phil0925 Posted January 20, 2014 Author Report Share Posted January 20, 2014 The buyer has now admitted to changing the mix as well (only after I explained I had photos of before and after the sale). He set the screw fully home then turned it out 1.5 turns. He did no other testing as I can see. He thinks that he has richened the mix! He did it because his friend told him to! He says that it was original set to 2.5 turns, I don't know, as I never set it because the plug was always a perfect colour. I live at 850 meters altitude and at home it was perhaps just a "little, tiny tiny" ?? lean (by response) but very marginal if not perfect (by my experiance) at 1100 to 1200 Meter altitude where I always ran it, except for 10 mins going and 5 mins back (its the free wheel down bit), I nearly always used it at quite slow revs appart from blips needed for obsticules, prefering technical difficulty than green lanes etc. Oh and of course the other difference is the change due to the pipe disconnection gearbox to carb that he made?..... Anyone the carb expert? Who would like to make a difinitive statement, it would be appreciated and help me a lot. Thanks ALL! Quote Link to comment Share on other sites More sharing options...
phil0925 Posted January 20, 2014 Author Report Share Posted January 20, 2014 Opps, he lives at approx 150 Meters so 1000 Meter different. Quote Link to comment Share on other sites More sharing options...
sectionone Posted January 20, 2014 Report Share Posted January 20, 2014 1.5 turns out is the starting point. Spanky's Jetting Guide: A correctly jetted carb makes a tremendous difference in the torque, midrange pull, top-end pull, and over-rev of your engine. If you have never jetted your bike correctly, you will almost certainly gain some performance at some point in the bike's powerband. A cleanly jetted pilot circuit can be the difference between having to clutch the bike out of a turn or not. The needle can make all the difference in the world for the power of the machine in most situations, as it controls the throttle range that most riders spend most of their time using. A correctly sized main jet could mean the difference between being able to rev out high enough to not have to shift one more time at the end of the straight, or the power falling flat on top and requiring you to make that extra shift. Are you fouling plugs? Many people will tell you all sorts of band-aid fixes, from running less oil, to running a hotter plug. Both are incorrect fixes for plug fouling. It's all in the jetting. The only way to know what jetting changes you will need is by trial-and-error. No one can give you jetting specs, because every bike is different, every rider has a different style, and jetting is totally weather dependent. Unless the person telling you what jets to use is riding an identical bike, on the exact same track, at the same time, his recommendations are meaningless. Jetting is fairly simple, and is a useful skill to learn if you ride a two-stroke and want it to perform at it's best. It's very important that you start with the pilot circuit. The reason is simple. The pilot circuit affects the entire throttle range. When you are at full throttle, the main jet is the primary fuel metering device, but the pilot is still delivering fuel as well, adding to the total amount of fuel that your engine is receiving. Before you start to rejet your bike, you need a clean air filter, a fresh plug (actually you need several plugs to do plug-chop tests for the main jet), and fresh fuel. One important detail: Make sure the engine is in good mechanical condition. If your engine has a worn top-end, fix it first. Trying to jet a worn out engine is a waste of time. The same goes for reeds that don't seal properly, and a silencer that needs re-packing. Worn reeds will mimic rich jetting, and worn rings will mimic lean jetting. Before you start the jet testing, Install a fresh plug. Set the float level to the proper specs, an incorrect float height will affect your jetting all across the throttle range. Warm the bike completely, and shut it off. As already stated, start with the pilot circuit. Turn the air screw all the way in, then turn it out 1.5 turns to start. Start the engine, and turn the idle screw in until you get a slightly fast idle, or hold the throttle just barely cracked, to keep the engine idleing. Turn the airscrew slowly in, and then out, until you find the point where the idle is fastest. Stop there. Do not open the screw any farther, or your throttle response will be flat and mushy, and the bike may even bog. This is only the starting point, we will still have to tune the air screw for the best response. Now is the time to determine if you have the correct pilot installed in your carb. The air screw position determines this for you, making it very simple. If your air screw is less than 1 turn from closed, you need a larger pilot jet. If it is more than 2.5 turns from closed, you need a smaller pilot jet. Once you have determined (and installed it if it's neccessary to change it) the correct pilot jet size, and tuned the air screw for the fastest idle, it's time to tune the air screw for the best