Runs great on muffs but not in lake

[Rensch1983]

Mercruiser 3.0 that I bought as a project. I rebuilt the engine and have it running great in the driveway on the hose with the muffs on however when I get to the lake and put it in the water it stumbles and dies and will not start back up.

Timing is set at the sticker on the valve cover which is 8° the idle is at around 700 RPM and it will start up in idle forever in the driveway and take throttle very easily once I put it in the lake it dies and does not want to start back up I would assume this has something to do with the back pressure from water in the exhaust and trying to overcome that. I tried giving it more idle screw but didn't seem care
It has a Rochester 2 jet carb

Any thoughts or ideas on what is going on.

 

[Lou C]

Fuel pump pressure?
Anti siphon valve on the gas tank?
Fuel tank vent?

 

[Rensch1983]

Fuel pump pressure?
Anti siphon valve on the gas tank?
Fuel tank vent?

Not sure on pressure, I'll have to get a gauge on it.. It has a vent because I overfilled tank and it spilled out. Then I'm not sure what an anti siphon valve is... Also how the difference between hose water and lake affects it

 

[tpenfield]

There are so many threads like this . . . 'runs great in the driveway' . . . running an engine with no load means nothing.

I would suspect the carburetor (it is carbed, right?) What was done, if anything, to the carb during the rebuild?

 

[Lou C]

Water is water your problem is no load vs load… yes the back pressure of water on the exhaust is an issue which is why final carb adjustments should be done in the water. The anti siphon valve is the barbed fitting on the tank that the fuel line attaches to. It had a spring loaded ball inside so if the fuel line were to leak it cuts off fuel flow. When the engine is running the suction developed by the fuel pump pulls it open. If it’s corroded inside or clogged with crud then you won’t have enough fuel flow with the engine under load.

 

[alldodge]

Do you know what the original serial number of the motor was?
If not, what year and what ignition system?

 

[Scott06]

When it won’t start back up do you have spark ? If not would check shift interrupt or disconnect it to test( won’t come out of gear though)

If you have spark dump a little gas down the carb if it fires for a bit then it’s fuel related

Just need to determine if you have spark issue or gas issue

 

[ROY WILLIAMS]

my boat had a high power in the water ... then the engine drops the lower power ...
I cleaned the gas tank vent ...then it runs high power !
my driveway runs good but I had to clean that gas tank vent ...engine runs great in the water ..

 

[Rensch1983]

its been raining so I took the time to pull motor back out to source a leak i had, going to test my leak repair and then put motor back in and head to lake for some on the water tuning.

The carb has a sierra marine tag on it and the previous owner stated it was new from them, but I obviously dont trust him. I bought a fuel pressure gauge and also going to bring a outboard engine fuel jug to bypass my tank/vent/filter and see if it runs good like that..

If it does then I know I have an issue with water or whatever on tank/filter side.

When it did die on the lake it would fire up after giving it some fuel and as soon as it started it would instantly start bogging and not recover, it would just die.

I suspect its a carb issue since it runs fine on the hose but introduce some water backpressure and it cant overcome it i guess..

I do see alot of people thought out the shift interupter.. would i have this issue if the controls are in nuetral and it dies?

 

[dubs283]

I do see alot of people thought out the shift interupter.. would i have this issue if the controls are in nuetral and it dies?

Not likely. Based on info provided I'd lean towards a fuel supply issue or carburetor issue. Sounds like you have a good plan to check that stuff out.

The shift interrupt switch when properly working only engages when coming out of either gear into neutral. If it engages going into either gear then there is an adjustment or cable issue that needs resolved. You can test the switch for operation by depressing it manually with the engine running in neutral, if the engine dies the switch works, if not it's faulty. Again I don't think there is an issue with the shifting, your problem is more likely fuel

 

[Scott Danforth]

You IDLE speed is 650 RPM in water in gear.

Out of water and in neutral, that is about 850rpm

Where are the idle mix screws set?

