Re: merc xr4 150 rpms question
If it has the original timing module on it, it's timed back to about 17 degrees at 6000 rpm. It won't hurt it, but it's actually backing off on power at that point.
The timing module has two functions. One is to mess with timing, advancing it from 19 degrees to 23 degrees at 5000 rpm, then backing it off to 17 degrees at 5500 rpm. The other is to stabilize idle by slightly advancing the timing if idle speed goes below a set point, usually 600 rpm.
Neither function is really needed, and in fact if the module should fail shorted at WOT, it'll grenade the power head in a heartbeat. Most recommend you remove it and set your wot timing to 23 degrees.
If you do that, you'll need a slightly steeper prop. If you choose to keep the module, prop it for 5300 - 5500 when heavy, and under 6000 when light. If you remove the module, you can go a little over 6000 lightly loaded without hurting it.
I went from a 22 high five to a 24 high five on an 18' Tracker fs1800. It turns about 6200 lightly loaded, and 5800 with 2 phat phishermen and their gear. Goes mid 60's lightly loaded, 57 or so fishing.
hope it helps
John
