3.0LX - slight stutter solution

[smartwork]

I've got a 3.0LX (merc 2 barrel) that I think runs excellent except for this slight pause that occurs about a second after "hit it". It doesn't seem to gasp, so I don't think it's starving. It does ride through it in short order and then gets right to business where it performs super smoothly.

There isn't a way to adjust the fuel for this part of the range, correct? If it's not fuel, is this something a timing adjustment will solve? If so, does the timing need advancing or retarding?

Thanks for any insights.

 

[Kainon]

Re: 3.0LX - slight stutter solution

I read a review today about the New Volvo Penta 3.0 Fuel Injected 150HP Engine, and they tested against a carb version 135/140 and they describe the same problem with carb'd 4cyls. And I think I've experienced the same thing, just that slight hesitation of a quick drop of the hammer.. If not then your accellerator pump needs to be replaced or the check ball is sticking.

ps.. I know you have a Merc, here's the link so you can read their observation.

http://www.trailerboats.com/output.cfm?id=2415655

 

[sltintexas]

Re: 3.0LX - slight stutter solution

tagging this as I am having identical issues.

 

[sickwilly]

Re: 3.0LX - slight stutter solution

If you remove the flame arrestor and someone pumps the throttle, do you see two nice, equivalent streams of gas going into the carb? The accelerator pump should produce two nice streams of gas when you drop the throttle to get the engine going until vacuum takes over. Old gas and time can clog up the passages, or the accelerator pump could just need to be replaced to restore it to its original level of performance.

 

[JustJason]

Re: 3.0LX - slight stutter solution

It's sort of the nature of the beast with inline 4's, and you have to live with it. They all have some sort of perceptable lag. There's not much you can do except for trying to dial in the accellerator pump a bit. It's trial and error, not really rocket science, but the lag will always be there.

 

[smartwork]

Re: 3.0LX - slight stutter solution

SW - I'll check that tonight and see how the gas streams look.

JJ - Why is it the nature of 4's to do that? What's inherent to them that causes it?

 

[fat fanny]

Re: 3.0LX - slight stutter solution

My rig suffers the same issue but it never really bothered me and have just learned to live with it. But when I was a kid racing moto x we use to rig this bottle to the carb and feed fuel line which required a npt fitting to be drilled into the throat of the carb below the slider and when the throttle was releast fuel would fill the bowl and when you cracked it back open it was like fuel injection vertually no hesitation but with all that said I don't see how this could be made to work on the mercarb affectivly to notice a difference worth canibilizing the cab for! but all in all just a thought.

 

[smartwork]

Re: 3.0LX - slight stutter solution

Okay, took a look tonight and I've got one REALLY good stream shooting into the carb with the throttle pumped. Shoots into the front most barrel. Before I go digging into it too deeply, we're positive there are supposed to be TWO streams, right? One shooting into each barrel?

 

[bondomerchant]

Re: 3.0LX - slight stutter solution

yes 2 might cure ur problem i think ur onto sumthing

 

[smartwork]

Re: 3.0LX - slight stutter solution

The way I'm figuring it is that the acceleration pump is going to shove the same amount of gas out once you throttle down, but the difference between one and two streams is how FAST it can get that quantity out of the bowl and into the carb/manifold. I guess as it is now, by the time the accelerator pump is finishing with the single stream, the engine is already taking over, so we may not really be benefitting from the last half of the stream. The two streams should cut that time in half which gives us the full shot earlier and helps reduce the lag time. Now, I just have to go digging to find the clog of that second stream.

 

[sickwilly]

Re: 3.0LX - slight stutter solution

You never say how old the carb is? I rebuilt mine at 6 years old and 152 hours. The reason I believe I needed a rebuild at this point in time is the original owner used the boat for 14 hours over its first 3 years.

I got a carb rebuild kit from advaced marine, it was a universal one. Then got some carb cleaner and compressed air. I stripped out a screw along the way, but with help from these guys here, I got it all done. Its made a heck of a difference.

Just get the manual here and follow the directions that come with the kit.

You can also buy a rebuilt carb and just swap yours out.

Its the same 2 barrel mercarb for the 3.0 and 4.3, just different settings and different jet sizes.

The accelerator pump has a rubber flange and spring. It wears down over time. It also has a small circuit under it, with a small check ball and a spring. It can get gunked up. Then there are small passages that feed the gas out. You have to soak it and blow air through it to clean it out.

