Merc 800 - Stumped.

Hey all, I have been getting lots of great info through this forum, I find it all very helpful and appreciate this wealth of knowledge. Unfortunately, I am having engine trouble and I don’t know what to search for because I cant figure out what is wrong! So here it goes…

I have a 1970 Mercury 800 outboard, 80 HP, inline 4 with Thunderbolt ignition. It is the distributor model. I picked up the motor from a buddy last year for nothing. He had been successfully using it for tubing his kids and fishing for the few summers he had it, so it mostly worked before I got it. I decided I was going to refurbish everything that needed it. I consider myself a very good mechanic, for what it’s worth. Here is my chain of events:

Purchased motor, replaced ALL wiring, replaced ignition leads, replaced spark plugs, performed link and sync, replaced starter motor, lower unit seals, impeller.
I took the boat out, and I found the RPM’s were unsteady at high end, and the idle was rough.
After further diagnostics I determined that one of my two fuel pump diaphragms was completely blown out, and the other had a pin hole through it. I replaced the fuel pump gasket and diaphragms, as well as the check valves. I then purchased and installed a carb kit with new needle valves, set float height per spec, cleaned out carbs, and reassembled. I performed another link and sync, and took it out. It idled pretty well, but would fall on its face every time I put the throttle down. I finally got it on plane, and was able to drive around fine for a few minutes, but after slowing down couldn’t get it to pick up again. It felt like I had one cylinder dropping out under load. The issue was seemingly intermittent.

So, I got it home, cleaned out the carbs again, just in case. Confirmed timing/link and sync. Spark is bright and hot looking. Now I turn the key, it starts up, runs for 10 seconds, and dies. With the start/throttle lever all the way forward, I can no longer get the engine to race, it simply doesn’t want to pick up. Fuel ball goes hard, bowls are full, fuel line is connected, tank vent open. I started the engine with the throttle near wide open, I could get it to run so-so at high RPM for about 20 seconds, but as soon as I slowed it down it was like someone turned off the key. Dead. After that it would stumble but wouldn’t start.
I am thinking it has to be fuel related. It is as if there is no fuel getting from the bowls to the engine. If I prime the ball hard plus a bit more, it seems to force some fuel into the combustion chamber and it will fire.

Any thoughts or advice or miraculous solutions would be much appreciated! I really want to go fishing!


Thanks guys.

I am having the same issue with the exact same engine, a '70 MERC 800. Did you ever find a solution?

mbb9674 said:

I am having the same issue with the exact same engine, a '70 MERC 800. Did you ever find a solution?

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I replaced my crankshaft seals, just need to put it back on the boat and test it out. Fingers crossed. But they were definitely original seals. I believe it was pulling too much air through the seals to draw fuel from the carb. Hopefully have it tested soon!

Let me know if it works. Was that a very hard job to tackle or just time consuming? When i pulled the lower unit to change the impeller the previous owner either sprayed white lithium grease all over the shaft or I think my lower crank seal may be leaking.

It is time consuming,you need buckets of patience and heat for the bolts,especially if it is a saltwater engine.
The seal I would be looking at is the crankcase seal,it may be the reeds,unlikely, but maybe a small bit of dirt/hardened oil deposit stopping them closing properly

mbb9674 said:

Let me know if it works. Was that a very hard job to tackle or just time consuming? When i pulled the lower unit to change the impeller the previous owner either sprayed white lithium grease all over the shaft or I think my lower crank seal may be leaking.

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Not difficult, I had it apart in an evening. 8 or 10 bolts holding power head on. I have built a dozen car and truck engines from scratch, but this was my first outboard work. Very non intrusive to the engine. Make sure you have a gasket kit though for the exhaust gasket and orings. Plus there is a seal on the top of the engine and 2 seals on the bottom stacked together.
The hardest part is splitting the exhaust gasket to separate the power head. Don't need a pry bar, just put shims on the top of the studs and loosen the nuts up. They jack the power head up off the lower unit. Works like a charm!
I'll let you know when I have it running! But in hindsight, I should have done theverything seal first thing. Penny wise pound foolish as they say.

Hey- so update as promised:

Got engine back together with new crank seals. Had a lot of trouble getting it started again surprisingly. Engine was sputtering along with starter motor but wouldn't come to life. So I disconnected the throttle linkage at the distributor, which allowed me to rev is up up bit without dropping it into gear. Then I used an old squirt pump style oil bottle to inject some fuel directly into the carbs. One touch of the key and she fired right up. I was able to keep it running at higher RPM for a minute or two before I dialed in the carbs and lowered the idle down.
Certainly seems better than before I took it apart. My only concern is that it seems to be maybe dropping out a cylinder, or something funny where it idles smoothly and then stalls for a moment and comes back, or dies completely. I am hoping it just needs a good run. Any thoughts on that one?

The only thing I am questioning is my coil, which is out of an old sports car. The wires on my OEM coil were in bad shape. This one seems to be working well though, I dont think it would contribute to a cylinder dropping out, but if the spark is weak, then technically it could.

Hope this helps with your path forward. Anyone with thoughts to help get mine to stable out a bit, please chime in. Appreciate any advice.

Thanks.