2012 3.0L TKS runs great with muffs, but slow on lake...

[cyclops222]

Post # 97 is the correct reason for the problem.

 

[GeorgioP07]

Installing the new anti-siphon valve tonight, re-testing fuel pressure and spark color.
One guy on Reddit had similar loss of power on the water past 3K RPM, after coming out of Winter storage.
His deduction was:......" Spiders built a nest in your vent tube".
I supposed I can shoot some carburator cleaner down there.
Would that explain that the performance restored when the boat made a sharp left turn a few times this Summer, or is that just a long shot? It was stored last Winter in a barn.
Most arthropods are killed by gasoline vapors. That would be strange indeed.
Thanks.

 

[alldodge]

Would that explain that the performance restored when the boat made a sharp left turn a few times this Summer, or is that just a long shot?

IMO no

 

[Scott06]

Installing the new anti-siphon valve tonight, re-testing fuel pressure and spark color.
One guy on Reddit had similar loss of power on the water past 3K RPM, after coming out of Winter storage.
His deduction was:......" Spiders built a nest in your vent tube".
I supposed I can shoot some carburator cleaner down there.
Would that explain that the performance restored when the boat made a sharp left turn a few times this Summer, or is that just a long shot? It was stored last Winter in a barn.
Most arthropods are killed by gasoline vapors. That would be strange indeed.
Thanks.

easiest way to tell is crack your gas cap open if it is under vacuum then your vent is plugged, and if you open it it will remove the restriction getting fuel out of a tank under vacuum

 

[GeorgioP07]

Hello,
I installed the new anti-siphon valve and dip/ pickup tube and ran at idle.
I get 6.5 PSI now just before the carburetor, as opposed to 4.3 PSI at idle with the old anti-siphon valve. The temperature was 72F when that was done. With the new ignition coil, I am only seeing blue in the sparks across all 4 wires. Very unlikely that I can test this updated configuration out on the water before Winter, as it's getting cold and raining a lot. I am getting all of my winterizing supplies ready, which by the way I noticed that my Mercruiser 3.0L service manual says that the spark gap = Plug Gap .035 in. (0.9 mm), but the label on the top of my flame arrestor says it should be .045in. I have been using the .045in since this the original flame arrestor.
Let me know what you think. Thanks.

 

[alldodge]

The manual has a misprint, the LTS plugs should be gaped at .045, use what is on the flame arrestor

 

[ESGWheel]

George,
The increase of pressure is a positive sign, and I am keeping my fingers crossed that all will be good. Its unfortunate will need to wait until next season, and I really am looking forward to that on the water test.

On another note: George you get my personal “Original Poster of the Year” award. You have stuck with this forum despite all the dead ends and frustration. More importantly you have never given up on finding a solution and followed thru on the recommendations and posting the results. You remained curious and willing. Your tenacity and constant communication are a role model for all Original Posters. My hat is off to you sir!

 

[Scott06]

Hello,
I installed the new anti-siphon valve and dip/ pickup tube and ran at idle.
I get 6.5 PSI now just before the carburetor, as opposed to 4.3 PSI at idle with the old anti-siphon valve. The temperature was 72F when that was done. With the new ignition coil, I am only seeing blue in the sparks across all 4 wires. Very unlikely that I can test this updated configuration out on the water before Winter, as it's getting cold and raining a lot. I am getting all of my winterizing supplies ready, which by the way I noticed that my Mercruiser 3.0L service manual says that the spark gap = Plug Gap .035 in. (0.9 mm), but the label on the top of my flame arrestor says it should be .045in. I have been using the .045in since this the original flame arrestor.
Let me know what you think. Thanks.

