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Runs rough at idle

J

Jakwi

Member
Hi there, I'm hoping someone can point me in the right direction.

I have a mercruiser 357 4A repower, with less than 100 hours on it. Carbureted.

Generally it is pretty reliable if I run it every week or every other week.

However if it sits 3 weeks without being started it has a really hard time starting and then runs pretty rough until fully warmed up. This morning it didnt even want to idle until it was fully warmed up.

Once its warmed up fully it seems fine. It might have the mildest miss at idle , but it might be my imagination.

If I start it every week it doesnt have this issue at all.

Anyway i was just wondering if this is normal, or if not any idea what i should look at? I'm thinking plugs, but like i say it's pretty new.

I'm in Florida so I dont winterize because I use it year round, but this time of year we do slow down a bit.

Thanks for the help
 
Does the carb have an electric choke? I would suspect that first. Sitting for a couple weeks probably doesn't have anything to do with hard starting.
 
It's been my experience that, with ethanol gas, leaving the boat for a week or two causes most of it to evaporate. What's left is some kind of sludge that doesn't burn too well. In my case, the solution was to add an electric fuel pump before the mechanical pump. A momentary switch allows me to refill the carbs with fresh gas, which not only dilutes the sludge, it allows easier starting which is good for the starter gear as well).

I doubt if it's a choke issue, but you can check that by pulling the flame arrestor off BEFORE starting the motor, and then seeing if the choke opens as the motor warms up.

Jeff
 
Ditto Jeff's comment re; evaporation.
The electric fuel pump and the use of a momentary helm switch cures this issue.
Using the momentary helm switch to pre-fill the carburetor's float bowl extends starter motor and starting battery longevity.

In order to be USCG approved, the electric fuel pump circuit must be interrupted by a N/O low oil pressure switch.
I prefer to install a relay for the actual fuel pump power.
The relay would be triggered by the N/O low oil pressure switch (once oil pressure is achieved).

The "start-bi-pass" circuit is not a USCG requirement, but is necessary if you do not incorporate a momentary helm switch.


.
 
The electric fuel pump which is on the 357 is activated during cranking.

If you are experiencing extended cranking to get it started..... I would change the plugs cap and rotor. While changing the plugs I would do a compression test. If you are hanging a valve you will have a problem starting a carb engine.
 
Docksidemarineservices said:
The electric fuel pump which is on the 357 is activated during cranking.
Click to expand...

Jakwi, the fuel pump would be powered via the "start-bi-pass" circuit.
Jeff and I are talking about a momentary helm switch (in addition to, or in lieu of the start-bi-pass circuit) that would prevent the need for starter motor activation.



.
 
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Hey guys, thanks for the ideas.

I use exclusively Non ethanol gas, so that shouldn't be an issue.

I will definitely check the choke, I never even thought to look at it, but it seems like an easy thing to verify.

The engine I have is a Mercury repower. Mercruiser 357 Alpha 4v. https://www.mercurymarine.com/en/us...emanufacturing/plus-series-alpha-sterndrives/

I hear you on the posibility of a hanging valve/low compression, I tend to think that isn't the issue because the motor is so new, but I know the PO was using Walmart brand 10w30 rather than the 25w40 it calls for so who knows. I'll have to hunt up a compression checker, I need to pull the plugs anyway and take a look.

It is relatively new, less than 100 hours. It does have a mechanical fuel pump, so the electrical pump is an interesting idea. I'm an avionics guy by trade so wiring one up with a relay to the oil pressure switch as a control, and a bypass switch for startup would be relatively easy.

Normally, if started regularly it doesn't require extended cranking to get it started. If I start it every week it will take no more that 15 seconds to catch and may idle a little rough for less than a minute, but generally settles right down and idles easily.

For some reason though if I wait three weeks it is much much harder to get it happy. My procedure is a couple of squirts from the throttle control, then set the throttle to mid way and crank the engine. This last time it took a good minute of cranking and several more squirts from the throttle to get it going and it didn't want to stay running at idle position. After a couple of restarts I found I had to keep the throttle up, 1500-2000 rpms to keep it going. Once it reached temp, 175, it would idle, but it was a little rough, I continued to let it run for another 5-10 min and it finally settled down. At that point it was normal, normal rev, normal idle. I shut it down, let it sit for a few min and started it up again, no issue started right up and idled easily.

