1987 Merc 180 (“470” 3.7 litre) won’t idle

[Kolbads]

My 180 won’t idle. It seems like it’s flooding. Its starts easily if I give it quite a lot of throttle, and sounds fine above 2000 RPM. When I ease the throttle back to 1800 or less, the rpm goes up and down a couple of times, then stalls.

Recent history: The two 180’s are new to my boat, so I don’t have an operating history, but I trust the marine mechanic who was maintaining them in the boat they just came out of and said they were in good running condition. I’ve only had the boat out for three sea trials. The port engine started easily and idle well for all three trials, But had very low power on the first two trials. I replaced the pertronix ignition and it had good power and ran fine initially on trial number three but started overheating so I shut it down and returned on stbd engine.

Since then, it won’t idle, as described above. Seems like it’s flooding at low throttle. I swapped the Rochester Quadrajet carb with the Quadrajet carb off a 488 (very similar but not identical; it was working fine on the 488). That didn’t change anything. Still sounds fine above 2000 rpm, surges and stalls if reduce throttle to 1800 rpm. So I’m thinking the problem isn’t in the carb itself.

All the tests have been at dock, in neutral.

My online reading suggests the (only?) other cause or to be high fuel pressure. But I haven’t touched the fuel pump since the engine was idling fine on the first two sea trials, and at the start of the third. Nevertheless, I guess my next step is to get a fuel pressure gauge.

I don’t think the engine overheated to a critical level on the third sea trial. And I also don’t see how overheating could have lead to the stalling at idle problem.

Help, please!

 

[Kolbads]

My 180 won’t idle. It seems like it’s flooding. Its starts easily if I give it quite a lot of throttle, and sounds fine above 2000 RPM. When I ease the throttle back to 1800 or less, the rpm goes up and down a couple of times, then stalls.

Recent history: The two 180’s are new to my boat, so I don’t have an operating history, but I trust the marine mechanic who was maintaining them in the boat they just came out of and said they were in good running condition. I’ve only had the boat out for three sea trials. The port engine started easily and idle well for all three trials, But had very low power on the first two trials. I replaced the pertronix ignition and it had good power and ran fine initially on trial number three but started overheating so I shut it down and returned on stbd engine.

Since then, it won’t idle, as described above. Seems like it’s flooding at low throttle. I swapped the Rochester Quadrajet carb with the Quadrajet carb off a 488 (very similar but not identical; it was working fine on the 488). That didn’t change anything. Still sounds fine above 2000 rpm, surges and stalls if reduce throttle to 1800 rpm. So I’m thinking the problem isn’t in the carb itself.

All the tests have been at dock, in neutral.

My online reading suggests the (only?) other cause or to be high fuel pressure. But I haven’t touched the fuel pump since the engine was idling fine on the first two sea trials, and at the start of the third. Nevertheless, I guess my next step is to get a fuel pressure gauge.

I don’t think the engine overheated to a critical level on the third sea trial. And I also don’t see how overheating could have lead to the stalling at idle problem.

Help, please!

(Does anyone remember my 2023 posts where I thought I’d be sea trialling in two weeks? HAHAHAHAHA!! First sea trial was this May. And still not cruising.)

 

[Scott Danforth]

have these carbs been sitting? how is the needle/seat/float?

 

[kenny nunez]

A compression test should be done on the engine in question.

 

[Scott06]

have these carbs been sitting? how is the needle/seat/float?

X2 I took a edlebrock off my 65 GTO that ran perfectly (changed manifold and carb set up to tripower), drained the gas out the vents and put it in a box for 3 years, put it back on and it would not idle until I cleaned it

 

[kd4pbs]

The only way to know that it is flooding is to physically look down in the carb's throat when it messes up. Tell us what you see when it does this.
As a fellow Merc 3.7 owner, my sympathies go out to you.

 

[Kolbads]

Thank you all for your replies. I’m replying to your questions here, and new info in my n xt post.

have these carbs been sitting? how is the needle/seat/float?

The first carb (“the 180 carb”) started easily and idled fine for each of my three Sea trials this summer, so hasn’t been sitting since it operated well. The flooding (?) problem started during the third trial. After 15-20 minutes of good operation on both engines, the port engine was overheating so I shut it down. After letting it cool down for a while, I tried starting it again a few times, waiting 10 minutes or so between each time. Each time it wouldn’t start, or only for a few seconds. I came back to the dock on the starboard engine. Port engine problem has been the same since.

