Fuel Starvation problem, 7.4L Mercruiser Inboards

[New350ec]

Hello - I am brand new here, and this issue is relative to a 1991 350 EC that I just purchased from a friend. He (we) have been trying to isolate a fuel starvation problem with the starboard engine. It runs fine at idle, but at speeds above 2500 rpm or so it begins to starve for fuel and lose power. If I switch the tanks so the starboard engine runs off the port tank feed, it runs fine. Here's what we've done so far:

Changed all fuel filters and water seperator, siphoned fuel from the fuel line to try and determine if the tank has water. We even rigged up a system so we could run the boat at 2500rpm on the port tank only and siphon fuel from the starboard, thinking that maybe when the boat went on plane their was water in the tank that was being sucked up. Note, there was no water in the seperators ever noted. This exercise resulted in pulling pure fuel from the tank. I have also removed the gas cap underweigh, to eliminate the possibility of a bad vent hose.

I just rigged up a jumper to jump out the Fram fuel filter, but left the anti siphon check valve in place. I noted that the valve worked freely. I haven't tested this yet, but until I can get out and check it I am wondering if someone here has had a similar problem?

A littler background: I purchased a 1989 300 weekender brand new in 1989, and I have also owned a 1989 340 Sundancer, as well as several Wellcrafts all with inboards. I am somewhat familiar with the merchanics of a vessel this size, but this has us stumped.

 

[SloBurn]

So it's isolated to the Starboard tank, possible clogged pick up screen in the tank?? Just guessin'.

 

[CliffA]

are you sure the problem is fuel starvation? problems with the ignition system can often give the same symptoms as fuel starvation. to verify fuel starvation is the issue try pouring a small amount of fuel into the top of the carb when the problem begins. if the engine picks up and runs OK for few seconds than it is being starved for fuel. if the engine does not pick up the problem is likely electrical. or you can use a 5 gallon gas can to supply fuel to this engine temporarily. if the problem goes away you have an issue with the fuel supply from the fuel tank. if the problem does not go away the problem is likely a weak fuel pump or is caused by an electrical problem.

two suggestions to check if fuel starvation is the problem:

1. double check anit-siphon valves. they are cheap so i would just repalce them. even though the check ball 'appears' to be moving freely you don't know for sure.

2. the fuel tank vent on the fuel tank that supplies the starboard engine may be clogged or partially clogged. As the engine runs at higher rpm's more fuel is sucked out of the fuel tank by the fuel pump at a faster rate than when at idle or low speed. if the vent to that tank is clogged there will be a vacuum created in the tank because fresh air cannot replace the space in the tank created by the fuel that is being sucked out. this will slow or stop the flow of fuel out of the tank if/when the vacuum gets strong enough. check both ends of the vent hose, the hose itself as well as the fitting on the hull. there is likely a small screen in the hull fitting for the vent hose that keeps bugs out of the vent hose. this small screen can become clogged over time. a fast way to check if this is the problem is simply remove or open the gas cap to that fuel tank when the problem starts happening. If the engine starts running normal again the vent line is clogged. if the engine does not run normal the vent line is not your probem.

cliff

 

[New350ec]

SloBurn and CliffA - thank you.

The engine runs perfectly on the port tank so I have ruled out electrical problems. I have cleared the vent and even ran with the fuel cap off to see if venting was the culprit, but no change there. I suspect the anti-siphon valve as well, but a clogged pick-up tube seems the most likely at this point. I will find it!

 

[Captain Allen]

Check for cracked fuel line from tank. Might get fuel at idle speed, but suck air at higher rpm demand.

 

[New350ec]

Update to anyone following. The problem turned out to be a collapsed fuel line. The line left the tank and made a sharp, over 90 degree bend. As the line aged, the outer layer deteriorated and allowed the line to kink. I replaced the line by taking it to a local shop called Fluid Systems in Fraser, MI. I will be replacing all of the fuel lines this week.

Now, to fix the shower sump, air conditioning, replace the hot water tank.......