Engine Roll...454...Port Engine

[Big Billy]

Hi Guys

Recently my boat engine has developed what is called an engine roll, not a smooth idle, but a surging roll type idle. Also, as I'm accelerating, the engine won't go past 2,500...I can push the throttle up underway, but it just won't go past 2,500, no breaking down or anything. Coming home yesterday Iwas running about 2,500, sneaked it up to 2,800 but after a few minutes it dropped back to 2,500. At idle I can do what I want, mechanical linkage 4 barrel, will open and roar powerful, but underway I just can't get past 2,500. I changed fuel filters, and checked spark plugs, everything looked fine. I guess I'll clean the carb, change the fuel pump and coil next, but was hoping for some good ideas...ran fine three weeks ago, but not now...I have new tanks and go through a fair amount of fuel.

Please help,
Bill

 

[northern]

Had similar problem on a V6 stern drive. One time it was a coil the other it was a bad spark plug.

 

[Big Billy]

:huh::huh::huh::huh:

I gather that you have a 454 carburated engine, how about a little more information about the year, boat, etc.

Yes, it's a 1979 Jersey Dawn Sportfisherman, I think the engines are 1985s with about 2,000 hours. They've run good, burn no oil, up until lately. I've owned the boat for 4 years, but I remember the roll when the previous owner used to boat by. When you are in the engine compartment, you really can't discern the roll, but when you are in the cockpit listening to the exhaust, it's there. New plugs every year, electronic ignition, new tanks due to ethanol, I don't think I have the ball valve t hing at the tank...

Bill

 

[Big Billy]

Thanks for the added information, I think that will help us give you some assistance. I don't have anything to add for you to try unfortunately, but hopefully someone will be along shortly.....

If it wasn't for the lack of RPM, I would say it might be a normal function of the exhaust, as my 340 has a very distinctive idle roll due to the way the mufflers are plumbed. But since you can't make RPM, it might be something more sinister.

Does the starboard engine still make the recomended range for WOT? Also I am assuming you have a clean bottom, i.e. no barnacles or marine growth.

Very nice looking boat BTW, always loved those sport fishermen since I was a kid.

Yes I had them both wound up to 4,000 a few weeks back, racing a thunderstorm, which is the top you strive for when putting engines and props together, so they were fine then. The starboard is fine, so I know fuel quality isn't an issue.

Thanks for helping,

Bill.

 

[JediJD]

Here's $.02 worth...

It was called a "hunting idle" when I was doing this stuff years ago...two places to look are vacuum leaks and fuel dripping into the intake manifold. To look at fuel drip, take the flame arrestor off and look down the carb bowl at idle...you can actually hear the drip if you listen (be SAFE! I did this on cars not boats). For vacuum leaks...take a small propane cannister and at your hardware store get a small nozzle for it that fits on the top of the cannister. On the nozzle, put a couple of feet of rubber hose (small, like vacuum hose). Let a little bit of propane out and put it next to the carb...idle should increase (if not, you are running too rich). Then run the hose around the intake manifold gaskets and the base of the carb. If idle increases, you have a small vacuum leak. Note...NEITHER is generally related to high rpm failure...that is usually a secondary ignition breakdown or fuel restriction...depending on the symptoms. If you are not getting a lot of misfiring at 2800, check fuel PRESSURE, not just filters.

 

[Big Billy]

Thanks, I'll try those things. Another thing that has been happening, on the same engine, since I've owned the boat (4 years) is most of my trips are around 15 miles. Everytime, since I've owned the boat, when I am about half way, I lose rpms on that engine. Assuming it was just the throttle cable moving, I'd simply move it up a hair to get back in synchronization with the starboard and off I'd go...if that helps in anyone's conclusions....

Bill

 

[JediJD]

Really sounds like fuel delivery in the mid range. I am really thinking you have two completely different issues. The midrange could be heat related to a primary (coil) delivery or secondary (plugs and wires) problem.

Do you know if you have electric or manual fuel pump(s)?

 

[Big Billy]

Really sounds like fuel delivery in the mid range. I am really thinking you have two completely different issues. The midrange could be heat related to a primary (coil) delivery or secondary (plugs and wires) problem.

Do you know if you have electric or manual fuel pump(s)?

It's a mechanical, the big $168 one.........

 

[Big Billy]

Turned out the coil was the problem, sounds like a new engine now...thanks for the ideas guys......