1982 Merc 305 - Stutters after warming up

[jordache16]

I have an 82' Century Sport 2000 with a Mercruiser 305 that is running rough under load after warming up. Engine runs great for the first 30+ mins at both low speeds and high speeds. However, after it gets warmed up, the engine starts to sputter and loses power. It usually won't die after it sputters like this rather it will return to idle and run just fine at idle. Then if I go to take off again, it will sputter and not want to go. I have found that the issue goes away and/or does not crop up at all if I take the rear engine covers off and let it run like this in the open air. This makes me think the engine is getting a little too warm, however the temp gauge never goes above 165 and sits right at 160 most of the time. The exhaust manifolds do seem fairly warm/hot if I touch them after it gets warmed up. Is this normal, or should the manifolds be cooler? Also, is there a way to test if the water is circulating through the manifolds and risers appropriately?

As a quick fix, I already tried changing out the ignition coil thinking it may be going bad / not delivering enough spark when warmed up, however that did not fix the issue (ignition coil being a 1.5 ohm Pertronix Flame Thrower). I haven't ruled out spark/electric side of things. I am wondering if the alternator should be replaced as it is the original alternator and only charges around 13.1V according to the cluster gauge.

Any other ideas of what it might be and/or how to diagnose the issue? I think the key is that it runs great / no issues with the engine covers off. With engine covers on, it will run fine at idle / putting speeds but when you give it throttle and put it under load it will start to sputter and want to die after it is good and warmed up.

 

[Bondo]

Any other ideas of what it might be and/or how to diagnose the issue?

Ayuh,....... Pull the fuel filter, dump it's contents into a clear vessel, 'n look for anything but clean fresh gasoline,.....

Go out, run it til it stutters, then loosen the fuel cap, 'n listen for a Woosh,...
If so, the tank vent is plugged,....

 

[jordache16]

Thanks for the tips. I dumped the fuel filter and it had maybe a dozen very small black chunks in it so that is a little suspect. It has been a while since it was replaced so I am going to go ahead and replace the fuel filter. I also checked the tank vent and it is completely clear. I am currently inspecting the cooling system and am going to test the temp sensor and gauge as I still think it is getting too hot. Should I be able to hold my hand on the exhaust manifolds once it is fully warmed up? I vaguely remember reading about that several years ago when we rebuilt the engine that you should be able to hold your hand on the exhaust manifolds and if they are too hot to hold your hand on then there is a circulation issue.

I already have a new recirculating water pump on the way that I am going to put on next weekend just to rule that out. My understanding is the recirculating water pumps very rarely go bad so I am not expecting that to fix it (worse case scenario is preventative maintenance). While I have the rear seats out I am going to go ahead and replace the alternator.

 

[1985 Century Mustang]

When was the last time your carburetor was changed or rebuilt? It seems pretty old it's an 82 correct.

 

[jordache16]

Correct, it is an '82. The carb was rebuilt in 2012 when the engine was rebuilt. The carb is definitely on the suspect list. It seems like the engine isn't getting enough fuel or air when I go to take off once it is warmed up.

Taking off the engine covers is either allowing more air to get in or is cooling things down enough that the issue doesn't occur.

 

[1985 Century Mustang]

Correct, it is an '82. The carb was rebuilt in 2012 when the engine was rebuilt. The carb is definitely on the suspect list. It seems like the engine isn't getting enough fuel or air when I go to take off once it is warmed up.

Taking off the engine covers is either allowing more air to get in or is cooling things down enough that the issue doesn't occur.

I'd take that old puppy off and rebuild it. If your carb is a Rochester or similiar, the kit is only $12. If you slowly watch this video at first and numerous times, you can rebuild it yourself like I did just a few months back. I never rebuilt a boat carb and had great success with my 1985 Rochester 2 bbl carb... You can do it. It's really not that hard. And you'll save a few hundred bucks. The trick is to watch it several times before you attempt it. Check these videos out below. This guy Mike is an awesome YouTuber.
Disassembly and Assembly. Part 1 and Part 2.

Part 2

 

[1985 Century Mustang]

This was my carb, some before and after pics. When I was done she purred like a kitty. I used a lot of carb cleaner and small pry/cleaning tools etc to remove the crap in this carb. I paid $11 on eBay for my rebuild kit.

Here https://www.ebay.com/itm/3733612555...o1aPjTwTrq&var=&widget_ver=artemis&media=COPY

 

[1985 Century Mustang]

Correct, it is an '82. The carb was rebuilt in 2012 when the engine was rebuilt. The carb is definitely on the suspect list. It seems like the engine isn't getting enough fuel or air when I go to take off once it is warmed up.

Taking off the engine covers is either allowing more air to get in or is cooling things down enough that the issue doesn't occur.

Interesting