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Did you check the timing? If the timing is off there will be a reduction in performance and power will be greatly reduced. If the engine is "pinging" or "knocking" under heavy acceleration, your timing is advanced, and pre ignition is occurring (that's when the spark has ignited the fuel in the combustion chamber prematurely), resulting in a knocking sound. When the piston is compressing the air/fuel mixture within the combustion chamber, the spark plug produces a spark from current it receives from contacts closing in the distributor cap. (Where all the spark plug wires are plugged into) if that spark come BEFORE the piston finishes it up stroke or compression stroke, the piston is then being slowed down from the kinetic energy produced by the exploding fuel. Thats where we loose our power. Idealy, the spark should come AFTER the piston has rotated over the top of its stroke, It has reached its limit in compression and is on its way back down then the spark should ignite the compressed fuel, thus giving the piston an additional "shove" back down to start its "Power" stroke, once its reached the bottom of the power stroke (stroke #2 of 4) it is then carried back up for the "exhaust" stroke pushing the burnt fuel out of the combustion camber and into the exhaust manifold, one it has reached the top of that stroke it begins its journey back down to start stroke number 4, the Intake stroke. Drawing in air/fuel mixture to again be pushed back upward to compress the air/fuel, and repeat the process over and over again. Now that I have thoroughly bored you to tears, Yeah, check the engine timing, if its not the timing it may be a vacuum leak.
DO NOT , I repeat do not use starting fluid on a 2 stroke chainsaw or trimmer. you will score the cylinder or the piston. When trying to diagnose a hard to start motor just give it a small squirt of mix in the carburetor. If it won't fire you have a compression problem or an ignition problem. when you test for spark make sure you have a good plug, that is well grounded and not in bright sunlight which can make it hard to see the spark. if you have spark, do a compression test.
You need three things. Spark, compression and fuel. Pull the spark plug and make sure it is not fouled and you have spark when cranking. Put your finger over the spark plug hole and feel for compression while pulling the engine over. There is a reed valve between carb and block that clogs and prevents fuel from getting to cylinder.
Have you tried pouring a little fuel into the plug hole and try for start? If it runs for a short time, you have fuel delivery problems. Check the fuel filter for plugging. Look under the carburetor bowl for a small solenoid that commonly sticks in one position. I'm not aware of an automatic compression release on these engines. As this is a 4-stroke engine, it's possible that one of the valve clearances has closed up during use. Check this with the piston at top-dead-center on the compression stroke. Hope some of this helps!
Gotta start with the simple things...
How do you know the compression is fine? Since you did not give a compression reading I do not know what method you used to test the compression.
How do you know the fuel system is fine? The plug is wet with what? Could it be water?
How do you know the spark is fine? Have you used a spark tester? Or did yuo use the spark plug on the side of the head? Did you know a bad spark plug can still spark under regular condition but wont spark under compression?
You left out valve timing- its a 4-stroke.
First put in a new plug. Then see if it hits on a prime ( see my tip on diagnosing a small engine).
If you have double checked all the basics and are sure we have fuel,compression, and ignition and it still wont start then check your flywheel key and valve timing.
If you have mixed your gas and oil to a 4 to one mix you have way too much oil in your gasoline.
Drain out your fuel tank and carburetor. -----------------------------------------------------------------------------Remove the spark plug and clean if necessary.Mix your fuel in a fuel container at 40 to one. Some sites even recommend a 50 to one mixture. (50 to one mixture is one pint of oil to 6 gallons of gasoline)Refill your fuel tank on the trimmer and try to crank, if it does not want to crank within 4-5 pulls then you may want to give it a very small shot of starting ether in the carburetor intake and try to crank again.Good Luck, I hope this helped,Please rate me, Thanks
Let's start with the easy stuff first. Remove and clean the spark plug. Look into the tank and fish out the fuel filter. It should be stuck in the end of the fuel line in the tank. I usually use a short piece of coathanger with a small hook bent into the end of it. Clean or replace the fuel filter. The next thing is to clean the small screen at the exhaust. Remove any carbon build-up there that you can. Sure hope this helped and best wishes.
Check the line head and cable, there may be grass or something like twine twisted between the shaft and housing. If nothing is found there take the spark plug out and see if it will turn over, possibly liquid gasoline has gotten into the cylinder somehow.