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Mag Chain Madness MK2

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Mag Chain Madness MK2

Posted by George Phillips at Thursday 12:39
My mag chain goes from slack (up to 1/2") to really tight as I turn the engine. I've fitted a new inlet cam bush so would guess that that end of the chain is running true so the only other culprit must be a wonky armature shaft? Or dodgy sprockets? It's a new chain. Any views? Cheers, George

Re: Mag Chain Madness MK2

Posted by ian_cordes at Thursday 13:29

Could there be a tight link on the chain, perhaps? That could cause the effect you are experiencing. Clutching at straws now really......

Re: Mag Chain Madness MK2

Posted by George Phillips at Thursday 16:47
That was what John thought on the Mk1 posting. I'll check it out. G

Re: Mag Chain Madness MK2

Posted by David Cooper at Thursday 17:06

This always confuses me.  If the total distance around the two sprockets is fixed then the chain cannot go tight and slack because the chain only has one length.  So if its tension does vary, surely it must follow that the distance around the sprockets varies?  In which case, the hole in the middle of one or both of the sprockets is not the middle of the circle described by its teeth.  So sprocket must be wonky.  I suppose uneven chain wear could be to blame if it is so bad that the chain won't sit down the same depth into the sprockets at different positions.

If the tight and loose locations occur with the sprockets at opposite 180 degree angles, then the sprocket(s) are to blame.  If they occur when the same bits of the chain are in the same place, then the chain is at fault.  Maybe the relative positions of the sprockets can be changed if it's a sprocket issue?  So the offset bulge (if they have one each) always has the same angle to the horizontal.

Re: Mag Chain Madness MK2

Posted by George Phillips at Thursday 18:23
Interesting David. I did mark the mag sprocket and noted that it always pointed in the same direction when the chain went tight. BTW, it's a new chain. I'll check the trueness of both sprockets tomorrow and go from there. Many thanks, George

Re: Mag Chain Madness MK2

Posted by les_howard at Friday 09:57

People often do not compensate with the appropriate back pressure rotation when measuring chain tension. This is profound with the high loading of the camshaft timing chain but still applies to the weaker rotational loading that a magneto creates, Trick is to rotate the DRIVEN sprocket/gear TOWARDS the turning force direction(to oppose it) that the chain is creating at at any time...then measure the slack....Les

Re: Mag Chain Madness MK2

Posted by jonathan_newton at Friday 10:25

Les, doesn't that translate to the other chain run though?   This could be the reason George is seeing a variation in tension if he is only testing at one point on one run.  Come on George you must have progressed by now or are you on an Easter break wink

I am currently rectifying what looks like the consequence of an over tight mag chain that created a fracture in the Mag platform when the motor heated up.   It had been repaired with a cold stitch weld and a supporting plate back in the 60's.  I am grateful for the methods and ability of modern welders.

Cheers

Jon

Re: Mag Chain Madness MK2

Posted by les_howard at Friday 12:21

Hi Jonathan. I know what you're saying. With the cam chain the slack is "hidden" by the chain tensioner and one tests on the top run which can feel bar tight but if you do the correct back rotation you get to feel all the available slack (dummy cover fitted incidentally)

The same mistake can be made on the mag chain but not so profound...It is easy to just feel one run do the the rotation and check the same spot thinking it has tightened or become more loose due to the loading change.

What one also has to remember is that any difference caused by sprocket wobble is amplified by a factor of 4 times!...Eg: a 1mm wobble is + and - 1mm = 2mm..... then there are 2 chain runs (top & bottom) so that makes a 4mm difference in overall length of the chain run between tight and slack so it doesn't need much of a wobble to cause a big variation.....Les

Re: Mag Chain Madness MK2

Posted by George Phillips at Friday 15:44
Update guys. Checked sprockets/chain with engine on retarded tick-over but before I could assess anything the pointer jumped and got caught between the chain adn the sprocket causing an instant stop. AS I fugured it had probably moved the mag I had to strip it all down and reset the chain tension and the ignition timing. Amazing how much quicker one does it each time! End result - couldn't detect any wobble on the sprockets although running the chain round the sprockets when both were off the machine it gave the impression of being a tad too big but it was bought from Andover Norton and has the correct part number. In the end I set it very slightly slack at the tightest point and will live with the over-slack on the sloppy part. It doesn't touch the chain-case so should be OK. Disappointing not to have got to the bottom of it though. George

Re: Mag Chain Madness MK2

Posted by George Phillips at Friday 20:30
Just lost an upload. To answer Jon's point about checking the tension at only one point, the chain was so rigid on both the top and bottom runs that the point is irrelevant. I'm just wondering if the renewal of the inlet camshaft bearing (which was allowing copious oil flow into the timing chain case) was the result of this chain tension? If so it's likely to happen again! George
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