Imminent Catastrophe?

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Expat
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Imminent Catastrophe?

Post by Expat »

Hoping that is not the case but can longer ignore the elephant in the room, so looking for reassurance from the collective.

It seems the old G3 has small end rattle as far as I can tell from researching here. Nice and quiet on tickover, not much different when revved up, no camshaft end float rattle now but.... on riding I can hear a rattle just as the revs build and it's gear change time. Then, when cruising on the flat at around 30-45 mph on a steady throttle, all is fine until throttle is opened slightly more, the light rattle starts, then quietens down if throttle is closed again.

So it's a engine speed tapping/rattling under light load. Not pinking, carb slide or something falling off.

What's the likelihood of something letting go, please? Got quite attached to the old girl and would hate for that to happen. :cry:

The obvious thing to do is to investigate and I will if necessary, but is it known that these old bikes will just keep on plodding if not abused?

Thanks for any and all comment,


Cheers, Steve
Keep shiny side up.

These are my principles, if you don’t like them, I have others. (Groucho Marx)
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clanger9
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Re: Imminent Catastrophe?

Post by clanger9 »

Small end rattle is highly unlikely. Big ends usually rattle more on light load.
Audible light rattle under increased load could be the primary drive. Main bearings tend to be a noticeably heavy knock and/or vibration.
Check everything is OK in the primary - could be just the chain is a little loose...
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Braaap
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Re: Imminent Catastrophe?

Post by Braaap »

A loose little end has a double knock Ta-tap, Ta-tap, Ta-Tap...

A loose big end will rattle under light load, especially with full advance.

Try cruising at a steady light throttle, (or even at idle), and playing with the advance/retard. As full advance is selected it will start to knock.

If it is a big end bearing... it will keep going for some time with no drama. AMC bikes are infamous for running sweetly when absolutely knackered.

My first G3L had a crook bottom end. Including a timing side bush so loose that the crank was starving for oil and the VERY loose big end was blue from heat.
Still ran 70mph with no problems.

The F. Neill book i have says, "8 to 10 thousand miles can be ridden with a loose big end".

These old girls are STRONG ☺

(In the case of my CS with a loose fitting big end pin in the flywheel.... an early strip down would have saved a lot of damage. But that motor was rebuilt by an idiot.
A typical engine will rattle slightly without hurting anything).
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Rob Harknett
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Re: Imminent Catastrophe?

Post by Rob Harknett »

Could be anything. Loose fitting like tank bolt even. For years I had a rattle and eventually discovered it was a loose rear light lens ( little round prewar rear lamp ) Another similar rally then started, not the rear light this time. Then one day I got almost home, dropped a gear, open up, bike never went any faster, it slowed down and stopped. Gear box sprocket had come off, the nut must have been off for ages. As when I put sprocket and nut back on the rattle stopped. I never considered engine rattle, as that had been rattling for over 20 years and never got any worse. Even front wheel rattle even turned out to be a loose faring fitting. I'd check all fittings and chains etc. Then see if you find the rattle. If not wait till you can hear it increase and able to trace what it is. These annoying rattles are often some silly.
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Re: Imminent Catastrophe?

Post by Braaap »

Good call Rob.
I once pulled the heads and barrels off a BMW R100S looking for a banging noise that came on with power. Turned out to be loose exhaust pipes :oops:
Last edited by Braaap on Tue Oct 02, 2018 11:01 pm, edited 1 time in total.
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Rob Harknett
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Re: Imminent Catastrophe?

Post by Rob Harknett »

I had a new car, the engine whistled from 50 mph up to 60 mph. The mechanic took it down the road, came back and said its your Radio aerial. This was the old type metal telescopic type, it had a split in it, acted like a whistle reed. He was puzzled, until he wound the window down. The aerial was fitted just in front of the window. So he heard it louder then noticed the split.
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Expat
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Re: Imminent Catastrophe

Post by Expat »

Thanks for all your speedy replies chaps. Now if only I'd asked you months ago, it would have stemmed the flow of my concern somewhat....a problem halved is a problem shared, or summat. :)

Interesting that I seem to be way off with my line of thought, but I freely admit I've very limited experience of engine rattle causes. Thought big end knock would have been a deeper sound under load, but then it's a small, low power engine and as with all unmeasurable things, it's all down to past experiences and knowledge gained.

I'm convinced the noise is coming from the engine and can't play with the timing on the fly as it has auto a/r, so will be gentle with her and monitor the severity, if I can! Meanwhile, will check the primary drive out.

Did once spend weeks trying to find the cause of a rattle from the rear of my wife's Peugeot 205. Turned out that the spare tyre had deflated in it's carry rack under the back of the car, so the tyre couldn't prevent the wheel and rack jumping around over bad road surfaces.


Cheers All for the advice.

Steve
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These are my principles, if you don’t like them, I have others. (Groucho Marx)
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Re: Imminent Catastrophe?

Post by aobp11 »

Steve,

The explanation for big end rattle seems to be as follows. Consider the piston movement near TDC. When there is no (extra) pressure above the piston then the piston+conrod is pulled down by the crank. The bigend will be constantly under load and will not rattle. When you slightly open the throttle, the cylinder pressure pulls the piston down and less force is required from the big end. At some throttle opening the bigend becomes more or less "floating" near TDC and start to rattle. At still more throttle the cylinder pressure becomes the major force source and the bigend will again be (near TDC) constantly under load.

Albert
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clive
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Re: Imminent Catastrophe?

Post by clive »

Have you been around the engine whilst running with a long screwdriver as a stethoscope. ( better still a real one). This will confirm where the noise is coming from. My thoughts carb side flutter, timing slide could be cams needing shims, tappet noise. In my experience small end Bush doesn't cause a noise when worn rather it allows the piston to rock side to side with a significant increase in oil consumption.
clive
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Expat
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Re: Imminent Catastrophe?

Post by Expat »

Hi Clive,

Yes, I do have a mechanics type stethoscope, two earpieces like a doctor's and has a steel rod to probe around with. Surprising how sensitive it is.
Trouble is, the engine is smooth and quiet running when stationary, even when you rev it up.

As I mentioned, it's not the carb slide, (new carb with duct to remote, oem filter), or camshaft end float,(just shimmed them as they were clacking, not the same rattle I'm hearing now), don't think it's tappets as adjusting them makes no difference.

Oil consumption.....hmmm. Oil level does drop steadily, no smoking noticeable at idle or when riding. Drip tray placed under the bike has maybe an eggcup full in it after 3/4 weeks with occasional commute riding during that time, more oil being left at work obviously!

Can only think oil is possibly being lost when on the move, maybe from the breather? Primary isn't filling up. Compression seems good, performance seems good, bearing in mind I have no comparisons (except a 1300cc four pot Yamaha!)

I have fitted a remote oil filter in the return line to the tank, but that was about two years ago.

I digress. The rattling concerns me more than oil consumption.

Thanks for your reply.

Albert, thanks also for your interesting reply.

Steve
Keep shiny side up.

These are my principles, if you don’t like them, I have others. (Groucho Marx)
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