Logo
FULL SITE MENU

50 Merc loses power at higher RPM

H

hotdogwater

New member
"Have a houseboat with twin 5.

"Have a houseboat with twin 5.0 mercruisers. Boat sat for many years until we bought it. Had outdrive service done and replaced plugs, oil, fuel separators and rebuilt carbs. Both engines ran fine for first hour or two after outdrive service now one is having problems at higher RPMs. Runs ok at below 2k but above it starts to die intermittently. If you back off throttle it wont stall but if you stay in it it dies. Replaced fuel sep. a 2nd time (dumped it once and had a debris and cloudy) but problem started again almost immediately after replacing fuel sep 2nd time.
Other owners think it's still fuel issue. After reading this board, I am lost to solution. If it was water in fuel (tank was filled up 60 gal + older 20 gal and treatment added) wouldn't it run like it did just after service when it was fine for 2 hours?

Any help is much appreciated.
-Daryl"
 
"dumped it once and had a

"dumped it once and had a debris and cloudy...Other owners think it's still fuel issue"

Listen to the other owners. Adding 60 gal. of fresh fuel to 20 gal. of who knows what has stirred up the sediment in the tank and created a witch's brew. May need to dump the carb bowls and definitely change filters. Pour a sample of the fuel in a glass jar and see what you get as it settles out. May be necessary to pump out the fuel cocktail and have the tank cleaned.
 
"those alumnium tanks like to

"those alumnium tanks like to cause hellova havoc with debries when sitting not full and having room to collect water, i agree with guy, best thing to pump them out and have cleaned"
 
"Guy and Brett are correct.
I

"Guy and Brett are correct.
I may add to your water problem, rotten fuel from sitting so long. Best not try to pump or add additives to remove water, you must pull tank and have it cleaned along with fuel lines and carb."
 
"Hey all, I'm one of the o

"Hey all, I'm one of the other owners. Thanks for the replies so far. Couple things...

The port tank, which feeds the problematic port motor, seemed completely empty when we started this project. It's an 80 gal tank, and we could slide it around easily. The "older" 20 gallons is only about a month old.

The other tank and its motor, which was probably about half full of 5 yr old gas, has stalled a couple times, but OTW not given us any trouble.

As said, we rebuilt both carbs - they were the worst gummed-up with varnish I've ever seen. We also installed new water separators before running them at all, and then changed both again after we started having the cutting out issue on the port motor.

We've been treating the gas with Seafoam the whole time. It's also worth noting that the generator has never had a problem, and it is tee'd into both tanks. I understand the generator pickup is actually located higher in the tank than the engine pickups (to help prevent getting stranded), which in my mind explains the genny not having bad gas problems.

Thanks again for the help.
Trace"
 
"Two hours is a lot of warm up

"Two hours is a lot of warm up, to then run bad. I'm assuming you are satisfied that the fuel is now ok. I was going to suggest the fuel/separator, but you alread did that. to get the fuel thing out of the way, cross connect the tanks and see if they still act the same. If so it isn't fuel,; sshoould be simple hose connections, no time or money.

If it is not fuel, I'd think oil or cooling system. check to see if the oil level is high in the poor operating one. Over filling can after a period of time running start to foam. this makes the valves (hydraulic lifters) collapse. simple check.

Over heating could also be a problem maybe the exhaust is blocked or the raw water pump is shot. IR Thermoter shoould find that quick. and you can use reading on the other identical engine to see if the weak sister is overheating"
 
"Thanks, yeah, we're actua

"Thanks, yeah, we're actually going to try swapping the fuel lines tonight. I'm not convinced the fuel contamination is not the problem. We don't know what the inside of the "empty" tank looked like before we started filling it.

For the record, the access in the engine bay is such that we would have to pull a motor to pull a fuel tank. Not going to happen.

The oil levels are good, and neither motor is running hot - exhaust manifolds just warm to the touch at all times. Both have fresh impellers, and just had the outdrives serviced. New thermostats too.

I really believe it will clear itself up pretty soon. It may take some more fuel filter/separators and Seafoam, but eventually we should filter out / burn through the problem."
 
Gasoline begins to degrade aft

Gasoline begins to degrade after 60-90 days w/o a preservative which can keep it stable up to 365 days. Then it begins to convert to varnish while loosing its volitility. The octane of the fuel falls off and leaves a low volital oil product that loves to foul your fuel system. Adding octane enhancers will help it burn but their side effect is quickly carbon fouled spark plugs. Good luck on the five year old gas.

RECOMMENDATION: PUMP IT OUT AND SAVE YOURSELF A SERIES OF MIGRAINS; otherwise pay your annual sea tow bill. The cost to pump it out will be worth it in the long run.
 
"Bad gas. I once restarted a c

"Bad gas. I once restarted a car engine after the fuel had been sitting for months. About 10 gallons of fuel system cleaner later, the electric f.i. fuel pump spit out it's guts from all the solvents, and I had to wire it back together with a coathanger to get her home. I've never seen so much varnish in my life. I had to pull the injectors three times and ultrasound clean them before it cleared up.

Is this enough for you? Drain the fuel and use it in the lawn mower..."
 
Check the inline anti syphon

Check the inline anti syphon valve at the fuel Tank for something hunging it up. It must be completly clear and have full travel or at higher RPMs the engine will hesitate from Fuel starvation.
 
Back
Top