Obviously more pressure is better only water test under load will.
luckily you have one of the easiest engines to drain , two drain plugs…

 

[GeorgioP07]

I want to thank you guys for all the help in the 2025 posts here in fixing my Mercruiser 3.0 liter engine from 2012 with TKS to running great again.
I haven't been on this site for over a year. Most of that time the boat was in storage, November -June - due to living in Ohio.
I got it back in June due to a longer running project that ran until June, only reviewed the harness on the trailer, then ran it in July.
It sounded great in the garage again on mufflers, so I ran it in August on the same lake with full throttle. It is back to full performance on the water. Finally!
Looking back, it looked like the main issue in all of 2025 season was that the pick up tube check valve ball coming from the gas tank became corroded/galled/frozen during the 2024-2025 storage period somehow.
This was limiting the fuel pressure in 2025 to 4psi reaching the carburetor, until it was replaced, - no matter what else was performed. It is much closer to 7psi now.
I also needed to adjust the throttle cable + two turns to advance, - so that the advance position was all the way against the throttle plate.
With that being done, the 3.0L does idle a bit higher now, closer to 1050 RPM's, as opposed to the 750RPMs, but here it starts every time. I get that loud clunk when I put it into gear now due to the higher idling, but I will keep an eye on that.
I noticed that with the flame arrestor pulled off, - I could see that some gas was continuing to run a little bit into the Venturi after turning off the engine, which then flooded the carburetor and was impossible to start anytime soon after.
We did a quick tap with the rubber mallet on the right side of the carburetor and that stuck needle stopped being stuck. It appears that somehow after the carburetor rebuild last year, the seat and the needle were sticking a bit to each other at the microscopic level, so the float did not cut off the fuel flow as it should have.
After that, I took it out on the lake, and the 2025 problems with the slow speeds, the hard starting after running it, and carburetor flooding are no longer issues.
I did not need to check anything else, just put six new gallons of 87 octane in with the old stabilized gas.
Thanks again for all the insights, I learned a lot.
Hopefully this post will help someone else. If they're having any issues with their Mercruiser fuel systems.
All the best!
George

 

[Scott06]

I want to thank you guys for all the help in the 2025 posts here in fixing my Mercruiser 3.0 liter engine from 2012 with TKS to running great again.
I haven't been on this site for over a year. Most of that time the boat was in storage, November -June - due to living in Ohio.
I got it back in June due to a longer running project that ran until June, only reviewed the harness on the trailer, then ran it in July.
It sounded great in the garage again on mufflers, so I ran it in August on the same lake with full throttle. It is back to full performance on the water. Finally!
Looking back, it looked like the main issue in all of 2025 season was that the pick up tube check valve ball coming from the gas tank became corroded/galled/frozen during the 2024-2025 storage period somehow.
This was limiting the fuel pressure in 2025 to 4psi reaching the carburetor, until it was replaced, - no matter what else was performed. It is much closer to 7psi now.
I also needed to adjust the throttle cable + two turns to advance, - so that the advance position was all the way against the throttle plate.
With that being done, the 3.0L does idle a bit higher now, closer to 1050 RPM's, as opposed to the 750RPMs, but here it starts every time. I get that loud clunk when I put it into gear now due to the higher idling, but I will keep an eye on that.
I noticed that with the flame arrestor pulled off, - I could see that some gas was continuing to run a little bit into the Venturi after turning off the engine, which then flooded the carburetor and was impossible to start anytime soon after.
We did a quick tap with the rubber mallet on the right side of the carburetor and that stuck needle stopped being stuck. It appears that somehow after the carburetor rebuild last year, the seat and the needle were sticking a bit to each other at the microscopic level, so the float did not cut off the fuel flow as it should have.
After that, I took it out on the lake, and the 2025 problems with the slow speeds, the hard starting after running it, and carburetor flooding are no longer issues.
I did not need to check anything else, just put six new gallons of 87 octane in with the old stabilized gas.
Thanks again for all the insights, I learned a lot.
Hopefully this post will help someone else. If they're having any issues with their Mercruiser fuel systems.
All the best!
George

make sure you get the idle speed fixed as that thunk is the dog clutch being hammered more than usual. You may also have issue coming out of gear at the higher speed as well. Bottom line dont leave it this way.

 

[airshot]

Did you use a factory rebuild kit when the carb was rebuilt ? Smacking the carb with a hammer is never part of a rebuild. Obviously you didn't get it clean or it would not have stuck. Be prepared to take it apart again, order the rebuild kit so you have it when it happens again. By all means get that idle speed down to 750 rpm or lower...otherwise you will be rebuilding more than a carb.