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Re: not running on #3 cyl' on my 1982 gs550
Not likely valve timing if it will run, cams are one piece so if one is right all are right. Have you checked compression with a gauge, or just by feel? Try swapping the sparkplug wire with the #1 cylinder and see if the dead cylinder moves with the wire. Also try moving the spark plug to another cylinder and see if the dead cylinder moves with it. If the compression is indeed good, the problem will be with the spark plug/ wire, or in the carb, these are the only things that can kill a single cylinder. The coils run a pair of cylinders so a bad coil would put out 2 cylinders at the same time. Start with the spark plug, I have seen a perfectly good looking plug, that would fire whun held to the head, but would not fire in the cylinder.
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It seems that you have done all the good stuff, so my advice is a shot in the dark. Sometimes when Lawn mowers are put away for the winter, the fuel left in them will become stale. Remember it is not the fuel that burns, but the fuel vapour. If the fuel is stale it won't vaporise properly. I would suggest that you empty all the old fuel out and put some fresh fuel in.
If you have a good spark and the spark timing is correct, also assuming you have sufficient compression, then the problem is almost certainly the carburettor. How did you clean it? Cleaning a carburettor is a delicate task and requires compressed air to blow through all the little holes and jets. You need to make sure that the float actually floats. If it is a metal float, they can fil with fuel and sink. The float turns off the fuel to the carburettor, thus the float valve is very important. If the float valve leaks the engine will over fuel and not pick-up as you describe. The float level must be set at the correct setting. If the float stops the fuel off too early the engine will run lean or not at all, since on some carburettors the fuel mixture will be further reduced by what are called Defuser jets. If the float level is too high, the engine will run too rich.
After starting the engine, when it stops again remove a spark plug and examine it. The spark plug may give you the clue you need:- If it is over wet with fuel, if it's oiled up, or if there is a total lack of fuel altogether.
I'm sorry to be a bit vague, but sometime diagnostics can be hard from a distance and without seeing it for myself. Nonetheless, I hope that I have been of some help.
If you are only getting 65lbs of compression, remedying that would be my next move!! .. I have seen some run as low as 85lbs..run very poor too.. but never at 65lbs. You should let the engine turn over for about 5 seconds to get compression level.
Not sure I am following your story, but you need compression, spark at the right time, and fuel for the engine to start. If you know for a fact the plugs are firing, and you have compression, you may not have fuel at the injectors.
You would need to figure out if it is a problem inside the cylinder or with something like the crank-cam sensor firing the coil. I would change the plug again to see if it works for a few days. Could be a problem with the fuel injector or a vac leak if the plug is firing but no power from that cylinder.
You have two coils. Each operates two cylinders in what is called the wasted spark system. The plug fires on the compression and the exhaust stroke. If the spark plug is wet you have either a shorting wire , a bad spark plug cap , or a fouled spark plug. Try replacing the spark plug cap and spark plug. If you still have spark on that cylinder and the plug is dry. You have a bad needle in the carburetor, a stuck float, or dirt causing the no fuel issue. You will have to clean and inspect the number three carburetor .
70psi compression indicates it's rebuild time; the compression should be double that. You can confirm what the compression should be with good rings by putting a few drops of oil in the spark plug hole and testing compression "wet". That figure will be what a new saw would have.
Since you are getting sparks to all the plugs thn the problem could be tht of fuel to those two cylinders 2 & 3.
What was the condition of the spark plugs in those two cylinders? ( 2 & 3 ), were they wet or dry?
Try te following,
Swap the spark plugs form the firing cylinders and fit them to the cylinder 2 & 3.
Try to block up the air intake side of those two cylinder carburator and start the bike's engine see if they would fire, if they do fire thn theres a problem to those two carburators, fuel is not going into the combustion chamber, the problem could be float level , or the jets may need to be blown with compressed air.
Hope this helps!
Check if the spark plug is getting sparks.
Check if the spark plug is wet with fuel after a few cranking.
Hope you have cleaned the carb well ..blowing compressed air to its jets and also checked the float level inthe float chamber.