Engine Rough Idle No change when plug is pulled?

[LowerBost]

So I have a 2005 Yamaha 115 2 stroke engine. It has a rough idle. When I pull one of the plugs off, the engine does not change sound but will with the other 3 plugs. New plugs just put in, cleaned fuel filter and ran seafoam thru the gas. Still has a rough idle. Checked the back pressure and seems okay. Good spark as well. Any ideas what this can be? I'm running out of ideas.

Thanks for the help!

[boscoe99]

Bad plug, bad spark plug cap, bad coil wire or bad CDI.

You need to verify whether or not there is no spark to the one plug that does not seem to affect the motor when it is not connected.

Then find out why no spark and fix that issue.

[TownsendsFJR1300]

When you say back pressure, does that mean compression?

What is the exact compression for all cylinders? They should be within 10% of each other...

The suspect cylinder, how's the spark plug look?

Also, when testing the compression with all the spark plugs out, you can also check spark at each plug. Plug in each plug in the cap, ground it to the head and while spinning the engine over, look for spark jumping from the electrode to ground.

Please post what you find.

[madharchod]

simple way to test the plug change the spark plug around and if it works you can say its the plug if it dosent then start looking else where but my gut feeling is its the spark plug
cheers

[99yam40]

check fuel pump
could be leaking into crank case and flooding a cylinder

[LowerBost]

I changed the plugs around and still the same issue. I even changed the wires around to make sure it wasn't that or the coil pack and still had the issue. Spark looks good. Compression seemed okay from when the guy tested it with his tester. I thought about the CDI but ruled that out because if that was the case then it wouldn't be getting any spark at all to that plug would it?

[99yam40]

good compression, good spark at the proper time, and the proper fuel to air mix in the cylinders is all it takes make it fire properly.

Too much fuel is as bad as too little when trying to get a cylinder to burn properly

[TownsendsFJR1300]

What are the low and high spec's for compression? Do we know what the compression of YOUR ENGINE is to compare to yamaha spec's?????



As posted above too much fuel will foul a plug. If you were to crank the engine over with the plugs out, are you getting more fuel/fumes out of the bottom cylinder than the rest?