It's been a while since I had a carburetor engine, and that one was manual choke, so maybe this is somewhat normal, but it just seemed strange. It's not the end of the world of course, but if it indicates a bigger issue I'd rather get it resolved now rather than have it rear up when I'm out on the water.

One more question about the electrical fuel pump idea. Do you run it in series with the mechanical pump? or do you remove the mechanical pump and replace it with an electric?

thanks all
 
...........................
Jakwi said:
Hey guys, thanks for the ideas.

I use exclusively Non ethanol gas, so that shouldn't be an issue.
Non Ethanol gasoline can still leak and/or evaporate from the carburetor's fuel bowl.

I will definitely check the choke, I never even thought to look at it, but it seems like an easy thing to verify.

The engine I have is a Mercury repower. Mercruiser 357 Alpha 4v. https://www.mercurymarine.com/en/us...emanufacturing/plus-series-alpha-sterndrives/

I'll have to hunt up a compression checker, I need to pull the plugs anyway and take a look.
Keep in mind that a hose and threaded fitting equipped compression gauge will be giving you an accumulative reading.
This means that in order to get an accurate cylinder pressure reading, you will be cycling each cylinder through at least 3 compression cycles.... or about 7 crankshaft revolutions.

It is relatively new, less than 100 hours. It does have a mechanical fuel pump, so the electrical pump is an interesting idea. I'm an avionics guy by trade so wiring one up with a relay to the oil pressure switch as a control,
This would be a N/O Low Oil Pressure switch.

and a bypass switch for startup would be relatively easy.
Two methods:
a.... the starter motor's S circuit will trigger the relay during starter motor activation ONLY.
b.... a momentary helm switch in lieu of the starter motor's S circuit that will trigger the relay. Key word..... "Momentary"

With b, you do not need to crank the engine in order to prime the carburetor fuel bowl.

Normally, if started regularly it doesn't require extended cranking to get it started. If I start it every week it will take no more that 15 seconds to catch and may idle a little rough for less than a minute, but generally settles right down and idles easily.
A well tuned and healthy engine should fire up in 5 seconds or less of cranking.
Any more than that, you will be over-burdening your starter motor and start battery.

One more question about the electrical fuel pump idea. Do you run it in series with the mechanical pump? or do you remove the mechanical pump and replace it with an electric?
You can do either. I would remove the mechanical pump.

thanks all
Click to expand...
 
Jakwi said:
Hey guys, thanks for the ideas.

I use exclusively Non ethanol gas, so that shouldn't be an issue.

I will definitely check the choke, I never even thought to look at it, but it seems like an easy thing to verify.

The engine I have is a Mercury repower. Mercruiser 357 Alpha 4v. https://www.mercurymarine.com/en/us...emanufacturing/plus-series-alpha-sterndrives/

I hear you on the posibility of a hanging valve/low compression, I tend to think that isn't the issue because the motor is so new, but I know the PO was using Walmart brand 10w30 rather than the 25w40 it calls for so who knows. I'll have to hunt up a compression checker, I need to pull the plugs anyway and take a look.

It is relatively new, less than 100 hours. It does have a mechanical fuel pump, so the electrical pump is an interesting idea. I'm an avionics guy by trade so wiring one up with a relay to the oil pressure switch as a control, and a bypass switch for startup would be relatively easy.

Normally, if started regularly it doesn't require extended cranking to get it started. If I start it every week it will take no more that 15 seconds to catch and may idle a little rough for less than a minute, but generally settles right down and idles easily.

For some reason though if I wait three weeks it is much much harder to get it happy. My procedure is a couple of squirts from the throttle control, then set the throttle to mid way and crank the engine. This last time it took a good minute of cranking and several more squirts from the throttle to get it going and it didn't want to stay running at idle position. After a couple of restarts I found I had to keep the throttle up, 1500-2000 rpms to keep it going. Once it reached temp, 175, it would idle, but it was a little rough, I continued to let it run for another 5-10 min and it finally settled down. At that point it was normal, normal rev, normal idle. I shut it down, let it sit for a few min and started it up again, no issue started right up and idled easily.

