75 Merc 850 ran great/then wouldn't start

[tony779]

I have a 75 Mercury 850, SN below 4366801, on a 1976 AMC runabout. I had a compression check done about a year and a half ago that was all cylinders above 120. I had the distributor replaced as well. It has really run great since then other than just being a little rough on occasion. I recently ran sea foam and deep creep through it. It seemed to make it run a little smoother. I took it out this past weekend and ran it pretty hard for about 30 minutes. I anchored for about 30-40 minutes and then tried to crank it and got nothing. I cleaned the plugs and checked the timing belt and tried to crank it. All I get now is a periodic backfire through the exhaust and an almost clunk sound (dont know how else to describe it) that stops the engine from cranking for about 2 seconds and then it cranks more and does it all again. I am baffled and afraid. I am afraid that something internal busted but it didnt give any indications while it was running. Do you have any ideas?

 

[tony779]

Re: 75 Merc 850 ran great/then wouldn't start

^Also, if I didnt break something internal, I have a new power pack on it too. Now my old tac doesnt read properly. Does anyone know of a good generic type I could get or do I need to match one to the power pack?

 

[Clams Canino]

Re: 75 Merc 850 ran great/then wouldn't start

You can match any generic tach. You pick the signal off the stator instead of the switchbox with those.<br /><br />You must have gotten a 6 cylinder switchbox? I've seen that before. Somewhere I had a four cylinder switchbox infesting my stack of 6 cylinder units. If it rears it's little head up in the next 30 days, I'd gladly swap you even. (assuming you get the rest back in order first).<br /><br />-W

 

[Chris1956]

Re: 75 Merc 850 ran great/then wouldn't start

Clams, My 1977 150HP Merc and my 1973 85HP Merc have the same kind of switchbox. It has a 4/6 cylinder marking on it.

 

[Clams Canino]

Re: 75 Merc 850 ran great/then wouldn't start

Odd... must be a later Merc replacement that was "generic". <br /><br />The original four ones were marked 4/8 even though there was no eight (unless it was used in an IO application too). The six ones just said 6 as I recall. the only difference between them is how they signaled the propriatory tach circuit. I use the later system with the tach signal picked off the alternator (stator).<br /><br />-W<br /><br /><br />-W