The second carb (“488 carb”) sat for a year. It was running well last summer. In August I removed it from the 488 along with the intake manifold and left them well covered until swapping it onto the 180 a week ago.

Haven’t gone inside to check needles/seats.

A compression test should be done on the engine in question.

Agreed. I haven’t done it yet. I will.

X2 I took a edlebrock off my 65 GTO that ran perfectly (changed manifold and carb set up to tripower), drained the gas out the vents and put it in a box for 3 years, put it back on and it would not idle until I cleaned it

I will likely take one of the carbs apart for cleaning next. Will be my first time, hope it isn’t tricky. A friend said he had the same problem a few years ago. It didn’t change after he installed two rebuilt carbs. He installed an Edelbrock and said it’s been fine since.

The only way to know that it is flooding is to physically look down in the carb's throat when it messes up. Tell us what you see when it does this.
As a fellow Merc 3.7 owner, my sympathies go out to you.

I hope the various issues will be cleaned up soon and I get a couple of years without new mysteries!
A friend was at the engine while I was at the helm during a recent attempt to start. He said he could see a drip when he was looking through the choke plate. I haven’t looked myself. I will next time I start it.

 

[Kolbads]

NEW NEWS

Last night I was flushing the 180 carburetor with gas as well as I could while leaving it assembled, hoping I might discharge any dirt at the needles/jets. I spotted a crack in the casing (lower right corner of photo). I don’t know where it runs internally, but I imagine that could be the problem. I’m about to reinstall it anyway and see if it runs better after the flushing.

I’ll also try cleaning the 488 carburetor while it is off again.

Thank you again for your replies. Your further thoughts are always welcome. I’ll keep you posted.

 

[Kolbads]

NEW NEWS
Oops. The photos:

 

[Scott Danforth]

You can't do anything without pulling the carb apart . It's like trying to change your underwear while keeping your pants on while wearing a snowmobile suit

The engine that overheated needs a new head gasket. Open deck aluminum block with an iron head means that as little as a 10 degree overheat will spit the head gasket

 

[Scott06]

Thank you all for your replies. I’m replying to your questions here, and new info in my n xt post.


The first carb (“the 180 carb”) started easily and idled fine for each of my three Sea trials this summer, so hasn’t been sitting since it operated well. The flooding (?) problem started during the third trial. After 15-20 minutes of good operation on both engines, the port engine was overheating so I shut it down. After letting it cool down for a while, I tried starting it again a few times, waiting 10 minutes or so between each time. Each time it wouldn’t start, or only for a few seconds. I came back to the dock on the starboard engine. Port engine problem has been the same since.

The second carb (“488 carb”) sat for a year. It was running well last summer. In August I removed it from the 488 along with the intake manifold and left them well covered until swapping it onto the 180 a week ago.

Haven’t gone inside to check needles/seats.


Agreed. I haven’t done it yet. I will.


I will likely take one of the carbs apart for cleaning next. Will be my first time, hope it isn’t tricky. A friend said he had the same problem a few years ago. It didn’t change after he installed two rebuilt carbs. He installed an Edelbrock and said it’s been fine since.


I hope the various issues will be cleaned up soon and I get a couple of years without new mysteries!
A friend was at the engine while I was at the helm during a recent attempt to start. He said he could see a drip when he was looking through the choke plate. I haven’t looked myself. I will next time I start it.

Get a harbor freight ultrasonic cleaner to soak it in. Most failed attempts at rebuilding carbs are because people don’t get it clean enough. Mikes carb parts has good quality kits

 

[Kolbads]

Thanks all for the further replies. I put a third carb on, from the starboard engine, which was running fine. No change -- the port engine still wouldn't idle below 1800 rpm. So, not a carburetor problem after all. I started thinking about weak spark possibility and tracing every wire I could imagine might play a part, measuring resistances and voltages of each. I discovered that the purple-yellow wire between coil positive and the slave solenoid was barely hanging on at the solenoid: it detached from its ring connector the little more than a touch from me. I reattached it with a new ring connector and continued my search, finding nothing else of note. When I started the engine, it started easily and came to a smooth idle around 700 rpm. Problem apparently solved. I don’t see why a poor coil-solenoid connection would cause the problem. I think it would only be active when the key is to Start.

