Rumble in the Jungle

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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Iron Head
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Rumble in the Jungle

Post by Iron Head »

Still not got My '48 rigid G80 on the road but am nearly there, however test rides around the garden are revealing an unusual intermittent rumbling/churning from somewhere in the engine (which, a bit worryingly, can be felt through the handlebars) and I am wondering if it is main bearings? It is the intermittent bit however, that to me, is a bit puzzling.

The bike hasn't run since the 60's. A top end job revealed the big end felt fine (by hand) and I don't have a regular big end noise - nothing present too under load from the big end. This is the first bike I have ever owned with a magneto chain - can these create noises, the tension is fine. I rebuilt the magneto and the bearings felt fine but can these create this effect? I also noted I have a bit of crank end float but not substantial, again can this cause rumbling/churning noises?

I do know the clutch hub rollers are pretty corroded as the bike stood for probably nearly fifty years with the primary cover off, similarly the primary sprocket sprung drive was pretty corroded and revealed plenty of rust (from WD-40) during early start-ups.

The bike is a keeper and is starting beautifully and I am quite prepared to strip the whole bottom end and get things sorted but any thoughts/ideas would be very much appreciated in advance.

Thanks.

Iron Head
SPRIDDLER
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Re: Rumble in the Jungle

Post by SPRIDDLER »

The 'intermittent' bit is confusing. Can't really offer much with any confidence as far as the rumbling/churning goes, but a correctly adjusted mag chain shouldn't cause any noise like that.
As far as the corroded clutch rollers are concerned, the rollers only 'roll' when the clutch is disengaged. A rusty shock absorber spring wouldn't be noisy.
Crank end float is unlikely to cause noises unless it's so excessive that it allows the flywheels to rub against the inside of the crankcase.............
Having been laid up for 50 years and bearing in mind that the primary cover was off, and that the drive sprocket assembly and clutch was rusted it may be that water has got into the main bearings (they don't have any form of seal) and rusted them, in which case they would be pretty noisy.
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Iron Head
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Re: Rumble in the Jungle

Post by Iron Head »

Thanks for some great suggestions Spriddler, very much appreciated. The bike was dry stored with the primary cover off so the rust is atmospheric but I suppose it could have got to the mains after all that time. I am wondering about your flywheel suggestion but expect that would be pretty constant once the crank was pulled over to one side during running, or will the crank 'float'?

Anything to do with the oil pump? Return flow is pretty good and steady and pumps don't usually wear much in my experience but who knows?

Regards

Iron Head
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Re: Rumble in the Jungle

Post by SPRIDDLER »

When the shock absorber spring is fitted it takes up the crank end float by pulling the drive shaft 'outwards' but it's highly unlikely that the f/wheels are rubbing as this would only be likely after a careless rebuild or the partial collapse of a main bearing.
Another noise source could be from the dynamo, but I doubt that it would cause any noticeable vibration, neither would the oil pump.
If the noise occurs whilst stationary maybe a prod around th'engine with a listening stick might enable you to home in more exactly to the source of the noise.
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REW
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Re: Rumble in the Jungle

Post by REW »

Check your various chain tensions (and health of the chains) as I have found similar odd resonances in my single in the past, removed or at least changed by adjustment.
Ron

1951 Matchless G3L thumping round the Durham Dales.
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robcurrie
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Re: Rumble in the Jungle

Post by robcurrie »

Check the chains (most likely the final drive chain) for stiff - rusted links.

Rob C
MikeM.
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Re: Rumble in the Jungle

Post by MikeM. »

Main bearings rumble when worn.
g80csp11
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Re: Rumble in the Jungle

Post by g80csp11 »

id say its a gearbox issues , layshaft bushes perhaps
my 46 has a heavy rumble under load and not engine or clutch related . one day i will pull the box apart and repair
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Iron Head
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Re: Rumble in the Jungle

Post by Iron Head »

Thanks for the feedback everyone. I can advise that every chain is new (I'm not running a dynamo) and, hopefully, correctly tensioned. I am getting the noise both stationary and under way.

I'd wondered also about a bent gearbox mainshaft creating a tight/slack situation on the primary chain which might cause what I (perhaps incorrectly) described as intermittent as it's a 'pulsing' rumbling noise that seems to always be there - if that makes any sense!

Would be interesting to try and run it without the primary chain - pretty hard to do I would imagine without some form of decent power supply to the crankshaft sprocket. Anyone got any ideas?

Thanks

Iron Head
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Re: Rumble in the Jungle

Post by SPRIDDLER »

Iron Head wrote: I am getting the noise both stationary and under way..........I'd wondered also about a bent gearbox mainshaft ..........Would be interesting to try and run it without the primary chain
Running th'engine fast enough with an external power source sounds a bit of a rigmarole.

Why not come at it the other way round, i.e. coast on a long downhill stretch in Neutral with th'engine stopped?
Or as an even wilder thought, downhill, engine stopped, in gear (say 4th) with the clutch pulled in. Maybe there'd be too much drag or noise from the clutch when coasting in gear with the clutch pulled in but worth a try before resorting any other more challenging methods.
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