Newbie Questions- timing

Information relating to the Matchless G9 or AJS Model 20 500cc twin
Sams Dad
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Joined: Mon Apr 19, 2021 6:49 pm
Location: Lancs UK

Newbie Questions- timing

Post by Sams Dad »

:cry: Hi All, recently got my first Matchless- 1958 G9. I bought it as a non- runner, but have had it running with some difficulty as parts were missing from the carb. Got the bits from the club and fitted them. Brilliant service. I repaired the A/R cable as the nipple had slipped off. Since doing that no attempt to fire. I am trying to get the timing right. I have pulled the pinion gear off with a puller, set points at 12thou and timed with a cigarette paper. When i check it it seems out after rotating the crank 720 degrees . Is this because there are two lobes on the cam ring ? Should I be checking the timing at that point to make sure both plugs will fire ? Or have I got this completely wrong ? I have followed the manual to find tdc - should the inlet valve just be closing as the piston comes up on that stroke ? If so i have that right. Anyone point me in the right direction.
Groily
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Re: Newbie Questions- timing

Post by Groily »

Discerning choice of machine! Congrats on acquiring a fine beast.

Both valves should be shut at TDC on the compression stroke.
When one cylinder is on compression, the other will be on the exhaust stroke - the pistons move up and down together.
720° of crankshaft rotation (2 turns) = one 360° turn of the magneto, as it goes at half engine speed.
The mag should be back to where you started after 2 turns of the crank, or half-way round (with the points opening on the other lobe of the camring to fire the other spark plug) after one turn of the crank.
With the pistons in the right place (x° or y fractions of an inch / mm before Top Dead Centre), and the magneto set at full advance (which is with the cable slack), then the fag paper thing should get you pretty close to where you need to be. After a full rotation of the mag the positions should all be the same! If they have moved, then the pinion will have slipped in the doing up, or the camring has moved.
It is always worth checking the points opening on both cylinders / lobes of the camring, yes, but most people find it easier to set things initially with the points just opening on the lower lobe because you can see the points gap a lot more easily in that position.
Bear in mind that the HT pick-up on the gearbox side of the magneto is fired when the points open on the lower lobe, and the engine side one on the upper lobe. If by mischance you should muddle up which cylinder is on compression, which on the exhaust stroke, swapping the plug leads over will sort that. There is no 'rule' as to which HT pick-up should go to which spark plug - your choice. I prefer 'rear pick-up to the off-side cylinder' myself, but it's completely immaterial.
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clive
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Re: Newbie Questions- timing

Post by clive »

I would suggest you should be checking the firing point on both cylinders because as you say there are two lobes on the cam ring. If they are much out you may have to go for a compromise timing point which halves the difference. When you have done that once you will know for future retiming.
clive
if it ain't broke don't fix
Sams Dad
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Location: Lancs UK

Re: Newbie Questions- timing

Post by Sams Dad »

Thanks for your help. Yep seems a great machine. So I set up on the right/offside cylinder then 360° and check the other lobe for the timing point? If set right or close to it happy days, if not a compromise on the two. Makes perfect sense. Set with fully advanced timing - i thought according to manual that is with cable slack , but the mag turns clockwise so surely that makes the spark later therefore retarded ? Have I got this back to front and advanced means later spark ?
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clive
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Re: Newbie Questions- timing

Post by clive »

The magneto is an anti-clockwise. it is named according to the rotation viewed from the drive end. Looking at the points end they are rotating clockwise so assuming the mag cable enters from the gearbox side it is tight cable for fully advanced. If it enters from the barrel side then it will be slack wire. Make sure the points are about 12 thou when fully open (again compromise if difference between the two lobes. As the bike has not run for some time clean the points with some very fine wet and dry as they do corrode (oxidise?) when left for some time unused.
clive
if it ain't broke don't fix
Sams Dad
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Location: Lancs UK

Re: Newbie Questions- timing

Post by Sams Dad »

Hi Clive, sorry i did not help myself there. It runs clockwise as I l look at it from the left/ near side of the bike arrggghhh makes perfect sense now. When the cam ring is pushed anti clockwise therefore it IS advancing. I think i got confused from one side of the bike to the other so I need to check everything again as I don't know where i confused myself and it would time up backwards but may struggle to run that way lol It is a slack wire as enters engine side of mag. I think i am getting there now. Is there supposed to be a timing cover gasket or just wellseal on it ? It has about 4 layers on it!
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dave16mct
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Re: Newbie Questions- timing

Post by dave16mct »

There isn't a gasket available and normally just Wellseal is used. On my 650 I had trouble with it leaking from the timing cover and I made a gasket which cured it.
Dave.
Sams Dad
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Re: Newbie Questions- timing

Post by Sams Dad »

Thank you , Dave. Cornflake packet plus wellseal ? Or proper gasket paper ?
Groily
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Re: Newbie Questions- timing

Post by Groily »

I always use Flexoid gasket paper for these (and I always make one), but I'm sure many other things would do. Whoever thought narrow mating faces and 2BA threads were a suitable approach to timing cover design wasn't thinking much about us 60 years later!

If it's as original your mag has the cable entry on the left of the cb unit. So slack for full advance per my previous, and points turn clockwise - as you've now confirmed.

If you find there is a significant difference in points gap, or in timing between cylinders (or both) come back online and some suggestions will be forthcoming. The worst thing with manual camring K2Fs is in this area, and often it isn't wear on the cam lobes that causes it.
Minor discrepancies can be 'averaged out' as Clive says, and that's fine. But it is not uncommon to see as much as 5° difference on the magneto, which is 10° on the crankshaft - and that is a bit too much for comfort even on a soft engine.
Sams Dad
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Location: Lancs UK

Re: Newbie Questions- timing

Post by Sams Dad »

Hmmm timed up the right cylinder and tested left to make sure the same. The points do not open on the other cam. Widen the gap ? Or set up on left cylinder and see where that puts the right one.
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