possibly running on 3 cylinders instead of 4

My motor is a 1985 120 johnson . I had taken it to a mechanic and had it checked for low rpms at wot. The boat was slow planing out. The guy said the motor was good(good spark, compression, and fuel).<br />I thought the prop was the problem since the motor had been checked. Today I took the plugs out and the top right plug had no gap at all.How could this happen?? Well I replaced the plug and hooked up to the water hose and cranked it. It sounded good. I took off the top right plug wire and there was no difference. All of the others would sound like the motor was bogging down when removed. Sounds like that cylinder isn't doing its job. What could be causing this and how much should it cost to repair? <br />Thanks for all help.

Re: possibly running on 3 cylinders instead of 4

What are the compression numbers?<br />Spark test, how large of gap will it jump, with plugs in?<br />What are fuel pressure numbers?<br />One of your post said that you removed VRO, what are you using for fuel pump?<br />What is stator output, hot engine?

Re: possibly running on 3 cylinders instead of 4

I am unsure about the numbers for the compression. The Plugs have good spark as i pulled them and checked them all. The guy pulled the vro off for me and put a nother fuel pump(he said it was a low pressure high volume pump) in boat with a water seperator in line. Unsure about the stator. I have no experience working on outboards. But I have a manual on the way. Thanks

Re: possibly running on 3 cylinders instead of 4

Electric fuel pump? Water seperator between tank and pump or pump and engine?

Re: possibly running on 3 cylinders instead of 4

yes it is an electric fuel pump. The water seperator is between the tank and pump.

Re: possibly running on 3 cylinders instead of 4

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Re: possibly running on 3 cylinders instead of 4

Bmann,<br /><br />You stated your mechanic said you had spark, compression, and fuel.....did he do a compression test? If you have spark and fuel, and you remove the plug wire and there is no change, it's probabally a compression problem. If the spark plug gap is closed, I bet you broke a ring and one of the pieces closed your gap. Have your compression checked on all 4 cylinders. Good luck.<br /><br />Zane

Re: possibly running on 3 cylinders instead of 4

Well I carried my boat to another shop today. We pulled the head and sure enough 1 of the pistons was bad. The guy said that it needed bored out. Should all cylinders be bored or just the 1? He said bore them all and put new pistons in. I want to do what is right, but don't want to throw any money away if it isn't nesecarry. Thanks Brett

Re: possibly running on 3 cylinders instead of 4

It is not necessary to bore all. But I would bet that once the cylinders are measured, they will be out of round past the spec and therefore have to be bored. If they measure good, then by all means just hone and re-ring.<br /><br />Good luck!