How To Knock Out Ping.
HOW TO KNOCK OUT PING.
SOME EASY AND SOME NOT-SO-EASY, WAYS TO SATISFY A DEMANDING MOTORCYCLE
Among the world’s pressing problems, the tinkling sounds that come from an engine under load are not the foremost. World hunger, nuclear war and maybe what diet Liz is using are all more important. But some motorcycles still make those irritating pinging noises whenever the throttle is whacked open at low to medium engine speeds.
There are ways to eliminate the problem. Some of those ways are simple and cost nothing, some involve considerable time and more than a little effort. But for the bike with high-octane diet demand and nothing but low-octane fuel available, desperation can set in.
To begin with, there are two reasons for an engine to ping, and they are not the same. There is preignition and there is detonation. Preignition, as the name implies, means ignition begins too soon, before the spark plug sparks. This is usually due to some hot spot in the chamber, often caused by a spot of carbon or by a casting irregularity on the chamber wall that acts like a diesel’s glow plug. Detonation occurs after the spark plug fires. As the flame front travels across the combustion chamber, temperature and pressure may increase enough to spontaneously ignite the mixture. This causes a much harsher explosion than the engine is designed for, and it’s noticeable as knocking or pinging. Either way, with detonation or preignition,you hear a rattle or a ping when the throttle is whacked open.
The starting point to eliminating the dreaded death rattle is to set all adjustments on the engine to stock figures. All the things that go into a normal tune-up are vital to eliminating detonation. Also, some adjustments you may not think about might affect the pinging. Valve lash, for instance, determines effective cam timing. Lash set to the maximum stock setting increases compression pressure, while minimum lash reduces compression pressure. It’s most important to have all cylinders absolutely identical. Carb synchronization can also affect combustion pressures and thus should be carefully checked.
If the cause is detonation, the easy fix is to retard the ignition timing, provided the timing can be adjusted. Retarding the timing lowers the combustion pressure and temperature in the cylinder. This can be a problem on some of the more recent electronic ignitions that have no adjustment provision. Even then, some electronic ignitions use centrifugal advance mechanisms that can be modified for slower advance by lightening the centrifugal weights or increasing spring tension. In any case, this is the normal first step to eliminating ping. If it works, you’re lucky.
A second solution that doesn’t require engine work is to find higher octane fuel. This has the benefit of extracting the most possible power from an engine, because solutions like retarding the timing or lowering compression tend to reduce power. The octane ratings on gas station pumps are not perfect at predicting what fuel most resists pinging. Try different brands of fuel and you might be surprised at the difference. Unleaded premium is normally> available at a higher octane rating than leaded regular, and there are few motorcycles that can’t run on it. It’s more expensive, but if the engine doesn’t self-destruct, it may be worth the added cost. For the next step up, try your local airport. Aviation gas is normally available in 80 and 100 octane. The 100 comes in two varieties, low lead and standard. Either 100 octane fuel will work much better than any available automotive gas, but the high lead content of any 100 octane av gas will shorten spark plug life considerably. Cost ranges from a low of about $ 1.60 to a little over $2 a gallon, but this is still cheaper than racing fuels or add-in octane booster. If you don’t use a motorcycle much and want the most power, or use a high performance bike with very high compression, av gas or the even more expensive racing fuels may be the best solution.
Some of those aviation fuels come with names like 100/130 octane. The first number is the effective octane number with a lean mixture, and the second is the rating with a rich mixture. That should be a hint that if we can make a slight change to carb jetting, we can reduce detonation. It doesn’t take much of a difference in jet size to make a big difference in the way a motorcycle operates, so don’t get carried away. Because the emission tests are run with the engine lightly loaded, the carburetion of most machines is leanest at idle and part throttle, not full throttle. Because of this, the main jet doesn’t need replacement as much as needle jets or needle position. If your carb needle doesn’t have circlip rings for adjustment, you can still raise the needle by moving washers around the locating clip.
Past this point, it’s an inside job. Something will have to come off so the combustion chamber can be cleaned out or the compression ratio lowered. On a four-stroke engine additional or thicker cylinder base gaskets are the best way of lowering compression ratio. Adding head gaskets increases the likelihood of a gasket leak because head gaskets contain high combustion pressures, while base gaskets only contain oil in the crankcase.
A better solution, but one that’s more difficult, is to open up the combustion chambers of the head. To do this correctly requires measuring the volume of each combustion chamber and careful grinding and polishing to make the volume larger, but equal, in each chamber. Any increase should be limited to no more than 10 percent. When cleaning up combustion chambers, small rough spots in the casting can be removed.
Other internal changes that can reduce compression pressures include replacing the pistons with lower compression pistons and installation of a higher performance camshaft. That’s right, the usual high performance cam has more overlap than the stock camshaft. The greater overlap reduces compression pressure at low engine speeds, while improving air flow at maximum engine speed. Because most pinging occurs at lower engine speed, usually at the point where maximum centrifugal advance of the ignition occurs, the performance cam can improve peak power while reducing low speed power and pinging.
Fortunately, most motorcycles don’t require such extreme solutions to pinging problems and a careful tuneup or, at most, decarbonizing the head will do the job. And then you can find something else to fix.E9
