1973 merc 20 hp ignition issue

[gwmark78]

I picked up a near new 1973 merc 200 and after putting it in the water found it was running on one cylinder. Swapped the coils and spark followed the coils, so I figured it must be the one coil and ruled out the switchbox. So bought a new coil and installed and fired right up as it should when running on 2 cylinders. Shut it down and came back a few minutes later to start and started hard and I could immediately tell by the sound it was running on one cylinder again. So got my Harbor freight spark light out and had very weak and intermittant spark one the same cylinder that had the bad coil before. I guess I need a DVA adaptor to check peak voltage on the components but I am hoping one of you out there might have some insight on what I could check without a dva. This is my first experience with CDI as I have 5 restored old inline 6's and I know how to troulbleshoot points/dist ignition, but this is new territory for me.
Thanks Gary

 

[Laddies]

Re: 1973 merc 20 hp ignition issue

Here's the test for a 73 20HP

 

[cmod777debian]

Re: 1973 merc 20 hp ignition issue

I am not too familiar with outboard 2 cycles......BUT.....I had this SAME exact issue with a 1979 John Deere Liquifier Snowmobile.

I replaced the CDI box with a brand new one and both cylinders were rocking away.

I would check compression on both cylinders. Your issue sounds like you may have bad piston rings or even a bad piston.

Check compression before you get a CDI box or try the electrical route.

cmod

 

[gwmark78]

Re: 1973 merc 20 hp ignition issue

Hi Bob,
I saw that page on CDI pdf but I have the 4911 switchbox and this tech show a 336-4516 swithcbox and points. I found another tech for 1974-1985 with my 4911 switchbox and it says:
NO SPARK OR INTERMITTENT SPARK ON ONE CYLINDER:
Check DVA output from switchbox while connected to ignition coil, you should have a reading of at least 150v or more. If reading is low, you can have a problem firing both cyl.. If the cylinder that is firing has strong spark, the problem is likely to be inside the distributor cap. The cylinder that is firing on mine has STRONG spark, but what I don't get is the statement about the problem being in the distributor cap...this does not have a dist cap. What are they talking about?
Gary

 

[CDI Tech Support]

Re: 1973 merc 20 hp ignition issue

Here's the test for a 73 20HP

Here's the updated one for that ignition:

Mercury 1 and 2 Cylinder - 336-4516

 

[CDI Tech Support]

Re: 1973 merc 20 hp ignition issue

Hi Bob,
I saw that page on CDI pdf but I have the 4911 switchbox

Mercury 2 Cylinder - 332-4911

NO SPARK OR INTERMITTENT SPARK ON ONE CYLINDER:
Check DVA output from switchbox while connected to ignition coil, you should have a reading of at least 150v or more. If reading is low, you can have a problem firing both cyl.. If the cylinder that is firing has strong spark, the problem is likely to be inside the distributor cap.

The cylinder that is firing on mine has STRONG spark, but what I don't get is the statement about the problem being in the distributor cap...this does not have a dist cap. What are they talking about?
Gary

Nice catch. Typo. Fixed.

 

[gwmark78]

Re: 1973 merc 20 hp ignition issue

I guess I'm still stumped because I did replace the coil on that cylinder and it worked on first start as I posted originally and then started again and now have gone to no spark again. There has to be something wrong to be loosing that same cylinder. The coil on this cylinder is the original that I swapped in testing. The secondary, ohmed out at 4500 on the 20K scale, now it checks at 1000. One other thing, when I swapped coils in testing I could not get the motor to stay running although I had good strong spark and no spark on the other . I checked the apparent bad one and sure enough it had no ohm reading on either primary or seconary. Anyway it seems I ruined a good coil after running maybe 5 min.

 

[CDI Tech Support]

Re: 1973 merc 20 hp ignition issue

Check that ignition coil's ground point. If it measures around 0.3 ohms to the battery's negative post, replace the switch box. Our replacement part# is 114-4911 List price is $250.00. We sell direct or you can purchase through iboats.

 

[gwmark78]

Re: 1973 merc 20 hp ignition issue

CDI Tech,
Thank you for the help in troubleshooting. My motor is not battery start, so cannot check as you have indicated.

 

[CDI Tech Support]

Re: 1973 merc 20 hp ignition issue

CDI Tech,
Thank you for the help in troubleshooting. My motor is not battery start, so cannot check as you have indicated.

You indicated earlier in this thread that your switch box is the 4911. This is the troubleshooting guide for that ignition & is not a battery start ignition...it's an ADI (alternator-driven) ignition:

Mercury 2 Cylinder - 332-4911