Rebuild or repower

[.44mag]

1983 Mercury 150 Ser # 6431959. After a day of fishing we get ready to go back to the ramp to go home. Crank up and idle about 70 yards out of a cove then attempted to get on plane. No dice front of the boat stood up and the motor sounded like it was hitting a rev limiter. Now yes I did notice all day that it was idling rougher than usual, after it had been sitting for about 3-4 weeks. Any way did a compression test today, (1) 105, (2) 118, (3) 115, (4) 118, (5) 115, (6) 119 [was touching not covering the 120 mark so]. So what do yall think? Yes I could rebuild it but it's still a 35 year old motor with new parts. Or bite the bullet and get some thing new or at least newer? I did notice this chunk out of the head gasket when I started putting stuff up so not sure if it's all the way through. Guess I will at least take that head off to see.

[leonreno]

I would definitely pull head to see what is happening, maybe just a bad gasket. Worth seeing first.

[jbh3]

Check prop for spun hub.

[.44mag]

I would definitely pull head to see what is happening, maybe just a bad gasket. Worth seeing first.

I pulled the head. Straight checks are great, surprisingly. Nothing catches my finger nail in the cylinder. Guess rings are froze or worn.

[.44mag]

Can't fix my typo Straight edge checks look good surprisingly.

[Gary Morgan]

I believe I would be checking that center cylinder. The combustion chamber looks like it isn’t firing.

[lpugh]

1983 Mercury 150 Ser # 6431959. After a day of fishing we get ready to go back to the ramp to go home. Crank up and idle about 70 yards out of a cove then attempted to get on plane. No dice front of the boat stood up and the motor sounded like it was hitting a rev limiter. Now yes I did notice all day that it was idling rougher than usual, after it had been sitting for about 3-4 weeks. Any way did a compression test today, (1) 105, (2) 118, (3) 115, (4) 118, (5) 115, (6) 119 [was touching not covering the 120 mark so]. So what do yall think? Yes I could rebuild it but it's still a 35 year old motor with new parts. Or bite the bullet and get some thing new or at least newer? I did notice this chunk out of the head gasket when I started putting stuff up so not sure if it's all the way through. Guess I will at least take that head off to see.

The compression readings as posted have nothing to do with the problem described. As mentioned above spun hub or prop fouled with weeds,
You should have done a leak down test before the heads were pulled. Was the motor warmed up, batteries fully charged and throttle wide open when performing the compression test? I do have a problem with the carbon in the center cylinder and lack of in the top cylinder. I doubt the 20 psi variation would cause a serious rough idle but would be noticeable. I would risk the price of a pair of head gaskets and put it back together. Do a good de-carbon treatment and recheck the compression properly before going forward. Check the prop hub for slippage, and then address the tuning issues that exist
No way I would consider rebuilding that motor, You could get a low hour Opti max in good shape for less money and you would be far happier with the results

[.44mag]

Yes motor was warmed up before the compression test was done, the throttle was wide open and batteries were fully charged. I plan on checking the plug wires, coils, switch boxes, trigger and stater once I get the new head gaskets in. Old gasket and head are back on and I have poured carb cleaner in all 6 cylinders and will soak them like that for the rest of the week till I get another day off. I pulled the prop off, mainly checking for fishing line and such, none, but couldn't tell any thing about the hub. Will do a white paint line across every thing on the prop and hub once it's back together. I didn't think about a leak test till after I had pulled the head off to so duh on me.
Yes that middle combustion chamber had me scratching my head along with the top (the one with the low compression reading).
Guess if go to an optimax I could go back to a 24v trolling motor to counter the weight difference, I think the opti out weighs this black max by 50 pounds. This one weighs 375-ish.
Just a little back track here. When It acted up I did shut down and raise the motor up to check for weeds. Nothing was on the lower unit. I did go get new plugs and put them in the next day and took it out to check. It was just me in the boat and no gear, it jumped on plane like normal but was still idling rougher than normal, ran about half a mile slowed down and turned around dropping to idle. I trimmed it all the way down, looked back to make sure. Went to get on plane and it stood up and sounded like it was hitting a rev limiter again.

[jarhead175]

Small gas container with 1/2 a gal of fuel, add 16oz seafoam, take your boat to the lake and unload. Leave the boat tied to the dock and run your fuel line into the small container. Start the motor and idle on the mix till the motor dies. Hook the fuel line up to the gas tank and go run at WOT to blow out the carbon. Then do your compression check.

[JOE54]

check for spark on all 6 ,make sure carbs are not flooding ,running rich on that cyl

[leonreno]

Also might check reeds.

[.44mag]

Will do guys thanks

[John Jackson]

Could also be a fuel delivery problem, since the first time is OK. Gas tank vent might be clogged.
So it yourself rebuild for those motors is relatively inexpensive. I would go that route, and include the carbs. Center head looked awfully rich.

[JOE54]

+1^^^^

[.44mag]

Went out and did some resistance checks with my volt/ohm meter, the number 3 coil which corresponds with the wettest looking combustion chamber I could only get a reading of infinity. So guessing that coil is bad. New head gasket should be here any day now will install it and check with my spark tester, once I find it or buy a new one. Dang thing grew legs I guess. Stator seams to be good according to readings and the chart I have. Ordered carb gasket kits, no needle and seats yet. Will see what they look like when I take the carbs apart for soaking in cleaner. Need to get a dva adapter too.

[.44mag]

Ok new head gasket on. Did spark test on all six, don't look in the picture but all six are blue and jumping a 7/16th gap. The top carb is off and soaking in carb cleaner, gonna let it soak till Monday then soak the middle one and so on.

[.44mag]

This is before cleaning. Does this look right for the floats? So I'll know when I put them back together?

[leonreno]

Looks good to me, I always blow into fuel inlet to see if there is any leakage when I hold them upside down.

[.44mag]

Ok.after new head gasket, fuel lines from inrichment valve to each carb and cleaning the carbs it idles a heck of a lot better. Ran it at the lake this morning gets on plane with way less effort than it has in a long time and got up to 52.2 mph according to my GPS and speedometer. Before it would barely get to 50.1 mph.

[OnTRAK]

Did you do a complete lync & sync procedure after reinstalling the carbs? If it hasn't been done already, it's not a bad idea to completely remove (trash it) the idle stabilizer box and set maximum spark advance timing to between 20*-22* BTDC.