Fuel line restriction?

[Don Gardner]

I have had an intermittent issue with my starboard 6bta 330 engine. When the bottom of the boat is clean (right after a haul out) it reaches full rpm (2950-3000). After a month or so it won’t reach more than 2500. Feels like it is starving for fuel and not able to push the extra load of growth on the bottom. No issue with the port engine. I have vacuum gauges on the primary fuel filters. The port gauge stays around 2 in/hg. The starboard reads 6-7 in/hg and this did not change after I put on a new filter. Am I correct in assuming that there may be a restriction between the tank and the filter and not between the filter and the lift pump? If I change the fuel lines between the tank and the filter, how do I get the air out of the lines?

Topic

[Rob Schepis]

To test it, at the dock with the starboard engine running, close the fuel valve between the primary filter and the tank and watch the gauge it should start to rise, open it before the engine sputters out.

[Don Gardner]

After I switched the vacuum gauges on the primary filters and ran the engines, it is obvious the gauges are faulty. Now at 1500 rpm the starboard gauge shows no restriction and the port shows 6 in hg restriction.

[Don Gardner]

Yesterday I pulled the pickup tube from the starboard tank, blew it out, blew out the line from from the tank to the selector valve, blew out the valve, replaced the line from the tank to the primary filter, and blew out the filter mount. I started the engine and ran it at 1500 RPM’S for about 10 minutes in neutral. I didn’t find any obvious obstruction. Also I swapped the vacuum gauges on the starboard and port filters. At 1500 RPM the gauge showed no restriction. I assume the restriction would only show while under way though.

[Don Gardner]

More pics

[Don Gardner]

Here are some pics of my engines. more to follow.

[Don Gardner]

I’ll try to get some pictures next week. I have checked the fuel solenoid. Changed fuel filters. Checked throttle linkage. Have not put a pressure gauge in the fuel line.

[Rob Schepis]

If you were really losing 450-500 rpms due to increased load (fouled bottom) you would have to be seeing black smoke.

As your opening post title suggested, sounds fuel related to me and perhaps it’s something that is occurring over time as is the bottom growth although the two are unrelated.

How about some good pictures of the engine room including the engines, the turbos/exhaust and the fuel delivery including the off engine filtration.

Have you gone through this: https://www.sbmar.com/featured-article/understanding-low-power-troubleshooting/

[Don Gardner]

Rob,
The filter is below the level of the fuel.The filter mount has a built in priming pump so I assume I can get the air out with that. Photo tach was used to verify the tach on the dash and it is within 2 rpm. Not getting any black smoke. I have not pulled the dip tubes but it is on my agenda. The fuel tank selector appears to have leaked at some time so I plan to replace it as well as blow out the fuel lines and clean the filter mount. I think I will also swap the vacuum gauges from one filter to the other to verify.

Can you think of another reason that it runs great with the bottom clean and then not reach rpms when it starts to get some growth? Prop?

Don

[Rob Schepis]

The vac gauge reads whatever is behind it, so yes the filter and back to the tank. 6-7 inHg should not cause the performance issue you are seeing. As to getting the air out – is the filter setup location higher or lower than the level of the fuel in the tank? You can install a squeeze bulb or you can pump away at the lift pump.

Is the 2,950-3,000 on Aetna’s or a phototach verified analog tach?

When you get this rpm drop are you seeing any black smoke?

Have you ever pulled the dip tubes from the tanks to inspect them as to condition and type/size?

[Author]

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