Engine 'rattles' at high RPMs

[jkenan]

The late model A4 in my 'new' Ericson 29 is finally running smoothly (with much help, advice, and parts from Don!), except when I run it under load at full throttle. Not that I usually run it this way, but motoring through head seas in a squall, you need that extra push. My concern is that I hear what seems to be a fairly steady rattling sound when running it this hard. At first I thought it might be one of the slide brackets that holds the compartment cover in place, but both were held fast. After removing the cover, I definitely ascertained the sound was coming from the engine itself, and not from any brackets or fittings on the exterior - it was definitely coming from inside. Lowering the throttle a bit caused the sound to stop, but with that also comes a loss of speed. Not a good thing when your only moving 4 knots at full throttle.

Any ideas on what this sound is, and how to stop it? I invested in Indigo's 3-bladed prop, and want to be able to count on the extra power when I need it, but certainly don't want to cause any problems by pushing it too hard.

Thanks in advance.

John

[Don Moyer]

John,

You might be hearing the rather unusual sound an Atomic 4 will sometimes make at high power settings when other engines might be "pinging". Before doing anything more serious, I would re-check the ignition timing under load. Set the power to just where you're starting to hear the rattling sound, and then slowly rotate the distributor a small amount in each direction to see if you can get away from the sound.

Best regards,

Don

[jkenan]

Don-

Thanks for the reply. I will follow your suggestion. I'm still a newbie at this, so please explain what you mean by pinging? Assuming for a moment that it's not a timing issue, do I risk throwing a rod? What would the next step be if the timing adjustment does not work? I don't mean to get ahead of myself before trying the simple fixes, but I am curious.

BTW, the new coil definitely resolved the shut-down problem. Thanks for all your help.

Thanks.

John

[Don Moyer]

John,

Good question! I'm sometimes guilty of using rather dated terms.

The term "pinging" was used quite a bit in the 1950's and 1960's to describe the sound that those large stroke, high compression engines would make when they were developing (for whatever reason) unusually high combustion chamber pressure in the cylinders.

Common reasons for the elevated combustion chamber pressure included use of too low an octane fuel (which tended to burn too fast), ignition timing set too far before top dead center, or lugging the engine down in too high a gear. In each of these cases, the fuel air mixture would burn at a rate that created more pressure within the cylinders than the engine could possibly transfer to the rear wheels, and a very distinctive metallic sound would develop within the engine which we usually called a "ping".

Interestingly, engineers disagree as to the exact cause of the sound. Opinions range from bearing clearances slamming together, valves being driven closed with more than usual gusto, and (the one I like best) the leading edge of the burning fuel/air ratio exceeds the speed of sound causing a mini sonic boom.

While the Atomic 4, with its low compression ratio, is at the opposite end of the spectrum in terms of designs that are prone to pinging, it does have its limits, mostly in terms of advancing the ignition too far. If you have an engine that has compression values in the neighborhood of 115 to 120 psi, and you slowly advance the timing while under load at close to maximum power, you might hear a subtle "rattling" sound just before you start to lose RPM. In most cases however, the Atomic 4 will simply lose power.

In my last response to your question of what to do about your "rattling" sound, I suggested that you rotate your distributor a small amount in each direction to see if the sound disappears. If you haven't yet made that adjustment, you might try first to rotate the distributor a very small amount in a clockwise direction to retard the ignition.

Whatever one calls the sound that generates from too much combustion chamber pressure (and whatever its cause), it's not good to operate within that range. Well built engines will tolerate a fair amount of pinging or rattling, but eventually pistons will suffer damage, and bearings will wear at an accelerated rate.

Regards,

Don

[jkenan]

Don-

Thanks for the explanation. I like the sonic boom myself, since the jury is still out.

I did notice a loss of power after the rattling sound was there for a while. I will make the adjustments the next time I'm on the boat, and let you know what happens.

This is all very helpful, and will help me take better care of it. Thanks again.

John