mercruiser 120 wont start

I have a 1976 Duo that i received for free due to a seized motor. I was going to rebuild the motor but cost was outweighing common sence.
So I found a OMC from the same era that i switched ALL the mercruiser stuff on to including wirring. The motor ran perfect when I went to look at it. It is now completed and in the boat. The other day i tried to start it and it woldnt start unless it was at full throttle. Then it would kind of run till i tried to move it out of throttle then would die. It also shoot gas from the top of the carb. Kind of like a jugular vein. So i took the carb off, took it apart to check the jets and all. And all seemed to move (the float) and blow through just fine. So i put it back together and now it is not shooting gas out of the top. It seems like it wants to start because it will pop bu tnot fire.
What could be my problem. It does get spark and compression. and it is getting gas.
Help would be very appreciated!!!!!

Have you verified the timing is correct? Is #1 cylinder at TDC on compression stroke and rotor pointing at #1 wire?

It also shoot gas from the top of the carb. Kind of like a jugular vein. So i took the carb off, took it apart to check the jets and all. And all seemed to move (the float) and blow through just fine. So i put it back together and now it is not shooting gas out of the top.

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That's all gobbledygook to me. Are you saying the accellerator pump was working and now it isn't? Is the carb base gasket leaking air?

Get the timing correct and the carb working first.

Ok so tonight i checked the timing, I lined the timing mark on the crank shaft pully up with the 0 on the timing plate.Took of the distributor cap and it seemed way off so i turned the distributor so it was pointing at the number 1 wire, and now it is firing in the carb. It make a poof sound then blows white smoke. So is the timing not right now?I am begining to be lost.

...and now it is firing in the carb.

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A Valve may still be open and backfiring?...timing should be between 6-10 degrees BTDC...don't know exact amount. Do you have a manual...you need one? Keep the flame arrestor on the carb while setting it up.

You wouldn't have had to change the dist. setting or any wires if you removed the wires from the old engine, pull the old engine out and then drop in the new engine if the new engine was a 4 cylinder be it a 140 / 3.0 or a 2.5 / 120 hp. You also would have not had to pull the bell housing just the engine. You would have had to pull the drive though.

The above being said.... say you dropped in the new engine and changed over the manifolds and such you wouldn't have had to even touch the starter even. All that would need to change over was the manifold and carb. all else is the same.

Am i correct ???? Please give us a little more info. to work with, we have seen it all... so we can help you to get it running safely.

You posted the engine was running before you installed it.

By the way welcome aboard.

I feel you need to start from scratch.

find top dead center of the timing mark to the timing tab just after the intake valve closes. (compression stroke). This is important because if you are on the exhaust stroke you will be 180 degrees out of time.

with the dist cap off but handy, confirm rotor is pointing to number one spark plug wire.

next confirm spark plug wires are going to the right locations. (firing order see attached picture.)

Let us know how you make out.

After exhausting your efforts and making certain that all else is OK, check the SI (shift interrupt) system.
Since you swapped engine components, this may be out of adjustment.

The SI is Merc's version of OMC's ESA (electronic shift assist), and if not working correctly, can play hell with your ignition.
Temporarily disable the SI, and give this a try.

DO NOT attempt to use the boat with the SI disabled.
You need this in order to come from gear back into Neutral.... especially during coming in to the dock after an outing.


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RicardoMarine said:

After exhausting your efforts and making certain that all else is OK, check the SI (shift interrupt) system.
Since you swapped engine components, this may be out of adjustment.

The SI is Merc's version of OMC's ESA (electronic shift assist), and if not working correctly, can play hell with your ignition.
Temporarily disable the SI, and give this a try.

DO NOT attempt to use the boat with the SI disabled.
You need this in order to come from gear back into Neutral.... especially during coming in to the dock after an outing.



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If it's attempting to fire would it be the shift interupt system? Still learning myself.

No. if the shift interuptor switch is functioning correctly and it was activated it would short the neg of the coil to ground there for no fire at all. now if the switch is faulty itself and may be shorting out internally and intermittenly then it could be possible. if that is the case then simply removing one of the wires going to the switch would temporarly disconnect it from the circut.

wow, sounds like a tricky problem to diagnose. My original engine had this problem, i just ended ripping it out and selling... this might have been the problem it was having. I checked timing, distributor, and everything. Wish I would have known this before.

On a mecruiser the shift interuptor MUST be in place and functioning otherwise the outdrive will NOT shift out of gear without over shifting the shift control and causing it to go past neutral and engage into the opposite gear you are shifting out of.. It mometarily kills the ignition when resistance occurs in the short shift cable when all else is good with the cable and linkage set ups to allow the clutch dog to seperate from the gear you are trying to shift out of.