Help

gublube

Cadet
Joined
Jul 13, 2012
Messages
20
Ok, I think it's my first time posting. I'm far from a mechanic but do have some basic knowledge. I bought a mercruiser 5.7 carb motor from a boat yard. Ran gd for about 8 trips fishing at sea all day. Started cutting out. So here is what it's doing. It runs gd for about 100 yards then cuts out. I've changed cap, rotor and upgraded sensor. Motor starts with tap of key. Idles great, revs hd and strong on ear muffs. Once under load it will cut out after about 100 yards at full throttle then has huge miss. I've checked everything I can. Once it cuts out at full throttle, you can push the outdrive button dow. Ir up or any electrical item and it picks up and runs strong. I was told alternator was bad. Changed and it ran about a mile gd and back at same issue. Has a tb4 ignition system. I've checked and cleaned every ground. Getting all current volts it should. Unless my friend missed something. It had a coil that said use with external resistor. But after alot reading g thise were only for point system . Mine is not a point system. So I bought a gm 12volt coil. Still does the same thing. I dont know what else to check a d I'm not very smart on tte voltage testing stuff. It runs great in neutral and revs gd and strong. Coil does feel a little hot that prob should. Now I had this motir installed by a boat mechanic. He used a low pressure electric fuel pump and connected tte positive to the + on coil. Said be has always done that. It ran gd for about 8 trips or so before having this issue. We only have one mechanics shop here and they only work on newer stuff and original. Since the motor is not original they wont touch it as they dont know what's been done. But like I said. It runs gd for about 100 yards the cuts out at any speed or rpm. But if you hold outdrive button it will run great til u let go. Also if I push bilge pump button it get lil more power. But the outdrive button seems to take away the issue the best. I know it's something shorting out. I'm lost. Been at it for almost 2 yrs with different mechanic minded friends. Anyone ever have an issue like this.

John
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
43,319
Holding the trim down causes the motor to pickup, that is different

Its a 5.7 but what's the serial number (best), or at least the year?
 

gublube

Cadet
Joined
Jul 13, 2012
Messages
20
Hi, when I put new alternator the ssue went away for a short time. The fuel filters, water separator, and gas was all replaced. New pick up tube, just about everythin . I'm thinking the coil is getting hot and short g out the ignition system. But then again I'm no mechanic. The numbers on the block show different years and use by gm motors. It shows a crate motor for car for around 87, and it shows same block used in mercruiser for around 97 to 2007. So I'm not sure what year exactly. It is a roller motir, changed from efi to carburetor. I dont see where fuel has anything to do with why it picks up the power and the cutting out goes away while holding outdrive button or any other electronic switch. I say outdrive button as it takes most power and makes it run stronger. If I push bilge pump it picks up slightly. But the outdrive button while holding it down makes it run normal and strong as lo g as you hold it.
 

gublube

Cadet
Joined
Jul 13, 2012
Messages
20
Also it dont take an mechanical fuel pump. Like i said I got it from boat yard and he said it was fresh rebuild and had about 70 hrs. Ran good for about 8 trips and I run All day from daylight to dark fishing.
 
Last edited:

gublube

Cadet
Joined
Jul 13, 2012
Messages
20
Sorry for spelling. I'm not illiterate. Small keypad and auto correct on this tiny phone. Lol
 

gublube

Cadet
Joined
Jul 13, 2012
Messages
20
no mention of a spin on filter or carb filter replaced.

Carb filter is clean and carb was rebuilt when I got motor. Freshly opened and checked. Filters all gd and new. Fuel pump pumping gd. It's a low pressure 4lb to 7 lb psi. What my marine mechanic had me get before he retired and moved back east. I even bought another and same issue. After all fuel issues were hone through I tried tilting power trim n outdrive for the heck of it. It picked up power and tgats when I noticed it has to be some short or something else. Runs too gd in driveway and revs up strong. No miss or anything. Timing dead on. Only after running full throttle for about 100 yards then it starts doing it like sonethinv heating and shorting. But holding the power trim on outdrive takes that away and runs gd. I'm confused.
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
43,319
Your saying everything is setup as it should and nothing is wrong, but ............ it doesn't run right.
If it was all dead on, then there would be no problem

The motor is somewhere between 87 to 2007 and timing is correct, not going to happen, any more then if it was 97 to 2007. The 97 to 07 had varying settings based on what the ignition system is used. You said its TB4 so that helps a bit

A carb that has been overhauled may not be correct if it was not done correctly

We need to know what is dead on, what the alternator output voltage?
What is timing set at?
What is the fuel pressure at WOT?

