Mercruiser 120 running rough (video)

[italianstal27]

Hi all,

1973 mercruiser.

Currently: 8 degrees BTDC, 1 1/2 turns idle mixture screws, oil sender is having grounding problems, but oil pressure (before it failed) was 30psi.

Engine will not start unless the choke is open (allowing air in), unless it's stone cold. Having issues finding just the right spring setting...

Pulling spark plugs individually doesn't seem to help *all* that much, but it's really hard to tell. Engine will rock if only 3 cylinders are firing...

Engine accelerates no problem.

Thoughts on what is causing that popping sound + the rock?

 

[italianstal27]

Compression
1: 125
2: 115
3: 115
4: 115

 

[Scott Danforth]

normally compression on those motors should be between 140 and 150. with compression that low and even across all the cylinders, its time for a bore/hone/new pistons

poping is from ignition (dwell and timing) or running lean (dirty carb)

did you set the dwell prior to setting timing?

 

[Bt Doctur]

reduce the idle speed
open the idle mixture jets untill highest rpm
check timing

 

[italianstal27]

Spark plugs are dry and pretty black. Idle mixture was backed out as much as possible before we started to get soot out the exhaust. The thermostat is opening to about the first notch on the fast idle cam. Should it be opening all the way?

We did not check dwell. Dwell would imply a bad distributor cap right?

 

[italianstal27]

Oh also, the distributor is solid state. No points

 

[Scott Danforth]

Spark plugs are dry and pretty black. Idle mixture was backed out as much as possible before we started to get soot out the exhaust. The thermostat is opening to about the first notch on the fast idle cam. Should it be opening all the way?

that says a few things

first, your idle circuits are plugged, time to pull the carb apart

second, your choke is not adjusted fully

 

[italianstal27]

Ahhh, so If I have been adjusting the idle mixture screw with the choke not fully open, then we're idling at the wrong air / fuel mixture for the engine operating temperature?

Phrased differently: If you have a perfectly tuned engine, at operating temp, and you then close the choke to say the first notch on the idle cam (half open), what does the engine do / sound like?

 

[italianstal27]

AH! I don't have the intake to the hot air choke metal spring connected to anything! perhaps the metal isn't getting hot enough to drop the choke plate open?

 

[italianstal27]

More googling and youtube video watching is leading me to believe our choke isn't hooked up properly! The metal is heated from the exhaust manifold. On our manifold does not have this port to connect a "heat tube" to the carb.

I believe our best option is to pick up an electric choke for a 2GC/GV rochester carb, install it, and then tune the engine that way.

Any further tinkering with an improperly connected choke bi-metal plate is an effort in futility.

Is that the correct line of thinking? The electric replacements are $15.

 

[mickyryan]

if its warm weather start it run it and don't even use choke keep it open adjust it to be open all the time,then back out idle till its highest you can get then readjust timing and see where you are, also id shoot for 900 idle max, also I bet carb is clogged if its just idle it does this. and yes use electric choke
also remember these arent well balanced engines at low rpm they shake rattle and roll a ton compared to a v engine , at least all of mine always did, atm mine idles at 700 and takes off just fine and runs good oil pressure is around 10 psi per rpm but at idle its between 20 and 40 till it warms up ,

 

[italianstal27]

Alright the electric choke is AWESOME! Finally got the choke to open! Engine runs pretty smooth at 1200 but still hearing that miss ocassionally especially at 800 rpm. I think it's time to pull the carb off and verify that I actually cleaned it properly back in March when I first rebuilt it...

Also found some white smoke and something bubbling on the exhaust manifold gasket. Has anyone ever seen that before?
-John

 

[Grub54891]

Also found some white smoke and something bubbling on the exhaust manifold gasket. Has anyone ever seen that before?

​Yeah, you more than likely have a valve cover oil leak, it runs down to that gasket and gets hot and burns off. Fix the leak.

 

[italianstal27]

Found oil on the headbolt eight next to where the smoke was coming! Definitely the valve cover; that solves one mystery

Will update tonight after pulling carb. The accelerator pump isn't squirting gas in like it used to (good stream) and it's a little sticky. I think something got gummed up. Anyone seen a carb get sticky / clogged after sitting for 4 months?

 

[italianstal27]

Well we got robbed last night and someone stole our dash...

So I'll have to give you guys an update in maybe a week from now :'(

Thanks for the help everyone, we haven't given up on this project yet