New (?) Comet Engine Noise

[Martyn Goodwin]

I have now put over 5,000 miles on my Comet since the major rebuild that did NOT involve splitting the cases, but did include a new piston and liner.

On a ride yesterday I thought I heard a new noise – a rhythmical knocking that changed frequency with engine speed but could only be heard on a neutral throttle – as soon as I rolled on even slightly positive throttle the noise was no longer audible (well not to me) and conversely, if I rolled off the throttle – engine braking, again the noise was no longer audible. In all other respects the bike is fine – runs very well, uses little oil and handles as well, if not better, than my 2013 Moto Guzzi V7.

It sounded like big end knock in a conventional slipper bearing motor. Oil level is fine, and the oil pump is doing its thing, likewise the tappets are at their correct (none) clearance when cold. Is this just me getting more and more accustomed to the bike – or could it be something to worry about? I have also checked the quill and it is intact and clear. I also removed the sump plug and inserted a magnetic wand which picked up a few wiskers of metal - though I think they came from the very worn valve stems - they were worn to a letter "D" profile due to misaligned guides - and this was fixed about 2,000 miles back.

Any suggestions???

Tempting fate – tomorrow I am heading off on a 400 mile 2 day trip. It is a shakedown for a 2 week-long tour in the second half of March when, along with 100 other pre 1955 British Bikes, I will be riding around Tasmania, with an expected total of 1,400 miles to be covered over 12 days (that EXCLUDES the ferry trip J).

Martyn

 

[Howard]

Noises are always difficult to describe, and even harder to diagnose from a description.

It does sound like a big end, but are you misleading yourself into describing a big end based on your assessment? ............... That's a bit deep for me, and I wrote it!!

How about small end bush?

H

 

[craig]

"a rhythmical knocking that changed frequency with engine speed but could only be heard on a neutral throttle"

In my experience, classic rod bearing failure noise in a Comet. The noise will become more pronounced with miles. I have seen a couple hundred miles completed with this noise.
Pull top end and you will feel the clearance by push pull on the rod. Cheers , Craig

 

[Big Sid]

Sounds like a light piston slap to me . What skirt clearance was allowed ? What type piston ? This was common in many original new Vincents , especially with 5 thou up skirt clearance . The piston pin can tap if a bit on the loose side also . Both lessen when the throttle is rolled on . Sid .

 

[clevtrev]

You have another (approx) 100 miles on your big end.

 

[vibrac]

You have another (approx) 100 miles on your big end.

about mid day on day 1 of the trip then trev....

 

[Big Sid]

Gonna be interesting to learn the follow up on this . Do let us know . Sid .

 

[Martyn Goodwin]

Took the bike out for a 20 mile run on Friday night - before my weekend trip and I was convinced the noise was worse than before - so the weekend trip was made on my new twin - A Moto Guzzi V7.

Just got back home and already I have the oil draining from the Comet. In the morning I will pull the primary drive cover to check the ESA and primary chain and if they seem OK then I will be pulling the head and barrel - stay tuned.

Martyn

 

[Martyn Goodwin]

Argh!! Just pulled the head and barrel and removed the piston. The con rod can be rocked SIDEWAYS about 5 degrees in either direction plus I can feel what seems like 2 to 4 thou of movement in the vertical plane. Strip down is underway.

Oh - of course nothing seems wrong with the ESA or primary chain.

Martyn

 

[Big Sid]

If the original crowded big end is still in place the side rock is expected , but not the up and down looseness . Thats proof of its being
clapped out . Sid .