120 HP Mercruiser 4 cyl - only 2 of 4 plugs sparking

Purchased used '85 boat. Sat in storage for 3 years with gas in tank. Seller specified that when just taken out boat felt like wanted to stall so advised the carb may need cleaned and rebuilt. Took out on the water, started, put in gear, full throttle and barely moving. Considered carb or fuel pump. Unhooked main fuel line to carb to see if fuel was pumping... it was. Checked gas for crude or particals, looked clean (added carb cleaner and dry gas prior to taking it out).

To eliminate anything minor before rebuilding the carb, replaced four spark plugs, cap, rotor, points, and wires. While doing this I realized that only 2 of the spark plugs are sparking, so now I think it may be electrical or have to do with the ignition.

This is my first boat. I am a car guy so most engine components and terms I am familiar with, but this not so much. Doesn't help I don't have a manual yet either. Any help is appreciated! The marina owner and I worked on this for 4 hours today and baffled by the spark plugs now. Thanks for any and all replys!

Re: 120 HP Mercruiser 4 cyl - only 2 of 4 plugs sparking

If you pull the cap off and remove the center wire from the coil and place it near the block (1/4 inch or so) and turn the engine over with the starter you should get a spark each time the points open as the point arm rides over the distributor cam. If not, the cam may be worn to the point where the points are not opening. Assuming of course the points are adjusted correctly. If you get spark there, you should get spark to the plugs.

Re: 120 HP Mercruiser 4 cyl - only 2 of 4 plugs sparking

Thanks for the reply.

Points are new and gapped properly (spoke with the service guy at marine shop next door and told him the suggested gap says 18-25 and he said gap at 20). Pulled the cap and getting spark there, put the cap back on, getting spark to both end cyclinders, little to no spark to the two center cylinders. Engine turns over and starts, but not firing on the two center.

We started the engine and pulled the wire off a center plug and no change in the motor. Did this to the other center plug and no change, and then did to one of the end and the engine bogged out. So, somehow the engine is running on just the two. Could this be a bad coil?

The engine starts up and runs but it's missing... like hopping which is indicating to us that it is not getting full power. A boat mechanic came by (only for a minute of course) and I showed him the plugs. The two center plugs were burnt out and the end plugs were perfect (replaced all today). He suggested that if the plugs were burnt could lead to something in the ignition system. He said that this was just a sign of something deeper but not expensive... whatever inexpensive means to a marine mechanic.

Hoping to be able to do a process of elimination to save money since this was suppose to be an inexpensive start to a new hobbie.... any suggestions on things to check are much appreciated.

Re: 120 HP Mercruiser 4 cyl - only 2 of 4 plugs sparking

bad coil?? - no, you have spark

points should be adjusted by dwell, not just gap

stony is right on, you need to check the spark coming from the coil tower when cranking the engine, this will tell you if you have good consitent spark every time the points open

if you have intermittent spark or every other spark is bad/weak then suspect points, worn cam lobe, wandering distributor shaft, etc...

Re: 120 HP Mercruiser 4 cyl - only 2 of 4 plugs sparking

Ok.... quick question.

What does it mean to adjust a point by dwell?

Second, if I am getting an every other spark would this mean a new distributor? My deliema is that I don't want to pay a marine mechanic if it is something I can do myself. But I also don't want to spend upwards in the hundreds replacing what I can. So I am torn.....
Is there a way I could identify the distributor needs replaced before replacing??? Or is it something that can only be known after replacing and seeing if the prob is fixed.

I will check tomorrow morning Stoney's suggestion... It's a new idea and since I am out of them a great start to the morning. Thanks!

Re: 120 HP Mercruiser 4 cyl - only 2 of 4 plugs sparking

Dwell is the number of degrees the points remain closed to saturate the coil .eg: 28-32 degrees.
You can check it by removing the dist cap, pull the plugs, turn the motor over slowly stopping at the high point of each lobe and measuring the gap of the points.they should all be equal.Firing order is 1,3,4,2

Re: 120 HP Mercruiser 4 cyl - only 2 of 4 plugs sparking

Anywhere but down said:

replaced four spark plugs, cap, rotor, points, and wires.

Click to expand...

Anywhere but down said:

We started the engine and pulled the wire off a center plug and no change in the motor. Did this to the other center plug and no change, and then did to one of the end and the engine bogged out. So, somehow the engine is running on just the two.

Click to expand...

Short Answer:

Swap the positions of the #2 and #3 spark plug wires at the cap and see if that makes any difference

Long Answer with explanation:

Take off the distributer cap.

Hit the starter so that the rotor turns.

Watch CLOSELY which direction the rotor turns.

From #1 spark plug wire, MAKE SURE that the firing order for the plugs 1-3-4-2 is correct in the direction the rotor turns.

It's very easy to get the 1-3-4-2 correct BUT IN THE WRONG DIRECTION.

What happens in that case is that the #1 and #4 wire are in the right place, but 3 and 2 are in the opposite position on the cap from where they should be. The engine will actually idle very well, but it will have no power and the 2 center cylinders will make no difference when you pull the plugs, because they are firing on the exhaust stroke.

Re: 120 HP Mercruiser 4 cyl - only 2 of 4 plugs sparking

Thanks again for everything! Worked all day at the marina and ended up correcting the firing order. Marine mechanic was working and popped over every once and awhile to assist. Once I switched the firing order it wouldn't turn over. Replaced the starter and now the boat fires up immediately each time. New problem.

When I went out it was running but I was still not getting any power. I was basically trolling and my linkage seems off. Mechanic said to get new carb and prob fixed. Called around and its $375! I found a marina that has a rebuild kit for $28... is rebuilding a carb difficult? I was planning to take off the old carb and soke it in gas for a couple days to get all the crude off. But, as simple a rebuilding a carb looks I am questioning myself because if it was simple then why is it such a popular service people get done.

Any thoughts?

Re: 120 HP Mercruiser 4 cyl - only 2 of 4 plugs sparking

soaking the carb in gas for a couple days will do nothing, that basically what happens when you aren't using your boat

you need to completely disassemble the carb and soak it in a cleaning solution for at least an hour but no more than three, then blow out all the passages with compressed air and install the new components from the kit

Re: 120 HP Mercruiser 4 cyl - only 2 of 4 plugs sparking

The Rochester 2 barrel has some internal passages that are drilled at several different angles then the carb body is plugged.

If they're clogged up, there is no way to get to them to clean them out. I have had no luck at all cleaning those internal passages with any of the carb cleaners on the market. I've soaked them for days, agitated them, heated them up, blown with compressed air, poked little wires around, etc.

Won't hurt to try a carb kit, you might get lucky and that's all it needs.