BRP Exhaust residue


2 replies to this topic
  • BugTeeth

Posted July 17, 2004 - 10:51 AM

#1

Anyone have any input on what the color of exhaust residue (the stuff on your finger when you touch the tip of your exhaust pipe). It´s my understanding that on a perfectly tuned motor this should be a coco brown color.The same should be on your spark plug. Mine is jet black and is slightly moist with what I think is oil. My bike runs great but I´m wondering if it´s rich or oil is blowing past the rings and causing this problem. Whaz up wit dat...


00 650R FMF Titanium 4 full system, K&N, Corkpopped

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  • qadsan

Posted July 17, 2004 - 02:49 PM

#2

I wouldn't put much faith in that theory alone because the exhaust residue & spark plug color won't accurately determine how well tuned your engine is. It’s good info (pieces to the puzzle), but certainly not the holy grail to determine if an engine is perfectly tuned. If combustion is overly rich, then you'll have the byproducts from incomplete combustion (carbon / soot) built up in your exhaust, or on your spark plug, etc, but that also doesn't tell you where in the throttle range you're running too rich or how rich you're running.

Checking the spark plug is just one of several tools that can be used to help you get your bike optimally tuned, but ideally you should also be looking at exhaust gas temperatures, gas percentages, etc, in addition to knowing & feeling the physical symptoms of how a bike that’s running too rich or too lean feels. An exhaust temperature of ~1800F is close to ideal for our bikes and it's yet another tell tale sign that your bike is properly tuned. When checking carbon monixide levels, you should see ~6% at wide open throttle, ~4 to ~5% during steady state throttle and ~3 to ~4% at idle when your engine is optimally tuned.

Spark plug color alone doesn't tell enough about the mixture to know if you're running too rich or too lean unless you're way too rich or way too lean. Also, the spark plug will only accurately reflect your bikes air/fuel mixture immediately before the engine is shut off (the operation of your engine & riding environment is dynamic). For instance, to accurately read your spark plug for too lean or too rich of mixture at WOT, you must run your bike WOT for a good 8 seconds or so under a load and then immediately shut off your engine by pulling in the clutch and pressing the kill switch before backing off the throttle so your spark plug will tell you what’s going on at that point in time (WOT under a load). You also want to make sure you’re not trying to read a fouled spark plug or a brand new spark plug. If you install a brand new spark plug, make sure and get at least 20 to 30 minutes of good hard riding on it before trying to read it. At this point, you can pull the spark plug to determine if your mixture is too rich, too lean or about right at WOT. If you don't immediately shut your bike off from WOT and ride back to your pit and then shut off your bike, then the spark plug will only reflect how the bike was running immediately before you shut it off and won't reflect what's going on at WOT. If you want to see what's going on at half or quarter throttle, then you must immediately shut your bike off at the desired throttle position to properly read the plug, but it gets tricky trying to figure out which of the overlapping circuits are contributing too much or too little to the mixture. Also make note that twisting the grip half way isn’t always half throttle. If you plan to do spark plug checks at given throttle positions, then mark your throttle grip positions according to the throttle slide opening for better accuracy. There’s a good deal of trial and error if you really want to get the most from your jetting or you can pay an expert to do it for you, but most people go with known good setups that are generally ‘good enough’ as opposed to optimizing their jetting because it can be very time consuming to do.

There are many factors to inspect when reading a spark plug, but the mixture ring (also known as the fuel ring) is one of the most critical for jetting and not insulator color as many folks might think. If you think about it, how can the insulator color be an accurate guide for the air/fuel mixture? What is the color of unburned fuel? The byproduct from the combustion process from too lean or too rich of jetting is the abundance or absence of carbon (more fuel burned = less carbon // less fuel burned = more carbon). If there’s too little fuel in the mixture, you’ll be running lean and there won’t be much of a presence of carbon on the insulator. Your engine will also be running hotter than what it should be if the mixture is too lean and you may experience pre-ignition/detonation depending on how lean the mixture is. If the mixture is way too lean, then you’ll see tiny black or silver aluminum flakes (like pepper) on the insulator, which is aluminum being blasted from your piston’s crown. Detonation is caused by the air/fuel mixture exploding instead of burning and you may hear the resulting knocking sound from this, particularly when the engine is under a load. The knocking sound heard is actually a shock wave that's disrupting the boundary layer of cooler gasses that cover the internal parts of the combustion chamber, resulting in incomplete combustion. This rapid rise in pressure and temperature exerts extreme force on engine components and can do very bad things such as crack your engines head, crack or put holes in your piston, blow head gaskets, break your connecting rod, damage bearings, seals, etc. This is why you should not base all your jetting decisions on color alone because it doesn't tell the whole story. If you’re running too rich, then you’ll see varying amounts of carbon (soot) on the insulator depending on how rich you are. If you’re insulator is black & wet, then perhaps you have an oil control issue to deal with (rings, valve stem seals, valve guides, etc).

The mixture ring is simply a carbon ring that’s formed on the insulator during the combustion process which indicates the presence of unburned fuel. The closer this ring is towards the electrode, the richer the mixture (the less fuel that’s combusted). The deeper this ring appears into the shell on the insulator, the leaner you’re mixture (more fuel is being combusted). You want the mixture ring near the junction where the insulator joins the shell, which means most of the air/fuel mixture is being combusted. You don’t want this ring to disappear; otherwise you’ll be running too lean.

Viewing the mixture ring can be a real problem because you can’t normally see it just by looking down the spark plugs shell with your eyes, so you’ve either got to cut off the spark plug shell or use some type of illuminated magnifier at about 10X to 12X to see what’s going on. You can find spark plug viewing tools at certain performance & specialty shops if you really want to get into it, but most people never take it that far and are happy enough with a general setup that’s good enough as opposed to being optimized. The Childs & Alberts CT407 which is also sold by Champion as the CT456A is my favorite spark plug viewing tool and it definitely works better than the less expensive flashlight versions sold at various performance shops. There’s plenty more to reading spark plugs, but that should cover some of the basics. A dynamometer is yet tool that can be helpful, but not everyone has one of these in their bag of tricks ;o)

If you don’t want to start jetting your bike from scratch, then check our Eric Foster’s “The Pig Pen” web site. He kindly formatted, spiced up & posted my jetting data/suggestions for the stock XR650R carb. Just remember to use this data as a guideline to work from, but it should be fairly close & good enough for many folks who want good grunt from their pigs.

http://www.xr650r.us/jetting/

  • BugTeeth

Posted July 17, 2004 - 06:14 PM

#3

:worthy:Qad, your level of knowledge humbles me. I am an appreciator of fine engineering. It`s amazing that these machines work as well as they do. Mechanically these motors are pretty straight forward, but when you get into chemical dynamics the playing field becomes fluid (no pun intended). Mixtures of fuels and lubricants, coolants all pose variants. I guess the question we all have to ask is what are the perfect fuels, internal lubricant and coolant to make our beloved BRP's the happiest,jetting is another chapter... I know this has been hashed out on this site and it is up for opinions. I have been running Bel-Ray Thumper semi syn 15-50, the highest octane pump gas I can get and Toyota red coolant. I will probably go to a full synthetic next oil change to see if some of the carbon goes from my exhaust residue, I guess that will tell me if I have a ring blowby problem. I almost hesitate to change anything since I have no start problems and this motor runs like the proverbial scaulded dog...The throttle response is instantaneous and the acceleration is "Oh, how I love it". I am a perfectionist but I have also served in the military..."If it ain`t broke don`t fix it" anyway thanks for the insight YOU ARE THE GURU!!!!! I will follow your instruction on spark plug literature.





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