Dennis B. and I took the boat out on Lake Carlyle in September (2011). Everything worked fine, but on continued high speed (WOT) runs, we heard some regular "popping." "Pops" were a few seconds apart. The popping wasn't loud, but how could it be, originating a few feet up a water-filled pipe? (I'm quite sure it wasn't via the intake.) I'm not sure if the following posts could explain our problem, but possibly . . . :

Calling the 427 Brain Trust
October 11 2006 at 5:23 PM Eric Jensen (no login)
About a week ago I developed this weird pop in my starboard exhaust that I can't figure out. I suspect it sounds like a backfire if the exhaust were more conventional. It occurs about once every 3 seconds. There is no indication from the carb side of a cough when it happens so I believe it is downstream a bit. The only thing I can point to leading up to this was a small shot of "Sea Foam" I had administered. I've used this product before and not had a problem. The pop is there when the engine is cold or hot. It seems to be there from idle up to 2000 rpm and it may be there past that, but I can't hear well enough to say. I have a sneaking suspicion it is at cylinder #8 as that exhaust temperature is 30 degrees less that all the others. My first concern was plug wires, so I replaced all of them. No change. Then I suspected a sticky valve. I did a check of all the valves (using the Paul Pletcher running style) and it seems to be right on spec there. I did not have a set of spark plugs with me but switched cylinder 8 with cylinder 5. No change. By the way I was careful in routing the #7 and #8 wires separately, although I am suspisious of the steel loom it runs through. Can anyone offer a suggestion on where to look next? Other than that it seems to run strong.
Eric

Thanks for all the suggestions
October 16 2006 at 10:28 AM Eric Jensen
Response to Calling the 427 Brain Trust
Lots of suggestions to consider. I appreciate the responses. There were several things I had not previously considered. I pulled the boat last weekend, so my ability to diagnose some of these problems is somewhat cramped now. I'm thinking of pulling the belt on the seawater pump and trying a few things. There are a couple of requests to elaborate on the sound it makes. It occurs every few seconds I'd guess I'd call it a weak backfire. You have to consider that it is occuring at least 10 feet up a water filled pipe and is much attenuated by that. It is not associated with a shudder, a cough or a change in rpm by the engine. You could live with it if it didn't seem to point to a more serious problem. The thing I found most interesting about it is the fact that it appears as soon as the engine starts, no matter how cold. This seems to point to a weird ignition problem to me. I think I will change the distributor cap as a simple precaution. By the way, I want to change the plugs on both engines after 3 years. What does everyone recommend? I took the spark plug numbers listed in the CC 427 manual to my local Auto Zone and got completely blank stares from them. Serves me right. They are not into 40 year old boat engines.
Thanks,Eric

Backfire problem solved - better late than never.
June 8 2007 at 3:30 PM Eric Jensen
Response to Calling the 427 Brain Trust
It seemed like the least I could do is to post a followup to this problem, because the solution has been found. In the original post, I mentioned that this all ocurred after using some Sea Foam carbon remover. I more or less dismissed this out of hand but it seems it may have played a part in the problem or in uncovering it. I had not pulled ALL the spark plugs because the PO told me that he had just changed all of them when I bought the boat. I did look at all the easy to get to plugs and they looked OK. I had sort of zeroed in on cylinder 7 because it appeared to be running a little cooler. This turned out to be a red herring. On the outside bank cylinder 2 of the starboard engine, the plug appeared to have virtually no gap. I'm guessing someone dropped it and never checked the gap when it was installed. The cylinder was apparently firing but not really contributing much as there was a great blob of carbon and tar on the electrode. I'm thinking the Sea Foam knocked enough soot loose to get the cylinder firing again but poorly. This would also explain a much earlier post where I had complained of 15 to 20% excess gas consumption from the starboard engine. Funny how close that comes to 1/8 of the cylinders being a dead load. Anyway, all is well now and I am kicking myself for not pulling all the plugs sooner. Sometimes the easy solution is the best solution. Thanks everyone for the suggestions.
Eric