1973 Mercury 850 - no spark all of a sudden

[Scott Rickard]

Hi all,

The other weekend I was cruising at about 30mph - motor was running fine (other than bogging every time I shifted from neutral to fwd, which is an idle speed problem I'm aware of.)

I hit a series of 4 or 5 fairly big waves - with a trihull - ouch! Motor kept running fine until a minute or 2 later. Then it just slowed down quickly (no bang or anything) and stopped. Couldn't start it again at all.

I've read the FAQ and the CDI troubleshooting manuals, so I know what I need to test (or have tested since I don't own a DVA meter). BTW, the starter engages but I have no spark at the plugs, even with the mercury kill switch disabled.

My question is, do you think it's more likely to be the trigger or the switchbox/coil? I can install a new CDI switchbox, but I'm not interested in tackling a trigger and distributor.

Any thoughts?

 

[Scott Rickard]

Re: 1973 Mercury 850 - no spark all of a sudden

I forgot to add that all wires and connections are in good shape and very clean.

Also, I'm not convinced that hitting the waves hard led to the failure, but the timing is awfully suspicious...

 

[Laddies]

Re: 1973 Mercury 850 - no spark all of a sudden

First check the drive belt to see it's not broken and the do this tigger test from CDI you don't need a DVA to check that engine.

 

[Scott Rickard]

Re: 1973 Mercury 850 - no spark all of a sudden

Thanks Laddies - do you mean simply test for 9 volts on the brown wire (while cranking) to determine if the switchbox is bad, or the test outlined in the picture (which should tell me if the trigger is bad)?

 

[Laddies]

Re: 1973 Mercury 850 - no spark all of a sudden

Do the test outlined in the photos.

 

[Scott Rickard]

Re: 1973 Mercury 850 - no spark all of a sudden

Thanks! I'll report back with the results.

 

[Scott Rickard]

Re: 1973 Mercury 850 - no spark all of a sudden

Laddies - I did the test in the picture:

Jumpered the red and white terminals on the the coil side of the switchbox, disconnected all trigger wires, jumpered the brown and white trigger terminals on the trigger side of the switchbox, put a ground wire on the black trigger terminal, unscrewed the coil HV wire at the distributor cap and and ran the coil HV wire to about 3/8" from ground.

I got a very healthy spark band from the tip of the coil HV wire every time I touched the black trigger terminal jumper wire to ground.

I cleaned and tightened all connections, cranked the motor, and still no fire to any of the spark plugs. Distributor cap was clean and in good shape.

Does this point toward a faulty trigger?

I found the CDI trigger replacement procedure here:

http://www.cdielectronics.com/Portals/0/installsheet/134-3736.pdf

Unfortunately, I don't trust my skills to replace it.

 

[emckelvy]

Re: 1973 Mercury 850 - no spark all of a sudden

When you bypassed the distributor you got a strong spark. This proves that nothing is wrong with the rest of your ign system. The trigger is shot, most likely the snap ring broke, fell down and ran around in the cap a while like in a blender, then snapped the trigger pickup off like a twig. Ask me how I know this!!!!

The CDI procedure is very detailed, full of warnings, and makes the repair sound a lot more complicated than it really is.

The dist is readily dismantled with hand tools. The only thing that really makes it easier is if you have a small impact wrench, to remove the rotor fastening nut. You also need a deep socket for this. But you can just as well hold the end of the rotor in a gloved hand, and use a ratchet to loosen the nut. I've very rarely found one so tight it resists hand-tool force. The impact is nice 'cause it zips it right out with no muss or fuss.

If you're at all handy with tools you should be able to do this. One thing that'll help immensely is if you make match marks on the flywheel so you can restore distributor belt timing when you're done. When you pull the cover off the distributor pulley (one, 5/16"-hex-headed screw), you'll see an arrow cast into one of the ribs. This is the belt timing indicator.

As you turn the flywheel, the arrow should point the belt timing mark on the flywheel, and be pointed directly at the centerline of the crankshaft.

If the arrow doesn't align perfectly to the timing marks on the flywheel, as long as it's lined perfectly to the centerline of the crank, take note of the position. Restore to this position when you're done.

Other than that, it's just basic disassembly, reassembly. You'll need to check/adjust maximum spark advance when you're done. See the "Link and Synch" instructions, posted elsewhere, for this.

Follow those incredibly complete CDI instructions, and it should be a piece of cake!

BTW if you're price shopping check here:

http://www.boatfix.com/searchcdi.asp

HTH & G'luck...........ed

 

[Scott Rickard]

Re: 1973 Mercury 850 - no spark all of a sudden

Thanks Ed! Very helpful info. Sounds like you have some experience in this department.

I've been wrenching on old cars for almost 20 years, but distributors were never my strong suit. Also, my Clymer manual is from 1977 and it doesn't have a section on distributor removal.

Do I need to pull the flywheel to access the rotor shaft to dist nut? Can it really be as easy as loosening that nut, disconnecting linkages and dropping the whole assembly down and out??

Do you know of a "step-by-step" guide floating around the internet somewhere?

Thanks again! I just might try this if I can find some good pictures.

Edit: I think I'm going to try it : http://www.myoutboardengine.com/distributor_1.html

 

[emckelvy]

Re: 1973 Mercury 850 - no spark all of a sudden

Scott, that's a pretty good page you linked to, you'll notice that the distributor in the pic isn't quite like yours, it's for a 3-cyl Merc. So most of the part #'s aren't gonna work for you. But the concept is the same. And at least he's got a good writeup and plenty of pics.

