Mercruiser 140 spark loss on 3&4

[mutiny]

I skimmed through the other topics and I failed to locate one like this so I apologize if there is another like it. If there is, I would love to have the link to it, or simply re-open the topic due to my lack of finding it.

I have an early 80's boat with an equally old Mercruiser 140 in it. After running it for a few times this season I noticed that I was experiencing a loss of power, and the engine bouncing around a bit at idol. After taking her home and looking it over I did the following:
Compression Check. Roughly 140 on all cylinders, and a fuel check showed clean fresh fuel. A spark check showed that I only have spark on #1 & #4. I had the AHHH HAAA!! moment I am sure we have all experienced. As a result of the spark issue, I did the following

Replaced Cap, Rotor, Condenser, Wires, and Plugs. I used all Mercruiser recommended parts including the asked for AC spark plugs. I made sure to gap them all to spec .035 and then made sure to set the points to the required space as well. I think .018 - .022 (I went by the service manual and my numbers may be off from my memory due to not having the book in front of me) After installing all the parts, I then checked for spark and found no change. My AHHH HAAA moment was quickly put on the back burner.

I took off all the ignition parts again and re-inspected. I checked the ground wire from the coil to the points by removing and using an ohm meter, then checked all other wires involved in the ignition. Nothing. At this point I changed the ignition coil due to just not knowing any other avenues but to no avail.

I got into the down and dirty and thought that my distributor cams may be worn, but after using a micrometer, they were all the same and showed no signs of wear or tear. Just for giggles, I dug out my old dwell meter, and found that my dwell was also within manufacturer spec.. I now turn my attention to all others who would like to throw some suggestions my way. Thanks for reading, and I hope someone knows the solution.

 

[achris]

Did you check the distributor shaft/bushings for wear? (just move the shaft side to side. If you have any movement, it's worn). The other thing I would do, move the HT leads on the 1 and 4 posts (on the dissy cap) to the 2 and 3 posts, see if you lose spark on the same cylinders.

 

[mutiny]

Did you check the distributor shaft/bushings for wear? (just move the shaft side to side. If you have any movement, it's worn). The other thing I would do, move the HT leads on the 1 and 4 posts (on the dissy cap) to the 2 and 3 posts, see if you lose spark on the same cylinders.

I have swapped out HT leads 1 and 4 and put them on 2 and 3. No change in the spark situation. I did wiggle the distributor shaft back and forth but did not find wobble or play from side to side. I did however find that if it grabbed the distributor shaft and tried to twist it clockwise, it would go about an eighth of a turn and spring back into its position. After looking into the distributor, I found spring loaded contraption and it appears that it is working as it should. No wobble on the bearings. Thanks for the reply Achris.

 

[achris]

So, the swap proved the leads and plugs for all 4 cylinders good... Next I would remove the dissy cap, pull the coil lead out of the cap and put a plug in it and lay it on the engine block. Crank the engine and watch for a small spark between the points faces for every cylinder, ie every time the points open, you should see that small spark. The rotational movement of the top section of the shaft is correct btw...

 

[MikDee]

I have swapped out HT leads 1 and 4 and put them on 2 and 3. No change in the spark situation. I did wiggle the distributor shaft back and forth but did not find wobble or play from side to side. I did however find that if it grabbed the distributor shaft and tried to twist it clockwise, it would go about an eighth of a turn and spring back into its position. After looking into the distributor, I found spring loaded contraption and it appears that it is working as it should. No wobble on the bearings. Thanks for the reply Achris.

That is your centrifugal advance! It should move freely, and more the 1/8th turn, then spring back! You should clean, & lightly lube the weights, and check the springs. By the way, it sounds like your cap, and, or rotor, maybe bad?