Stihl MS290 Problem

This probably isn’t the best forum to ask this question, but I saw another guy ask about Stihl saws, so I figured I’ll give it a shot.

I’ve got an MS 290 that won’t start. Carb works fine; fuel line and fuel are good; air filter is clean; plug has a spark, motor isn’t seized up. I’ve spent hours tearin the thing apart and checkin every part I can think of; I even put some parts on another 290 to verify. I’ve reached a dead end, and I’m lookin for some suggestions.

Thanks,
Kyle M

How is the compression?

How do you know the carb works fine? What are you basing that on?

When was the last time it ran?

If the machine truly does have spark then the number one suspect has to shift to fuel.

Grouse

Compression feels like it’s right when you pull the cord. I put the carb on another 290 and it ran fine. I’m usin the same fuel in both of em. I even took the fuel line and filter from the one that works and put it in the one that doesn’t run. It worked two days ago with no problems. I set down after cuttin a few trees; it wouldn’t run the next day. Makes no sense to me.

Check the muffler. The spark arrester screen will plug up. Poke a hole in it see if that is the problem.

Kyle Moeller wrote:

Compression feels like it’s right when you pull the cord. I put the carb on another 290 and it ran fine. I’m usin the same fuel in both of em. I even took the fuel line and filter from the one that works and put it in the one that doesn’t run. It worked two days ago with no problems. I set down after cuttin a few trees; it wouldn’t run the next day. Makes no sense to me.

Hmmm. I’m starting to wonder about the coil. You could still get spark with a bad coil, but then they fade as they heat up.

You’re right, this is a strange one.

Grouse

Spark arrestor was fine, but I just compared the two saws with the exhaust manifold off, and the one that works doesn’t leak any fuel when you run it through a cycle, but the broken one leaks a little bit around the piston ring. New piston rings maybe? Would that small leak let enough fuel escape the cylinder and prevent it from firin up?

Pull the flywheel and make sure it didn’t snap the “key”. That would put the timing way off.

quote=1958526]and the one that works doesn’t leak any fuel when you run it through a cycle, but the broken one leaks a little bit around the piston ring. New piston rings maybe? Would that small leak let enough fuel escape the cylinder and prevent it from firin up?[/quote]

Yep. That’s why my first idea was compression. If you’ve done basic fuel and spark checks, then this is always the suspect on small 2 stroke engines.

It’s not that fuel is escaping. It’s that you are not achieving enough compression of the fuel-air mixture to ignite it with that tiny spark.

Do you have a compression tester? If not, I’d have a shop test it. A comp tester is a handy tool to have, though. I think I got my tester for about $30.

Grouse