Issues at idle

Cap'n Morgan · **Posted:** Wed Jul 30, 2014 9:22 am

ric wrote:

How am I wrong? Volvo Penta discontinued the 3.0 in 2011. I stated correctly that Mercruiser still offers the 3.0 in fuel injection. Quit being a jerk and learn how to read.

Because VP did not phase it out because of lack of power, it was a purely economic decision, and they had plans to replace it with a new engine after the 4.3L V6 phases out. See the link I posted below from before you ever even owned a boat. And the little 12ft. aluminum boat with little OB engine you had back then does not qualify as being much of a boat.

viewtopic.php?f=11&t=6797

*Profanity removed by mod

rpengr · **Posted:** Wed Jul 30, 2014 9:54 am

LouC wrote:

Well Ray, the physical restriction causes the air velocity to increase which lowers its pressure, in the entire throat of the carb. While the whole operational principal of how a carb works is based on Bernoulli's law, the choke merely accentuates that effect temporarily, to keep a cold engine running because of fuel droplets condensing out of the mixture. Its not dependent on precisely where the jets are and in fact in this pic you can see the main nozzle below the choke. The idle jets are below the throttle plate. This is from my Chilton's manual American cars 1970-1977 lol....ancient stuff.

Lou, I hope we can call this a debate and not an argument, as I highly respect your expertise and experience.
However, I still represent that the extra vacuum caused by a choke is not caused by Bernoulli's Principle

From your Chilton's manual:
"When the choke valve is closed, only a small amount of air can get past it. When the engine is cranked, a fairly high vacuum develops in the air horn. This vacuum causes the main nozzle to discharge a heavy stream of fuel."

This helps prove my point. The high vacuum is caused by the highly restricted inlet, not by an increase in velocity. Bernoulli's Principle is the exchange of energy from potential energy (pressure) to kinetic energy (velocity). The choke reduces pressure without increasing velocity past the main jets by any significant amount. I am quite familiar with Bernoulli's Principle, Universal Gas Law (PV=NRT), and other thermodynamic priciples occurring in a carburetor (latent heat, evaporation, etc).

Ray

LouC · **Posted:** Wed Jul 30, 2014 10:30 am

Is vaccum not a lowered air pressure and the way the fuel discharges from the main nozzle is because of the greater atmospheric pressure in the fuel bowl? The gas is not physically sucked out of the bowls via vaccum rather the greater atmospheric pressure in the bowl pushes it out. I actually think Chilton is somewhat wrong in their explanation there . I just wanted to show the close proximity of the nozzle to the choke.

In any case if you have a boat with a carb, they take maintenance. The choke shaft gums up, the linkage can gum up, the springs can lose tension. Easy stuff to fix if you keep the linkage clean, clean the choke shaft where it goes through the air horn and make sure it closes, that the choke pull off (vaccum break) pulls it open a fraction after the engine starts and that the thermo spring opens it once it reaches 160 or so.

rpengr · **Posted:** Wed Jul 30, 2014 11:37 am

LouC wrote:

Is vaccum not a lowered air pressure and the way the fuel discharges from the main nozzle because of the greater atmospheric pressure in the fuel bowl? The gas is not physically sucked out of the bowls vaccum rather the greater atmospheric pressure in the bowl pushes it out. I actually think Chilton is somewhat wrong there . I just wanted to show the close proximity of the nozzle to the choke.

That's all true. I was just saying that the lowered air pressure was not caused by accelerated air stream (Bernoulli), but rather by physical restriction.

LouC · **Posted:** Wed Jul 30, 2014 11:48 am

rpengr wrote:

LouC wrote:

Is vaccum not a lowered air pressure and the way the fuel discharges from the main nozzle because of the greater atmospheric pressure in the fuel bowl? The gas is not physically sucked out of the bowls vaccum rather the greater atmospheric pressure in the bowl pushes it out. I actually think Chilton is somewhat wrong there . I just wanted to show the close proximity of the nozzle to the choke.

That's all true. I was just saying that the lowered air pressure was not caused by accelerated air stream (Bernoulli), but rather by physical restriction.

