Slack timing chain Issues?

I am about to open up the timing chest on my 1972 Commando to check the slack in the timing chain.
I have put about 8,000 miles on the bike since I bought it, and have never checked the timing chain tension. Yeah, I know, my bad.

One reason I will be checking the tension is I am having some misfiring issues and wonder if this might be due to a slack timing chain.
Any thoughts on what poor running symptoms might arise from a slack timing chain??

Stephen Hill

Would not a strobe tell you if your ignition was wandering?

Worth checking, but I am guessing that that a strobe would not recreate the issue. The bike is misfiring when the engine is loaded at higher revs. I will open up the timing chest and the chain will probably be slack. Just curious what it any issues this causes.

Carbs could need a service, ignition timing out, needles need lifting up one notch could be lots of things, a lose timing chain usually makes a noise, jets and needles could be worn from vibrations, timing chain should be adjusted every 10k miles by the book, what you running points or EI, maybe just needs a good service.

Why hesitate?Who knows how long it has been since it was checked.

If you fit the Merc Big Pin timing chain from Jason the Chainman there is no real need to go in and check the tension, pre-worn as delivered and the big pin means the bush surfaces are the largest possible and the biggest diameter allowable within the spec. In a Merc in duplex form (as used on Chainman's Norton Primary chain) they easily last for 100K+ miles

EstuaryBoy said:

Would not a strobe tell you if your ignition was wandering?

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Yes, it would. Even at 3000 RPM you'd see timing jitter. That would not produce misfire however unless it was really nasty, and you'd see that at all load conditions.

kommando said:

If you fit the Merc Big Pin timing chain from Jason the Chainman there is no real need to go in and check the tension, pre-worn as delivered and the big pin means the bush surfaces are the largest possible and the biggest diameter allowable within the spec.

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Very good point kommando,
If you do go this route, then add one of these: https://andover-norton.co.uk/en/shop-details/16339/cam-chain-tensioner-rubber-faced-06-1094-
This gives the chain and sprockets a bit of cushion as the 72's had the solid steel chain tensioner. You will notice grooves cut in the steel tensioner from the chain. They changed to rubber in MkIII's now I think andover only sells the rubber bonded chain tensioner and did away with the steel units.
I use this chain and have many mile on my Interstate. I give it a thumbs up, for an upgrade and piece of mind.
Cheers,
Tom

Might be a dumb question , but is it fairly straightforward to change the chain without stripping the engine ?

I appreciate the feedback. And I admit I am coming at this in a back-asswards manner. As in I am asking what symptoms to expect from a slack timing chain. Versus just adjusting the adjusting the slack or changing the chain. Different thought process, and different advice, hopefully. I will dig into the timing chest soon.

Without getting into the gory details, I have gone through every trouble shooting approach anybody would care to think of (which I don't want to get into here) to address a misfiring issue. So I am now at a point where I am thinking outside the box. My two theories, until I rule them out, are a slack timing chain or low compression.

Stephen

phippsy said:

Might be a dumb question , but is it fairly straightforward to change the chain without stripping the engine ?

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Yes, but you will need to remove the AAU or EI rotor fitted to the end of the cam so the engine will need retiming.

Stephen Hill said:

I appreciate the feedback. And I admit I am coming at this in a back-asswards manner. As in I am asking what symptoms to expect from a slack timing chain. Versus just adjusting the adjusting the slack or changing the chain. Different thought process, and different advice, hopefully. I will dig into the timing chest soon.

Without getting into the gory details, I have gone through every trouble shooting approach anybody would care to think of (which I don't want to get into here) to address a misfiring issue. So I am now at a point where I am thinking outside the box. My two theories, until I rule them out, are a slack timing chain or low compression.

Stephen

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Your strobed timing mark will be a blur as the camshaft will vary its relationship to the crank as the slack moves from the top to the bottom run of the chain as the cam followers move over the cam lobes. Spark timing will vary accordingly.

phippsy said:

Might be a dumb question , but is it fairly straightforward to change the chain without stripping the engine ?

Click to expand...

You will need to stop the crankshaft from turning if you want to do it in situ. https://www.classicbritishspares.com/products/norton-commando-clutch-locking-tool-06-1015
Remember the timing pinion that drives the oil pump is LH thread. cam is normal. By the way, I have never done it this way before but I think it would work.

