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Mercruiser 470 runs for a while and cuts out. then runs fine for a while.

P

pump.tech

New member
Looking for help please. driveway mechanic still scratching his head. that would be me.


runs well in driveway. put in water run down the river about a mile starting and stopping, all ok. Idles fine and will jump out of the hole with plenty of power.

continue down the river about 3-5 miles and will start to miss. I try to give a little gas and will not take the fuel. Runs like it is starving. If I go to idle the engine cuts right off. It takes 20-30 mins of col down time and then the engine will start right up and run ok. I have confirmed this on three separate runs as I look at fuel being the problem. I have replaced the carb, fuel pump, removed and cleaned the tank, replaced the fuel line and moved away from the engine. I have replaced the anti siphon valve. the points, cap, wires are 2 years old. Pulled 2 year old plugs and they are fine. they look good nice off white color, no gum or presence of oil or water. all plugs are good and fire.
voltage regulator runs at 14v constant. I am thinking the coil is breaking down. anyone got any ideas??? help please.
 
Not sure if it is your coil but it is possible. The coil could be degraded and crapping out after it gets hot. Is the coil hot when this occurs? Is it the original oem coil? Can't just use any 12 volt coil. There is a purple resistance wire that goes to choke. Is it attached? I think the oem coil is 1.5 ohms with points setup. (don't hold me to that!) Did you change the condenser? Could also be crapping out. Could also be fuel related. Did you use the oem carter fuel pump? It should be 4-6 psi and no more. Did you use the spacer between the block and the face of fuel pump? Did you clean tank before or after you installed fuel pump,carb,etc.? If you did it after installing then you will have to check filters in fuel pump,filter that screws into carb, and change water seperator filter again depending on whether the tank was all nasty with bad fuel,water,sludge,etc. You gotta take one thing at a time and by using the process of elimination. Was the carb rebuilt or brand new and is it the correct carb? Are your fuel lines the correct size? It matters! I know I am asking you alot of questions but just trying to find out more from you so I might be able to help you. I have a 1988 3.7lx 180 horse (470) with a four barrel carb, which is basically the same as yours . I have all the upgrades that can be done on this particular engine and will try and help you. Did you use a timing light to set initial base timing. You must use a timing light! You have to be able to get the engine at low idle or say 600-650 rpms. You cannot set it at say 800-1000 because the 470's have a rapid yet short timing advance curve and you will not get base timing if the engine is running fast as I'm guessing it is. Maybe that is not the case but figured I would throw it at you. Did you look under the points at the timing advance weights? Are they clean? Are the springs intact? Could be many things so I am giving you alot of things to carefully look at before you buy any parts. Almost forgot! make sure your choke is operating properly! That can cause you all sorts of grief and the engine will choke itself out if not opening properly after warmup. I had to adjust mine. The manual will explain this in detail and it has to be done when the engine is cold of course. It is a pain in the ass but it has to be right or you will have carb related symptoms that you are possibly experiencing. Possibly! Don't rule anything out just yet! Are all of your wires and or wiring free of corrosion? Correct plug gap? That is enough for now and should keep you busy for awhile! good luck, Tom
 
Check the distributor shaft for wear. It sits on an angle and prone to wear which can kill the engine as it wobbles.
 
To answer questions:

1. As far as I know it is the original coil.
2. Purple resistance wire is attached
3. I have not changed the condenser, however it is only 2 years old. Points and condenser were changed as a preventive measure two years ago.
4. this is not an OEM fuel pump. However after changing the fuel pump I still have the same problem. I think I have eliminated all fuel possibilities. New fuel pump, new rebuilt carb, new anti siphon valve, pull fuel tank and there was nothing in it. no water, no trash to speak of, nothing to note.
5. fuel line 3/8" is the correct size.
6. I would think if timing was a problem then I would have problems all the time. Not after getting hot. I can check it again however.
7. Choke works correctly it seems. When engine starts to miss I can pull the engine cover back and while still running down the river, I can look at the throat of the carb. The carb is open as it should be. The throat is closed when cold and I can watch it open as it should in the drive way test.
8. I pulled all plugs and inspected. All plugs are normal according the the book. All plugs and wires were replaced two years ago, along with points, condenser, distributor cap, rotor button.

