Engine Knocking....

[Denis Sweeney]

My Ruby has developed a disturbing knocking noise.

Having completed some 2100 miles in the last 5 months including the Eurotour and attending Moreton, a few issues had presented themselves involving worn bushes in the distributor, leaky fuel pump and carburettor problems. All these have, in recent weeks, been rebuilt and re-installed.

On completing these adjustments Ruby ran, albeit she felt and sounded a bit chuggy and clanky, but after several adjustments to the ignition first retarding then tweaking it back and forth, she sounds a feels so much better than she has ever been, I actually find myself having to take my foot off the accelerator as she just wants to go.

However since this recent work, I can now detect a definite knocking noise.  At tick over, it cannot be detected even when revving the engine whilst stationary it is not there, it is not noticeable accelerating in first and second but appears at the top end of revs under load in third and all the time in forth. The rate of knocking increases with revs.

I have never heard this knocking before and is causing me concern as my experience in these matters is very limited. I do not want to ignore it, but do not want to start stripping down for no good reason.

Ruby has a 2 bearing engine with a high comp head and was rebuilt many years ago but has only done 3100 in total. The rebuild included new main bearings, new white metal bearings, bores re-sleaved, new pistons, valves  etc. but it is the original crank and cam shaft.

Can anyone offer any advice or does anyone with experience of A7 engines reside in the Kent area who could have a listen or could point me in the direction of a trusted A7 mechanic or workshop dealing with A7's in the Kent area who could have a listen and offer some advice.

Any thoughts much appreciated
Cheers Denis S        

[David Stepney]

Little end? I had a similar problem with my RP. It sounded a bit like a big end rattle. I took the sump off and examined the bearings and all was in order. So, more in desperation than diagnosis, I pulled the rods and pistons out and examined them. No: 4 little end had just the merest amount of 'rock' in it. I replace the pistons and the noise disappeared.

[John Mason]

It could be a number of things but start first with the easiest to examine. I would first check the camshaft end float. If that’s excessive it can produce a noisy tapping sound. I am not ruling out a little end but it’s easier than check the camshaft first.

John Mason.

[Ruairidh Dunford]

Try shorting each plug in turn.

[Bob Kneale]

A loose flywheel could also be the problem - I've had that on my box saloon, and at least the Ruby engine is easier to remove the engine than the former.
Certainly worth investigating that before disturbing the bottom end.
Good luck with your investigation.
Bob

[Howard Wright]

Hi Dennis

I may be asking Granny to suck eggs but are you sure the timing is right? Too advanced and the engine will knock but will seem to be performing well. One definite advantage of manual advance and retard is that you can easily retard whilst driving  .

Could be worth retarding the ignition a degree or two?

Cheers

Howard

[Mike Costigan]

A loose flywheel could also be the problem - I've had that on my box saloon, and at least the Ruby engine is easier to remove the engine than the former...

Bob

A loose flywheel usually knocks on the over-run rather than under load.

[Denis Sweeney]

Howard, cannot be certain the timing is spot on, but started from a point of being too far advanced with the engine sounding like a tractor thro to too far retarded and the engine kept cutting out so back again till it ran well.
I will try advancing in small increments again in the coming days and also try Rhuairidh’s shorting the plugs and let you know.
Cheers for the comments

[flywheel1935]

If it is the cam, you will find the timing will be all over the place as the helical gear alternates at the extreme ends of the wear. (don't ask how I know !!!!)
Also if its flywheel ? then noise should stop when clutch is pushed .

[John Cornforth]

Hi Denis

Regular engine noises can be more or less divided into two types, those that only depend on RPM regardless of load, and those that also depend on load i.e. throttle opening / combustion pressure in the cylinders. The first category can be heard if you gradually rev up with the car stationary.

From your descriptions, the noise would seem to be in the second category. This points towards big or little ends. The ignition timing will affect combustion pressure, with more advance leading to higher peak pressure and more loading on the big and little ends. An HC head will also increase pressure. Pressure is highest at around the torque maximum at roughly 2500 to 3000 RPM, but will drop away at higher RPM for a standard inlet/carb due to restricted breathing. The general racket made by an A7 engine at high RPM may also mask noises to some extent ! Colder/thicker oil before the engine is warmed up can also temporarily quieten this sort of noise.

If you have any slack in the distributor or the various gear pairs between crankshaft and distributor, this can cause the timing to have scatter, with some firings having slightly more advance than others. I suspect this gives an uneven slightly "clattery" characteristic. A timing strobe light shone on a mark added to the camshaft pulley can reveal this as a blurred rather than a clear mark. The only cure other than eliminating wear is to back off the advance a little. Bear in mind that to retard the timing by 4 degrees you need to rotate the distributor clockwise by 2 degrees, which isn't very much. The DK4A body has a circumference of about 225 mm, so 2 degrees shift is 1.2 mm. If you still have the little plate and pointer, this is one division.

The only other thing I can think of is that the HC head has a flat area that overlaps with the piston crown. The clearance is very small, I have no exact data but I believe the pistons are very slightly below the top of the block at TDC, so it's not much more than the thickness of a compressed HC gasket, perhaps 30 thou. This assumes the top face of the block hasn't been lowered by refacing. Clearly, if you have any extra clearance in the big/little ends or any crank whip at high RPM, it might just be possible to run out of clearance especially on cylinders 2 and 3. I don't know if there is a safe and reliable way of checking clearance without dismantling, e.g by putting soft solder wire into the gap and seeing how far it is squashed down - maybe others could comment ? If the clearance is too small, a metal spacer shim between crankcase and block could be useful but would of course require dismantling.