Well, despite the weather being less than ideal this weekend (freezing temps... snow Saturday), I managed to record the ignition curve of my distributor and compare it to the two ideal timing curves for this engine that I found on the web. I made two passes, ramping up slowly to certain engine speeds, holding it steady and noting down the amount of advance. Between 1st and 2nd tests, I re-adjusted distributor's initial timing of 5deg. BTDC hence a slight difference in the curves, but results are pretty much the same.
As you can see I'm pretty darn close to the "prestolite" curve (green), after 1500RPM it starts to accelerate a bit more rapidly towards total advance of around 26-27deg. adjusted.
What is a bit confusing is the two ideal timing curves specified from Merc (red & green) for this engine are quite different. The red curve doesn't achieve total timing until ~2800RPM, while the green curve achieves total timing closer to the 2K mark. The path to total timing on the red curve is more gradual as well, the largest gap is about an 8deg. difference @ the 2K mark. How much this actually makes a difference? I don't really know but it might be significant.
Here is another look, this time I compared Test #2 data VS. the minimum and maximum limit curves for each type of distributor:
As you can see if I were to have the prestolite distributor, I'm right in the ballpark and would certainly give it a pass. Only thing is I'm almost certain that I have a Delco Remy. But should it even matter? It's interesting how the curves are quite different and has me wondering why? Even with all this data I'm not 100% convinced that these curves are right for my engine anyways.
For example if you look at this link for where I got the "prestolite" curve (
http://www.boatfix.com/merc/bullet/95/95_11.PDF)
You will see pages 1-6 are all for the 3.0L inline 4 with varying ignition setups. Just look at how different each of those curves are!! I mean pages 3,4,&5 show the total advance coming in wayyyy later at 4K RPM. So I guess I'm having a hard time understanding why there is such a difference when engine displacement is the same???
Maybe the answer is in different ignition setups you might have "hotter" spark with these new digital setups vs. an older points based system. Maybe that has an effect on what the curve should look like? Another possibility I can think of is that the advance curves for the merc 165 is outdated and based on leaded fuels which were more resistant to pre-ignition?
To me, I still say that my advance is coming in wayyy too quick for my application, to have total timing come all in at what is effectively a fast idle of only 2-2500RPM does not make any sense to me. The boat hasn't even had the slightest chance to plane at those engine speeds. My next course of action is still to pop open the carb just to make absolutely sure the correct jets are in there. Then I'll try throwing some stiffer springs in the dizzy and try to get a less aggressive curve.
One more thing - thanks for putting up with my crazy...lol it never seems to end.
