Marine Abnormal Combustion & Piston Failure

George Van Parys

Mar 1, 2000
RO Number
The Leading Cause of Premature Engine Failure

In this article we will discuss the causes of premature engine failure in two stroke engines. Many causes will result in a failure before the engine has left your shop. All too often in the rush to get the boat back in the water, the cause of the original failure is either overlooked completely or diagnosed incorrectly. How many times have you replaced an engine because it had so many hours, it was worn out?. I almost never see an engine that just wore out from too many hours. Almost always they fail from neglect, abuse, or some other preventable cause. What this means is, if you don't diagnose and correct the original failure correctly it is likely that the replacement engine will fail for the same reason. Therefore it is worthwhile spending a little extra time on diagnoses to make sure the condition no longer exists.

The single most common cause of premature two-stroke engine failure is detonation and/or preignition. When the spark plug ignites the fuel mixture in your engine there is a controlled burn that lasts between 1.5 and 4 thousandths of a second. This burn must occur at exactly the right time so the piston can convert the expanding gas into usable energy. Anything less than a controlled burn at exactly the right time is considered abnormal combustion. The likelihood of abnormal combustion is greatly increased in today's two stroke engines due to extreme combustion temperatures and the removal of lead from gasoline. To avoid abnormal combustion all the components must be functioning correctly: fuel mixture, ignition timing, fuel type and octane rating.

Detonation: A form of abnormal combustion, which occurs after spark ignition. When the spark plug first ignites the fuel mixture it tries to expand. Since the burning fuel is trapped between the piston and head it cannot expand. Therefore it escalates' the pressure and temperature of the unburned mixture high enough to cause them to ignite prematurely. Since the piston had not yet arrived at the proper position to convert the expanding gases into useful energy the energy is dissipated as extra heat and knocking noise in the engine. The most common damage caused by detonation is ring land and piston crown erosion. Severe detonation can result in thermal fatigue and breakage of the lower lands or fracture off a section of the top land and/or crown.

Preignition: This type of abnormal combustion occurs when stale fuel mixture is ignited by an unintended source prior to spark plug firing. Preignition or Surface ignition is aggravated by the fact, that each rotation of the engine in which ignition is initiated by an uncontrolled source in the combustion chamber, the energy released raises the temperature at that point. Each subsequent cycle is more likely to preignite from that unintended ignition source. This situation will rapidly develop into "runaway" preignition and be self-sustaining with preignition continually advancing with every combustion cycle. Preignition will act like a very advanced spark, increasing the combustion temperature and pressure and increasing the tendency for detonation. Damage from this stage of preignition will appear similar to that of detonation and can occur in only minutes.

There are many causes of abnormal combustion. If present, they must be identified and corrected in your service dept. prior to delivering the boat to your customer. The most common causes of detonation and related preignition are fuel related. Such as low octane levels, poor quality fuel, and lean fuel mixture. The first two causes can be virtually eliminated by using only premium brand high-octane fuels. In recent years due to the elimination of lead from gasoline oil companies have begun using a variety of additives to establish octane level. These additives vary in quality from brand to brand. So use only the best. The third cause is lean fuel mixture and will not be eliminated so easily. I recommend that on all engine exchanges you include the following steps into your normal procedure.

Disassemble, cold dip clean and install carburetor kit on all carburetors as usual. This is where most people stop but it's not enough. Next write down all jet sizes and confirm with latest O.E.M. service bulletins all jetting updates. As fuel quality changes quite often the factory recommended jet sizes will change. In other words, the size recommendation when the engine was new may no longer be correct. Replace jets as necessary, the remaining jets must be cleaned thoroughly. Chemical and compressed air cleaning is not sufficient, you must clean the jets with either a small jet brush or run a length of heavy denim thread through the I.D. of the jet. While suspending the thread on both ends, traverse the jet back and forth across the thread to polish out the I.D. This makes the jet act like a smaller size. So, even if you can blow through the jet it doesn't mean it's clean. Also, you need to blow through each air and fuel passage in the carburetor while the jets are out. I like to use those spray cans of carb cleaner with the nozzle extension to apply extra pressure at each passage. Then follow up each passage with compressed shop air using a blower that will apply pressure at each passage as well. Generally a brush won't be necessary for each passage because they are so much larger than the jets. Now assemble the carb's as usual.

The next reason for lean mixture is a vacuum leak. You will want to check all gasket surfaces including block half surfaces, vacuum ports and cracked hoses sometimes you will even find leaks at loose throttle shafts. There are several different ways to find a vacuum leak. Always bring the engine to a low idle before you start checking. Take the engine outside, for good ventilation, and remove the cowling completely. Next spray a suitable product around all possible leak areas. The RPM's will change if a leak is found. Many different products have been used to find vacuum leaks, such as WD-40, carburetor cleaner, starting fluid, propane and many others. Whatever your shop uses remember, these products are flammable and extreme caution should be exercised. Checking for vacuum leaks should be done on all engine exchange jobs as part of a final check even if you don't suspect a leak. Otherwise small leaks can go undetected initially and develop into more serious trouble later.

The following causes must also be included in a discussion of abnormal combustion. Although less common than fuel related failure, the consequences of ignition related failure can be just as dramatic. Any ignition component that fails and allows a cylinder to cross fire to another can result in detonation. The component can be as simple an unprotected wire.