 

[QBhoy]

Apart from all of the above sound advice…I’d pull the plugs and check they aren’t wet or that there isn’t water ingress to the cylinders…caused by one of a few common sources. Like manifolds or risers…jacket or head etc etc. usually caused by frost damage.
When out the water on trailer…it may be easier for residual water to take the path of least resistance, instead of into the oily bits.

 

[airshot]

If your sure the carb is clean and in good working order, then fuel adjustments and idle speed " must be set in the water while in gear and underway" this is a quote from the factory service manual!! Adjustments will not work when on muffs or a test tank. Get everything working properly, fuel flow and ignition befire setting idle and mixture screws. Again from the service manual...

 

[Rensch1983]

You IDLE speed is 650 RPM in water in gear.

Out of water and in neutral, that is about 850rpm

Where are the idle mix screws set?

I guess I can crank a turn in the throttle screw to see if I can get it to light. I couldn't get it to remotely stay alive in the water the other day. It would catch and bog then die... Both mixture screws set about 1.75-2 turns out timing at 6-8* per sticker on valve cover.

 

[Scott Danforth]

I guess I can crank a turn in the throttle screw to see if I can get it to light. I couldn't get it to remotely stay alive in the water the other day. It would catch and bog then die... Both mixture screws set about 1.75-2 turns out timing at 6-8* per sticker on valve cover.

your mix screws are way too far out (rich). start at 1-1/4 to 1-1/2 turn out and bring your idle speed up

 

[Rensch1983]

your mix screws are way too far out (rich). start at 1-1/4 to 1-1/2 turn out and bring your idle speed up

Interesting... I don't know since it runs great on muffs then dies on the lake I'd assume to opposite it would need more fuel with the water back pressure I couldn't see it wanting leaned up but when I go back to lake I'll see if That makes a difference.

 

[Scott Danforth]

Interesting... I don't know since it runs great on muffs then dies on the lake I'd assume to opposite it would need more fuel with the water back pressure I couldn't see it wanting leaned up but when I go back to lake I'll see if That makes a difference.

it could be running on 2 cylinders on the muffs and you wouldnt know it

do you have points or electronic ignition? if points, make sure to set your dwell with a meter

 

[Rensch1983]

I'm pretty sure I would know if it's running on two cylinders on the muffs

, it was converted to electronic ignition by the previous owner it has a pertronics igniter on it

 

[mkeenan4]

I'm pretty sure I would know if it's running on two cylinders on the muffs

, it was converted to electronic ignition by the previous owner it has a pertronics igniter on i

2 questions out of curiosity, since your engine is out, check to make sure the exhaust flap is going the correct way and are free. second, when you back it in the water, are you leaving the transom straps on or are you allowing the boat to float freely? if we're suspecting an exhaust flow issue, i'm trying to troubleshoot that. if i'm reading correctly, you're just backing the boat in the water, not touching anything and having your issue...no shifting, no backing off the trailer, no load on the engine. if that's the case, all of these other guesses are kind of irrelevant in my opinion.

 

[Rensch1983]

2 questions out of curiosity, since your engine is out, check to make sure the exhaust flap is going the correct way and are free. second, when you back it in the water, are you leaving the transom straps on or are you allowing the boat to float freely? if we're suspecting an exhaust flow issue, i'm trying to troubleshoot that. if i'm reading correctly, you're just backing the boat in the water, not touching anything and having your issue...no shifting, no backing off the trailer, no load on the engine. if that's the case, all of these other guesses are kind of irrelevant in my opinion.

I do not have an exhaust shutter valve on my exhaust pipe and when the issue occurred when I had it in the water the transom straps were off and I was even out in the middle of the lake when it died the only reason I'm thinking exhaust back pressure is because it runs fine on the muffs with no load obviously but idles and revs just fine as soon as you introduced water where it has the back pressure to overcome from the exhaust the engine struggles and dies in won't stay running