rough idle/stall

ok so Ive got nine hours on the boat since the engine went back in and it has been running great. Idleing perfectly, running strong no problems at all 100%. Towards the end of the day on saturday it very suddenly start idleing rough, it was loping barely hitting 500 rpm on the tach and when you put it in gear it would die. If you kept it revved over 1000 no problems and when you put it under power it stumble til you got it over 1000 and then just take off like a rocket, full power up through the band.

You could also tell it was really rich at an idle, mind you this was a sudden change as well, running perfect and then bam next time I slowed down I had this issue and it stayed that way all the way back to the trailer. Any ideas? Could the float be stuck open in the carb and dumping too much fuel in at an idle?

I am having the same exact problem. I have an 87 with the 351 in it, with the electronic ignition conversion. Engine has 112 hours and I can not get it to idle right to save my life. I've changed plugs, wires, fuel filter, cleaned the anti siphon valve, and even tried to adjust the carb. No Luck. I want to see what others say on here.

Chad,
Have you checked the timing? How about the mechanical advance? Even with the EI conversion, the advance is still mechanical.

Jllogan wrote: advance wouldn't affect the idle though. if its in time the advance wouldnt affect until revs were up.

It wil if the advance is hanging up and not returning all the way to base timing.

I have not checked the timing yet. I have never messed with timing before so I am "learning" ... Some of the symptoms mine is doing are:
*Rough Idle when under a load (in gear wants to shut off)
*When you give it gas under load then it gives a chug chug and them jumps up and runs.
*If I adjust the idle up to almost 1k it does ok but I don't like it idleing that high.
*When I do get idle adjusted right, after I run it around the lake and back it doesn't stay that way (its either too hight again or too low when I come down off plane)
*When idle is around 650 - 700ish then it wants to run rough

Chad,
When the EI conversion was done, was the timing checked/adjusted?

watrski wrote: I have a spare carb if you need.....

A good working one? Why do you have a spare?

Chad I suspect you have a vacuum leak and/or the idle fuel orifices are boogared up in the metering block.

Justin,

One other remote possibility, but a quick easy check.

I'm thinking one of your electric choke wires could have popped off, and the choke might have closed up on you.

It doesn't make total sense, where you're running fine up top, but chokes are mechanically forced open at higher throttle.

is your PCV hose port connected to anything? My local guru had me block it off.

mine is a 25 year old carb didnt have those fancy pcv ports back then. Its goes into the spacer. Ill fiddle with it tonight if I have time and see if I cant spot something otherwise, I may just install your "spare" and then Ill know my issue.

Plus I need to come over and get some more evening chippewa skiing in, even on your "rough" water. I need a few days for my toe to heal though.

seafoam wont help. Are you sure your linkage is tight/adjusted properly? I also doubt its a fuel pump, if anything I would think it would idle ok but then fall on its face when it needed alot of fuel/wide open if the pump was weak.

watrski wrote: is your PCV hose port connected to anything? My local guru had me block it off.

I'm pretty sure that diagram is an automotive carb.   If that is what you have, it would be best to get a marine one.

I had the orig fuel pump slowly shred bits of red paper gasket into the carb fo.r two seasons before it failed
Caused lots of rebuilds inbetween.
If you see red sediment in the bowl, jets or needles, that's what it is

Wow, was this timely. I have a 1990 351W PCM with a Holley carb. Last fall it started doing the exact same thing, very rough idle, running flooded would only idle good at 1200 rpm anything below caused loping and flooding, I could not adjust it out. So this spring I attempted to reset the timing with a cheap eBay light and could not get it right. Choke is functioning properly, I replaced plugs, wires, coil, points, condenser, and I also adjusted the air metering screws per instructions. With a new timing light I was able to make the point setup work very good. But when I had the timing issues I ordered a Pertronix Ignitor conversion so I went ahead and installed it. At first it started harder than the points and slightly flooded, otherwise it ran well, but now after maybe 20 hours I can not keep the idle set. At first start unless the idle is set high it will lope and stall, after a while it will be running at 1200 rpm, I then set the idle back and it runs fine for a while, then all of a sudden it will start idling very slow and loping, again I set it up. I am getting ready to reinstall the points. The Pertronix starts harder than the points for sure and I don't like that but I am not sure that the points will solve this. I am starting to think something is stuck or plugged in the Carburetor?