5.0 surge and stop, fuel or spark or what?

[MTBoat]

1993 5.0 mercruiser 2 barrel,,,,, surge n stop....fuel or spark?
If I give the symptoms perhaps someone has an idea of where to look first. This 5.0 just had the carb supposedly rebuilt by shop...has all new coil/plugs,cap,rotor,etc, and wires. All guages and other key indicators are running where they should....

If fires up nicely and runs smoothly. Then after about 5 miles all of a sudden it will start to stumble and if you give it more throttle it tends to bog....back off and stop and it'll tend to settle at idle and you can run it a while at idle and sometime it will simply surge up a thousand rpms and fall back,,,,usually after 10 minutes of not being able to add throttle without bogging,,,it will die. Then won't restart, but, if you wait a little then it will start and run a few seconds and die. Or if you throttle all the way forward, it will want to start, but die.

I checked the water fuel separator and dumped it out....some fine stuff in bottom of gas,,,did not seem to have water in it.....

Then after a half day it started up again and we ran about 2 miles this time and it started to stumble and the only solution is back off the throttle to keep it running,,,,another 10 minutes at idle in gear moving and eventually die again,,,,same scenario.

Is there a fine screen in the 2 barrel carb that can plug,,,or does it sound like something is shorting out and not getting spark? Does it sound like a water in fuel thing? I'm frustrated to find the culprit...
Any ideas and key checks before I take to the shop with guys that don't seem all that competent to me,,,unfortunately not a lot of choices in my area,,,,no mercruiser people believe it or not.

Thanks

 

[Rick Stephens]

Not getting enough fuel. Sounds like a pump or a poor rebuild. Certainly could be the inlet filter on the carb, but a rebuild should have gotten that.

First thing to try, pull the line off the carb and stick in a soda bottle. Crank the motor and see if you have plenty fuel flow.

Next, replace filters if you haven't already. Dump yours out again.

Last is redo the carb. Don't feel too bad. Happens more often than any other job. Takes attention to detail to do a carb right.

 

[alldodge]

When it starts to stumble look down the throat of the carb and see if its flooding (probably isn't just checking). If not flooding I would suggest putting a fuel pressure gauge connected at the carb. Need to make up a Tee fitting with connectors and fuel line, or use a adapter fitting and pressure should be between 4 to 7 psi.

Could use a Merc part 91-18078

 

[salty3rd]

If you have the old coil I would swap it back in to see if it makes a difference, I have seen a bad coil do similar things to what you have going on.

 

[MTBoat]

Today I pulled the tiny mercard 2 bbl filter/screen out of the carb. I was concerned that perhaps the mech put it in backwards....after I got it out, i researched diagrams and exploded views to make sure I put it back in correctly. Unbelievably, I found two distinctly opposite diagrams,,,one showed the spring going into the nut, then the filter to the carb....the other, and seemed more accurate one shows the spring going into the carb, then the blocked end of the filter, with the open end of the filter adjoining a small seating washer inside the large screw in nut from the fuel pump side. I fired it up and ran like a top, but,,,,haven't got it under sea conditions yet.
Does anyone think they have the definitive answer to proper positioning of these items?

The sad truth, is I cannot remember when I took it out if the guys had the little filter turned or not,,,, I did that in a GM 454 and it ran good for a while and then surged and lost power..., am hoping that might be it.

Took gas out of water seperator and let it sit in a jar...zero water, zero contaminates....so not sure if there is something floating in the tank getting periodically sucked into the gas line causing the surge.....

 

[Rick Stephens]

Run it. Proceed from there.

 

[MTBoat]

Thanks all,
I posted under title Rochester or Mercarb, and AllDodge got it right for me....my mechanics did put put the spring in the carb...and the mercarb shows it goes between filter open end and nut with fueline in......I am sure that's gonna let my carb function finally.....but....nearly everyone thought it should go the other way, which Rochester's do.....
happy camper!