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honda outboard 20hp misfire at high rev

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weejohn

Member
I have a honda 20hp 4 stroke it starts fine pumps water ok . I have just replaced the sparkplugs put new fuel in tank drained old fuel from system also done oil change and filter . When the engine in neutral l gave it a quick high rev there was a few back fires . any ideas what could cause this :confused: . I dont want to go out fishing just in case it breaks down :(
 
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(This engine has a rev limiter for keeping you from overspeeding the engine in neutral) if i rev to high does it make a popping sound . no not done compression test yet when hot , i stripped the jets out of carb blew everything out , carb is very clean
 
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i have done a compression test 160 psi on both cylinders engine hot and throttle open . still popping at high revs 3/4 revs and below is fine . any ideas ?
 
I'm guessing you don't have a tach. I mean...if you KNEW what the rpms actually are then it would be easier to figure out what is going on. The rev limiter will cause the engine to sort of "bark" and, yes, it can sound like it is popping. But it should NEVER hit the rev limiter if the outboard is in gear and has a properly sized, undamaged propeller.

Could your prop hub be starting to spin? That would cause you to hit the rev limiter and performance would be down.

If it's not that then it may be a VALVE timing issue. Your compression (160psi) is ok for a well worn engine but is far below the "nominal" of around 200 psi. A loose or jumped timing belt could account for the lowered compression and the performance fall off and popping...

The popping and performance fall off could also be because of an IGNITION timing issue. The ignition firing could be affected by a loose pulsar coil or a slipped magnet in the cam sprocket.

Another possibility is a weak or broken valve return spring or springs. This would keep one or more valves from fully closing and might get more noticeable at higher rpms.

Carbon build up around the valve stems can also cause valves to "hang" open. This is a problem on older engines that do a lot of trolling.

There are a few things to consider...especially if this is an older 20.

Good luck.
 
thanks for your reply the year of the engine is 2004 . i will get someone to have a look at it the only good thing is i can still use it at half throttle and it should run all day
 
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