It's been a while since I had a carburetor engine, and that one was manual choke, so maybe this is somewhat normal, but it just seemed strange. It's not the end of the world of course, but if it indicates a bigger issue I'd rather get it resolved now rather than have it rear up when I'm out on the water.

One more question about the electrical fuel pump idea. Do you run it in series with the mechanical pump? or do you remove the mechanical pump and replace it with an electric?

thanks all
Click to expand...

If this is a Mercruiser 357 complete drop in, I would double check to see if it has a mechanical pump. Especially if it is the same one in your link. I would also not modify anything with the fuel system.....Find the problem. It will probably not be fuel delivery to the carb.
 
Ricardo, thanks for the clarifications. My preference would be for the momentary switch to prime the Carb, as for the 5 seconds I need to go back and actually time it. My comment about 15 seconds is really just a guess based on my memory, it could very well be within 5 seconds when regularly started.

Chris, It definitely that exact motor, and has a mechanical pump, with the transparent line to show if the pump has failed.

My inclination is to:

First Check the choke operation, it's easy and could definitely impact cold starting. Although it seems like it would affect any cold start, not just one after three weeks.

Second try starter fluid or another "alternate fuel source" to see if is simply a situation of evaporated fuel from the bowl, and the delay is just proper priming of the carb.
If it's just evaporated fuel, then really it isn't that big of a deal. I always start the boat on muffs before taking it to the ramp so that when I get to the ramp it starts easy. Certainly an electric pump with a priming switch would cure this, but I debate whether it is worth the effort.

Third check the compression/plugs. This is really why I started this thread, If there is a deeper issue I want to resolve it. I guess I could understand a compression issue occurring after sitting for a while if the rings were worn, or if a valve is sticking, but it seems like I would get some smoke on startup if that were the case, and I don't see any. As an aside when I first bought the boat it had an issue where it would completely die when hot. It took me a bit, but I figured out that the coil was shot, once I replaced the coil the problem went away. In that trouble shooting process though I pulled the distributor cap and inspected it and the rotor, Everything looked basically new. That was last July. I've put no more than 10 hours on it since then.

The difficulty is that I only really get one shot at troubleshooting it after letting it sit for three weeks.
 
......................
Jakwi said:
First Check the choke operation, it's easy and could definitely impact cold starting. Although it seems like it would affect any cold start, not just one after three weeks.
Correct!
Also, these should actually be called an "electric Un-Choke" in that they default to a choke chimney "closed" position. Then rely on the helix heating element to open the choke plate/chimney.

Second try starter fluid or another "alternate fuel source" to see if is simply a situation of evaporated fuel from the bowl, and the delay is just proper priming of the carb.
If it's just evaporated fuel, then really it isn't that big of a deal. I always start the boat on muffs before taking it to the ramp so that when I get to the ramp it starts easy. Certainly an electric pump with a priming switch would cure this, but I debate whether it is worth the effort.

The difficulty is that I only really get one shot at troubleshooting it after letting it sit for three weeks.

Using a safe means of priming the float bowl (via the vent tube),

chemical dispensing bottle .jpg
...... add some gasoline to the fuel bowl.
Now see if it responds as it should.
Click to expand...
 
This is a great idea, I'll have to figure out where the vent is, but it will either prove or not that it is a fuel issue.

Thanks
 
That engine being less than 100 hours should catch fire in half a spin. Cranking for 15 seconds is a nope.
I personally dislike starting fluid except as a testing method to rule out spark as a problem. The squeeze bottle with some gas in it is far more benign and works for priming just as well as "ether"
 
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o2batsea said:
That engine being less than 100 hours should catch fire in half a spin. Cranking for 15 seconds is a nope.
I personally dislike starting fluid except as a testing method to rule out spark as a problem. The squeeze bottle with some gas in it is far more benign and works for priming just as well as "ether"
Click to expand...

And it definitely does that when it has been started within a week or so, it's after two weeks or definitely by 3 weeks of sitting that it has this issue.

I agree with you on the squeeze bottle, I'm going to get one from amazon and give it a try.
 
FYI...... the squeeze bottle would be used to fill the carburetor’s fuel bowl......... not for priming into the two primary throttle bores!
 
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