That is not what’s being said. According to wwillf01 and Rolling 20’s both are on this very thread claiming they have good working SmartCarb’s with different CR500 configurations, as do others on other forums. Currently the main discussion on this completely derailed thread is in regard to CR500s and the mixed bag of results people are having. So what’s really going on here?
Antagonism clearly. But the real issue is what is being done or can be done to take advantage of high atomization fuel systems on CR500’s. CR500 people know that Honda systematically detuned the liquid cooled CR500 during it’s nearly 20 year run from the glory days of 1988 and the anemic end result is what led to a fairly thriving aftermarket of modifications and improvements. I don’t have all the facts or all the known nuances about that but I can say that they were the first anomaly we ran across as we started getting a sizeable amount of carburetors in the field. What I mean is the first set of bikes that needed progressively richer metering rods than the standard metering rod covering the majority of products we were putting carbs on, including snowmobiles, Karts and primarily the Enduro market we had targeted. They were all over the map, some responded well to the standard Q11/.080” metering rod, others would try to spit the piston out on the floor regardless of what metering rod/click you tried.
I believe one if not the most widely misunderstood part of a well-built/tuned two stroke is the head, or perhaps the head of the tuner, I don’t know.
A two stroke combustion chamber appears pretty straight forward but plays a major role in how the engine turns heat into good clean roost. There are actually quite a few components that interrelate to how well the head can do its job but I’ll keep it simple.
The three main things to look at are squish, quench and surface area to volume ratio. Inside the combustion chamber, the squish band and quench area comprise the same features, are closely related, but do two totally different things.
1. The squish band acts to create turbulence, or a mixing of the air/fuel charge as the piston is compressing it (hence the term squish). This makes a more homogenous mixture that requires less ignition advance. Faster combustion means we can run later ignition timing, which results in higher combustion pressures and thus more power. Think of it as fanning the flame.
2. This same area is also designed to trap significant amounts of raw fuel to cool (quench) the air/fuel charge and thereby prevent detonation and or pre-ignition. (As a side note this is a really important aspect of quench combustion control because it relates to excessive unburned hydrocarbon emissions in hi-po two strokes, and is one reason KTM and others are starting to show up with new larger “open chamber” minimal squish band combustion chambers.) Anyway we're learning new ways to do it but the idea is to deliberately concentrate and trap excess fuel between the piston crown and cylinder head and hold it there during the combustion process. This boundary layer effectively cools the outer edge of the piston crown and combustion chamber with raw fuel. The evaporative cooling effects provided by the more or less liquid fuel, quickly stabilize the atomized end gases that surround the combustion chamber thereby preventing detonation and/or pre-ignition. (A typical squish/quench band in a modern two-cycle engine comprises 50% of the bore diameter and is on the average of .055” thick.)
3. Surface to volume ratio is the relationship between the total CC’s of combustion chamber volume, relative to the compression sweep of the cylinder after exhaust port closing. The importance of keeping this area to a minimum is to limit the size of the area so as to not allow too much time for the combusted mixture to reach the ends of the chamber. This is especially critical in a large bore, over square engine, such as a CR500. 84.5mm X 89mm I believe. Contrastingly, Kawasaki’s KX 500 is 86mm X 86mm.
Again, I personally do not know enough about CR500’s to know how much they changed the head over the years or all the varieties of stroker kits, port jobs or aftermarket or modified combustion chambers but it’s clear this is still one of the most prolific and emulated engines around and that some of the better ones are over 25 years old.
To Mtn-Doo’s point there have been strides made in combustion chamber design over the last 25 years and a majority of manufacturers have made changes for no other reason than to just deal with fuel quality and differing seasonal blends that continue to evolve, others, like KTM are clearly beginning work with DFI as evidenced by their latest combustion chamber designs that will allow for very high speed plume injection and the faster burn rate that a highly vaporized mix creates.
So what have they done? They’ve minimized the squish/quench area to both slow down the flame front (less fanning and turbulence) and opened up the surface area to take advantage (someday) of high speed DFI injection and move power up because of the faster, cleaner burn provided by the finely atomized fuel. While worrying less about detonation as the fire is already out by the time the flame front reaches the edges of the combustion chamber and the piston begins to move away from the head.
An additional benefit of a highly vaporizing carburetor is that a fine fuel vapor pulls more heat out the engine especially if aspirated through the crankcase, than a wet mixture, further atomizing the fuel in the process.
To get the best resullts out of a SmartCarb on your CR500, or any bike for that matter, then the short answer for most is this:
Pick a fuel and stick with it.
Pick an oil and stick with it.
Make sure the intake is free of obstruction that can cheat the venting/float bowl pressurization circuit.
Tune your carb.
If modified:
Loosen up the squish a little from what you would normally consider running (this will vary of course) and/or be prepared to run race fuel or additives. If you have a way to calculate your squish velocity and modify your heads then shoot for 25m/s max when running highly atomized fuel/oil mixtures.