throttle response. Again, make sure the bike is at full operating temperature. Set the idle back down (the bike should still idle, despite what you read in the Moto Tabloids), and ride the bike, using closed-to-1/4 throttle transitions. Turn the air screw slightly in either direction until you find the point that gives you the best response when cracking the throttle open. Most bikes are sensitive to changes as small as 1/8 of a turn. The air screw is not a set-it-and-leave-it adjustment. You have to constantly re-adjust the air screw to compensate for changing outdoor temps and humidity. An air screw setting that is perfect in the cool morning air will likely be too rich in the heat of the mid-day. Now, it's time to work on the needle. Mark the throttle grip at 1/4 and 3/4 openings. Ride the bike between these two marks. If the bike bogs for a second before responding to throttle, lower the clip (raising the needle) a notch at a time until the engine picks up smoothly. If the bike sputters or sounds rough when giving it throttle, raise the clip (lowering the needle) until it runs cleanly. There isn't really any way to test the needle other than by feel, but it's usually quite obvious when it's right or wrong. Last is the main jet. The main jet affects from 1/2 to full throttle. The easiest way to test it is to do a throttle-chop test. With the bike fully warmed up, find a long straight, and install a fresh plug. Start the engine, and do a full-throttle run down the straight, through all gears. As soon as the bike tops out, pull the clutch in, and kill the engine, coasting to a stop. Remove the plug, and look deep down inside the threads, at the base of the insulator. If it is white or gray, the main is too lean. If it is dark brown or black, the main is too rich. The correct color is a medium-dark mocha brown or tan. Once you have a little bit of experience with jetting changes, and you start to learn the difference in feel between "rich" and "lean", you'll begine to learn, just from the sound of the exhaust and the feel of the power, not only if the bike is running rich or lean, but even which one of the carb circuits is the culprit. The slide is also a tuning variable for jetting, but slides are very expensive, and few bikes need different slides, so we won't go into that here. Keep in mind, even though this article is intended primarily for two-strokes, four-strokes also need proper jetting to perform right, although they are not quite as fussy as their oil-burning cousins. The only real difference in the two is with the pilot circuit. Two-strokes have an air screw that you screw in to make the jetting richer, ansd screw out to make the jetting leaner. Four-strokes, on the other hand, have a fuel adjustment screw that you screw in to make the jetting leaner, and out to make it richer. 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jse Posted January 20, 2014 Report Share Posted January 20, 2014 The buyer has now admitted to changing the mix as well (only after I explained I had photos of before and after the sale). He set the screw fully home then turned it out 1.5 turns. He did no other testing as I can see. He thinks that he has richened the mix! He did it because his friend told him to! He says that it was original set to 2.5 turns, I don't know, as I never set it because the plug was always a perfect colour. I live at 850 meters altitude and at home it was perhaps just a "little, tiny tiny" ?? lean (by response) but very marginal if not perfect (by my experiance) at 1100 to 1200 Meter altitude where I always ran it, except for 10 mins going and 5 mins back (its the free wheel down bit), I nearly always used it at quite slow revs appart from blips needed for obsticules, prefering technical difficulty than green lanes etc. Oh and of course the other difference is the change due to the pipe disconnection gearbox to carb that he made?..... Anyone the carb expert? Who would like to make a difinitive statement, it would be appreciated and help me a lot. Thanks ALL! I have more than serious doubts that a mixture change of one turn on the fuel screw had any overall effect on the engine damage situation (it has the most effect in the idle circuit metering to needle-draw transition) . The 150 vs 850 meters (jetting wise, a change of about 2300 ft., and a much bigger gap at the 1100 to 1200 altitude) could result in an overall lean mixture (if running well at 1100/1200 meters) and this may likely be a contributing factor. Experientially speaking, engine damage of this sort is usually a combination of several issues, a "perfect storm" you might say, of miss-adjustments (lean mixture due to main jet/needle adjustment, old fuel etc.). It would also seem that it is the responsibility of the new owner of any used bike (knowing-or should have known, for instance, the change needed in jetting due to altitude) to set up the new bike before use. Knowing the above new information, detonation may be the primary cause. Detonation usually announces itself with dramatic engine noise, which in this case was possibly ignored and/or misunderstood. Jon Quote Link to comment Share on other sites More sharing options...