Ignition related abnormal combustion: A form of abnormal combustion, which occurs as a result of an incorrectly advance spark ignition. The ignition system tries to ignite the spark plug of a cylinder when the piston in the correct position. The defect which leads to the cross fire condition incorrectly ignites the spark plug of a cylinder that the piston had not yet arrived at the proper position. The fuel mixture tries to expand, since the burning fuel is trapped between the piston and head it cannot expand. Therefore it escalates the pressure and temperatures of the unburned mixture high enough to ignite a second flame front. Since the piston had not yet arrived at the proper position to convert the expanding gases into useful energy, the energy is dissipated as extra heat and knocking noise in the engine. The most common damage caused by detonation is ring land piston crown erosion, with severe detonation. Thermal fatigue can result in breakage of the lower lands or fracture off a section of the top land and/or crown.

As with fuel related failure, most causes of ignition related failure can be identified at your shop prior to delivering the boat. Ignition failure can stem from a variety of sources. Our experience indicates, that more times than not, the culprit isn't component failure, but human error. Most OEMs provide voltage and ohms specifications for checking the major electrical components. Although failure of these components are rare the checks are quick and easy and should be a part of every prestart check-list. In the long nun this will save a lot of hair pulling once the boat is in the water. By far we find many more failures caused by improper wiring than defective components. It can be a very simple yet costly mistake to reverse a pair of wires coming from the trigger to the switch box or from the switch box to the coils. If a pair of wires is reversed, the trigger will fire the wrong cylinder at the wrong time. This can lead to detonation failure by igniting a cylinder that the piston had not yet arrived at the proper position. This situation can be duplicated even if the wires have been connected correctly. If the insulation of either a primary or secondary coil wire is melted or defective it's possible for one coil to cross fire with a nearby coil. In fact I've even seen coils cross fire across coil mounting brackets. Unfortunately this problem may not be apparent while the engine runs at the dock or in the tank. Cross firing in most cases only occurs after the engine reaches operating temperature, moisture has developed under the cowling and the engine is under load. So you run the engine at your shop with no problem, thinking everything is fine, you deliver it to our customer. While your customer is using the boat and the conditions are just right the engine cross fires and detonates. Naturally he will be as far from land or civilization as possible. Of course when your customer returns breathing fire, you can't find any reason for the failure. That's when the real creative thinking usually begins. By now your frantic, you need a reason any reason, any excuse at all, because you have to tell the volcanic customer something. You're desperate and grabbing at straws then out of nowhere you hear someone who looks a lot like you say, Oh, I know it's a, it's a, soft pistons, or maybe defective rings. Yeah, that's the ticket... Well, we've inspected a lot of pistons over the years. They've been sent out for metallurgical analysis, spectrographs, density tests, melt down tests, hit on, beat on, cut dropped, scratched and cussed out, And this is what we've learned, it's more likely the boating gods were unhappy that fateful day, than soft pistons.

Other causes of abnormal combustion are excessive or glowing carbon deposits on the piston or in the combustion chamber, hot spots in the engine caused by overheating and boiling the coolant, spark plugs with too high a heat range, cracked or broken spark plug insulators.

Piston Failure in Marine Engines:

Let us review two common types of piston failure, the cause and prevention's. Pre-ignition and detonation are the most common causes of piston failure. Pre-ignition occurs when the combustion process starts before the designated time needed for peak performance. When the fuel vapors ignite premature, the molecule expansion is trying to push the piston down when the piston is on its way up. This creates a shock wave onto the piston crown and forms excessive heat. The heat created is so intense the piston starts to melt (weaken) eventually leading to major engine damage. Detonation is when the temperature and or/pressure in the combustion chamber is above normal, after the spark plug fires, a portion of unburned mixture will violently explode before the wave like front passes through it. This violent explosion colliding with the flame front is called spark knock.

Conditions that cause pre-ignition/detonation are too much ignition timing, excessive carbon deposits in the combustion chamber, improper heat range of spark plugs, lean fuel condition and inadequate cooling. How can these destructive processes be prevented you may ask? Lean conditions are the most common of all premature failures. Look for partially blocked fuel filters, plugged carb jets, improper carburetor mixture screw adjustments (use a tachometer when setting for best results) and vacuum leaks, do not forget on two strokes the crankcase can create a vacuum leak. Also, keep in mind, two strokes and gasohol do not get along.

Incorrect ignition timing, too much advance should be reset to manufacture's specification. Ignition advance is a calibration of spark in relation to piston speed, as the rpm increases the spark needs to take place sooner to allow for the time needed to ignite the fuel vapor. In turn, the combustion pressure created is pushing the piston down providing peak performance.

Excessive carbon deposits are contributed to poor quality oil, which can glow in the cylinder that can ignite the fuel mixture. Improper cooling can be from plugged water passages, malfunctioning water pump or a defective thermostat. Improper engine RPM range from too large or too small of a propeller along with overloading the boat will cause overheating.

This leaves us with spark plugs, use the type specified according to the manufacturer and set the gap to their specification. Each manufacturer's experience with different heat range plugs has resulted in the selection of the best performing plug for each engine. A "too hot" plug, under continuous heavy duty operation, can produce pre-ignition. A "too cold" plug, under light duty operation, can cause carbon deposits which short or foul the plug.