 

[kenny nunez]

That wire is being fed by the ignition switch in the “run” mode. After the engine starts the “bypass” circuit is no longer feeding 12 volts to the coil. The voltage at the coil is now provided by the ignition switch. Since the purple/yellow wire is connected to the + or Bat side of the coil it will read voltage.

 

[Kolbads]

… It's like trying to change your underwear while keeping your pants on while wearing a snowmobile suit

lol. Now that’s an analogy that a Canadian can relate to!

 

[Kolbads]

That wire is being fed by the ignition switch in the “run” mode. After the engine starts the “bypass” circuit is no longer feeding 12 volts to the coil. The voltage at the coil is now provided by the ignition switch. Since the purple/yellow wire is connected to the + or Bat side of the coil it will read voltage.

That wire is being fed by the ignition switch in the “run” mode. After the engine starts the “bypass” circuit is no longer feeding 12 volts to the coil. The voltage at the coil is now provided by the ignition switch. Since the purple/yellow wire is connected to the + or Bat side of the coil it will read voltage.

Thanks. The Mercruiser manual (#8, p 4A-4) says that wire supplies coil only in Start position, which makes sense to me.

Which wire provides the voltage to the coil from the ignition switchwhile the engine is running?

I’d like to find a diagram that identifies all wires and voltages to and from the coil and distributor in the different modes. I’m having trouble sorting it out for myself.

 

[Scott06]

Which wire provides the voltage to the coil from the ignition switchwhile the engine is running?

Purple powers the coil when in run not in diagram linked below it is a resistance wire, everything but this resistance wire will be 12 v, the resistance wire drops it to 9 v at the coil when in run. 12v is only supplied off the purple yellow during cranking.

get the factory engine manual for your engine serial number will have the wiring diagram

ignore the pertronix chit in this thread halfway down is a wiring diagram

https://www.marineengine.com/boat-forum/threads/mercruiser-470-pertronix-wiring-question.416076/?srsltid=AfmBOop1vpTPqU3DVlxom-Y2mCxB_iOuMd-nXd8VKaQtL0NlUsEYghXH

if you are not sure how points work Uncle Tonys garage ion you tube has a couple great videos of how points ignition systems work

 

[Kolbads]

Thanks. The Mercruiser manual (#8, p 4A-4) says that wire supplies coil only in Start position, which makes sense to me.

But it makes me wonder how that bad connection could have been the cause of the idling problem, since that wire wouldn’t carry any charge on the key is in the Run position.

 

[Kolbads]

Purple powers the coil when in run not in diagram linked below it is a resistance wire, everything but this resistance wire will be 12 v, the resistance wire drops it to 9 v at the coil when in run. 12v is only supplied off the purple yellow during cranking.

get the factory engine manual for your engine serial number will have the wiring diagram

ignore the pertronix chit in this thread halfway down is a wiring diagram

https://www.marineengine.com/boat-forum/threads/mercruiser-470-pertronix-wiring-question.416076/?srsltid=AfmBOop1vpTPqU3DVlxom-Y2mCxB_iOuMd-nXd8VKaQtL0NlUsEYghXH

if you are not sure how points work Uncle Tonys garage ion you tube has a couple great videos of how points ignition systems work

Thank you. I have the factory manual and have been poring over it! As I posted a moment ago, that leaves me wondering why fixing the connection of the purple yellow at the coil solved the idling problem. Coincidence? Then I wonder what really was the problem.

 

[Kolbads]

Moving on to the overheat problem. Checked everything I could think of. Had what seemed a good coolant temperature drop across the heat exchanger, so thought the flow must be okay. Out of ideas, I pulled the riser and it was pretty restricted. Visually, I can only see the space between the sleeve and the casting at the outlet. I cleared what I could with a knife blade, but it didn’t improve much.

I thought I would cut the sleeve out altogether, as I’ve heard others have done with good result. But an experienced marine mechanic has told me not to, that the sleeve is needed. Do others here have experience with that question?

 

[Kolbads]

… It's like trying to change your underwear while keeping your pants on while wearing a snowmobile suit

lol. Now that’s an analogy that a Canadian can relate to!