Interesting, does the air flow not have to speed up to get past the choke plate? More velocity=lower pressure. Can the restriction alone lower pressure?

rpengr · **Posted:** Wed Jul 30, 2014 12:11 pm

LouC wrote:

rpengr wrote:

LouC wrote:

Is vaccum not a lowered air pressure and the way the fuel discharges from the main nozzle because of the greater atmospheric pressure in the fuel bowl? The gas is not physically sucked out of the bowls vaccum rather the greater atmospheric pressure in the bowl pushes it out. I actually think Chilton is somewhat wrong there . I just wanted to show the close proximity of the nozzle to the choke.

That's all true. I was just saying that the lowered air pressure was not caused by accelerated air stream (Bernoulli), but rather by physical restriction.

Interesting, does the air flow not have to speed up to get past the choke plate? More velocity=lower pressure. Can the restriction alone lower pressure?

Yes, it speeds up as it squeezes around the edge of the choke plate, but immediately slows down just past it.

Can the restriction alone lower pressure... Yes. The pressure energy in this case is converted to heat (friction). (The heat energy would not be noticeable because the lower pressure also causes a temperature drop per PV=NRT)

Capt. Hoss · **Posted:** Wed Jul 30, 2014 12:20 pm

Everyone take note this is how you are supposed to argue. Valid points backed by facts. I LIKE IT!

LouC · **Posted:** Wed Jul 30, 2014 1:08 pm

I agree we are having a good discussion and everyone learns something from that.
One thing I've been teaching my son is that you have to understand how things work in order to fix them. Some of his acquaintances just rip things apart changing things with no idea how they work, why they don't work or how to fix them.

schoolsOut · **Posted:** Wed Jul 30, 2014 1:40 pm

rpengr wrote:

First start of day/week, I pump throttle twice, then set throttle a little above idle. If it doesn't keep running, then one more pump and works on second try. I can idle and go into gear within 10-15 seconds after that.
After the first start of day, all other starts are just turn the key at idle.

We always did this, exactly, with my dad's carborated boats. Mine if FI so I don't have any recent experience with non-FI engines.

fi.na.tine · **Posted:** Wed Jul 30, 2014 5:56 pm

I guess my biggest problem that I have found is the problem when it is started, and runs in neutral - after a good warm up (140 degrees) I shift into forward - and it dies. Then I start all over again. It has warmed up fro about 5 minutes at that point - and takes about 10-15 to get going from cold.

When it's warm it starts fine after one pump and runs fine as well.

LouC · **Posted:** Wed Jul 30, 2014 6:10 pm

OK the boat's a 2008, when was it tuned up last, including adjusting the carb (idle mixture, idle speed, choke etc), when were the plugs done, etc. That's got to be the simplest engine ever. I'd advise getting a workshop manual for it and learn yourself.
My old '88 will start from cold on the first crank or second if I give it 3-4 pumps. This is after sitting on the mooring for a week or longer. Usually it starts on the second crank over even after winter storage.
So a tune up and carb adjustment is in order, including changing fuel filter(s) if that has not been done.

fi.na.tine · **Posted:** Fri Aug 01, 2014 2:39 am

Took it out today. I forgot to say - "straight" gas (w/o ethenol) is very hard to come by here. Most people just use stabilizer. So I bought stabilizer from Lucas Oil and put it in. Boat was much easier to start. Three pumps and idle for 4 minutes and I was off and going. The stabilizer as is supposed to help clean the carb as well.

rpengr · **Posted:** Fri Aug 01, 2014 6:58 am

fi.na.tine wrote:

Took it out today. I forgot to say - "straight" gas (w/o ethenol) is very hard to come by here. Most people just use stabilizer. So I bought stabilizer from Lucas Oil and put it in. Boat was much easier to start. Three pumps and idle for 4 minutes and I was off and going. The stabilizer as is supposed to help clean the carb as well.

I don't know which carburetor you have, or where to tell you to look for the idle jets... but it sounds like you could benefit from opening the idle jet(s) another 1/8 turn.

ric · **Posted:** Fri Aug 01, 2014 7:13 am

Holley carb.

I had the same 3.0 with starting issues. It needs a full tuneup. There's a thread of mine in here somewhere.

Don't use sierra caps/rotors. They are boy built using brass. They corrode and will miss fire in months.

fi.na.tine · **Posted:** Mon Aug 04, 2014 3:01 pm

It's been out 3x now since the original problem. Each time it starts when using the procedure outlined in the owners manual, and keeps running - ever since I put in a stabilizer with carb cleaner. An experienced friend of mine also had hands on with it this weekend and said he would be surprised if it needed anything but cleaning.

www.iFourWinns.com

Issues at idle

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