Cold Starting RPM: What is normal?

[ratdude747]

This is something I noticed last season (my first season of owning a boat)... it seems I had to run the idle awful fast on cold start.

Engine is a 1976 Mercruiser 888 (302) that has been swapped to a Holley 4160 (4 barrel) marine carb.

It would only start if I gave it enough throttle to get up to 2000RPM... and wouldn't stay running at actual idle until a couple minutes after. Once warmed up, it was fine. The boat's manual shows that some amount of throttle is needed to cold start (simulate what a fast idle cam does on a car carb)... but is this excessive?

I had the electric choke tuned a bit rich to help... which didn't. I then reverted it to "neutral" adjustment (when cold, just enough tension to close the choke) which still didn't change things.

My concern is blowing up the impeller since I did fry one last spring by running it too fast on insufficient muffs, but that was 3000RPM or so. On the last outing of the season I didn't overheat, so presumably my impeller is still good, but before I do my first test firing on muffs this coming week (or even tomorrow if the rain stays away), I'd like to make sure there isn't something else I need to check... and I really don't want to blow the impeller. I may do the dish soap solution down the impeller hose trick... still...

I'll note that it didn't do that on muffs last spring. But it did do it every time I was was on a lake/river.

 

[Lou C]

Thinking back to when I had cars with carbs & auto chokes they all had fast idles of at least 1500 rpm. When I crank over my 4.3 cranks over from cold it will start but needs 1500 rpm to keep running.

 

[Pmt133]

I have a few holleys and they can be temperamental on cold starts but typically you can tune around that and correct it. It doesn't sound unreasonable to me other than length of time. Unless it's frigid out, I would think 2 pumps let it come to 2k once it fires off and back down to 1500 for 10 seconds or so should be all it needs. That usually is all I need on anything I've had. I would avoid 2k on muffs for longer than a few seconds only because of impeller concerns like you said.

 

[cyclops222]

You are not up to date on adjusting a 4 barrel carburetor. a 2 barrel is very simple. Was your 4 barrel brand new ? Never bought and tried out by someone who returned it ? Used But said to be excellent.
Idle screws adjusted to the leanest & slowest speed when fully warmed up ?
The list of causes of high cold idle is long. Linkages bent to get more power at top speed. Wrong jets inside the carb? What time of day ? what was the temperature of engine and air temperature.
You were sold a factory reject ? Happens A bad run of new parts / carbs.
Was the intake manifold brand new ? Gaskets new ?
Do you have the FULL automatic choke adjustment procedure ? did you follow every step ? Or hop around to only do what you needed to do ?

 

[cyclops222]

I did a web search on your Holley 4160 carb.

It is the ABSOLUTE worst carburetor for someone with no training about ALL THE POSSIBLE ADJUSTMENTS. Hopeless for DIY person. I would return it if possible.

 

[Lou C]

However there is a ton of info on Holleys everywhere. I tried one, it gave me fits so I went back to my original Quadrajet and it’s been far less trouble. Why? Well GM had to make them at least pretty good because they were OEM on millions of their cars!

 

[ratdude747]

A quick history on this boat engine... a lot more details are in various threads of mine, but of particular note is this one.

• The boat, as received in fall 2023, had the original intake and carb (2 barrel) replaced with an Edelbrock Performer 289 intake and a cobbled 4160 off a 1970's Ford Super Duty (no choke control, etc).
• As part of the work done to get the boat back on the water (2023-2024 offseason) I kept the intake but replaced the carb with a used marine 4160, specifically one off of (and factory tuned for) a Mercruiser 5.0 (engine had spun bearings twice, eBay seller was selling parts off of it after deciding to engine swap). I rebuilt said carb after having way worse running issues. I also redid the fuel and PCV system to fix several cobble jobs that were unsafe and/or failing. The people worked on it last were experienced auto mechanics... the gaskets/sealing were done well.
• I have some limited carb tuning experience... my daily driver is carbed. Just going off what I had been taught. I did set the idle mix when it was on muffs.
• All times were in the morning, not super hot, but not cool/cold.

 

[cyclops222]

I am old enough to be happy with a 2 barrel. All my marine, auto and power equipment with carbs will only go to 1200 rpms on freezing weather. BUT I do not set the idle mixture for the leanest setting. I set it for the best running COLD engine. All chokes are set to be fully closed at a cold start. A few seconds after starting I can open the choke to 1/2 closed. Or the automatic choke will go to a similar amount of opening.
The cam shaft timing can really cause a very rough cold high speed setting to be needed.

 

[ratdude747]

I am old enough to be happy with a 2 barrel. All my marine, auto and power equipment with carbs will only go to 1200 rpms on freezing weather. BUT I do not set the idle mixture for the leanest setting. I set it for the best running COLD engine. All chokes are set to be fully closed at a cold start. A few seconds after starting I can open the choke to 1/2 closed. Or the automatic choke will go to a similar amount of opening.
The cam shaft timing can really cause a very rough cold high speed setting to be needed.

The bottom end has never been apart to my knowledge... and it's relatively low hours. Not to say that there isn't chain stretch, but I'd be surprised if there was anything significant.

The original carb/intake is long gone (before I got the boat, at least a couple decades ago), so 4 barrel is what I have.

I set the choke to on a bone cold engine, just close the choke and no more.

 

[Lou C]

One other thing to make sure of is that the choke unloader or vacuum break as it is sometimes called is working. This is a vacuum operated linkage that pulls the choke plate open a fraction of an inch right after the engine starts to allow enough air in to keep the engine running. If this does not function cold starts will be very difficult.
And if you have an electric choke, that would be more properly called an electric un-choke. Meaning that, if there is any interruption in the current supply to the choke it will close most of the way and the engine will run rich and rough.

 

[cyclops222]

Most chokes are fully closed when cold enough. As engine creates a vacuum in the carb throat that pulls the choke plate to a partial opening condition. That is the time and setting that I adjust the idle mixtures for a strong steady mixture. Done

 

[Scott Danforth]

I run a holly 4150 without a choke. initial start is usually about 1500 RPM for a minute or so.

no reason you cant get the 4160 to run at 1500 RPM or so for a few minutes.

the choke should hold the carb shut, until you start it, and then it should open to about 3/8" wide when running, and slowly start opening from there

you may have to adjust the choke a bit more to the rich side

also, if you are running lean all the time at idle, it can make starting a bit difficult. read the plugs. are they white, black or caramel colored? if white.... back the idle mix screws out just a tad (about 1/16 of a turn) and see if that helps.

 

[ratdude747]

One other thing to make sure of is that the choke unloader or vacuum break as it is sometimes called is working. This is a vacuum operated linkage that pulls the choke plate open a fraction of an inch right after the engine starts to allow enough air in to keep the engine running. If this does not function cold starts will be very difficult.
And if you have an electric choke, that would be more properly called an electric un-choke. Meaning that, if there is any interruption in the current supply to the choke it will close most of the way and the engine will run rich and rough.

As far as I know the unloader works fine. We shall see when I muff test it.

Electric choke does have good power (I did have to close up the spade connections through, they were a bit loose). I actually did some rewiring to prevent the opposite issue which was an early suspicion; I moved all of my instrument cluster minus the oil pressure buzzer light from "RUN" to "AUX"... the issue was I'd have the engine off but key on for trim gauge and the like purposes, and since the choke was also getting power, I wondered if that was part of the problem (choke opening too soon); this isn't like on my truck where the choke power comes from the alternator's field directly (no choke power if alternator not turning). Didn't make any change, but IMHO it's a better setup (if nothing else, less battery drain when using gauges with the engine off)