Mercury 7.5 HP (1969 model year) Idle Speed Spec ?

I just finished a rebuild on my 1969 Mercury 7.5 hp motor and was adjusting the idle speed.

The engine seems to like to idle around 1100 RPM in neutral and 900 RPM in gear (per my induction tachometer). I can set the idle a bit lower, but the chances of it stalling seem to be there if I lower it too much.

I am just wondering if there is a specification for proper idle speed for these motors, as I have not been able to find one on the Internet? I only could find a spec for a much newer 8 HP model that states "850 RPM in gear". Just wondering if the older Merc's would be about the same.

Allways adjust the slow needle in the water in forward gear. After it is up to operating temp turn the slow needle out until it coughs and then back in until it stumbles. Then adjust it half way between those two points. Turn the needle 1/8 turn at a time and wait about 5 seconds for the crankcase to stabilize. You should get it to 700 rpm in forward gear and always adjust it a tad rich especially if you do alot of slow speed operation. It helps for cold starts and it protects the engine. Too lean is the same as running low on oil.

I just checkd my book you should get 650rpm in forward gear on the water.

OK, thanks.

Couple of follow up questions. . .

There is adjustment for both the throttle linkage (idle stop ) and the idle mixture, for which you outlined the adjustment procedure. What would be the adjustment for the idle stop on the throttle linkage.?

Should I get the idle speed in the ballpark with the idle stop and then adjust the mixture, then make a final adjustment of the idle stop?

You will find the sweet spot I found it best to final the adjustments on the water after running it WOT for about ten minutes. Go back to slow adjust the needle first then adjust idle speed and go back and check the slow needle again.

OK, will do. Thanks.

Did you install new piston rings? If so be sure and do a 8 hour break in period before running WOT for extended periods. Run it up to 3/4 throttle and at variable speeds for the first 8 hrs and short spurts of WOT.

Yes, new main bearings and piston rings. I have run it on a stand and water barrel so far. Too cold to be using it on a boat this time of year. I have run the engine for about 3 hours total at various speeds, but only at the lower speeds while in gear.

The maximum throttle position while in neutral yields about 3200 RPM. I also ran a bit heavier on the oil maybe about 35:1 instead of 50:1.

The engine seems to run well, but the full in-water use will tell the real story.

I rebuilt the engine, because it was sucking in some water through the lower seal where both the gasket and crankshaft seal were feeling their age.