1988 mercruiser 5.0 alpha one

Just picked up a 1988 mercruiser 5.0 in an old Thompson boat. Got it up and running in the driveway and everything seemed great. Took it to the lake yesterday and it fired right up and ran full tilt for about 15 min (after proper warm up). It stated to die and surge back to life a few times and then died. I did get it back up and running once and the same thing happened again. I spent the next 2 hours trying to get it running to no avail. It would start up every time but then idle for between 3 and 15 seconds and die. I tried nursing it with starting fluid but didn't change the results. When the throttle was pushed all the way forward, i could visually see fuel squirt into the carb. I also cracked the fuel line open and fuel leaked out when being cranked. Not sure where to go from here. The previous owner said that the carb was rebuilt last season but i have no real way of telling.
More details:
It did have some gas left in it from last season but i added more than double what was in it. Didn't smell bad at all.
Its carbureted (MerCarb 2-bbl)
Its a 22 foot cuddy cabin Thompson
Ran good for 15 min but topped out at 30. Felt like it could do better
best 15 min I've had in a couple years!!! I need to get this thing running good!
Really appreciate any help. Im happy to answer any questions. Thanks

Welcome

tried nursing it with starting fluid but didn't change the results.

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Check for spark

Could use a motor serial number but being a 1988 it's most likely you have TB4 ignition

"Bad gas mixed with good gas just equals more bad gas" Its amazing how much carb engines hate bad gas. I didnt believe it till I experienced it on my own years ago. Had lots of engine problems, fixed them all when I stopped messing around and pumped all of last seasons fuel out. Added fresh treated fuel and everything was golden.

JonBoat55 said:

"Bad gas mixed with good gas just equals more bad gas" Its amazing how much carb engines hate bad gas. I didnt believe it till I experienced it on my own years ago. Had lots of engine problems, fixed them all when I stopped messing around and pumped all of last seasons fuel out. Added fresh treated fuel and everything was golden.

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That’s true. I tore the carb apart to clean and rebuild and everything looked good. I didn’t see any water in the bowl or debris in the filter. I agree that it could just be the gas so I’m going to drain it and replace with new NE gas. I am also thinking that the fuel pump could be going bad. Would that make sense? It does to me but I don’t have the experience with these mechanical fuel pumps that some of you guys do

alldodge said:

Welcome

Check for spark

Could use a motor serial number but being a 1988 it's most likely you have TB4 ignition

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It does have the thunderbolt IV ignition. I just assumed that I had spark because it would fire up every time but doe shortly after. Could that still be the TB4? It does have a new button and cap on it. I did order a new coil as well because the old one looks rough inside the connector. Thanks for the help

You said starting fluid would not help to keep it running, which tells me your loosing spark

It's not the coil

Might be the ignition switch. Measure voltage at the coil to see if 12V is dropping

check the trigger assembly in the distributor

alldodge said:

You said starting fluid would not help to keep it running, which tells me your loosing spark

It's not the coil

Might be the ignition switch. Measure voltage at the coil to see if 12V is dropping

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Now you have me thinking....I did find two ignition switches under the seats that were bad. What could cause them to burn out?

Scott Danforth said:

check the trigger assembly in the distributor

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I will check that tonight and post my results. Thanks!

Ok...Thank you all for the suggestions. I have tested and corrected/replaced everything that has been suggested so far. I had it running like a top on the hose at home. Took it to the lake this weekend (94 deg) and it fired up and ran amazing for about 10 minutes. It then seemed to stumble and then die just like last time. I checked all the usual stuff and then cracked the fuel line loose to check for fuel. I had no fuel at all while cranking. I did spray some starting fluid in the carb and it barks right off but stalls within seconds. When it did start on the fluid, i got a gush of fuel but then back to nothing. I'm assuming it is vapor locking in the fuel pump or filter somehow.? I sprayed some lake water on the fuel pump and fuel lines to cool them off but nothing changed. Its a brand nw fuel pump. Anyone have any ideas? Should i put an electronic fuel pump on it? Oh, and the fuel tank vent is not clogged. I did try to run it without the fuel cap on as well. same results...

My '88 MerCruiser 4.3LX had two fuel filters. One was a small paper cylinder that fit inside a housing that screwed into the carb. The other was a porous stone filter that fit on the top of the fuel pump, under a cover.

You might check them. Maybe the one on the carb is partially clogged.

the filter in the carb is not needed if there is a canister filter.

ok fuel issue
There is a fuel restriction, the antisiphon valve, collapsed fuel line or tank pickup tube

The antisiphon valve is on top the tank. Remove the fuel line then the valve. There is a spring and check ball inside

alldodge said:

ok fuel issue
There is a fuel restriction, the antisiphon valve, collapsed fuel line or tank pickup tube

The antisiphon valve is on top the tank. Remove the fuel line then the valve. There is a spring and check ball inside
View attachment 409020

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Ill take a look at this asap Thank you!

Scott Danforth said:

the filter in the carb is not needed if there is a canister filter.

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I did check the small filter in the carb inlet. Perfectly clean...

Chris1956 said:

My '88 MerCruiser 4.3LX had two fuel filters. One was a small paper cylinder that fit inside a housing that screwed into the carb. The other was a porous stone filter that fit on the top of the fuel pump, under a cover.

You might check them. Maybe the one on the carb is partially clogged.

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I checked the one on the carb inlet. No debris at all.

Remove fuel fill cap next time it stumbles

flashback said:

Remove fuel fill cap next time it stumbles

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I did remove the fill cap but no change. Thanks