Could it be Pinking?

[Martin Baker]

My mildly tuned 1938 Pearl (3 bearing crankcase, Phoenix 1.5 inch crank and rods, Paul Bonewell cam and followers, Dave Flake ported block with o/s inlet valves, Nippy manifold and downdraft Zenith, Accuspark ignition, Dave Dye Ricardo head, 5.25 rear axle, Nippy gearbox) exhibits a dreadful, penetrating noise, (a bit like something loose around the battery box), when on power between about 3 and 3,500 revs. Below 3000 revs very smooth and above 3,500 smooth again.

I've checked that there isn't anything loose under the bonnet. I timed it by feel at tick over. Could it still be too far advanced? If it is extreme pinking, how sensitive to timing is it?

Suggestions/advice please?

[Duncan Grimmond]

IF it's extreme pinking the way to check is to retard it a little from the sweet spot you found at tickover? Then try it again but avoid running with too much pre-ignition or you'll do serious damage...
Acuspark should behave smoothly, in my limited experience anyway.
I've never come across valve bounce on a side-valve but that may be good fortune!

[Nick Salmon]

Martin. Do not exceed 2,999 revs or very quickly get to 3,501. Problem solved. Simples. (Apologies for flippancy. Hope you find it).

[Alan]

Was the crank/flywheel assembly balanced when the engine was built?

[Bob Kneale]

Martin
If retarding the ignition still produces the same symptoms check the secureness of the camshaft front bearing. I had the same on my ruby 3 bearing engine and solved it by deleting the 1/4" dia dowel and square headed set screw with a 3/8" BSF bolt threaded through both crankcase and bearing similar to what has been written in other posts recently.
It could well be your high spec camshaft that has caused the square headed set screw arrangement to give up.
Good luck
Bob

[John Cornforth]

Hi Martin

With A7 type compression ratios on modern 95/98 octane fuel I'd have thought pinking would be extremely unlikely. Harshness from too much advance can happen of course, but it doesn't match your description.

I would be more inclined to think that engine vibration is exciting a resonance in something loose around the bulkhead area. Another thought is that the exhaust pipe doesn't have quite enough clearance somewhere along its length. It could be something as trivial as the number disc at the base of the gear lever.

[Chris KC]

Martin, there isn't anything in your post to indicate what compression ratio you are running or how much advance is actually set. Given that you have set the latter 'by ear', you may reasonably not know this of course, but it would be fairly easy to check.

The noise of pinking is quite distinctive and although I've heard it described as the sound of a hammer hitting metal, I remember it more like the sound of a wine glass breaking. I say 'remember' as I haven't heard it since the 1970's, despite on at least one occasion running a ridiculous amount of advance in error. Modern fuel simply doesn't do it at any plausible advance setting for a road-going A7 (others please feel free to correct me!)

I would certainly check where your ignition is currently set as a point of reference, but my guess would be you are looking for something else.

Is the car new to you? Or has this noise developed over time?

[John Mason]

Going back to the 70s recall the pinking sound as something resembling that of a sewing machine under the bonnet. Martin is you sound similar and does it increase in speed with the increase in engine revs.

John Mason.

[Bob Culver]

Few moderns have heard pinging. Here it was also called "bottling" because it sounded like some one shaking a crate of beer bottles. My car at 7:1 used to climb at full throttle and revs in the gears for about 2 miles evry afternoon on my way home. A short flal at the summit was fowed by a sharp upwrd turn so had to back off momentarily in 3rd. When planted again the unburned oil, caused a short burst of pinging (and run on). Prolonged pinging however faint can be very damaging. Brief pinging is mainly a  cause of broken rings, although malleable irons very resistant.

[Martin Baker]

Thanks everyone for prompt suggestions.

The noise is definitely more like a loose panel resonating but I can't find anything. I have checked that the floor screws are tight, I have in the past tried isolating the flywheel cover. One new idea is to make sure the exhaust pipe has enough clearance from the chassis.
Other points:

• Always try to use high octane low ethanol fuel.
  
• no run on.
  
• plugs show running rather rich and poor consumption
  

In answer to the various points raised:

• crank & flywheel where balanced
  
• front cam bearing has standard dual O ring and cap screw mod
  
• tried emptying battery box/wrapping tools in rags
  
• compression ratio - I undertsand the Ricardo head is 6.5:1
  
• car not new. Have experienced previously but came to fore during 700 miles on TOTIE last week, especially the 140 miles each way.
  
• once in resonance range, noise increases with engine revs.
  

I'll keep exploring but grateful for any more ideas.