3.0L Spark Plug Gap and Timing Questions

[stormulus]

I'm a new member to this forum and also new to power boating. I appreciate all the knowledge that has been shared here and have spent many nights recently pouring over its pages in order to gain as much knowledge as I can. So thanks to you all.

My boat is a 2004 model, however the sterndrive was likely manufactured in 2003. Serial number 0M679646. I have some questions regarding spark plug gap and timing. I have printed versions of the owners manual (2003 copyright), service manual (1999 copyright), and parts manual (revised 5-2-2019), all of which I ordered from Mercury Marine recently.

Spark plug gap:
- decal on spark arrestor indicates .045
- owners manual indicates .045
- service manual indicates .035
- I have also seen (online) a service bulletin from I believe 2006 that listed the gap at .045.

To start the engine, it had been taking 5 or 6 throttle pumps in order to get it to start firing upon cranking it over, and then required more pumps to keep it running, for about the first 10 to 15 seconds. The idle was kind of rough too. The morning the manuals arrived I picked up new spark plugs, gapped them at .035 per the service manual, and put them in (I had not yet noticed the info on the decal and in the owners manual was different). It still was hard to start (using the same 5 to 6 throttle pumps procedure) and after the engine was warm, starting it was worse. I started wondering if I was flooding it every time I tried to start it. The next day, with a cold engine and no pre-pumping the throttle, I turned the key and it fired right up. Warmed it to operating temperature, turned it off, waited 10 minutes and it fired right up again without any pumping of the throttle. The idle was also much smoother, although it could have been a little better. On the water the boat ran great.

The next day I noticed the .045 gap indicated on the decal. I checked service manual again (which says .035) then the owners manual. Conflicting gap info. I looked online and found the service bulletin indicating .045. So I re-gapped the plugs to .045. I'm convinced that .045 is the correct gap, but with that gap it starts hard like it did before I replaced the plugs, and idles a little bit rougher than it did.

So I'm wondering two things: 1. Is .045 in fact the correct gap? 2. And if so, I'm curious why it would run better with the wrong gap. Would that be because of fuel mixture and/or timing?

(side note - is it too much to ask that when spending $79 on a service manual, that they would include a supplement with any updated information?)

Timing:
- decal on spark arrestor indicates 1* ATDC
- owners manual indicates 2* ATDC
- service manual indicates 2* ATDC for serial numbers 0L341000 and above, which this one is.
- I have looked but have not found online any service bulletins relating to timing.


Thanks in advance!

 

[alldodge]

Howdy
The 3.0 has been made for many years but the manual has not been updated between later years until the MPI version came out. This is another reason for the bulletin. When in doubt always go with the decal on the motor, it will have latest info when the motor was replaced. The only thing the would take precedence over the decal is a bulletin.

Could be your main issue is the carb needs a rebuild

Have you dumped the fuel out of the filter into a clear container to see if there is anything but clear gas?

The spark plug bulletin I have list .045 for plugs as u say

 

[alldodge]

What plugs are you using "MR43LTS" ?

 

[stormulus]

Thanks for replying alldodge.

I used NGK BPR6EFS plugs, which is listed in the service manual.

By the way, it seems I must have mistakenly deleted a paragragh of my original post before I posted it, which went something like this:

When I mentioned the conflicting info I have regarding timing, I meant to ask if anyone knows which is correct. I plan to adjust idle, fuel mixture and timing this weekend (I have not done any of these yet), but I wanted to confirm the .045 spark plug gap before doing so. And if nobody knows which is the correct timing, i'll probably go with what is on the decal and adjust from there if necessary.

I guess the part that confuses me is that when I improperly gapped the plugs at .035 and did not pump the throttle prior to starting it, it started started right up and ran pretty good, there was just a slight rough spot in the idle. Such as about every 1 to 2 seconds there was a little shake in the engine, otherwise it was running pretty smooth. When I re-gapped to .045, it ran more like it did before replacing the plugs. However it seems clear the gap should be .045.