Runs great on muffs not in lake

boernerg · 28 July 2020, 00:03

I have an older 75 hp mariner 2 stroke 4 cylinders (I think its an 86 model) On muffs in the driveway starts right up and runs like a champ. idles great goes into both forward and reverse and revs up no problem.

As soon as the boat touches the lake it becomes much harder to start, might run ok for a few minutes steady at 4k rpms sounds like it might be missing.. loss of power for a split second then revs back up then repeats.. If I throttle down after running a few minutes it has more trouble picking up speed and getting up on plane until the point where it wont go on plane at all and I barely make it back to the dock..

I ran seafoam through the fuel system didn't really seem to help. There doesn't appear to be a fuel filter anywhere?? when I squeeze the bulb it gets tight but I can keep squeezing and fuel will come out of the bottom carb but not the top.

Plugs have been checked and are fairly new they are gapless type..

Any advice on where to start next? Thanks in advance!!

Peter_C · 28 July 2020, 03:31

Carb rebuild with new seats (They usually come in the rebuild kit), and possibly floats. Something is wrong with the bottom carb as the fuel is not shutting off. The jets could well be plugged to. Fix those issues and try a wet test after.

boernerg · 28 July 2020, 03:46

Quote:

Originally Posted by Peter_C

Carb rebuild with new seats (They usually come in the rebuild kit), and possibly floats. Something is wrong with the bottom carb as the fuel is not shutting off. The jets could well be plugged to. Fix those issues and try a wet test after.

Do I need to dig deeper to find the fuel filter or was there just not one for these older models? I'll see If I can get that bottom carb rebuilt..

Peter_C · 28 July 2020, 04:10

Rebuild all carbs not one. Otherwise you will be rebuilding the others soon enough, maybe even before putting it back on the water.

There should be a tiny fuel filter on the fuel inlet. I would recommend installing a fuel water separator on the transom. They are cheap enough.
https://www.amazon.com/Sierra-18-777.../dp/B000N9MTFC

Nos4r2 · 28 July 2020, 17:29

Compression test it.

Do it before you even think about messing about with carbs, if the compression is shot it'll run out of water but not in.

Unknown · 28 July 2020, 19:21

Mine does the same, I call it lake fever

ShinyShoe · 28 July 2020, 22:27

too deep in the water makes exhaust harder to escape on a seagul...

Peter_C · 29 July 2020, 03:06

Quote:

Originally Posted by Nos4r2

Compression test it.

Do it before you even think about messing about with carbs, if the compression is shot it'll run out of water but not in.

^That is always a good start

boernerg · 29 July 2020, 05:17

UPDATE:

Did compression check today all 4 cylinders checked 135-150 so moving on to carb rebuilds and install water separator..

Peter_C · 29 July 2020, 19:41

I would also recommend changing all the fuel lines due to age and modern oxygenated fuels that eat fuel lines, along with changing the fuel pump diaphragm, depending on design. That is an older motor that needs a little refurbishing, but nothing major. The compression readings aren't great, but once running properly and opened up, they may well come closer together.

Let us know the results...

boernerg · 30 July 2020, 04:33

Update:

Pulled fuel tank and drained (was some water in the bottom) checked all fuel lines everything looked good but I did add a loop to my air vent line just in case water was getting in through that. Added some fresh gas mix then pulled both carbs and did a good cleaning on both, the bottom one had some trash in the bowl and in the little valve. Top carb looked ok but did a good cleaning on it anyway. Could tell a big difference as soon as I got it started The Idle went from 900-1800 so I figure I might need to adjust that but going to wait and see how it does in the lake first under a load.

boernerg · 30 July 2020, 21:49

Final Update:

Boat on lake today ran great!! Idle around 1100 rpm when in water 1800 on land so I was kind of surprised by that.. Ran it for a couple hrs it died once when throttling down but other than that performed pretty well (35 mph at 4k rpm). Thanks for the help and advice you guys are awesome!!

A1an · 30 July 2020, 22:15

Always set your idle with the leg in the water not on muffs.