The real funny thing is this. When I put the boat away last fall it ran great. When I got the boat out this spring it did not. One more thing. I had a local marine mechanic install a new lower unit last fall because I had lost reverse. Could that have anything to do with this?







1973 Browning Marine said:
Not sure if it is your coil but it is possible. The coil could be degraded and crapping out after it gets hot. Is the coil hot when this occurs? Is it the original oem coil? Can't just use any 12 volt coil. There is a purple resistance wire that goes to choke. Is it attached? I think the oem coil is 1.5 ohms with points setup. (don't hold me to that!) Did you change the condenser? Could also be crapping out. Could also be fuel related. Did you use the oem carter fuel pump? It should be 4-6 psi and no more. Did you use the spacer between the block and the face of fuel pump? Did you clean tank before or after you installed fuel pump,carb,etc.? If you did it after installing then you will have to check filters in fuel pump,filter that screws into carb, and change water seperator filter again depending on whether the tank was all nasty with bad fuel,water,sludge,etc. You gotta take one thing at a time and by using the process of elimination. Was the carb rebuilt or brand new and is it the correct carb? Are your fuel lines the correct size? It matters! I know I am asking you alot of questions but just trying to find out more from you so I might be able to help you. I have a 1988 3.7lx 180 horse (470) with a four barrel carb, which is basically the same as yours . I have all the upgrades that can be done on this particular engine and will try and help you. Did you use a timing light to set initial base timing. You must use a timing light! You have to be able to get the engine at low idle or say 600-650 rpms. You cannot set it at say 800-1000 because the 470's have a rapid yet short timing advance curve and you will not get base timing if the engine is running fast as I'm guessing it is. Maybe that is not the case but figured I would throw it at you. Did you look under the points at the timing advance weights? Are they clean? Are the springs intact? Could be many things so I am giving you alot of things to carefully look at before you buy any parts. Almost forgot! make sure your choke is operating properly! That can cause you all sorts of grief and the engine will choke itself out if not opening properly after warmup. I had to adjust mine. The manual will explain this in detail and it has to be done when the engine is cold of course. It is a pain in the ass but it has to be right or you will have carb related symptoms that you are possibly experiencing. Possibly! Don't rule anything out just yet! Are all of your wires and or wiring free of corrosion? Correct plug gap? That is enough for now and should keep you busy for awhile! good luck, Tom
Click to expand...
 
Mercruiser 470 runs for a while and cuts out......... then runs fine for a while.
Click to expand...
Just do your fishing in between run times! :)

I think that you've been given some good suggestions here.
Use the P of E...... (process of elimination), and one item, and one item only at a time until you find the problem.


.
 
You know me well because is that exactly what we do. Caught a nice 30lb stripper in the James river waiting on engine to cool off.
 
Pump tech,
You stated you replaced the fuel pump. Did you re-use or keep the thick gasket between the pump and block? If you did not your fuel pressure will likely be too high. That gasket is at least 1/8 to 3/16th thick. Mercrusier manual spec. for orginal Carter pump says 3-6 psi, the diagnostic section says 5.25-6.5 psi at idle to 1,000 rpm. This is an easy check as you can see it and measure it quickly at the pump base.
When a mechanic replaced mine he used a thin gasket and it would flood and die at low rpm and not run so great at cruise, a bit opposite of your problem but something to check as it will cause problems.
 
I have exactly same problem with my 3litre. I have changed everything except carb (distributor, coil, fuel pump, condensor, inline resistor). Mine will run perfectly for about an hour, then gets worse until engine quits. OK again after about 20 mins. Not fuel tank as it did the same thing with a temporary portable tank hooked directly to fuel pump. L
Let's keep in touch if we find solution.
 
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