3.0L Mercruiser rough idle when starting cold

[jaa241]

Hey guys,
Thought I'd throw this out there. I am having some cold start issues. It is a 84' or 85' mercruiser 140 serial number 6920111 and mercarb 1389-8490 which is a rochester I believe. The issue is with a cold start on the trailer in the water. It fires right up, with a throttle prime and throttled about 1/4 way. The rpms come up to about 1200 -1500 and I let it idle there for 2 minutes or so. I then try to go to neutral and it dies. It takes about 10 to 15 minutes to get the engine warmed up enough to get it into neutal and to put it in gear to get off trailer. I do not believe it should take that much time but i dunno. Once the engine is warm it runs great and I can stop and shut engine off and such and runs great. Its just the cold start.

I have set the idle around 700 because it seems to not like going down much lower. The timing is set at 6 BT and such, like i said it runs great when warm.
It has new fuel pump, new spark plugs, rebuild carb, new electric choke, electronic ignition.

Any suggestions on why it would take so long for the engine to warm up or is something else not set up right?

Thanks for your input and help!

 

[jaa241]

Re: 3.0L Mercruiser rough idle when starting cold

Any help would be much appreciated. Thanks

 

[Levinz11]

Re: 3.0L Mercruiser rough idle when starting cold

Sounds like your choke isn't working right. Pop off the flame arrestor and take a peek.

 

[jaa241]

Re: 3.0L Mercruiser rough idle when starting cold

Did that, when cold start and give full throttle to prime, choke closed except for a slight gap

 

[Another retired guy]

Re: 3.0L Mercruiser rough idle when starting cold

Hi. First guess would be she's running too lean when cold - but you say it starts easily so that doesn't quite explain it
Have you checked the position of the choke plate - should be almost closed when cold
What does the temp gauge show after running 2 or three minutes? It should be starting to move up. If it isn't, your problem may be a bad thermostat.

Edit - choke issue covered while I was typing

 

[jaa241]

Re: 3.0L Mercruiser rough idle when starting cold

I believe the thermostats is working because when it runs it increase to abt 130 to 140, but after 2 minutes it does not move. Would that cause a rough cold start? But if it seems to increase when running, I would assume that it working correct? Thanks for the help

 

[Don S]

Re: 3.0L Mercruiser rough idle when starting cold

Re-adjust the idle mixture. At 1200 to 1500 rpm you are above the mixture setting abilities of the screw.

 

[jaa241]

Re: 3.0L Mercruiser rough idle when starting cold

Re-adjust the idle mixture. At 1200 to 1500 rpm you are above the mixture setting abilities of the screw.

I adjusted the idle mixture screw when I was around 650-700 rpm. Is there something else I need to do or am I doing something incorrectly? Or could it be the age of the engine? Thanks for all your help!

 

[Another retired guy]

Re: 3.0L Mercruiser rough idle when starting cold

I believe the thermostats is working because when it runs it increase to abt 130 to 140, but after 2 minutes it does not move. Would that cause a rough cold start? But if it seems to increase when running, I would assume that it working correct? Thanks for the help

130/140 is low, and you should be seeing a temp increase after a few minutes. If you don't know when it was last changed, a new one might help to warm her up more quickly.

 

[jaa241]

Re: 3.0L Mercruiser rough idle when starting cold

130/140 is low, and you should be seeing a temp increase after a few minutes. If you don't know when it was last changed, a new one might help to warm her up more quickly.

What thermostat should I get if I would replace it. What should the normal run temp be? Thanks again

 

[Another retired guy]

Re: 3.0L Mercruiser rough idle when starting cold

What thermostat should I get if I would replace it. What should the normal run temp be? Thanks again

I think 140 is recommended, so running temp would be 140+, but I'm thinking that yours may be stuck in the normal operating position, so the engine is taking a long time to reach that temperature. It's worth a try - they don't cost much

 

[jaa241]

Re: 3.0L Mercruiser rough idle when starting cold

I think 140 is recommended, so running temp would be 140+, but I'm thinking that yours may be stuck in the normal operating position, so the engine is taking a long time to reach that temperature. It's worth a try - they don't cost much[/QUOT

thanks I think i will give that a try. I just feel embarrassed at the dock waiting for my boat to warm up cause I cant get it off the trail for like 10 minutes. I could always use a rope but that probably just as embarrising. If anyone has any other suggestions please let me know. I also check for leak around carb and intake manifold and there are no leaks. I am thinking if it is not the thermostat that it is just the engine. The engine must have low hrs on it because when I replaced the fuel pump I put a light in to check out the inside and it looked brand new. Beats me though. All help is appreciated. Thanks again.

