On one of my engines is a chain driven magneto. At the moment the engine is running nicely, but the chin is flopping around everywhere, the trouble is that when I increase the tension on the chain to what is correct, I give the engine around =BD a turn an the chain has become extremely tight, turn it back again and it is at the correct tension. I cannot understand why this is happening, as the sprockets appear to be in line, and no adjustment the other way improves the situation.
Does anybody know what the problem could be? Is it the chain?
Either the chain has a tight spot or one of the sprockets is running out of true.
On one of my engines is a chain driven magneto. At the moment the engine is running nicely, but the chin is flopping around everywhere, the trouble is that when I increase the tension on the chain to what is correct, I give the engine around ½ a turn an the chain has become extremely tight, turn it back again and it is at the correct tension. I cannot understand why this is happening, as the sprockets appear to be in line, and no adjustment the other way improves the situation.
Does anybody know what the problem could be? Is it the chain?
At last, something I can offer in return for the odd bits of help I've had over the last couple of years ;-)
I've had similar problems with bike and motorbike chains.
The suggestion that the chain has a tight spot is good one. To check for this, take the chain off the sprokets, lay it on its side on the bench and stretch it gently. If there are a pair of links sticking up or down slightly when it is pulled reasonably tight then they're probably the problem. Wriggle the two suspect links with your fingers, and compare them to other pairs of links. If they feel tight you've definitely found the problem.
How to fix it? If the tight links just feel gritty or rough a damn good cleaning may shift any grit or fine stuff jamming the sideplates. Let it soak in a solvent for a while and see if loosens up.
If they're just tight *careful* use of a rivet punch or screw operated 'chain breaker' will let you 'ease' the link a tad and all will be well. A combination of both soaking and easing will save an otherwise good chain from the scrapheap.
If your chain is an old Imperial size, but has a connecting link which is newer than the rest of the chain, it may be that it is a 'metric equivalent' size which is a tad tight. In this case get a proper Imperial link. The situation could of course be the reverse, a metric chain with an imperial link. These mis-matches between 'should be okays' can be a real bummer ;-)
The simple solution is to just buy a new chain, but it won't last long if there is also a sprocket problem.
Moving on to the sprockets. Just because they are neatly aligned with each other as regards the chain run doesn't mean they are running centrally on their shafts. They only have to be a little off centre to constantly change the effective length between centres, tightening and loosening the chain. Very few sprockets are ever perfect, which is why you always need that bit of slack in a chain even if it is running between fixed centres, and is also one of the reasons why many motorcyclists run a spring loaded chain tensioner.
Cure? Sometimes twisting the sprockets around a bit on their shafts will help. Sometimes this will make it worse, which is just added confirmation of the diagnosis. If they're keyed on then this isn't a viable solution.
If new parts are difficult to obtain there are firms who will turn off the old teeth on a lathe and fit a new toothed rim. (I'm sure someone on here will be more up to date on likely providers of this service than I am. (There used to be an excellent firm called 'SuperSprox', no idea if they're still in business.
Finally, the sprockets may be running dead centre and in perfect alignment but still have 'hooked' teeth if they have been previously abused. If the chain has been run too loose for too long the teeth develop a definite 'set' as a loose chain pulls on the tips rather than bedding down properly and pulling against the whole tooth. In extreme cases they look like a crooked finger. The problem here is that these 'crooks' tend to grab the chain and tug it downwards rather than releasing it cleanly to 'flow' onto the other sprocket. if this is the case replacement or re-toothing is the only answer. Hooked sprockets will destroy a new chain in record time.
Have a similar probelem on Scott SE; crankshaft and mag sprockets appear to run true and chain (1/2" x 1/8" (O81?)) is brand new, but it still runs tight and loose over the course of a revolution. I shall look more closely at the condition of the sprockets.
PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.