What did I do wrong?

[hillsidechild]

Hello.

I am a bit new to all of this, but I'm doing (I think) OK. I have 3.0 1999 i/o Merc. 4 cyl. The boat is a Larson 1986.

I've started the engine and she started fine for the first time since winterizing last fall. I proceded to then change the oil and plugs, gapping them at the recommended .035.

Since the plugs have been changed she is rough, misfiring, and cannot hold on at a low idle. I removed the plugs and reinserted them to make sure they were not installed incorrectly.....same thing. What happened??? I can' find the problem and do not want to put her in the water until this is resolved. Any ideas?

 

[Haut Medoc]

Re: What did I do wrong?

Are you sure that you have the wires right?
1-3-4-2....
Or you have a wire that is bad or not fully connected?.....​

 

[DHPMARINE]

Re: What did I do wrong?

Since it started and ran okay before the plug change,first guess has to be a plug wire on the wrong sparkplug.

DHP

 

[bruceb58]

Re: What did I do wrong?

Maybe you damaged one of the spark plug wires taking it off the plug. You can pull one wire off at a time and see that each plug causes an idle RPM change.

Also, run the engine in the dark and see if you can see any arcing occuring between the wires and the block.

 

[Gary H NC]

Re: What did I do wrong?

Could be a defective plug but my bet is a damaged plug wire.
Do you have an extra laying around you could try one at a time?

 

[Zackman]

Re: What did I do wrong?

With engine running, try pulling one wire at a time. When the engine runs the same with or without that cylinder, you found your problem cylinder. Swap wires and try again...

(Oops, samething Bruce said, sorry about the repeat, but can't find how to delete the post.)

 

[Jsiples]

Re: What did I do wrong?

I saw this and had to chime in, I have the same engine. I read an update from Mercruiser that said the new gap was to be .045..... I have an '84 would this be different than a '99?