Re: 75 Mercury 850 No spark
It's not really that difficult, but your best bet would be to pick up a Mercury Service Manual for your motor. Biggest deal is getting the distributor apart with out ruining the rotor, it's quite an expensive piece in itself.
You don't need much in the way of special tools; you can either purchase an inexpensive Merc flywheel puller off eBay (one that screws into the center threads of the flywheel); pull (2) opposite bolts off the center hub of the flywheel and use an automotive harmonic balancer puller (I'm sure you've got lotsa pullers in your line of work!) with Grade-8 5/16-NF puller bolts; or you can just pull all the bolts off the center hub of the flywheel, separating the outer ring and flexplate from the center hub. Reassemble with one drop of Red Loctite on the bolt threads and torque to 25 ft-lbs.
Probably the easiest to deal with is to pull the flywheel bolts but you've got to be extra-careful to align the distributor and flywheel to TDC so you don't have to move anything when you pull it apart. Note where the flywheel went so you can put it on the same way.
I can't recall but on your motor, it may have thru-bolts to hold the distributor to the block. If that's the case, you don't have to pull the flywheel to get the dist down. But of course there's a lot of small nuts and bolts to deal with!!!
Have any skinny-fingered mechanic friends????? You'll need one for the little bolt that holds the belt guard onto the distributor pulley! Once that is removed, gently pull the belt off the pulley and the pulley itself is pried up with pressure evenly applied to both sides. Before you pry away, note that there's an arrow cast into one of the ribs of the pulley. This points to the TDC mark/centerline of crankshaft and is a critical alignment point.
Once the distributor is out, there are lock tabs in the top "nut" that pry inwards; the "nut" unscrews to reveal a small ball bearing; 2 holes in the sides of the housing are pry points. If you're very very careful, with a couple of "tweaker" screwdrivers you can pry the bearing out without ruining the metal shield. Pry to the outer race and it'll come right out.
With the upper ball bearing out, get a 3/4" deep socket on your butterfly impact; hold the rotor in the other hand in a leather glove; and 'hit' the nut which is deep into the dist shaft with the butterfly. She'll knock right loose.
After the nut is a spacer tube. At that point the only thing keeping the rotor in the housing is Sheer Will. Keep a good hold on the copper "chopper" disk that sits under the rotor, and tap the rotor shaft with a plastic mallet. The rotor assy should come right out, leaving a larger ball bearing in the bottom, held in by a snap ring. You'll want to renew both the bearing and the snap ring when you reassemble.
Be sure you mark the "chopper" disk so you know which side faces outwards; you can install backwards and that wouldn't be good!
Assy is the reverse of disassembly, most important thing is to get the dist. belt reinstalled correctly, and to check/adjust ign timing when you're done.
There's your quick synopsis, but I'd recommend getting that manual anyway, to fill in all the Scary Details!
Check out the store at this site for parts, if you don't find what you need there's several other places you can try such as the following:
http://www.boatfix.com/searchcdi.asp
http://www.seilermarine.com/rapair/mercury-mariner-85-hp.htm
They seem to have the best prices but may or may not be in stock. This trigger is also used on Inline Sixes and is a very common replacement part. You might also find a Hot Deal on eBay or on your local
www.craigslist.org
HTH & Happy Repairs.................ed
