, would it be a good idea/easy thing to unscrew the plastic bit that is attached to the magneto body (where the HT lead screw in ) or would this cause trouble inside?

I cleaned the end of the HT lead and the metal bit the hole inside the magneto body (with brass wire brush/alcohol). After reassembly it started but i think the miss fire is slightly worse. This makes me think the issue is related to the magneto/cabling.

Cheers

email (option): nlhclarke@gmail.com

Two things if you decide to remove the high tension pickup from the magneto body.
Be careful not to let the brush itself (inside the pickup piece) fall into the magneto. Remove the pickup slowly & you'll see the carbon brush.
When the pickup's removed you can clean the rotating slip ring on the magneto shaft (which the carbon brush contacts) by inserting a cleaning rag into the mag body on the tip of a suitable stick & rotating the armature. The stick must be non conductive, preferably wood, or you'll get a very nasty electrical jolt.

All that corrosion on the lead and in the pickup wants cleaning. The corrosion will cause a weak spark due to the insulating properties of it .

email (option): ian.clare1@virgin.net

Hello chaps thanks for all the extra comments, i have assaome minor updates.

I ended up taking the pickup part off the side of the magneto and it came away quite easily, bolts not tight or rusted. The pickup itself intact with quite a length of spring loaded graphite in place with nice eaven wear mark on it from the slip ring. The slip ring looked quite clean as it was but cleaned it anyway (alcohol rag on piece of dowl). After this running was definely affected advserly (again !). Bizarely it started 1st time but was misfiring at low revs as before. It stopped and i could not restart it. Putting the plug against the head i have to say the spark is very weedy and hard to see in the garage in daylight with door open. I saw some utube video of some chaps testing a lucas megneto after service. They spun it with a drill holding HT lead to body. Spark looked nice and blue and fat. I know its subjective but to me i think the spark is too feeble. Its ashame i cant post a vieo on here.

Is there a step by step guide to removing the magneto? is there an insitu guide to testing it?

cheers

Neil

email (option): Nlhclarke@gmail.com

Neil,
If that magneto has not ben rebuilt then for sure the capacitor has well & truely past it's use by date .
There is a relatively simple and not too expensive way to check this is to fit an easy cap from bright sparks magnetos
I do not like the idea of running with one all the time but they are not expensive and take about 10 minutes longer to fit than replacing the points .
OTOH all of the people I know who have fitted one are more than happy with them .
Add to this you do not need to have any electrical skill or knowledge & the only tools needed are the BA points spanners & a flat head screwdriver .

It's worth trying another spark plug. There seem to be a lot of knockoffs around... Also, try snip a 1/4" off each end of the HT wire to remove corroded ends.

Neil

I'm not a motorcycle mechanic but I've seen some great Lucas Magneto rebuild videos.

This one sparks with the twist of the finger after the restoration, no drill required.

https://youtu.be/WkPzRAMy-zk?t=3260

I think a properly restored magneto will go a long way to making sure your bike runs well.

The guy in the video is in the UK.

Cheers

Al

Did you unblock the tiny hole at the base of the venturi just aft of the slide? If not that'll do it.Clear it with a piece of electrical copper wire which is softer than the carb body. I'm not all sure half an hour of ultrasound is anything like enough. Just because loads of crap came out doesn't mean there isn't more in there. I'd leave it in for hours. In my experience even a knackered mag that will start[with good points] will still idle ok. Have you tried closing the spark plug gap to ease starting?

email (option): jeremy@clogmaker.co.uk

Yes, the plug gap should be .018" for a magneto, not the .025" the plug has from new...A faulty mag will usually start the bike OK but will get weaker and weaker as it gets hotter and often won't generate a spark at all at higher temperatures....
However, on the basis it's much easier to work on the carb than the mag, I'd concentrate on that with Jeremy's comments in mind and also check that everything else in the carb is set up correctly....
Then check the ignition timing...It doesn't take long just to confirm it's correct and then you can forget it...Lastly come to the mag...(bearing in mind you've already checked the points gap and improved the HT connection)...

A low speed misfire is frequently caused by a blocked carb drilling....Ian

email (option): ian@wright52.plus.com

As for the plugs. Well I've had a number of useless NGKs in the last ten years. Used to be my go to plug. Last year we were told to use them in a old DT2 race engine. They lasted twenty minutes, consistantly.... Had to change them every race. Switched to same heat range copper core Champions, the first plug is still in there.

email (option): jeremy@clogmaker.co.uk