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I finally got my first case of detonation in my Suzuki 100cc. Yeah I know it isnt a motorized bike but it is a small 2 stroke engine and will serve as a good example in this case. Detonation is similar to pre-ignition and these are the main factors contributing to it:
1) excess cylinder pressure
2) overly lean fuel mixture
3) excessively advanced ignition timing
4) high air temperature
5) fuel being lower in octane than is required for existing engine compression
Reasons for my case of detonation: I had increased the cranking pressure from 120 psi to 150, there was an air leak at the cylinder base gasket causing a lean mixture (evidence for which was blackened oil deposits on the underside of the piston crown), I was running WOT in the middle of a hot summer day, and the octane ratings here in South America are always questionable.
Signs there was detonation: erosion of the piston in the center, tiny globs of aluminum on the spark plug. (see photo)
The reason for this post is that many people are buying those high compression cylinder heads while keeping the stock CDI which has abnormally advanced ignition (which is marginally OK before you modify with higher compression and better carburetor). And some of those heads give ridiculous pressures, often around 185psi. Hey, motocross bikes only have 150 psi. If you already have one and dont want to trash it just buy a dremel rotary tool (or Black&Decker in WalMart) and use a fiberglass disc to grind away at the center dome to make it wider and allow more compressed area (which lowers the pressure). I highly recommend buying a cylinder pressure tester. Adding another head gasket is undesireable because that defeats having a small squish area. Also the stock carb is impossible to jet correctly for all three RPM ranges and gets air leaks very easily where it mounts to the intake manifold.
2 stroke engines are very finicky and can go bad easily once you start to modify them for increased power. I dont want people to get scared and leave them pathetically stock but rather to be informed and maintain balance between the different engine systems when modifying. After modifying you should ride it hard for a short time and then dissassemble the engine and look under the piston to make sure it isnt black. A lean fuel mixture or an air leak causing a lean condition is the most pernicious cause of problems in a two stroke engine. Also oil on the top cylinder air fins indicates an air leak where the head and cylinder mate. Use sandpaper and glass to plane both surfaces and then use a fiber gasket (home-made). Also checking the crank seals every few months is a good idea. And check the conrod bearings and crank bearings for any up/down play.
reference: www.tricktuners.com/detonation_explained.htm
A quote from that site was new info for me: "Engines are much more prone to detonation while "lugging" when at high loads and low RPMs. This is mostly due to the extended burn times at low rpms". That situation exists for me when I give my room-mate a ride in town.
In the pÃcture you can see the detonation crater in the center of the piston and the speckles of dirty aluminum on the spark plug:
1) excess cylinder pressure
2) overly lean fuel mixture
3) excessively advanced ignition timing
4) high air temperature
5) fuel being lower in octane than is required for existing engine compression
Reasons for my case of detonation: I had increased the cranking pressure from 120 psi to 150, there was an air leak at the cylinder base gasket causing a lean mixture (evidence for which was blackened oil deposits on the underside of the piston crown), I was running WOT in the middle of a hot summer day, and the octane ratings here in South America are always questionable.
Signs there was detonation: erosion of the piston in the center, tiny globs of aluminum on the spark plug. (see photo)
The reason for this post is that many people are buying those high compression cylinder heads while keeping the stock CDI which has abnormally advanced ignition (which is marginally OK before you modify with higher compression and better carburetor). And some of those heads give ridiculous pressures, often around 185psi. Hey, motocross bikes only have 150 psi. If you already have one and dont want to trash it just buy a dremel rotary tool (or Black&Decker in WalMart) and use a fiberglass disc to grind away at the center dome to make it wider and allow more compressed area (which lowers the pressure). I highly recommend buying a cylinder pressure tester. Adding another head gasket is undesireable because that defeats having a small squish area. Also the stock carb is impossible to jet correctly for all three RPM ranges and gets air leaks very easily where it mounts to the intake manifold.
2 stroke engines are very finicky and can go bad easily once you start to modify them for increased power. I dont want people to get scared and leave them pathetically stock but rather to be informed and maintain balance between the different engine systems when modifying. After modifying you should ride it hard for a short time and then dissassemble the engine and look under the piston to make sure it isnt black. A lean fuel mixture or an air leak causing a lean condition is the most pernicious cause of problems in a two stroke engine. Also oil on the top cylinder air fins indicates an air leak where the head and cylinder mate. Use sandpaper and glass to plane both surfaces and then use a fiber gasket (home-made). Also checking the crank seals every few months is a good idea. And check the conrod bearings and crank bearings for any up/down play.
reference: www.tricktuners.com/detonation_explained.htm
A quote from that site was new info for me: "Engines are much more prone to detonation while "lugging" when at high loads and low RPMs. This is mostly due to the extended burn times at low rpms". That situation exists for me when I give my room-mate a ride in town.
In the pÃcture you can see the detonation crater in the center of the piston and the speckles of dirty aluminum on the spark plug:
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