3.0 merc timing question

[airshot]

I recently bought a 1983 Starcraft Islander with 140 hp, 3.0 merc. Engine runs great, a little difficulty in re-starting after warm. Choke was converted to electric, still tinkering with it. My question is...the distributor was converted to electronic (point plate removed and replaced) and does not look like examples shown in forum pics, no colored wires to connect... Looks like stock distributor but has a plate with with electric impulse gizmo in place of points. Motor calls for 6 degrees BTC which we set with son-in-laws fancy programmable timming light, (he is a former auto mechanic), we were able to check total advance it is 27 degrees. With all this baseline discussion, did we do it right? Compression is good, idles excellent but only gets 3400 rpm at wot, however runs great at wot. The prop is a 14 1/2" x 19 pitch, I have found a 16" x 15" pitch to try as soon as weather cooperates, does it sound like I am on the right track or should I be doing something else. Have been an outboard man for 40 years, this is my first I/O. Thanks in advance for I-boats usually great help.

Airshot

 

[Don S]

Re: 3.0 merc timing question

You are too far out for props to be the problem, what's the gear ratio in the outdrive?

Just an FYI, increasing prop diameter is like increasing pitch. So if you increase the diameter by 1" and reduce pitch by 1" you basically stay the same. No change in rpm.

There are also a lot more things that can cause low WOT rpm than just a prop or timing. Have a look here.
http://forums.iboats.com/showthread.php?t=295070

 

[airshot]

Re: 3.0 merc timing question

Thanks for the reply, yes I have scanned that post and many others, have eliminated most of them, if the timing is correctly set I am down to prop.
or gear ratio, possible fuel pressure but does not act like a fuel problem.
Operating temp is good, no over load, boat is trailered, no growth, linkage allows full opening of throttle plates, etc. Checked each one down the line, only ones I am not sure of are which prop and gear ratio. Possible carb rebuild however carb appears to be functioning fine. The boat speed is up around 31-32 mph at 3400 rpm and tach matches the fancy timing light reading. How do I know what gear ratio I would need? Thanks again for your help.

Airshot

 

[EddiePetty]

Re: 3.0 merc timing question

......How do I know what gear ratio I would need?

Airshot

...here ya' go.

http://www.seilermarine.com/info/mercruiser-drive-ratio-chart.htm

(Google is your friend...."Mercruiser gear ratios")

 

[Don S]

Re: 3.0 merc timing question

How do I know what gear ratio I would need?

It's very simple, 1.98 been that way for ever. In other words, the crankshaft of the engine turns almost 2 complete turns for every complete revolution of the prop.
That should also give you some idea how to check the ratio.

 

[airshot]

Re: 3.0 merc timing question

Yep..read that before just didn't soak in...Well I was able to get out this am and the 16x 15 pitch gained me around 600 rpm. When running with the wind I was at 4100 and into the wind I was at 3900 rpm. Still think I need a carb rebuild as we have found that when it doesn't want to start if we pump the throttle a couple times it fires right up. Will see if carb job helps the WOT.
Thanks for your input !!

Airshot