Advice on Venturing into my G80 Engine

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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Advice on Venturing into my G80 Engine

Postby Iron Head » Fri Oct 19, 2018 11:10 am

Having put about 70 miles on my new to the road '48 G80 and in spite of excellent performance I am going to have to 'bite the bullet' with regard to the unsettling (possibly half engine speed frequency) mechanical, pulsing, rumbling I am experiencing (most noticeably) at tick-over which sounds something like the noise that Dr Who's tardis makes?!

I understand that whatever I reveal I will probably have to go on into the engine further but I was wondering if it might be something like timing gears, camshaft lobe wear, bushes or even, having read recent posts, a broken oil pump shaft. Sorting my worn out CP clutch hub certainly took one of the parts of the noise away but it still doesn't sound good and can even be felt through the bars on tick-over. My main thought is rumbling/damaged mains, the bike stood for probably 50 years but there would have been oil present and also the big end feels and sounds fine to me. No swarf is revealed from the sump plug but I don't know if that always means a lot particularly if it is knackered bearing races. Can the gearbox mainshaft generate noises like this in neutral and in drive?

Any thoughts on where to start first would be appreciated, it would be great if I stumbled upon something relatively exterior rather than having to go the whole hog, plus the bike appears to be pretty low mileage from the condition of all sprockets etc.

Thanks

Iron Head

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Re: Advice on Venturing into my G80 Engine

Postby Rob Harknett » Fri Oct 19, 2018 12:49 pm

Some noise can be heard/felt simply by not having steering head bearings adjusted up correctly. Also tank rubbers tightened up. Rubbers should not be compressed at all. Bolts nipped up then wired on. If you have not wired up bolts, then I guess they have been tightened. Check all chain tensions are correct. Having done the simple things, your bike will still make some odd noises that will worry you. It will, try not to worry. If the bike is clean inside, some noise will quieten once carbon builds in engine and exhaust.

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Re: Advice on Venturing into my G80 Engine

Postby Mick D » Fri Oct 19, 2018 12:50 pm

Hi

It does sound like a worn big end or mains from your description, however my first action would be to get a long screw driver and place the tip on various points around the engine whilst resting your ear on the handle end - this will act as a sort of stethoscope and allow you to more accurately pin point the source of the noise.

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Re: Advice on Venturing into my G80 Engine

Postby clanger9 » Fri Oct 19, 2018 12:56 pm

My G2 did something similar after it had been left standing for a couple of decades. I ran it for a whole year before stripping it down: it turned out that the inner main bearing track had broken up. Probably the rollers had corroded while it was standing.
There was was no other engine damage, other than the usual wear & tear.
Good luck!
1966 AJS Model 14 250 CSR "cafe racer"
1993 Ducati 750SS

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Re: Advice on Venturing into my G80 Engine

Postby Iron Head » Fri Oct 19, 2018 6:00 pm

Many thanks for the feedback guys. Something like cracked inner races on the main bearings may well be the culprit! Will report back on my investigations.

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Re: Advice on Venturing into my G80 Engine

Postby Rob Harknett » Fri Oct 19, 2018 9:04 pm

Ah many years ago after a winter lay up I had a noisy engine, you could hear some was not right just turning over the engine. That did turn out to be rusty main bearings. As mentioned above Now this noise was something different that was not there prior to its 3 months off. A few years prior to hearing that engine noise when just turned I heard a similar noise after my trailer had not been used for a while. There was nothing in the wheel other than bearings it could have been. I have also had front wheel bearings make that same noise back in the 60's. I would describe it as a grinding rumbling / sound. I would add, since my engine bearings went rusty over winter, they have not do so ever since, That's over 20 winters ago now.

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Re: Advice on Venturing into my G80 Engine

Postby Iron Head » Sat Oct 20, 2018 8:55 am

Thanks Rob some very good points and observations there. Also the bike had been barn stored with its primary chain cover off and there was considerable rust on the shock absorber, clutch basket etc. May well be that rust transitioned through to the roller bearings in the light of your winter layoff.

Iron Head

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