 

[Levinz11]

Re: 3.0L Mercruiser rough idle when starting cold

I think 140 is recommended, so running temp would be 140+, but I'm thinking that yours may be stuck in the normal operating position, so the engine is taking a long time to reach that temperature. It's worth a try - they don't cost much[/QUOT

thanks I think i will give that a try. I just feel embarrassed at the dock waiting for my boat to warm up cause I cant get it off the trail for like 10 minutes. I could always use a rope but that probably just as embarrising. If anyone has any other suggestions please let me know. I also check for leak around carb and intake manifold and there are no leaks. I am thinking if it is not the thermostat that it is just the engine. The engine must have low hrs on it because when I replaced the fuel pump I put a light in to check out the inside and it looked brand new. Beats me though. All help is appreciated. Thanks again.

I always warm up my boat at home on the muffs before dropping in the lake. Just an idea.

 

[bonzoscott]

Re: 3.0L Mercruiser rough idle when starting cold

Hey guys,
Thought I'd throw this out there. I am having some cold start issues. It is a 84' or 85' mercruiser 140 serial number 6920111 and mercarb 1389-8490 which is a rochester I believe. The issue is with a cold start on the trailer in the water. It fires right up, with a throttle prime and throttled about 1/4 way. The rpms come up to about 1200 -1500 and I let it idle there for 2 minutes or so. I then try to go to neutral and it dies. It takes about 10 to 15 minutes to get the engine warmed up enough to get it into neutal and to put it in gear to get off trailer. I do not believe it should take that much time but i dunno. Once the engine is warm it runs great and I can stop and shut engine off and such and runs great. Its just the cold start.

I have set the idle around 700 because it seems to not like going down much lower. The timing is set at 6 BT and such, like i said it runs great when warm.
It has new fuel pump, new spark plugs, rebuild carb, new electric choke, electronic ignition.

Any suggestions on why it would take so long for the engine to warm up or is something else not set up right?

Thanks for your input and help!

Are you starting it in gear?

 

[jaa241]

Re: 3.0L Mercruiser rough idle when starting cold

Hmm thanks. Never thought of really starting it up at the house and letting it warm up. Do a lot of people do that with carb boats?

No it is not gear when I start. I always press in the throttle button, prime it once, and then back down to 1/4. It always fires up on turn of key. It just seems odd to me that I have to leave it at that 1/4 throttle which is around 1300 rpms or so for like 10 minutes. If I bring the throttle back down into neutral (idle) before its warm it stall out.

I am in a rush because of a lot of people, I usually try to put it into netural and then reverse right away, in which I will get a few turns of the prop to slide the boat off, but i then need to fire her back up.

Like I said though once warm-up, no issues. Its just the cold start

 

[bonzoscott]

Re: 3.0L Mercruiser rough idle when starting cold

If you hold 1/4 throttle, 1200 - 1500 rpm for about 1 maybe 1.5 minutes, then shift into reverse and go back to 1/4 throttle that should be good enough to get you off the trailer. At least that's my experience. The only other thing is maybe your shift assist in malfunctioning. I don't quite understand "when I shift to neutral it dies".

 

[Levinz11]

Re: 3.0L Mercruiser rough idle when starting cold

Hmm thanks. Never thought of really starting it up at the house and letting it warm up. Do a lot of people do that with carb boats?

No it is not gear when I start. I always press in the throttle button, prime it once, and then back down to 1/4. It always fires up on turn of key. It just seems odd to me that I have to leave it at that 1/4 throttle which is around 1300 rpms or so for like 10 minutes. If I bring the throttle back down into neutral (idle) before its warm it stall out.

I am in a rush because of a lot of people, I usually try to put it into netural and then reverse right away, in which I will get a few turns of the prop to slide the boat off, but i then need to fire her back up.

Like I said though once warm-up, no issues. Its just the cold start

A lot of people do this with any type of boat so they aren't "that guy" at a busy ramp with a cranky boat.

 

[jaa241]

Re: 3.0L Mercruiser rough idle when starting cold

If you hold 1/4 throttle, 1200 - 1500 rpm for about 1 maybe 1.5 minutes, then shift into reverse and go back to 1/4 throttle that should be good enough to get you off the trailer. At least that's my experience. The only other thing is maybe your shift assist in malfunctioning. I don't quite understand "when I shift to neutral it dies".

When I shift to neutral, I mean, I have it 1/4 throttled to warm up and then I bring the control arm back to neutral as before I would put it in reverse. But at that time of going from 1/4 throttle to neutral for idle, the toms drop and the engine just does or stalls however you would like to call it.

Shift interrupt switch works, have check that out and tested it.

Maybe starving it of fuel, but everything on fuel side is new and it run great once warm. Puzzling to me.