Why do warm starts need throttle?

[skydiveD30571]

I'm posting this here because it is my understanding that most carbed engines need a crack of throttle to warm start and that it is normal, so there is probably no repair needed.

I'm wondering why this is required though. In my case specifically with a 2006 5.0GL-H with Holley 2bbl 2300, I have always had great luck starting both cold and warm with a little throttle. Even 3 or 4 days of sitting without running, just a 1/4 to 1/3 throttle and turn the key and it starts right up. Throughout the day it's the same way. Just for kicks yesterday I tried several times without giving throttle. I noticed that it gets progressively worse the longer it's been off. After running engine to operating speed then turning off:

1) wait only a couple seconds then turn on again - fires right up and idles smooth.

2) wait 30 seconds to a couple minutes - fires up but idle settles a little low then speeds up and smooths out like normal

3) wait a couple minutes to 30+/- minutes - more cranking needed and then fires up, but more of a stutter start with low loping rpm's. Usually doesn't clear unless throttle is given then backed down to idle again. Then runs smooth.

4) wait more than half an hour - doesn't want to fire unless throttle is given.

I also noticed that the longer it sits off, the more the throttle needs opened to start. I usually start at 1/4 throttle and turn the key, then advance throttle slowly until it catches then back it off. Like I said, I understand that throttle is usually needed for warm starts, but what is the reason? Evaporating fuel so that fuel from transfer slots is needed to start? Carb slowly leaking fuel/vapor so more airflow is needed to clear slightly flooded condition? Not enough air flow at idle? Something going on in the engine itself? These are general questions since this is a common occurance it seems like. Any theorys or ideas or facts are appreciated!

 

[Don S]

Re: Why do warm starts need throttle?

It lets in more air. Carbs are like that, they don't get the precise air/fuel mixture like efi engines do when starting warm.

 

[skydiveD30571]

Re: Why do warm starts need throttle?

You'd think they could come up with something that increased airflow on warm starts, kind of like how a choke adds fuel for a cold start. Maybe that's my ticket to becoming a millionaire!

 

[Brewman61]

Re: Why do warm starts need throttle?

If all you need to do is slightly open the throttle on a warm start, doesn't seem necessary to "fix" it.
Love my MPI, turn the key and go, just like a car.

 

[skydiveD30571]

Re: Why do warm starts need throttle?

I come from times after carbed cars were the norm. I.e. drivers license in 2004. I haven't known anything but MPI in cars, and haven't driven a boat besides mine so its hard to compare. But learning this is normal for carbed engines and and is just the nature of the beast, I practically don't even notice that I'm cracking the throttle to start it anymore. Just hop in, throttle only button and 1/4 throttle, let it sit for 5-10 seconds while everyone gets seated, off we go. The only thing I gotta do is explain it to a new driver but its not like its complicated at all once its known. My stress level went from 8 to 2 after figuring this out.

Having said that, from April until August I went on a wild goose chase trying to figure out what was wrong. But once I messed with something I had to adjust more. In the end I ended up in the exact same place (i.e. a great starting and running engine) but now know this engine inside and out. Read the service manuals a dozen times, learned to check timing and compression, learned the parts and fuctions of carbs, how the cooling system works, etc. Although none was needed, it's nice to have that confidence so if something goes wrong I have that knowledge base to figure it out. Many thanks to Don and Maclin and everyone else for all their help.

I only posted this here to figure out exactly why throttle helps with a carb warm start, and if there were ways to compensate and help it out. If it's because the engine needs more air for the combustion process during a warm startup, I don't see a way of changing that, and am happy knowing I don't need to mess with anything.

 

[Don S]

Re: Why do warm starts need throttle?

You'd think they could come up with something that increased airflow on warm starts, kind of like how a choke adds fuel for a cold start.

They did, it's called a thottle lever, back in automotive times, it was called the gas pedal.

 

[skydiveD30571]

Re: Why do warm starts need throttle?

Well yes but I mean along the lines of the automatic choke versus setting a choke manually.

 

[Don S]

Re: Why do warm starts need throttle?

Cars actually had a stepped high idle cam that was attached to the throttle lever, the marine version is not stepped.
When the choke was closed, the throttle plates were more open with the stepped cam. and there were progressive steps to slow the engine as the choke opened. But with a boat you can't do that. No brakes to hold against the higher rpm if you want to go even if the choke isn't open, and you really don't want to go into gear at those higher rpms.

 

[John_S]

Re: Why do warm starts need throttle?

Well yes but I mean along the lines of the automatic choke versus setting a choke manually.

I'm betting your carb has an electric choke. Merc has what you are asking for: TKS (Turn Key Start) on their carbs. In my mind, its just more crap to go wrong, but makes people happy that grew up with comperterized injection.

 

[skydiveD30571]

Re: Why do warm starts need throttle?

John yep my choke is electric. I've read about Merc's TKS and looked around for a VP equivalent, but apparently they don't think its necessary or worth their time.

