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Reviving a Merlin and getting it back on the road.

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Re: Reviving a Merlin and getting it back on the road.

Post  FB8 on Sat May 03, 2014 10:36 am

Hello Matt

Some quick thoughts.

Fault finding on engines is usually a matter of eliminating the possible sources of trouble one at a time. The 10D engine really is very simple, one of the weaknesses can be the magneto. However the ignition system does have other components in it which I would always check first before committing to a new magneto coil or getting someone else to do the job.

As your bike has already run for a time I personally doubt that it is the magneto coil causing the problem. Can I just ask how you are testing your spark? (the easiest way is to take the plug out attach it to the HT lead rest it against the cylinder head and turn the the bike over on the starter by hand) You should be getting a spark at regular intervals about 3-6 each swing of the starter. Note;-The sparks are not as massive as those from an electronicly driven coil operated car system.

To test further I would remove the plug cap make sure you have a few strands of cable sticking out of the HT lead hold the end of this say 1/16 of an inch from the cylinder head and get the Mrs to swing the starter you should see sparks if not:-

Check the points are clean
Check the gap which should be 15thou at TDC
If they are showing heavy pitting or corrosion change them

Another component which can cause problems is the capacitor. This is harder to change on this engine as it means taking the back plate off (having removed the flywheel). A short cut is to cut the black wire for the capacitor and use a much smaller modern one mounted on the front of the points housing.

Ok if you now do have a regular spark get a non resistive plug cap and correct plug and fit them, test the spark again to be sure.

If you don't have sparks then it looks like the magneto coil about 40 of kit this is more difficult to change. Again the back plate has to come off. Apparently the resistance across the coil should be about 3,000 Ohms. You might as well put a new HT lead on as well if you are going this far.

For the bike to run correctly the timing must also be correct. There is a timing mark (an Arrow) on the brass fly wheel this lines up with a slot in the back plate. Must be at top dead centre use a rod of some description through the plug hole to feel when you are right there. Might be worth checking this in any case in case a previous owner has had the flywheel off and not put it back correctly.

Incidently the plug gap should be between 18-25 thou I would use 22thou.

Your tank issues worry me a bit are you getting a good flow of fuel to the carb when you tickle it? Is it a new fuel mixture?

Sorry if this all sounds a bit complicated but in actual fact it is quite straight forward. If you want to chat about it send me a private message and I will be happy to talk you through it some more.

Kind regards



Posts : 14
Join date : 2014-04-14
Age : 65
Location : Amersham

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Merlin conversation

Post  MrTweedy on Sun May 04, 2014 4:12 pm

Thought I'd share this PM conversation between myself and Adrian as a follow on from the above posts.

Thanks for the great reply, Adrian.
It's most appreciated.
I'm going to have a look at the bike just now with some of the things you mentioned in mind and report back.
The fuel is new and is definitely getting to the carb.
What is the method for removing the flywheel nut?



Hello Matt

You have to take off the gear lever and footrest. Then the cover; 3 clips

Use a large 1/2 BS/ 7/16 Whitworth spanner preferably ring (there is a special Villers Unit tool) must be a good fit.

The nut is right hand thread.

Hold the flywheel manually and give the spanner a sharp tap with a mallet two or three might be necessary. ANTICLOCKWISE as you are undoing it.

It should come loose. What ever you do do not try to jam the flywheel up with something like a screwdriver.

keep turning the nut until the flywheel comes off.

Take off straight and make sure when you put it back that the taper is good and clean as is the taper in the flywheel its-self. Keep everything nice and square to avoid damaging the taper or the body of the magneto.



Adrian...good news! I got her going...not great but it's a start. The screw that you use to adjust the points was loose, I think the thread on the points unit might be almost stripped. I've managed to tighten it enough just now but I suppose it will need to be replaced at some point.
Anyway, I managed to take her half way round the block before she died again. Not the points this time, probably a fuel issue.

Thanks again for your help!


Hi Matt good news. I checked with my original handbook as best I could and think that the housing for the points and condenser comes a separate part. ( original no M.1872) This might be available from Villiers Services I suspect you would have to remove the back plate to fit it. Having said that there is nothing worse than slipping points as they will let you down at the most crucial moment!

Alternatively you might be able to re-tap the hole possibly for something slightly bigger as long as it does not touch the point bracket. You used to be able to get a thing called a heli coil to repair threads but I don't know whether they ever came this small.

Anyway good luck with that. It occurred to me that as this thread has been so popular with fellow enthusiasts, that it would be a nice idea to post this private correspondence to sort of finish the thing off. What do you think?


Posts : 108
Join date : 2014-01-21
Location : East Lothian, Scotland. 1949 Francis Barnett Merlin. 1953 Ambassador Supreme. 1969 BSA Starfire (in bits).

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Re: Reviving a Merlin and getting it back on the road.

Post  FB8 on Thu May 15, 2014 8:10 am

Hello all

I have not written for a while as I have been waiting for things to happen including the registration of the bike. The V55/5 took a long time to come through. With the help of John Harding I completed it and have sent it off together with the certificated paperwork from the data officer and a copy of the old log book. Nothing yet.

One decision I have to make is which type of finish I am going to go for and would welcome peoples views. Basically there appear to be 3 options:-

1. Stove enamel I am particularly keen on this for the tank.

2. Powder coating I am thinking frame mudguards etc. But I am not sure I completely like the slightly plasticity effect.

3. A smoother thinner version of plastic coat which I am assured looks more like the stove

The bike will of course be black. Some things I am not sure of for example the wheel rims the chain guards top of the forks etc. Were these originally chrome?

Finally how do I get the trim detail sorted out?


PS I joined the club.


Posts : 14
Join date : 2014-04-14
Age : 65
Location : Amersham

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Paint Options

Post  ChrisTurner on Thu May 15, 2014 10:10 am

I agree that powder coating would probably look out of keeping. The additional problem with powder coating is that any minor damage is impossible to repair effectively. Stove enamelling is a good option if you can get it done but my choice would be 2K synthetic - it's what the trade uses. It is nasty stuff so the appropriate precautions need to be taken but your local paint shop will advise. It is easy to spray, gives a very durable high build glossy coat and is not particularly sensitive to temperature whilst spraying. It's also resistant to petrol which makes it ideal for fuel tanks. Chris


Posts : 131
Join date : 2011-04-29
Location : Plymouth Devon

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Re: Reviving a Merlin and getting it back on the road.

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