Mercruiser 3.0 runs inconsistently at speed

drummerdaveb

Cadet
Joined
Apr 25, 2017
Messages
10
Mercruiser 3.0 in a 2001 Rinker 180.
Starts fine hot or cold. Idles fine. No issues there.
Wide open throttle runs fine.

However at partial throttle, it hunts. Especially, in the range of let's say 2750 rpm or so down to above idle, it acts like something isn't quite right. With a steady throttle, it clearly is hunting or surging. Not sure what to call it. But it it acts similar to a motorcycle that is running a little lean.
When taking off, just opening the throttle there is also a bit of a dead spot. Almost like you would get if an accelerator pump isnt firing.

My questions are:
1) Is this normal?
2) Do you have any guesses what would be causing it? Am I due for a carb rebuild? Or does it sound more like an issue w/ a Vacuum leak, distributor issue, clogged fuel filter, etc?

Thanks!
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
43,351
I would think vacuum leak or carb
Which carb is it, Merc or TKS?
 

Rick Stephens

Admiral
Joined
Aug 13, 2013
Messages
6,118
Sounds like time to rebuild the carburetor. But not knowing whether you have TKS or Mercarb makes it hard to answer. Other factors present with TKS system. I can see plugged ports or float level low, something like that, causing your problem.
 

Rick Stephens

Admiral
Joined
Aug 13, 2013
Messages
6,118
I'd do a careful rebuild. Midrange ports are small and easily plugged. Take your time, a Marcarb is really simple and easy to work on. Run a wire or fishing line through every port. Use the right kit and replace needle valve and seat. Be really anal about float height.
 

TurtleTamer

Petty Officer 2nd Class
Joined
May 10, 2018
Messages
143
I have found that my carb is particularly sensitive to float level. The beginning of its climb out of the hole is the most susceptible spot for hunting/surging which happens to coincide with the 2000rpm or so transition between idle and mid circuits. This is also the most pitch the boat will see while in the water. The fuel sloshes to the back of the bowl and the float geometry changes such that it requires less fuel in the bowl to cause the needle to seat. Combined with a bit of the accelerator pump stroke and the bowl level seems to drop just enough to expose one of the main jets to air.

What led me to this theory, is the a strange condition in which I put the throttle to wide open as I climb out of the hole and leave it until top speed. When I do this, the engine begins to detonate over 4000rpm and I have to pull back on the throttle. Then, after five or ten seconds, I can go back to WOT and reach top speed with no detonation. (Timing and ignition components have been ruled out as an issue). This is why I think that the fuel level in the bowl goes down and doesn't fully recover for a bit until after the boat's pitch equalizes. I also had quite the surging issue at one point the directly correlated to the pitch of the boat when at part-throttle or slowly climbing out of the hole and hitting waves.

I use the spring-loaded needle which calls for 14mm float height. I've found that 12mm works better for me all-around. Some use the idle mixture adjustment to allow a bit of extra fuel for the transition, and others adjust the accelerator pump arm position (if they have the three-holed pump arm) to enrich the mixture on acceleration. I have found both will give me undesirable conditions. My belief is that the spring in the needle assembly causes inconsistent float heights, and when I use the solid needle it seems to hang occasionally and flood the engine at idle.

I haven't checked fuel pressure, but the 2mm rise in float height cures my issues.

Sorry for the long read, but what I'm saying is it's possible to do a perfect rebuild and then still have to tinker with it to tune it for your boat. Do a very careful and methodical rebuild and make sure you gain an understanding of the workings inside the carb in order to get it fine-tuned.
 

prc03sx195

Petty Officer 2nd Class
Joined
Nov 3, 2007
Messages
153
I have a similar issue with my 4.3L in that at a constant throttle position my rpm,s hunt very sporadically. A few times while at 90% throttle I can hear a strange loud clank coming from the engine bay. I don't have a clue what this could be so I'm wondering if it is related? I changed the plugs and the same thing happens with the rpm, but it is not consistent. Otherwise it runs great. Let me know.

paul
 
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