Don I was wondering why marine carbs don't have fast idle cams thanks for mentioning that. As often as people turn the key and go without hesitation, it's a good idea to not be idling at 1200 at that time. Not only would the drive gears be unhappy, but anything close to the boat like docks/trailer/other boats wouldn't be happy either. Make the driver do it manually to prevent shifting during high idle. Makes sense!

 

[Don S]

Re: Why do warm starts need throttle?

I've read about Merc's TKS and looked around for a VP equivalent,

The TKS is a lot more trouble than it's worth, and it doesn't work like you want it to work either. It was a good idea, just hasn't worked so far. You still have to play with the throttle. Volvo has better sense than to try to copy that POS setup.
What you want is called EFI.

 

[Silvertip]

Re: Why do warm starts need throttle?

You apparently are not old enough to have ever driven a carbureted automobile. A little throttle was standard starting procedure on a warm/hot engine. On a cold engine the automatic choke and a couple of stomps on the gas pedal provided the fast idle and rich mixture just as the boat does/should. Make sure the choke is working and adjusted properly. Some engines don't need throttle and some do. Life's a mystery.

 

[kawakx125]

Re: Why do warm starts need throttle?

the choke essentially closes the intake of the carburetor off so as to allow more vacuum in the carburetor when the motor starts. More vacuum pulls more fuel. you don't NEED more fuel when starting a warm engine, starting it with the throttle open a tad allows more air into the engine thereby reducing the vacuum and leaning out the fuel mixture. once it's running sufficiently the choke opens and normal operation of the carb ensues. A choke is only really needed for cold starts to richen the fuel mixture and get the engine running. once its warmed up less fuel is required to run the engine. with the automatic chokes you don't really get the option of controlling it yourself, and they basically work off of heat. as the mechanism in the choke control heats up it opens the choke. cools off choke closes. by letting the engine sit for 30 min it could be enough to cool choke mechanism into closing but the engine still warm enough to not need that much fuel. the rough running you are experiencing after that is due to a too rich fuel mixture and the engine struggling to burn that off, essentially too much fuel not enough oxygen.

 

[Oshkosh1]

Re: Why do warm starts need throttle?

You'd think they could come up with something that increased airflow on warm starts, kind of like how a choke adds fuel for a cold start. Maybe that's my ticket to becoming a millionaire!

Carbs are obsolete technology...they are what they are, no one is going to invest money to try and re-invent the wheel. I still have carbed sleds...one lungers(Walbro), two and three lungers(Mikuni). They ALL fire with a flick of the key(or 1-2 pulls)cold but also require a little "coaxing" when warm.

Some carbs will act as a heat sink(especially on inline engines where the intake/exhaust manifold are one) and get so hot it can superheat/boil the fuel in the bowl. I installed a phenolic spacer between the manifold and carb which acts as insulation between the two....it cooled the carb significantly. Easy install...just had to get taller studs.

DECADES ago, I had a 64 Bug with a 40Hp motor and a Weber carb which REALLY hated two things...hot starts and strong headwinds Did have cruise control though...I drove across West Texas/New Mexico/Arizona with a brick on the accelerator and both feet out the window. Got real strange looks from passing vehicles...and never went over 70!

 

[Lou C]

Re: Why do warm starts need throttle?

EFI is great with new boats and cars but if you keep stuff till its old like I do it can become an expensive headache, even though the trouble shooting is easier because of scan tools. With a carb boat, the worst you will ever have to do is replace the fuel pump, carb, and that's it. EFI you got a $700 fuel pump (lots of failures on Volvos) sensors, and ECU and injectors. Big $$.

Carb engines are notoriously individual, each one needs a slightly different cold and warm start technique. Besides the boat I have 9 mall engines with carbs, a new Suzuki DF 2.5, a Briggs&Stratton generator and the rest is lawn equipment. Each one is learning curve. I taught my son to start the Zuki since it's his, and the genny incase we need it and I'm not home. He's use to EFI cars so that took some practice....

 

[kawakx125]

Re: Why do warm starts need throttle?

reason for this being that no two carbs are EXACTLY the same and neither are any two engines in their requirements. Tuning 2 identical EFI vehicles, they could require different fuel ''maps'' to get top performance. Carbs have a very linear fuel curve and cannot measure things like air temp, pressure, engine temp and adjust the fuel curve accordingly like an EFI setup can. Not that you use the boat in a wide variety of temperatures but if so you'd likely find that different temps require different starting procedures as well. A cold start on a cold day may be different than a cold start on a warm day. with a carb, at a given throttle opening you are going to pull X amount of fuel. with EFI, at a given throttle opening you are going to pull the amount of fuel determined by the computer based on what the sensors tell it to. Yes it is quite a bit more expensive but a properly tuned EFI system will outperform a carburetor any day and make a substantial improvement in fuel consumption, power, and driveability

 

[skydiveD30571]

Re: Why do warm starts need throttle?

What you want is called EFI.

Maybe on the next boat. I briefly looked into conversion kits but the headache of retuning it (after spending $1200+) is not worth saving myself 5 seconds a few times a day.

Thanks for everyone's inputs here. My question was definitely answered, and Im especially happy that there's not too much I can do about it but get in the habit and enjoy the boat. Just in time for cold weather to move in!