Engine Rattling/Loss of RPM issue

[jkenan]

:confused:

I don't know if these two symptoms are related to the same issue, and would appreciate anyone's thoughts on the matter:

Issue 1: Most of the time when starting to motor under load, my A4 initially purrs along at 2000 RPMs pushing the Ericson 29 at a very comfortable 6 knots, and everything feels and sounds great. Inevitably, a rattling sound develops from the engine within a few minutes. Sometimes, immediately after starting the engine and engaging forward, the rattling starts. And finally, when I give her full throttle, the pinging always starts. I've tried rotating the distributor to adjust timing, but the rattling continues. Don explained the various theories on why engines ping (rattle) and the bottom line is that it is not good, can cause excessive wear to the bearings, and may require rebuilding to correct it. Before going to such extreme measures, I want to explore every other option.

Issue 2: Not inevitably (but with enough frequency to be concerning), the engine will slow to about 1500-1600 RPMs from 2000 RPMs and no amount of throttle will increase it. In calm waters, she slows to 5 knots, and in choppier waters, only about 2 or 3. If I shut down and motor again a little later, even in a short timeframe, I can get 2000 RPMs again. I did have a problem with one of my Batteries, and now wonder if it was causing an extreme load on the alternator (Mando). I've replaced the battery, and havn't had the opportunity to motor long enough to see if the problem persists. I've also cleaned the carb several times, and found nothing of concern there. I add MM Oil with every fill-up, and check oil regularly (there does seem to be a small oil leak coming from around the water pump, but only a minute amount - I've not had to add any oil yet because of it).

Engine Features:
Late model
Temp and OP normal, even under extended operating
Strong Compression (Last Check in April)
Steady Fuel Supply/Clean Carb/Electric Fuel pump
New Flamethrower Coil
New Electric Ignition
New Cap, rotor, cables, plugs
Indigo Prop (starting to get some barnacles, but minimal)
Crankcase Ventilation
FWC

Electrical:
2 SeaVolt 105's charged by a new Mando Alternator (one recently replaced)

Thanks in advance for any ideas or suggestions you may offer.

John

[Don Moyer]

John,

This is a bit of a long shot, but we did have one engine some years ago that developed a vacuum leak between the manifold and block, and the RPM slowed down about the same amount that you're reporting. In the case of that engine, however, once the leak manifested the customer never could get the engine back up to the original RPM. I can easily see, however, that as the temperature goes up and down, expansion and contraction between the block and manifold could open and close a vacuum leak in this area.

The way that your pinging could relate to a vacuum leak between the block and manifold is if the leak were caused by a faulty (leaky) manifold stud. Such a condition sometimes allows a small amount of water to be drawn in with the air fuel mixture and "over-boosts" the combustion process in that cylinder. Water injection was sometimes used to boost the power in airplane engines, but it had to be done very carefully or the process would damage an engine very quickly.

You might see if any of the manifold stud nuts are loose. If not, the only sure way to check the manifold gasket is to remove the manifold. If a stud were defective, you should see some evidence of water around the base of the stud.

Again, this is a long shot and I hate to cause someone needless work, but I really don't like the fact that your engine is continuing to have this distinctive pinging sound at high power settings. Perhaps someone else can come to your aid with a similar experience and propose a more straightforward remedy.

Don

[jkenan]

So if a small amount of water is being drawn in through a leaky manifold stud, and my engine is FWC, then I should notice some drop in my anti-freeze level over time, right?

Thanks Don.

John

[Don Moyer]

John,

Yes; I forgot that your engine was fresh water cooled.

However, even if you don't discover a coolant leak, I wouldn't give up on a leaky manifold gasket. A small leak would have the effect of leaning out the nearby cylinders, and a lean mixture of just the right ratio can cause pre-detonation and knocking.

Don

[jkenan]

Don-

I think that is what is going on. I have noticed a drop in my antifreeze over the last several months. The plastic container storing the excess antifreeze over the heat exchanger lost about half it's contents during that time. I havn't had to add any, but the level has dropped.

If I remove the head to check the gasket and leakage around the studs, what should I expect to see if there is a leak? While the head is off, is there anything else I should do as a matter of course given the opportunity?

Thanks.

John

[Don Moyer]

John,

I was actually thinking of the manifold gasket, not the head gasket, although I suppose that a tiny leak in the head gasket could allow just enough antifreeze into one of the cylinders to cause the plug to misfire. Are any of your plugs showing any sign of coolant?

I think I'd do a compression check, and if one of the cylinders shows noticeably lower than the rest and the plug in that cylinder looks different than the rest, I'd pull the head. If the compression is OK in each cylinder and the plugs all look OK, I'd pull the manifold first.

Don