dadof2 Posted January 20, 2014 Report Share Posted January 20, 2014 (edited) # 26 identifies hot spot # 48 identifies worn rings as cause of weakness (lean fuel air mix) # 49 identifies detonation and leaning of mixture due to altitude change. To me the engine looks pretty dirty and worn, with a fair bit of carbon and blowby stains. I suspect there has been some debris / carbon on the head where metal is now missing. These debris will have glowed and acted as a source of ignition as the piston approached TDC. When the spark plug fired the mixture there will have been two flame fronts approaching each other, one from the spark plug and one from the glowing debris. As these flame fronts converge there is a violent increase in combustion chamber pressure that causes something to give way. In the days of aircooled engines with thick heads and cast pistons it was usually the piston crown that gave way and holed. With modern high strength forged pistons, and thin heads due to the water jacket, it is just as likely, possibly more so that the head will give way. The detonation will have been exacerbated or made more frequent due to leaning of mixture due to change of altitude. Edited January 20, 2014 by dadof2 Quote Link to comment Share on other sites More sharing options...
totalshell Posted January 20, 2014 Report Share Posted January 20, 2014 screwing the pilot jet in 1 turn does not this mechanical effect produce.. and 1.5 is the agreed starting point and for many will be the finishing point as well.. what does french law expect you to do? THAT is the only question you need an accurate answer to .. sadly us low living Brits cant comprehend running a bike a 1000m above sea level Quote Link to comment Share on other sites More sharing options...
dan williams Posted January 21, 2014 Report Share Posted January 21, 2014 I think the simplest answer is the most likely. Somebody played motocrosser on a trials bike. Probably with straight gas once the tank ran out of whatever you had put in. The kind of cascade failure you are all trying to diagnose is pretty rare unless an engine is going like hell when it fails. 1 Quote Link to comment Share on other sites More sharing options...
aussiechris Posted January 21, 2014 Report Share Posted January 21, 2014 Phil, Something else you might like to check. While the cylinder is removed, take a look at the plastic/nylon flywheel stuffers to see if there is visible heat damage. That sort of damage has nothing to do with cooling water. It is caused by a lack of lubrication causing the big end bearing to overheat. Some time back, I had to repair a 125cc Gas Gas moto-cross bike. My friend’s son had forgotten to put oil in the fuel. The piston didn’t seize but it destroyed the big-end bearing. The lad stopped riding when it didn’t seem to perform right. In this case, the plastic flywheel stuffers were melted and had turned brown in colour (they should normally be off-white) and the crank pin, rod, and bearing were toast. It may be a clue as to whether the new owner had failed to put oil in the petrol (you’ll obviously not get an honest answer to such a question). If run long enough it would cause the whole engine to overheat and boil dry. By the way, changing the idle mixture would not cause this damage. And 1.5 turns out is right for a Keihin but if it has a Dellorto then 3 turns out would be closer to the mark. You replaced the water pump a long time ago so it’s unlikely that was the initial cause of this event. If I understand correctly, the guy rode it the first day (probably on the original fuel), then took it out for a trail ride the next day. I’m guessing he re-fueled it at that point. It’s really easy to overlook adding the oil. Either that, or he crashed and the bike revved out for too long. Chris Brisbane Australia. 1 Quote Link to comment Share on other sites More sharing options...
canada280i Posted February 2, 2014 Report Share Posted February 2, 2014 Any resolution or update on this string? Quote Link to comment Share on other sites More sharing options...
jfc Posted February 3, 2014 Report Share Posted February 3, 2014 Bump Quote Link to comment Share on other sites More sharing options...
steve_earle Posted February 10, 2014 Report Share Posted February 10, 2014 Just seen this and heres my input for what it's worth. From what I can work out the gearbox breather was connected to the carb causing a vacum in the carb that would make the bike run very lean especialy at higher revs as the vacum would starve the float bowl. The bike probably suffered a meltdown through running lean that lead to the other damage. The new owner should have noticed but didn't and just kept on riding it to destruction. A gearbox breather should not be hooked up to the carb in any circumstance on a 2 stroke trials bike. So It looks like this issue is knock for knock so to speak (excuse the pun). Quote Link to comment Share on other sites More sharing options...
canada280i Posted February 11, 2014 Report Share Posted February 11, 2014 Any update fron Phil, the original poster? I would like to know how or if this was resolved Quote Link to comment Share on other sites More sharing options...
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