70hp johnson wont go over 4000 rpm

[brands]

i checked to see if the spark plugs were firing and they all were but while the engine ran it didnt make a difference when the middle spark plug wire was off. the middle spark plug also is getting dirty and wet fast. it's either fuel or spark right? i have spark so what can i do to check what is wrong with it? thanks a lot<br /><br /> Justin

 

[DJ]

Re: 70hp johnson wont go over 4000 rpm

brands,<br /><br />That cylinder is not pulling its weight. If there is spark and fuel, I would think that the missing component needed for combustion is compression.<br /><br />Do a compression test. The cylinders should be over 100 psi and within 5-10 psi of each other.

 

[walleyehed]

Re: 70hp johnson wont go over 4000 rpm

Just because a plug sparks "OUT" of the hole, doesn't mean it will under compression.....sure, we can test, or simulate compression by making it jump the gap on a plug tester, but you CANNOT simulate heat.<br />I would first try a compression check of all 3, and if that reveals a problem, you know where you're at, if not, and comp is reasonable, try a new plug, if that doesn't do it, start looking at wire and coil.