Small Engine Troubleshooting Problems? Ask a Mechanic Now.
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Have you taken a look at the cylinder and sides of the piston? Your compression reading sounds good but in my experience it sounds too good. I have seen these machines with damaged piston and cylinder walls that allow crankcase pressures to bypass the sides of the piston and cylinder and give you really odd compression readings. I would remove the exhaust and take a look at the side of the piston, and maybe even remove the spark plug and try to get a look down into the cylinder walls.
The 50-60 on the old one is definitely an issue and I could understand it not running. I have never seen one of these that would run with less than 90 psi which being said you would think that the replacement would run since you say it has 125+ but I have seen times when the cylinder becomes damaged badly enough that it would give you false compression readings and it throws you off on troubleshooting starting/running problems.
Can you see down through the spark plug hole to be able to view the sides of the cylinder wall? You should be able to view damage from there.
Sometimes Ebay sellers aren't always the most honest but it very well could be a take off short block, just makes me wonder why it was taken off. I definitely understand the mission, I have had the same thing happen to me from time to time. You get way more time involved into the machine than what it is worth but it is nice to figure out the problem and once you do it helps to keep the same thing from happening again.
If the plug is dry then you aren't getting fuel to the cylinder. Try removing the air filter and spraying a little bit of fuel directly into the throat of the carburetor and see if it will start and run until it burns that off.
There is no way to adjust timing on a 2 cycle. The only thing that may have happened is the flywheel key sheared. If you haven't already done so you might want to check that. It really still doesn't explain the dry cylinder, even if you aren't getting spark at the right time you should be getting fuel all the time as long as the carburetor is functioning and the engine is pulling the fuel in. The cylinder being dry makes me go back to a damaged cylinder that is not creating the vacuum to pull the fuel in when it needs to or should be. By spraying directly into the throat of the carburetor you have somewhat eliminated the rest of the fuel system.
It is very aggravating when you get these machines that give you so much trouble like this one. I have been in the service and repair business for almost 30 years and it does happen once in a while, you hate to give up on it but with all the other things that you could be doing it is sometimes better to move on to something else and then come back to it at a later time. You hate however to just keep putting time and money into it because you know that you can never get the customer to pay you what it is actually worth or what you have invested into it.
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