Hideous Noise With Increased Revs

[Badgertadger]

Hi,

I have a horrible noise on my '67 Li 125 Special.

The bike was a non-runner when I bought it but it was only a sticking float was causing it to flood, so that was easy enough.

However, after a couple of minutes of riding, there was a horrible metallic rattling/clunking noise as if a large piece of something was being thrown around inside. - I was not thrashing the nuts off it, just pootling around on local roads.

When the revs dropped the noise stopped, so I thought it might be the chain whipping against the casing or something.

On removing the engine side cover, there was no obvious signs of contact anywhere, but I replaced the chain and it's guides (from undamaged original to the green nylon ones), also I replaced the clutch and replaced the gearbox end-float shim as this was towards the upper end of the allowed tolerance.

However, Once back together and running for a few minutes, the noise was still there.

If I'd thought about it I would have recorded it, but I really didn't want to run it again as it does sound nasty.

Fortunately, I found someone on Lambretta Club USA who described the same problem and who did record it (see below).

[youtube-share]https://www.youtube.com/watch?v=mu0se2ckZfA[/youtube-share]

Unfortunately, no solution/cause of the problem was posted on that forum.

I now have the engine stripped down again, including the top end and also crankshaft out, but still there are no signs of any damage/contact/loose bits of debris.

The barrel and piston have obviously been fairly recently replaced (with a 150cc setup) as they look in good condition with no scores, lips or signs of over-heating.

There are no sings of play in the crankshaft main bearings.

The big-end end-float is a little over 0.4mm and the small-end end-float is about 2.6mm. There are no shims at either crankshaft or piston end, but my understanding is that the big-end is tight enough for the top end not to need shims. (happy to be corrected!) There is no play in either bearing in the longitudinal direction and the lack of damage to piston or bore makes me doubt it is that.

Two things concerned me during the strip down;

The first was that after removing the circlips, I was able to push the gudgeon pin out with my finger. Don't get me wrong, it wasn't 'really' loose and certainly doesn't look worn, but I'd read that often a special tool was needed to get them out as they were almost an interference fit.

The second was that there are some marks on the crankshaft taper that correspond to some marks inside the flywheel boss, but there is no damage to the woodruff key. There does appear to have been some rubbing of the flywheel against the stators, but I find it hard to believe that this would cause this horrible noise.

Sorry for such a long first post, but I am pulling my hair out.

Any help will be very gratefully received.

Regards,
Daren.

[Eden]

are the rivets on the clutch spider ok? no movement?
bearing under the gear tree ok?

[Scooter Paul]

I had exactly the same thing on a 125 special. It was the fins on the flywheel. When the engine was cold there was no noise. Once it had got hot it sounded like there was something seriously loose inside. I put on a spare flywheel and it was cured! The fins could be rerivetted I guess.
Paul

[Wack]

If it has a Dansi or Filso flywheel with the cast in boss bin it anyway as the boss comes loose in the alloy. Whilst the crank is out I'd get it checked for true and change the big end bearing at the least if you don't want to upgrade.

[Adam_Winstone]

Most modern pistons have a finger push fit tolerance on the gudgeon pin so you've nothing to worry about if it is not loose. Indeed, I no longer tolerate the sort of crazy interference fit that original Lambretta pistons (incl. aftermarket) had and will always increase clearance if I'm fitting one of these so that it becomes a firm 'by hand' fit.

Adam

[Badgertadger]

Thank you all for such quick replies.

If it is flywheel related (and the closer I look at it, the more signs I'm seeing), then perhaps this is an opportunity to move to the 'dark side' and go for electronic ignition?

Getting the crank checked for true and replacing the big-end bearing sounds like a very sensible idea.

In an attempt to rule out the gearbox as a cause, I put the rear sprocket and clutch cage back on and spun it up to 2000rpm with the help of some cable ties and my drill. Pleased to say that it all sounded fine.

I'll do some research on where to take the crankshaft, and also on electronic ignition systems.

Thanks very much again.

[Eden]

Most modern pistons have a finger push fit tolerance on the gudgeon pin so you've nothing to worry about if it is not loose. Indeed, I no longer tolerate the sort of crazy interference fit that original Lambretta pistons (incl. aftermarket) had and will always increase clearance if I'm fitting one of these so that it becomes a firm 'by hand' fit.

Adam

yes ALL good pistons have a pin that fits properly, if it needs nocking in or out its too tight imo.