Engine running rough

Sbushman

Seaman Apprentice
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Jun 19, 2016
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36
Hey guys. I need some help. I have a 77 slickcraft ss 235 with a omc 190 hp. I just did a motor swap due to a cracked block. My issue is that when I run the engine in my driveway it runs great. No sputtering or hesitation. But today I took it out for the first time and had some issues. First I noticed the whole boat would shutter at around 700-900 rpms. And then every time I would try to go over 1000 rpms it would die. I could get it to go over 1000 rpms if I gunned it. But it would still have that hesitation at 1000. Amd then when I hit 3000 it would start to fluctuate the rpms. So it did this right away on the water. But if I took it out of gear and just ran the throttle it would run just fine again. So I know it has to do with having a load on the engine. Any ideas on what I should check or do. I'm at a loss here and could really use some help. Thanks.
 

Bondo

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Ayuh,.... Sounds like it's starvin' for gas,....
 

alldodge

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Agree its sounds like duel issue. Check fuel pressure (4-7 psi) from idle to WOT. If that's good then rebuild the carb
 

Sbushman

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Jun 19, 2016
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36
I'll check the pressure first. What do you think about those in line electric fuel pumps? I have one and was debating installing that and bypassing the mechanical pump. I'm not sure it that would be better or not.
 

alldodge

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Nothing wrong with mechanical or electric both will supply all the fuel you need. Only thing with electric is you will need to add some wiring and an oil pressure switch to use electric.
 

bruceb58

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Mar 5, 2006
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What is the status of your ignition system? New points, condenser, cap, rotor, wires and plugs? Make sure your ignition system is perfect and then troubleshoot the carb.
 

bruceb58

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Mar 5, 2006
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What do you mean oil pressure switch?
You need an oil pressure switch with an electric fuel pump so the pump doesn't run with the engine not running. You can't just wire it up to the ignition switch.
 

alldodge

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To power an electric pump it needs to have an oil pressure switch inline so if the motor looses oil pressure it will turn OFF the pump. You also need a bypss wire so when the motor is being started the pump gets power to supply fuel to the carb. Something like this

Fuel pump, electric.jpg

A factory install is like this

Fuel Pump Wiring.jpg
 

Sbushman

Seaman Apprentice
Joined
Jun 19, 2016
Messages
36
I get it now. Maybe I'll just check the pressure with the mechanical pump and go from there. As far as the ignition components, I have done anything with those. Like I said, in the driveway it runs great. So I didn't think I need to do a tune up or anything. But the age and condition is unknown to me.
 
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