Cheers,
Tom

Stephen Hill said:

.........So I am now at a point where I am thinking outside the box. My two theories, until I rule them out, are a slack timing chain or low compression.

Stephen

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Forget slack timing chain for the symptoms you are talking about. As noted it would be noisy, and a strobe should tell you if the ignition timing is wandering about, of course it won't tell you if the valve timing is wandering about, but....

To some degree I feel you are clutching at straws.

The timing wander would be worse under no load conditions, idle or on closing the throttle, rather than the heavy loading you mention.

You don't mention what ignition you are using, various set ups have their own quirks, for example Boyer sensor connections tend to fail, or at least become intermittent and cause ignition issues.

I should add that timing chain wear isn't necessarily even, you may have tight or loose spots in it, and you will need to tension the chain and rotate and recheck several times to fully determine it's condition.

CanukNortonNut said:

You will need to stop the crankshaft from turning if you want to do it in situ. https://www.classicbritishspares.com/products/norton-commando-clutch-locking-tool-06-1015
Remember the timing pinion that drives the oil pump is LH thread. cam is normal. By the way, I have never done it this way before but I think it would work.

Cheers,
Tom

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I have recently fitted a new cam that took a bit of dialling in, so I had the chain and sprockets off several times. I was also being driven mad by a chain that had tight spots, so I ended up changing it even though it looked in good general condition.

At TDC two valves are closed and the cam will only have spring tension on one side, and can be fairly balanced, though this may be influenced by the actual cam you have installed.

It is then possible to slip off the cam sprocket and intermediate gear assuming everything is fairly loose and free, and you already removed the worm/nut. Removing the oil pump is optional, but less fiddly than not doing so when removing and replacing the worm.

It is of course much more comfortable to do this with a motor on a stand on the bench and with the head off! But from where the OP is sitting we have just turned a cam chain change into a full motor overhaul!

Our OP has some new experience in store anyway if he has to renew the cam chain, loosening the LH thread worm/nut for one and properly holding the intermediate pinion is another, He needs either the AN tool or a cutaway timing cover. He will also need the tool for reinstalling the timing cover over the cam seal. He may also need a bolt for pulling the advance retard unit or EI rotor and a puller for the cam chain sprocket.



If I was him I would also be armed with a new timing cover gasket, an oil pump seal, and oil pump gasket and the two timing cover seals. (I always have stock of these!)

I would probably also have a new chain and tensioner pad to hand! Which reminds, me I need to restock with those!

CanukNortonNut said:

You will need to stop the crankshaft from turning if you want to do it in situ. https://www.classicbritishspares.com/products/norton-commando-clutch-locking-tool-06-1015
Remember the timing pinion that drives the oil pump is LH thread. cam is normal. By the way, I have never done it this way before but I think it would work.

Cheers,
Tom

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If done insitu the crankshaft can be stopped from turning if gearbox is placed in gear and the rear brake firmly applied .

HRD said:

If done insitu the crankshaft can be stopped from turning if gearbox is placed in gear and the rear brake firmly applied .

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That is certainly going to show up any clutch slip you might have, and your brake better be good too....

Alternatives include the correct clutch sprag...which may be used in conjunction with a the left footrest depending on your set up...

On the bench, or in the bike with the alternator rotor/engine sprocket/pulley removed I have a broached piece of alloy hex bar that forms a tool that fits a 52mm socket with a breaker bar!

I am still running the 50 year old original chain. Like you, I had neglected it for a while, and sure enough, it was loose. I picked it up when I was changing from a Boyer to a Trispark, and I was having trouble keeping the timing set point during installation. I had no miss fire, but the engine was not running as sweetly as it should.

When is the miss fire occurring? Under acceleration? Under part throttle? at idle? In a specific rev range? Other?

Also, what ignition system are you running?

I doubt very much that installing a new timing chain alone is going to fix your problem...

Time for exploratory surgery. Fitting a new chain or adjusting the old one properly won't make it run worse. But I gotta say, it sounds like a carb issue!

HRD said:

If done insitu the crankshaft can be stopped from turning if gearbox is placed in gear and the rear brake firmly applied .

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yes. that would work too.