Bought the boat with the problem, just wondered what causes so much damage to only one cylinder. Had both rebuilt since. Thanks
Cafe Racer, let us know what you find, good luck. BC

combustion
 

Your question about the burnt spark plugs is really about gasoline combustion - two basic phenomenon associated with this are Pre-ignition and Detonation of gasoline also called "knock."

Both of these conditions are related to the gasoline refining process and the anti-knock rating or as it is called "octane."

In this situation - gasoline combustion must be a controlled burning that enables the latent heat of the gasoline to do the work it is supposed to. When this process gets out of control you get "knock" and destruction of your engine.

Engines are designed to operate on a given octane of fuel - as the fuel varies so does the requirement of the compression ratio of the engine. Racing engines and aircraft engines using high octane fuel generally can run a compression ratio of 12:1 to 10:1 - this is 100-110 octane gasoline. Compression ratio is designed into the engine by the size of the combustion chamber and the volume of he cylinder. Standard automotive engines and marine engines built before World War II - and engines built after the gasoline crisis of the 1970's have a compression ration from 7:1 to 9:1 this allows a lower grade of fuel which means lower octane fuel to be used.

High octane and low octane fuel have no difference in heat value or power they differ in the rate and way in which the fuel burns under pressure. High octane fuel resists detonation and pre-ignition because of the "lead" content of the fuel or the way in which the fuel has been refined and the fuel molecules reformed by refining process. You cannot get more power from your low compression engine by using high octane fuel - it has the same heat. However you can destroy a high compression engine by using low octane gasoline.

Now given all of this several tuning factors contribute to and making this a very critical problem. First is spark timing - the distributor must have a straight shaft and run "true" or an internal wobble will fire the plugs differently in different cylinders. Basic ignition timing must be within design limits to produce maximum power. this ignition timing is extremely critical and is governed by its initial basic timing setting and the maximum amount of timing advance built into the distributor or provided in the engine control computer. 36* being a general amount determined by engine testing or by factory engineers. Too much ignition timing will destroy an engine very quickly usually leaving a telltale deposit of aluminum beads on the spark plug insulator.

Spark plugs must be of the correct heat range which varies with the length of the spark plug insulator. Hot plugs burn off the fuel that can foul them and cause the plug not to fire. Too hot spark plugs allow the plug tip to become incandescent which will fire the gasoline before the spark occurs - causing detonation and burning away of the spark plug electrode. Plug heat range is generally determined by temp measurement by engineers and you can pick it by starting with a cold plug and going warmer until spark plug "fuel fouling" or "misfire" is eliminated.

This brings us to the general issue of "fuel fouling" that the plug heat range is designed to eliminate - if the engine is receiving too much fuel it will "foul" the plugs and produce black smoke in the exhaust - and waste fuel. Fuel air ratio of 14:1 or 16:1 is provided by the engine fuel system - this can be checked with an in exhaust device called an "oxygen sensor" and is basically designed into the engine by the manufacturer. Fuel injectors that are clogged will cause an engine to run hot and lean raising cylinder temps, so can computer malfunction. Engine carburetor design from earlier days could malfunction also with incorrect adjustment of flooding floats. Improper jetting of the main fuel delivery circuit and or power circuit and or a stuck choke. Lean conditions in both fuel injection and carburetor systems can be caused by air leaks in engine manifolding and gaskets.

Finally, engine operating temperature and or overheating from improper coolant flow and operating temperature can cause all of this to become critical - mud in water jackets, steam formation in heads during operation all can cause engine spark knock in one or more cylinders. Engine operating temp should be about 160* to 220*.

Finally, supercharging an engine causes all this to become even more critical. Basically you need a good mechanic to sort all this out.

Fixx
 

+1 heat will do that,,i have seen a few in my day due to detonation..

yes, me to...just finished helping a friend with a new(rebuilt) S/B with vortec heads...killed itself in less than 20 hours due to the balancer slipped and even though he set the timing per specs....it was way off because the balancer issue....killed the brand new heads, broke off a exhaust valve, went thru the piston, which in turn sent the wrist pin thru the cylinder wall..reduced the whle engine to door stop material !!! so timing is critical !!!!

Fixx
 

Also dont trust the mark on the ballancer, i alway's zero out the mark on the ballancer with a degree wheel..get rid of the stock pointer,,i will add a billet adjustable timing pointer..once thats done i will mark the ballancer for the advance,28-36*..

After looking at the pictures of your engines I feel we need more information. I'm seeing crossover cooling, single plane mainfold, you say its a .060 over 502 with 36 total timing, and then it has stock exhaust. Do you have water pressure guages on the engines, are you using flat top pistons, do you run a thermostat, if so what is the engine water temperature and pressure running hard. What are your camshaft specs, and are your still using the non-adustable rocker arm set-up. I see MSD ignition, have you inspected the reluctor wheel in the distributor for corrosion, how old are the plug wires. Finally perform a leakdown test on the engine and post the results. Custom engines are just that, you may need 28 total timing or 42. Post the sparkplug you are using now to get an opinion on the heat range. If the engine runs and carries proper oil pressure, I would not tear into it yet. Finally, what octane fuel and how old is it.

The water preassure gauges are not installed. thermostat shows around 220 when running at 4500, the rocker arm are crane adjustables, MSD dist and cables are brand new, pistons are domed a bit (therefore we get a high compression) Heat range on spark plus are 6, Accel. I dont have camshaft specs but im sure that it is a marine cam and not agresive enough to cause reversion, even with stock exhaust. the oil preassure is fine, 60 at idle/cold, 30's afrter running. New valves, .The heads should be coming of today, hopefully i will have some pics and more info, thanks for all the help.

PS octane rating is 95 with a few gallos of plane fuel (110 octane).

220f water temp would be a major red flag :eek:

I agree...will never live @ 220 !!!

You must run water pressure gauges when you use a cross over. The 220 temp is a red flag.