Merc 305 Running rough when warm

[bronko69er]

Greetings everyone, first time poster but not new to wrenching or tech forums. I've searched quite a bit but have still not convinced myself that I know the source of the problem. Boat in question is a 1980 Formula Thunderbird and has the 305 merc with the alpha one outdrive. The problem started to come on last season and seems to have gotten progressively worse since then. I noticed it first after the engine warms up (symptoms don't exist when cold) and after idling for a bit (like trying to teach a new skier or coming in and out of no wake zones). After idling for a minute or 2 the motor will die. I can get it to fire back up and rev it in neutral without much trouble, but it continues to struggle to stay lit at idle. It will also die if I try to put it in gear. The only way I can get going is to rev it, then put it into gear and get off idle quickly before it dies. Once it's off idle, it will stay running but can't get above 1500rpm or so no matter how much or little throttle I give it. It runs rough like a few cylinders are dead. During this rough running, I can sometimes coax it back to health by very slowly giving it throttle, If I give it too much throttle too fast it will start sputtering again. Sometimes this technique doesn't work at all, I'll keep modulating the throttle for several minutes when all of the sudden its like a switch is flipped and its back to normal.

In my research, I came across a lot of info on vapor locking, so I experimented with both leaving the blower running constantly, and leaving the top hatch open to let all the heat escape. Doing these things didn't seem to change the bad behavior.

-Maintenance to note: Plugs have only one season on them, wires, cap and rotor have 2-3 seasons, fuel filter is brand new (only 1 tank run through), winter storage gas (stabilizer added) is virtually burn through now, I ran it down to 1/8 and refilled with fresh, problem still exists.

-Things i've eliminated: Ignition interrupt switch (for getting into neutral). This gave me troubles when I first bought the boat but I have since adjusted it to proper functionality. Maybe vapor lock as it seems to behave the same in 70deg weather with the hatch open as it did in 85deg weather with a hot engine compartment.

-One other symptom I just noticed last night for the first time that is likely unrelated: Heading back in for the night, I coaxed it up to a happy 3500rmp and cruised across the lake heading back to the dock (10 min transit). After idling through the no wake zone, it died a few feet from the dock. When I tried to restart, it would barely crank. I have not investigated any further, but I think this is just my alternator going bad and not related to the other symptoms (but for the sake of full disclosure, I figured I'd include it). Battery is only 2 years old.

I also ran across threads about ignition modules under the dizzy cap going bad and having hot start issues, but I'm not convinced my symptoms suggest that.

Thanks in advance for any help with the troubleshooting, I've only got a week to solve this before a boat camping trip so I'd like to get after it quick.

 

[alldodge]

Howdy

I would like to know the health of the engine, compression and leak down test.
At what rpm are you idling at in gear?
What's the dwell set at?

When I tried to restart, it would barely crank.

Are all connections clean and tight?

 

[bronko69er]

Howdy

I would like to know the health of the engine, compression and leak down test.
At what rpm are you idling at in gear?
What's the dwell set at?


Are all connections clean and tight?

Engine health is good. I head the heads off last year to replace a bad valve (bought it this way). My compression test found the bad valve. It ran great after that repair.These symptoms started occurring some time after the head work.
I'll have to check again on the idle speed, I remember fiddling with it last summer so that it wasn't too low when in gear. I feel like it was 750-800 in neutral and 550-600 in gear. I'd have to get it back in the lake to get a current reading.
I don't know the dwell. I don't recall if its point or electronic, nor do I have a meter to measure the dwell. Does the year indicate?
Connections should be good, but I'll double check.
Are you leaning towards electrical over fuel at this point? One thing I wanted to try was spraying starting fluid at the arrestor when its sputtering, to see if it smooths out.

 

[b.lee]

If the alternator is going bad, maybe the engine is trying to create the spark just off the battery power by itself? Maybe causing weak spark which would make it almost impossible to gain RPM's or stay running easily... just a thought.

 

[alldodge]

If you have an engine serial we could verify, but the 1980 range could be a points distributer. The issue I'm barely leaning toward electrical is because you mentioned it cleared up for a while and then came back. It could also be fuel related to a carb issue just trying to figure it out.

Fuel pressure should be 3 to 7 psi
Timing is 8 BTDC
Idle is 650-700 rpm
If point dwell should be 29-31

Any issue with taking the shifter out of gear and into neutral.

 

[bronko69er]

If the alternator is going bad, maybe the engine is trying to create the spark just off the battery power by itself? Maybe causing weak spark which would make it almost impossible to gain RPM's or stay running easily... just a thought.

I considered that but the problem existed long before there were any symptoms of a bad alternator. The alternator could be compounding the issue though, if it is indeed electrical.