Your distributor is held to the block by (3) mounting bolts. If I recall these bolts go all the way thru the dist body and are nutted on the ends. If you find yours is built this way, all you have to do to remove the dist is undo these (3) nuts, pull the dist pulley belt out of the way, and drop out the dist. A few linkages/connections I've omitted but you'll get the idea once you look at it.

If you should find yours doesn't have the (3) nuts, your distributor mounting collar is threaded and the bolts thread right in. So, you'll have to pull the flywheel since access to some of the bolts is blocked by the ring gear.

If you have a harmonic balancer removal tool, get yourself (2) Grade 8 or better 5/16"-NF puller bolts. Remove (2) opposite flywheel hub bolts, loosen the flywheel nut at center of crank, and pull away. An impact wrench on the puller makes the flywheel pop that much easier. The flywheel hub bolts go back in with a drop of Loctite Red and approx 24 ft-lb torque.

If you want to get real fancy, get a Merc flywheel puller which will screw down into the internally-threaded center hole in the flywheel and pulls the flywheel off lickity-split. Available many places (probably find one here) including eBay.

Last and Least Preferred, is to remove all the flywheel hub bolts and pull the ring gear/flexplate assy off, leaving the center hub. You can get away with this since all you're doing is gaining access to the dist bolts.

BUT, better be extremely careful if you do this, you've got to make sufficient match marks to be able to reinstall that flexplate in the SAME position on the flywheel hub.

All that Flywheel Fun, of course, doesn't apply if you've got thru-bolts holding your dist to the motor.

BTW, the dist pulley belt guard pops off with an itty bitty 5/16" combination wrench, it's a little tight in there but that center bolt/screw does clear the flywheel!

Very Very carefully pull the timing belt off the dist pulley. With (2) flat-bladed screwdrivers or other suitable prying devices placed opposite each other, GENTLY pry the dist pully upwards. If it won't go freely, use penetrating oil or perhaps a careful application of heat. Be sure to pry evenly from both sides or the pulley will bind.

Anyway, between the CDI instructions, Zman's writeup, and what you're getting here, you should be done already!

HTH and Happy Wrenching...........ed

 

[Scott Rickard]

Re: 1973 Mercury 850 - no spark all of a sudden

You're the man, Ed! I'm printing this thread to PDF and putting in my online repair file.

Thanks again to everyone for the helpful information.

 

[Scott Rickard]

Re: 1973 Mercury 850 - no spark all of a sudden

Well, I got the old distributor out and the new CDI one assembled and mounted.

The problem is none of my old screw settings (spark advance, throttle stop, idle) work on the distributor.

Attached are pictures showing the position of the distributor in Neutral and in full forward.

In neutral, you'll note that I have the idle screw screwed all the way IN and it still doesn't even come close to making contact with the trigger body. Link to pic below:

http://home.comcast.net/~scottrickard/Neutral.jpg

In full forward, I have to have the spark advance screw screwed all the way OUT to allow the throttle to move all the way to full forward. Link to pic below:

http://home.comcast.net/~scottrickard/Forward.jpg

What did I do wrong?? Can I be off by 45 degrees or something?

 

[Laddies]

Re: 1973 Mercury 850 - no spark all of a sudden

Scott, I don't believe the belt is lined up on the marks properly.

 

[Scott Rickard]

Re: 1973 Mercury 850 - no spark all of a sudden

Hi Laddies - Thanks for the response.

I took great pains to line up the arrow on the pulley with the timing mark on the flywheel, so I'm fairly sure that's not the issue. (It was lined up that way when I removed the old distributor, too.)

Is there anything else I might have screwed up? Could I have the plastic pickup plate pieces aligned incorrectly?

I think I just need the distributor to start its range of motion "a little more to the left" so my idle/spark advance/throttle stop screws actually do their jobs.

I'm stumped.

 

[daveswaves]

Re: 1973 Mercury 850 - no spark all of a sudden

Well, I got the old distributor out and the new CDI one assembled and mounted.

The problem is none of my old screw settings (spark advance, throttle stop, idle) work on the distributor.

Attached are pictures showing the position of the distributor in Neutral and in full forward.

In neutral, you'll note that I have the idle screw screwed all the way IN and it still doesn't even come close to making contact with the trigger body. Link to pic below:

http://home.comcast.net/~scottrickard/Neutral.jpg

In full forward, I have to have the spark advance screw screwed all the way OUT to allow the throttle to move all the way to full forward. Link to pic below:

http://home.comcast.net/~scottrickard/Forward.jpg

What did I do wrong?? Can I be off by 45 degrees or something?

Check and make sure the carb throttle arm on the top carb is on the correct side of the distributor cam. Sounds like something is interfering with the movement of the distributor.

 

[Scott Rickard]

Re: 1973 Mercury 850 - no spark all of a sudden

Daveswaves - We think alike.

I'm pretty sure I found the problem - the spring band pin (for lack of a better term) was located on the wrong side of a ridge on the distributor housing.

Before (incorrect):

http://home.comcast.net/~scottrickard/Incorrectpinplacement.jpg

After (correct):

http://home.comcast.net/~scottrickard/Correctpinplacement.jpg

The only reason I discovered it was I saw a wear mark on the distributor housing that looked like it belonged to a spring band pin!

I ran out of daylight - will try buttoning it up in the am and will report back.

Thanks guys! I really hope this was the problem, but I'm also bummed that my idle/throttle stop/spark advance screw settings all will have to be reset. Damn.

 

[Scott Rickard]

Re: 1973 Mercury 850 - no spark all of a sudden

SUCCESS! Thanks to everyone for the patience and for the great information and diagnosing. The old trigger was the problem and the new CDI trigger (once installed correctly) works.

I will start a new post asking about timing issues.