Reving in neutral is meaningless
 

gublube

Cadet
Joined
Jul 13, 2012
Messages
20
Well. I'm just saying that it will run perfect after it starts cutting out while holding outdrive button. Let go it cuts out. I can hold it as long as I want and it runs strong and fine. Let go and it cuts out. I'm not a mechanic and dont know the timing ect. Just trying to see what causes it to run good while holding a power switch and letting go cuts it out. The guy I got motir from has huge boat yard in san Diego. From giant yachts to small fishing boats. I assume he knew what to add to sell and not get bad rep. Boat ran good for many hours til this. Only difference now is I took out coil he had tgay said use with external resistor and put regular 12v on. I seen some systems had resistor in wiring harness. And some dont. Not sure what to believe lol.
 

gublube

Cadet
Joined
Jul 13, 2012
Messages
20
Well. I'm just saying that it will run perfect after it starts cutting out while holding outdrive button. Let go it cuts out. I can hold it as long as I want and it runs strong and fine. Let go and it cuts out. I'm not a mechanic and dont know the timing ect. Just trying to see what causes it to run good while holding a power switch and letting go cuts it out. The guy I got motir from has huge boat yard in san Diego. From giant yachts to small fishing boats. I assume he knew what to add to sell and not get bad rep. Boat ran good for many hours til this. Only difference now is I took out coil he had tgay said use with external resistor and put regular 12v on. I seen some systems had resistor in wiring harness. And some dont. Not sure what to believe lol. He did say motir was a 2007 but not sure I believe that. After research on the block numbers it varies. I can put the numbers on here.
 

Rick Stephens

Admiral
Joined
Aug 13, 2013
Messages
6,118
Sounds like a bad ground. Take the ground wire off at both ends from the battery to the motor and use a steel brush to make connectors, bolt and contact areas on back of motor and battery shiny bright clean.
 

gublube

Cadet
Joined
Jul 13, 2012
Messages
20
There isnt any marine mechanics here to look at all the settings and timing. I guess I can try and find an auto mechanic. I was just hoping someone has experienced this and could give me ideas what to try and fix. I said it was all dead on as it fores right up with Tao of jey. Runs perfect and smooth for about 100 yards or so. Or at less throttle it runs perfect for longer. So basically something is getting warm I assume and then heating up and shorting it out. I can run at lower rpm and speed alot longer. I can let odle all day and it dont cut out.
 

gublube

Cadet
Joined
Jul 13, 2012
Messages
20
Sounds like a bad ground. Take the ground wire off at both ends from the battery to the motor and use a steel brush to make connectors, bolt and contact areas on back of motor and battery shiny bright clean.

I have checked, cleaned and wire brushed every ground I can find. From battery to all connections on starter, alternator, solenoids and everything.
 

gublube

Cadet
Joined
Jul 13, 2012
Messages
20
I would also like to add that when I would clean the grounds I noticed the black ground wire from battery would spark when I touched it to bolt on block of motor. Only with battery switch on in any position 1 or 2 or both. I unplugged my radar power and the spark went from big to barely seen. Also if I unplug the wiring harness it dont spark at all. Sonething is creating current to the bock I assume. Forgive me if I'm not as mechanically inclined as many of you or even close. Just dont have alot help in this big city that knows anything. All mobile mechanics want alot to just test wires. I wanna make sure I have a marine mechanic familiar with this motor and ignition system first.
 
Last edited:

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
43,319
The TB4 doesn't use a resistor or resistor wire.

Unless you can get some tools, voltmeter, fuel pressure gauge and adapters to do some testing and let us know what you find, I don't see how anyone can help
 

gublube

Cadet
Joined
Jul 13, 2012
Messages
20
The TB4 doesn't use a resistor or resistor wire.

Unless you can get some tools, voltmeter, fuel pressure gauge and adapters to do some testing and let us know what you find, I don't see how anyone can help

Yes , I agree that I need to do some testing. I was just hoping someone had experienced this issue before. I'm sure someone may have but not sore if they will see this or chime in. Was hoping for things to test and check since its gotta be simple. Runs too gd except when it shorts. And runs too good while holding the power trim button on outdrive. Thank you all for your help and advice.
 

djmakko1

Cadet
Joined
Aug 21, 2016
Messages
24
when you press the trim button the boat runs great? and when you let go dies? so the drive goes up and stay up??
 

gublube

Cadet
Joined
Jul 13, 2012
Messages
20
when you press the trim button the boat runs great? and when you let go dies? so the drive goes up and stay up??

When it cuts out I can press the outdrive up or down and the cutting out goes away for a bit while I hold it. Or I can push bilge pump, engine fan, wipers, ect and it will run better. I use the tilt fir example as it draws the most power so ot makes a bigger difference. I dont press up. Usually down so it stays in down position. Other than that I. At loss. I've had few boat mechanics test all the grounds and positive leads. They are confused as well.
 

gublube

Cadet
Joined
Jul 13, 2012
Messages
20
When it cuts out I can press the outdrive up or down and the cutting out goes away for a bit while I hold it. Or I can push bilge pump, engine fan, wipers, ect and it will run better. I use the tilt fir example as it draws the most power so ot makes a bigger difference. I dont press up. Usually down so it stays in down position. Other than that I. At loss. I've had few boat mechanics test all the grounds and positive leads. They are confused as well
 

gublube

Cadet
Joined
Jul 13, 2012
Messages
20
When it cuts out I can press the outdrive up or down and the cutting out goes away for a bit while I hold it. Or I can push bilge pump, engine fan, wipers, ect and it will run better. I use the tilt fir example as it draws the most power so ot makes a bigger difference. I dont press up. Usually down so it stays in down position. Other than that I. At loss. I've had few boat mechanics test all the grounds and positive leads. They are confused as well.
 
Top