 

[bronko69er]

If you have an engine serial we could verify, but the 1980 range could be a points distributer. The issue I'm barely leaning toward electrical is because you mentioned it cleared up for a while and then came back. It could also be fuel related to a carb issue just trying to figure it out.

Fuel pressure should be 3 to 7 psi
Timing is 8 BTDC
Idle is 650-700 rpm
If point dwell should be 29-31

Any issue with taking the shifter out of gear and into neutral.

I'm hoping to get a chance to look at it again tonight or tomorrow. When I do, I'll verify charging voltage, and pop the cap to see if its point or electronic. I'll also check connections, throw a can of seafoam in the tank, and have a bottle of starting fluid handy to see if varying the combustion gasses has any affect.
The issue of timing is a bit mysterious. After I rebuilt the head and when to set the timing, I discovered an issue with marks. Based on how it ran and a vacuum gauge, I do not believe the marks are in the right places. I timed it by vacuum and how it ran. Doing that, I was able to get it to run great.
So neutral idle should be 650-700?
In regards to the shifter question. I do occasionally miss neutral coming out of FWD, but this is an issue with the lever itself, as I can avoid it by applying a bit of lateral pressure. When I bought the boat the ignition interrupt switch wasn't adjusted properly, so I fixed that, and it was worked well ever sense.

 

[bronko69er]

More info. Just checked and it is electronic ignition, no points, connections all seem tight and corrosion free. Jump started it and confirmed that the alternator is not charging so I pulled that to go get a new one. Hoping these symptoms are all related to a slowly failing alternator.

 

[alldodge]

Should be 650-700 in forward gear in the water, not on muffs.
You should be able to get 8BTDC idling, and by not being able to do this there could be other issues. I would suggest doing a compression test.

I was wondering if you had any difficulty coming out of gear into neutral, which could be another issue. Your not having that problem so no issue.

 

[keith2k455]

If it were me, I'd be looking at the carb. If you can verify ignition timing, I'd probably take apart the carb and give it a good soak and cleaning.

 

[bronko69er]

So I replaced the alternator, and took it on the lake last night, made some idle adjustments and the problem seems to be gone. I guess it was a combination of a slowly failing alternator and idle being set to low. Fingers crossed it doesn't return. Thanks for all the help. One other thing for sanity checking. In order to achieve 600-700rpms at in gear idle, The neutral idle is 900-1000. Is that to be expected?

 

[alldodge]

The 900-1000 is a bit high, my guess would be that you might have some low compression. Could be the engine is just getting tired

 

[bronko69er]

Time to revive this. Still having problems. Things I have done this year include general maintenance tasks such as oil and filter change, outdrive oil change, replacing the water separator/fuel filter, resetting the base timing, adding a carb spacer, additional blower and ducting to reduce heat soak in the carb. After getting up to operating temperature (180F), and idling or sitting for a minute or more it struggles to get out of the hole, sputtering at mid through full throttle for anywhere from 5 to 30 seconds before it comes back to life instantaneously. After that point it runs strong and as is should, I can come off plane then re-plane with no issues, as long as I don't stop and idle for very long. During this behavior, I can definitively say that the carb is not hot. Its barely above ambient temps. The only thing left I can think of doing is rebuilding the carb (rochester/quadrajet 4bbl), but I really don't like the idea of throwing parts at problems as a means of diagnosis (even though I just did that with the carb spacer and blower).

 

[alldodge]

Keith may be right about the carb, but I'm thinking the float is sinking and causing the motor to flood a bit.

 

[bronko69er]

How would float issues manifest only when up to operating temp?

 

[alldodge]

When your running your using a lot of gas. When idling the pump is pumping and if the float sinks just a little to much, then you start to flood. Your comment

can come off plane then re-plane with no issues, as long as I don't stop and idle for very long

When a motor is cool the gas puddles a bit more, when hot there is more atomization but to much with maybe dropping extra gas and it has a hard time running until the extra gas is burned off. Motor runs great around 12 to 13 Air fuel ratio, but get it 10 and it will be a dog. Could try to adjust the float level lower then spec to see if it starts getting a tad better. Don't keep it that way because if I'm wrong at high rpm it could get to lean

 

[bronko69er]

I like that train of logic. I'll get a carb rebuild kit on order and dive into investigate.

 

[harleyman1975]

In my minds eye I see the carb bowl full of cloudy gas with a heavy or out of adjustment float or fuel leaking through the idle circuit.

 

[bronko69er]

That reminds me of occasionally seeing faint whiffs of fuel vapors coming from the flame arrestor after shutting the motor off. I think we are on to something.

 

[wahlejim]

Have you checked to make sure your choke is functioning properly? Almost sounds like it is stuck and not opening all the way.