My procedures for diagnosing chainsaw/brush-cutter two stroke problems.
There have been a lot of differing ideas on how to find a given problem in two strokes, below are a few things that I check first before going deeper in to the problem.
1 Check your fuel, if it smells off then discard it (do this after you go through the rest of these procedures).
Put in fresh mixed fuel ie a make up a fresh tin, Don't use the old stuff from the same tin. Unleaded only has a shelf life of 28 days from the time of manufacture, not when you buy it from the pump. This will increase if you use a fuel stabilizer. I have some information from the Echo (Koritz) work-shop repair manual about fuel shelf life and fuel phase separation. At the moment I can't find it but when I do I will start an other thread about that.
2 Remove the spark plug and check for spark by putting a known working plug back in the plug lead (or a spark tester) laying the plug on the head and turning over the engine by the pull starter. If no spark, then check that the kill switch is working properly by disconnecting and using an ohm metre. Then check that the wires from the coil to the switch-switch to earth are not broken or worn and that they are not rubbing on anything. Next check that the coil lead isn't cracked or worn and that the plug terminal is connected to the lead. Some of these lead can be change by unscrewing them from the coil itself, others you can't ! If you still don't have spark and its electronic ignition and you will possibly be the coil, but check the earths under the e-bar and from the coil to the e-bar for proper grounding (scuff them up with sand-paper)
3 Remove the muffler and check to see if there is any carbon build up in the exhaust port and the spark arrestor screen. Clean the screen by heating it up and burning off the carbon using OXY, MAP or LPG torch and replace, otherwise hot carbon can shoot out of the muffler and start a bush fire. The bottom line is responsibility rests with you, so please replace ( think about this! A person was charged with starting a fire in the recent Victorian bush fires by using an angle grinder outside. You can clean mufflers by removing them from the saw and heating up the unburnt oil-carbon inside till red hot and blowing compressed air into it to keep the carbon burning. It billows out heaps of smoke so do it away from the house and make sure the wind isn't blowing towards the house, I didn't one time and the Mrs came out and gave me a fear-full rev. Also check to make sure it hasn't run on straight fuel and the piston and rings aren't melted together. If it has don't bother with the rest of this at the very least you will need a new piston and ring set, at the worst and piston and barrel kit
4 Next comes the carby and fuel systems. Check to see if the fuel and pulse lines aren't perished or cracked and that the vent line isn't perished or cracked, also some systems don't have a separate vent hose but have a duck-bill nib as a tank breather in the fuel cap or on the fuel tank itself. Check that there is no dirt or debris in or around the outside of the duck-bill. Dirt is not your friend in this case so try to keep these areas clean after use .Check clean or replace the fuel filter. If it has a felt type filter and this is discoloured replace these as the discolouring is either dirt or dried oil /fuel dyes.
5 Last comes the carby. I lay a clean rag on the bench for visibility and strip the carb and lay out the parts in the sequence that they where disassembled. If the machine hasn't been started for a while you will see a tarnish like film on the parts. This is dried out two stroke oil and the dyes that fuel manufactures use to distinguish between different fuel types. This is
what causes most carby problems, dried out dye/oil in small hard to get at passages and orifices and needle holes. I use an Ultra-sonic cleaner to clean the carbs, but you can use plenty of carby-cleaner and let it soak for some time, gently blowing out out the carb. If the diaphragms are ok (they are not stiff, if they are replace with a full carby kit ) reassemble the carb in the reverse order that it was disassembled. Set the high + low speed mixture screws at 1 1/8 -1/4 out from seated and refit the carby.
I have a small squirt bottle with fresh two stroke mix that I use to prime the carb to assist in starting . Once you have the saw started and idling, blip the throttle. If the saw bogs down or lags on throttle response ( this means that it is lean on the low speed side of the carby ) richen up the L adjustment screw out until it responds smoothly [you should not have to take the adjustment any more than 2 1⁄2 turns (on some carbs). If any more it may indicate that you have an air leak a some point ie crank seals ,crank-case gaskets,carby flange gaskets (emulsion blocks). If so you will have to remedy that problem before you go any further, after you do this you will also have to adjust the idle.
Next comes the high speed adjustment. I use a ratchet strap to hold the saw to the bench (note the bar and chain have to be fitted to give the saw the correct load for high speed tuning) hold the throttle at full revs WOT ( wide open throttle )and screw in the H speed screw until you hear the saw reach max revs (by ear or by Tach) then richen out until you hear it four stroking - a change of note, or reach the max rev range recommended by the manufacturer. After that you should be good to go. I test the saw in a log that I have to make sure that it is performing right .
There are two excellent reference web sites for chain-saws
The Chainsaw Collectors Corner http://www.acresinternet.com/cscc.nsf
Madsen's Online http://www.madsens1.com/MNUsawmaint.htm
These are two of the best for referencing carb, types points settings rev range bar and chain types. Madsen's has a wmf sound file for high and low speed tuning of the carbs .
I hope this help and I'm sure that I have left some thing out if so feel free to comment or correct me
Cheers 741Baus (Peter)
