Sorry, a bit of a read but I think the more info the better
1984 Mercury 75 hp outboard
2 stroke 4 cyl
S/N 6504041
Last few years my dad’s boat motor was unable to maintain idle. Had to kind of go from a somewhat fast idle into gear and get going to keep the engine running. So this year I took over the boat and here’s where I am.
I knew the fuel pump needed some work as it’s never been pulled apart in 35 years and it leaked at the inlet. That was fairly easy and surprising the diaphragm was still in decent shape (replaced anyway). The thing is the rebuild instructions show to install a spring and cap that actuates the diaphragm. There wasn’t any when I took apart the pump, nor a divot in the check valve retainer for the spring to go in. So I just didn’t include it. Fired up the motor but no difference in the idle.
Compression check gives me 80/105/115/115 (1-4), and yes I grounded the plug wires
. So I was a bit hesitant to try a carb rebuild based on those numbers but decided to try anyway.
After a full rebuild (disassemble and soak) I have the motor idling decent at about 1000 rpm in a test tank (55 gal heavy plastic drum). I can get it in gear but the drum isn’t quite big enough. It pushes out some water looking like it causes cavitation so I don’t think I’m getting proper load on the prop. Still get a strong pee though from telltale though.
I’m off to the lake tomorrow where my dad is so we can play around with this thing in a proper body of water. I’m at the link and sync stage and here is where I hit a bump with info. The factory manual tells me the primary pickup should be set at 7-9 deg BTDC and max timing at 23 BTDC. However a sticker on the cowl silencer cover says that pickup is 4 ATDC and max at 27BTDC. I’ve read in a few places about setting max back a few degrees due to fuel quality and whatnot but not sure what to do about the primary pickup. I also understand primary should be done in the water, forward gear idling.
Thank you all in advance for your genius work. I’m 35 and believe that if forums to this extent were around 20-25 years ago I would be much more competent.
1984 Mercury 75 hp outboard
2 stroke 4 cyl
S/N 6504041
Last few years my dad’s boat motor was unable to maintain idle. Had to kind of go from a somewhat fast idle into gear and get going to keep the engine running. So this year I took over the boat and here’s where I am.
I knew the fuel pump needed some work as it’s never been pulled apart in 35 years and it leaked at the inlet. That was fairly easy and surprising the diaphragm was still in decent shape (replaced anyway). The thing is the rebuild instructions show to install a spring and cap that actuates the diaphragm. There wasn’t any when I took apart the pump, nor a divot in the check valve retainer for the spring to go in. So I just didn’t include it. Fired up the motor but no difference in the idle.
Compression check gives me 80/105/115/115 (1-4), and yes I grounded the plug wires
. So I was a bit hesitant to try a carb rebuild based on those numbers but decided to try anyway.After a full rebuild (disassemble and soak) I have the motor idling decent at about 1000 rpm in a test tank (55 gal heavy plastic drum). I can get it in gear but the drum isn’t quite big enough. It pushes out some water looking like it causes cavitation so I don’t think I’m getting proper load on the prop. Still get a strong pee though from telltale though.
I’m off to the lake tomorrow where my dad is so we can play around with this thing in a proper body of water. I’m at the link and sync stage and here is where I hit a bump with info. The factory manual tells me the primary pickup should be set at 7-9 deg BTDC and max timing at 23 BTDC. However a sticker on the cowl silencer cover says that pickup is 4 ATDC and max at 27BTDC. I’ve read in a few places about setting max back a few degrees due to fuel quality and whatnot but not sure what to do about the primary pickup. I also understand primary should be done in the water, forward gear idling.
Thank you all in advance for your genius work. I’m 35 and believe that if forums to this extent were around 20-25 years ago I would be much more competent.
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