3.0 Mercruiser Dies On Acceleration

[Ng321]

1992 3.0 Mercruiser will start fine.

Idle is at 750 rpm at idle at 2 turns in on the idle screw. And at 1 1/4 turn out on the mix screw the vacuum is only 5inches. I should be idling forward to check rpm and vacuum but I cant stay in forward without the boat dying.

In this condition, the engine idles terribly and adjustments to the mix screw do nothing to vacuum. When accelerated to 2500 rpm in neutral vacuum increases to 10 inches and runs very smoothly.

When shifted into idle forward vacuum drops to 1" then immediately to zero then the boat dies.

Notes: turning the mix screw all the way in has no affect and turning it out to more than three turns also has no effect.

Right now im thinking I need to check compression first, then check base timing and verify timing advance at 2500 rpm. I dont know how to check for leaks in the manifold but that could also be a problem.

 

[alldodge]

Either have a vacuum leak or the motor is worn out. Vacuum at idle should be closer to 14 Hg

 

[kenny nunez]

You may have a vacuum leak between the head and the manifold. With the engine idling squirt some motor oil on the gasket that seals them together and see if the vacuum changes. Also around the base of the carburetor. It is normal to have a slightly lower vacuum in the water than on the trailer because of the back pressure of the exhaust underwater. If the compression is even between all 4 cylinders and the leak is between the manifold and head you may have to pull the head and have the surface milled along with the manifold, also a good time to have the valves ground and the head resurfaced.
Have you checked the timing? It should be 6* at around 600 rpm. Check that the mark on the balancer lines up with the “0” mark on the timing cover flag when the #1 piston is at TDC. It is possible for the outer ring of the balancer to slip and cause wrong settings.

 

[achris]

Having the serial number of your engine would help determine what the initial timing should be. 1992 is part of the era when Merc has about 4 different timing specs. The serial number will tell us which particular 3 litre it is and from that we can tell which timing spec to recommend.

Chris...

 

[Ng321]

I rebuilt the engine last April and it ran well all summer. Ill check the compression and try the oil trick around the sealing surfaces of the manifold tomorrow morning. I really doubt it the timing because I haven't messed with it in a while but it does act like it's stuck in base timing mode or something

 

[Scott Danforth]

a fresh rebuild should be 150psi on all 4 cylinders

 

[Ng321]

a fresh rebuild should be 150psi on all 4 cylinders

I believe it was 165psi each right after rebuild, hope it didn't fall off

 

[Scott Danforth]

I believe it was 165psi each right after rebuild, hope it didn't fall off

Chech your gauge, its reading high

 

[Ng321]

Checked compression and they all held 175 psi for over 5 minutes and while I waited i cleaned the #4 spark plug because it had alot of dry carbon fouling. Started the boat and got 10 inches of vacuum now. But this is in the drive way so the increase in vacuum is probably due to no exhaust backpressure from the water. Will check timing later today when I get a helper. Ordered a kit with all the gaskets for the manifold for $20 because its probably a vacuum leak.

 

[alldodge]

Gauge is a bit off but no issue, they are all the same
Now thinking timing
Cam/Crank timing marks installed correctly, or ignition timing

 

[Ng321]

Timing was at 1 degree ATDC when it should be at 1 degree BTDC. Not sure how it moved on its own. Correct timing, and clean spark plugs gets me to 15 inches of vacuum. I will still replace the gaskets on the manifold but it should run fine now

 

[Bt Doctur]

Checked compression and they all held 175 psi for over 5 minutes
I wonder how that possible because there is a check valve in the tester

 

[achris]

Checked compression and they all held 175 psi for over 5 minutes
I wonder how that possible because there is a check valve in the tester

An engine with measurably no blow-by (perfectly sealing piston rings). Where do I buy one of those?

 

[1960 Starflite]

An engine with measurably no blow-by (perfectly sealing piston rings). Where do I buy one of those?

I have an air compressor that will do that but only 130 psi.

 

[achris]

I have an air compressor that will do that but only 130 psi.

Is it 'Unicorn' brand?

 

[nola mike]

Checked compression and they all held 175 psi for over 5 minutes
I wonder how that possible because there is a check valve in the tester

Tough crowd. OP, your tester stops at max pressure and stays there, regardless of what the pressure in the cylinder does after that point. Cylinder will always lose pressure once you stop cranking. A better test of dynamic sealing ability would be a "real" leak down test that tells the rate of leakage from the cylinder. But not needed at this point

Also, my engine pulls the same vacuum at idle on the water as on muffs (20 inhg) fwiw. Is it a steady 10 on yours? This should be measured out of base timing mode. Did you check what you're timing is when out of base timing mode at idle?