 

[smartwork]

Re: 3.0LX - slight stutter solution

it's a '97, but I don't know if there have been any replacements prior to our ownership. I actually bought a carb kit last year and was going to rebuild at the end of last season, but that got delayed because for the most part it runs excellent. Late last night, I popped open the kit to examine it and I believe it's the wrong kit (ugh!), so I'll have to resolve that (probably by just buying another one). It looks like the main gasket between top and bottom of the carb is just slightly different in one spot.

SW - yea, it's probably one of those circuits plugged. It's amazing those don't plug more often for how small they are in any carb. Hopefully, resolving that one passage will provide a solution for the lag. Thanks for the great info!

 

[smartwork]

Re: 3.0LX - slight stutter solution

Well, I pulled the carb tonight and disassembled much of it and had some luck. Once you remove the top of the carb, there is the "V" piece that feeds the gas to each barrel. That piece is also the second half of the circuit that feeds gas from the accelerator pump to the manifold. I removed the three screws and took the piece off. I shot carb cleaner through the bowl of the carb and through the 1/2 of the circuit from the accelerator pump to where I removed the "V" piece. All was clear. I then worked on the "V" piece with carb cleaner and was able to clear the plugged side. Cleaned everything and gasket surfaces. Reassembled, and YES... I now have TWO streams spraying from the accelerator pump. Hope to give it a test run sometime over the weekend. I'll post results.

 

[smartwork]

Re: 3.0LX - slight stutter solution

Update: I finally made it to the lake. I can't say that having the two streams made much of a difference in performance. What I'm not looking at is the accelerator pump lever. I didn't realize that it actually has several settings (3) that determine how much the pump shoves into the carb/manifold. While at the lake, I moved it to the leaner pump setting and I believe it made a little difference. We got rained out, so I didn't get to test it a lot, but will soon. Just thought I would post the update. If that doesn't improve it much, I'll start researching timing.

 

[sltintexas]

Re: 3.0LX - slight stutter solution

For what its worth, I was having identical issues. I rebuilt my carb as well.

I better results from making sure my idle fuel mixture was adjusted correctly than adjusting my accelerator pump. Now out of approx 20 full throttle down starts, I may get 2 or 3 that have any noticeable lag... and those are extremely brief.

Here is a link to my post http://forums.iboats.com/showthread.php?t=415919 . I took ChevyJeff's advice and just did 1/2 turn at a time. I screwed down all the way, and then unscrewed 1 1/2 turns out... it wouldn't even start. by unscrewing it 1/2 turn at a time, then trying to start it, I got it started (took an extra full turn)...I kept doing half turn & wait a minute to see how engine ran. At some point it runs well, and I kept turning it and it would start to bog down again, then I screwed it back in a little bit to where the RPMs leveled back out and was running well.

 

[smartwork]

Re: 3.0LX - slight stutter solution

Just an update for anyone subscribing: I took the boat out today for some attempted tuning. I began by topping off with premium gas, and then headed to the lake. After rebuilding the carb earlier this summer (with little solution), I started adjustments like slintexas mentioned. I found that mine liked the adustment about 1/2 between where it bogs down from too lean and where it bogs down from too rich. My accelerator pump is set to to the middle setting on the linkage. This helped a little, but it still needed some work. I brought my timing light, but it was tough to use on the lake (bright sunlight). I tested a few timing changes. Advancing it worsened things, but retarding it brought positive results, and it didn't take much. I may do some further refinements, but dialing the timing back a little made the engine run better over the entire range. I still have a very slight hesitation when I hammer it, but I've got a higher pitch prop on it, so I feel now it's more like a "I (a 3 litre) can't rocket this boat instantly" pause than a "I'm falling on my face" pause. Hope to get out this week or next weekend to give it another run.

 

[JustJason]

Re: 3.0LX - slight stutter solution

I tested a few timing changes. Advancing it worsened things, but retarding it brought positive results

You don't want to "test" timing changes. You want to set it to factory spec and that's it.

Like I said, it's the nature of the beast.

 

[smartwork]

Re: 3.0LX - slight stutter solution

Did a little more testing and research today - off the lake and in light where I could more accurately see the timing light. Using the timing procedure whereby you eliminate the shift-switch (connect those wires) followed by connecting the white module wires together, I'm unable to distinguish any changes in timing which means I don't think I'm able to completely force it into base timing to set it. In the process, connecting the shift-switch wires also kills the engine (although connecting them is supposed to be part of the timing process and allow the engine to run). Researching it turned up one thread where a fellow had this same series of events and concluded a problem in the EST module. Anyone know of any exceptions where the shift-wires should not be connected together for this timing process? With them apart, the engine will run, but connecting the whites still does not force the engine into base timing.