rat-a-tat-tat-tat -- pre-ignition?

Jon B

I recently tried to time my 212 splasher 6 (1937) using a vacuum gauge. I hope to get better gas economy by advancing the timing a bit from the UDC flywheel setting, which was established at a time when gasoline was much inferior to today's gas.



After preliminary adjustments, I drove on the highway to test for pre-ignition 'ping'. When accelerating, I heard nothing I could identify as 'ping', but as the engine further accelerated to its shift point, I began to hear a rat-a-tat noise coming from the engine. (This would disappear as soon as I upshifted.) Later, after I re-set the timing back to almost UDC, I still heard the noise at increased speeds.



In fact I've actually heard this noise for several years, when the engine speeds up just before an upshift. I had actually put it out of my mind, since I heard it all the time. This is why I wonder if it actually IS 'ping'. I don't suspect anything internal in the engine, like loose rods, since the noise comes and goes. Might it be a loose exhaust damper shaft, or could this even be something like 'floating' valves (loose valve springs)?



If this rat-a-tat is a pre-ignition sound, wouldn't that occur during acceleration -- when the engine is under load? My car doesn't have a vacuum advance; does that have anything to do with determining when I'd hear the 'ping' of pre-ignition? Maybe there is a 'late' ping on cars without vacuum advance?



If the rat-a-tat-tat is caused by something besides pre-ignition, then at least I can advance the distributor again, and get a bit better mileage!



Anyone have experience with a noise like this?

Geoff

Jon, I recently heard a new corolloray to Murphy's Law - "If it aint broke, fix it until it is". It sounds to me like you have a slightly loose connecting rod. It will not be pre-ignition, this would only be under load at high speed going up hill, for instance.

Unknown

I agree with Geoff. Slightly loose rod are babbit begining to chip out. My old splasher 8 would tell me when one was going bad when I would wind her up in Second and overdrive.

PAULARGETYPE

now whats a guy to do! he tryed to save a few pennys on the gas and now has rod bearing problem it will only cost a few thousand $ to fix this what a savings !!!

Jon B

Possibly it was pre-ignition. I set it way back to UDC and drove it about 4 hours today, and the rat-a-tat noise is almost non-existent. A friend with a Hudson 8 told me today that, pre-vacuum advance cars 'knock' in the higher ranges of speed rather than in the lower ranges (which is normal for most cars) so, pre-ignition may indeed have been the culprit.

Geoff

"Almost non-existent"???? If it was pre-ignition, then retarding the timing would eliminate it. By retarding, you have lightened the load on the bearings, and lessened the noise. Check with the engine cold, this is when the noise will be most noticeable. Advance the timing, and bring the revs up slowly. If you hear a knock, short out each plug in turn. If one makes it go away, you have the source. Good luck,

Geoff.