Mercruiser RPM Question

[eldplanko]

Background: 1974 Mercury 888 (Ford 302) w/ 1200 hrs. Compression ~105-115 on all cylinders except one is low at ~85.

When I'm in the water, I can start and idle out of gear ~1900 RPM (vac at intake manifold steady at 20). I know this is high but here's why. When I put it in gear (ie FWD) rpm at idle RPM drops to ~1100 (vac drops and steady ~10-12). If I set the idle rpm when out of gear any lower than 1900 rpm, engine stalls when I shift into gear.

Also, recently (i.e. within a few hrs) went through general tune-up, points, dwell, base timing, idle mixture set w/ vacuum gauge.


I suspect low power due to low compression, but just wanted to check if I'm missing anything.

Thoughts? Thanks in advance.

 

[dockwrecker]

Re: Mercruiser RPM Question

Motor sounds done to me. Tough to get proper combustion under 110 lbs.

 

[Black as]

Re: Mercruiser RPM Question

I would say its time to change that motor out

 

[haulnazz15]

Re: Mercruiser RPM Question

You're gonna chew up that outdrive if your idle is that high. It needs to be between about 600-700 rpm to allow the outdrive to slip in/out of gear easier.

 

[eldplanko]

Re: Mercruiser RPM Question

Thanks for your comments. I agree with both of you and plan to doing some major engine work when $ allows.

I just wanted to double check though, is the drop in rpm due to the poor power output from the engine, or could it be something else?

 

[180shabah]

Re: Mercruiser RPM Question

Sounds like you have carb/choke issues as well. Even with the low compression, you should still be able to get that engine to idle BELOW 1100.

You need to get this addressed ASAP before you destroy the drive.

 

[eldplanko]

Re: Mercruiser RPM Question

Agreed on fixing ASAP. Just to clarify, it will idle in neutral down to ~800 RPM, so I'm not sure if there is a carb problem. I have the idle set so high, as that is the minimum I can set so that it can shift into gear without stalling out.

Thanks for your suggestions.

 

[EddiePetty]

Re: Mercruiser RPM Question

...now this may sound off-the-wall to the more professional folks on here...but: here goes!!!
I just today overcame a quite similar problem, though not as severe. It seems that the vertical shift shaft in my outdrive had accumulated enough crud that its rotation was creating enough drag to close my shift interrupt switch just enough to "chatter" the switch close-to-open repeatly. Sometimes the engine would die out, other times it would not accelerated past idle. I, as you, advanced my idle to get back in (the shock of the shift engagement must have been causing the shift shaft to complete its cycle.
Just a thought guys and gals so don't throw rocks.
BTW, after freeing my shift shaft.....'ol V-20 shifts first-time, every-time without dying and at 650 rpms!!!

FWIW...Ed in 'ol Virginny