Failed Cylinder Drop Test

[Slacker411]

Ok so I have been having problems with lack of power at WOT and intermittent bog and surge. It is a 1978 Mercury V-1500 V6 150hp 2 stroke outboard. Cylinders 1 and 4 seem to have no effect when removing spark plug wires during cylinder drop test. Also using a multimeter RPM gauge cylinders 1 and 4 register about 135 and its relatively steady on multimeter. With the other cylinders 2,3,5,6 the rpms are hardly readable but show to be around 1200-1900 but the numbers jump around so fast its very hard to read. These other cylinders produce a noticeable effect when removing their plug wires. Using a spark gap tester all plug wires put out what seems like good steady spark to the naked eye.

What I have done so far:
Replaced Fuel tank, Primer Bulb and all fuel lines, thoroughly cleaned carbs and new gaskets, replaced fuel pump diaphragm and new gaskets, new spark plugs, swapped coil packs on affected cylinders, cleaned all electrical connections and points, Checked the resistance of all plug wires at 20k ohms and all checked out around 0.99 except for one that had 0.63 ohms. Positive and negative leads on all coil packs reads open circuit with 0.00 resistance using both engine block as a ground and the negative and positive posts on the coil packs respectively.

I just took the boat to a mechanic and I don't really think he did any real work or enough troubleshooting. He was telling me he couldn't even get the boat started unless he sprayed fuel directly down the carb and then it died. When I got home the boat started up easily so I am not really sure he tried to hard. On cold starts I have to use carb cleaner spray or starting fluid to get it to fire up (like immediately). Any subsequent starts after it has run for a minute or two it starts immediately with 1 turn of the key. The mechanic told me the motor is probably just worn out and thinks its crankshaft seals that have gone bad producing lack of fuel suction. This would not explain the electrical issues however.

 

[Silvertip]

You would get far better response had you posted this in the Mercury & Mariner portion of the forum. You posted in the electrical section which is for "Boat and Accessory" type of electrical issues, not engine electrical issues. You either have spark or you don't. If not, troubleshoot. You either have fuel delivery or you don't. ​Again, if not you need to determine why. Now to what you are measuring. You said you were doing a drop test using a multimeter and measured 135. 135 what??? You should start with a "compression test". Bad cylinders will show up quickly if there is internal engine damage. Cylinders with zero or very low compression produce no power and would not react to pulling a plug wire.

 

[Slacker411]

I forgot to mention that Cylinders 1,2,3,4,6 have 92lb compression and cylinder 5 has 89 lb compression. I am attaching a link to a video that might help show what I'm talking about using the multimeter to test rpms. Pardon the poor cameraman skills. The video shows me first testing cylinder 1, then I test cylinder 3 below it (although you can't really see me attach the probe).

<iframe width="420" height="315" src="https://www.youtube.com/embed/V2uC9Yq-OO4" frameborder="0" allowfullscreen></iframe>

 

[Slacker411]

[No message]

 

[Chris1956]

I think cylinders 1 and 4 share a trigger. You might check it for proper operation. Spark plug wires should have zero resistance, as they are stranded stainless steel.

Your compression is even so it is fine. I suspect your gauge reads low.

 

[Slacker411]

Forgive my ignorance, but can anyone explain how to test the trigger?

Also, today I switched the power packs around to rule those out and the problem did not move to any other cylinders. This leads me to suspect a trigger or possibly stator? I know I read somewhere about an attachment for multi-meters that helps to get a more accurate reading for RPM? I suppose since the electrical output is at such a high frequency? Any thoughts?

I have a feeling my problem is electrical since cylinders 1 and 4 read the way they do on the multimeter. Although both of those cylinders jump the spark plug gap tester, maybe the output is erratic or incorrect in comparison to the other cylinders and I just cannot notice it with the naked eye?

 

[Slacker411]

http://www.maxrules.com/oldmercs/Wiring/1966ona/52.jpg

This is a diagram that shows the wiring setup to all of my coils. I am wondering if anyone can confirm that cylinders 1 and 4 share a trigger? I have a spare trigger from an identical blown powerhead that overheated. I am not positive that it works 100% but I would imagine that if the problem moves to another cylinder or stops then I can narrow it down to the trigger.

 

[Slacker411]

Here is the pic

 

[Faztbullet]

1 and 4 share the same trigger coil, 3 an 6 share a coil and 2 an 5 share a coil.

 

[sutor623]

Trigger is EASY to diagnose. What you can do is take all the spark plugs out. Index the flywheel for each cylinders TDC (top dead center) with a white mechanics pencil. Basically, spin the flywheel by hand and watch each piston hit TDC. When it hits TDC, mark the cylinder # at the timing marker on the flywheel. So when this is all done, you will have 6 numbers spaced evenly around the whole flywheel.

Then, hook up a timing light when the motor is at idle. When you point the light at the timing mark, you should ONLY see the number of the cylinder that you are hooked to flashing repeatedly. Do this for each cylinder. If you see other numbers flashing, the trigger is BAD. Just because you pass this test, doesnt mean the trigger is not 100% faulty, but most that are faulty will fail this test off the bat. Good luck!

 

[Slacker411]

I have great news. I got home this morning and pulled my spare trigger assembly out of my old powerhead. I slapped that puppy in there and took the boat for a spin and the boat is now running better than I've ever seen it run and goes what feels like twice as fast. It practically jumps out of the water! I replaced my old powerhead after replacing my transom, stringers, and deck which had rotted out BAD (gone). With stringers and a deck that actually contain wood and complete encapsulation under the deck with closed cell foam I figured I added some weight and the motor just wasn't big enough to go as fast as it once did. Little did I know I was running on only 4 cylinders. Thank you all for your input on helping me solve this problem!