Mercury 650 problems on the 4th of July.....sad day

rsamsel

Seaman Apprentice
Joined
Jun 2, 2014
Messages
44
Ill get right to it.

1974? Mercury 650 3 cyl 2 carb

Was running great in Puget Sound yesterday and lost 1/3 of its power. Was missing badly on that cylinder after that. Got it home and SEEMED to be running fine on the hose (i bet it wasn't but whatever). Took it out to the lake today for 4th festivities...only running on top 2 cylinders. Cant get the 3rd to fire at all.

Trouble shooting so far
-cylinder IS getting spark...removing the plug and running the engine with plug up against block has big white/blue spark
-cylinder does have compression (compression tester is 115 top 115 middle 100 bottom. Yes the bottom cyl is lower than the rest. I know that's not good but is that enough for it to not fire at all? It ran perfectly up until it didn't and I have a hard time believing it went from 100% fire to 0% fire by just getting wore out.


Boat runs fine on top 2 cylinders whether the bottom plug is connected to plug wire or not. Connecting or disconnecting the wire changes nothing.

Next things to check?

Its summer and the cost of outboards up here is terrible. I had a good running Johnson that I got rid of for this Mercury thinking it was the right move. Now I'm very upset with my decision. Hope its not a fatal issue.

Rob
Maple Valley WA
 

emckelvy

Commander
Joined
Jan 16, 2004
Messages
2,506
What number of plug are you using? Have you tried a different spark plug in #3? Just 'cause it's sparking outside the cylinder doesn't mean there's enough spark energy to spark under compression.

To prove that #3 is firing, you can use either an inexpensive "firing indicator" (usually available at auto parts and lawn mower-type stores), or just clamp your inductive timing light around #3 spark plug wire and see if it's getting a strong spark.

Sudden failures like that sure point to the ignition system. Bottom carb feeds cylinders 2&3 and top carb feeds cylinders 1&2. So, if you were to have an issue with #2 carb, it may affect the bottom cylinder more than middle, because #2 can still draw some fuel from the top carb.

I suspect you'll find your problem either with something obvious in the ign system, or fuel-related to the bottom carb.

An easy way to check to see if the bottom carb is feeding #2 is to block off the carb with your hand or a rag and if rpm drops, you're having an effect on #2.

If you find rpm kicks up when you block off #2 carb for a second, it's got a fuel problem and by choking off the carb you're pulling fuel up from the float bowl.

Another thing you can do is spray fuel/oil mix from a squirt bottle into the bottom carb and see what happens.

Compression in the bottom leaves something to be desired, but it should still run with 100 psi. Maybe after you get it all sorted out you can run some Seafoam in the fuel, to give the engine a decarbonization treatment. If the rings are stuck due to carbon, it'll help.

BTW, any water droplets on #3 spark plug? That would usually indicate a bad lower crankshaft seal, sucking in water. Or an exhaust plate leak, but you'd most likely see water on at least #2 plug as well.

Give it a shot and let us know what you find. HTH................ed
 

rsamsel

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Joined
Jun 2, 2014
Messages
44
Wow...amazing post even in the middle of the night. The only thing I know from your post is the spark looked VERY strong and I did try swapping plugs with cylinder 2 and still was completely missing #3(bottom). The rest of the things you asked will be updated tomorrow.

I appreciate the help so far...so torn up about the old girl I can't sleep.
 

rsamsel

Seaman Apprentice
Joined
Jun 2, 2014
Messages
44
What number of plug are you using? Have you tried a different spark plug in #3? Just 'cause it's sparking outside the cylinder doesn't mean there's enough spark energy to spark under compression.

To prove that #3 is firing, you can use either an inexpensive "firing indicator" (usually available at auto parts and lawn mower-type stores), or just clamp your inductive timing light around #3 spark plug wire and see if it's getting a strong spark.

Sudden failures like that sure point to the ignition system. Bottom carb feeds cylinders 2&3 and top carb feeds cylinders 1&2. So, if you were to have an issue with #2 carb, it may affect the bottom cylinder more than middle, because #2 can still draw some fuel from the top carb.

I suspect you'll find your problem either with something obvious in the ign system, or fuel-related to the bottom carb.

An easy way to check to see if the bottom carb is feeding #2 is to block off the carb with your hand or a rag and if rpm drops, you're having an effect on #2.

If you find rpm kicks up when you block off #2 carb for a second, it's got a fuel problem and by choking off the carb you're pulling fuel up from the float bowl.

Another thing you can do is spray fuel/oil mix from a squirt bottle into the bottom carb and see what happens.

Compression in the bottom leaves something to be desired, but it should still run with 100 psi. Maybe after you get it all sorted out you can run some Seafoam in the fuel, to give the engine a decarbonization treatment. If the rings are stuck due to carbon, it'll help.

BTW, any water droplets on #3 spark plug? That would usually indicate a bad lower crankshaft seal, sucking in water. Or an exhaust plate leak, but you'd most likely see water on at least #2 plug as well.

Give it a shot and let us know what you find. HTH................ed

Well....I have some answers and more frustration and puzzling things going on.

Plug is NGK BUHW...permagap plug.

Working on it today I can only get it to run at significant rpms. Covering the bottom carb makes it die. It seems that spraying fuel into the bottom carb does not make it run better. It seems that it floods it and kills it.

New think I noticed today. The #3 cylinder definitely is not firing but now it also seems that #1 is iffy. It was able to run and idle down on 2 cylinders the other day. Now it seems like its only running on #2 and catching #1 every once is a while. Pulling the plug on number 3 does nothing to the way it runs BUT when I plug it back in it seems to fire a couple times then fall right off.

Anything I can pull apart and check to diagnose if its major mechanical? This motor is new to me and ran great the first 10 or so days I had it out.

Running 50:1 with a new impellor. Should point out that it can take at least minute for the tell tale to start peeing...normal?

Any info helps. Thanks guys
 

rsamsel

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Joined
Jun 2, 2014
Messages
44
Ok....so now I wonder if I'm using the right plug. I am using that plug because that is what was in the boat when I bought it. I did some looking and im having trouble finding anyone else using that plug. That being said, the boat ran great for 10+ days out or so I hate to put the blame solely on that.
 

rsamsel

Seaman Apprentice
Joined
Jun 2, 2014
Messages
44
Does anyone have a suggestion to get some traction on this thread. I am stumped and cannot for the life of me figure out why this motor wont run right. Please!
 
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