MORE WOT problems on mercruiser

[DGIFF396]

I posted the other day about my wot problems and I appreciate the help but Ive yet to diagnose it and I have more information as to how and when its happenning. Ive read the 3 or 4 other wot posts and although mine seems similar in some ways, its got other symptoms far different. For starters, I have a 1978 mercruiser 470. The boat was running good for a few trips after I bought it and replaced the cap bearing in the outdrive. Now the boat has problems coming to high rpm or even cruising rpm after THE BOAT HAS BEEN ON THE WATER FOR A HOUR OR SO. It also seems to act up and "BOG" down when I cross someones wake and the boat rocks side to side, but again, only after running it for awhile. The temp gauge hangs at 140 to 150.. The boat it self will take off fine than after a couple seconds bog down and not wanna pull over 3 grand....below that its just fine. I had the chance to take it out on the mississippi yesterday so I can now tell you that it only seems to do it under load. When it was at its worst i pulled it out of gear at a stop and revved it up and the tach quickly and smoothly touched 5000 rpm, as soon as i put it in gear it accelerated hard it picked up for a second or two then bogged, although when I was power loading the boat to leave it would hol higher rpm/s fine without bogging?, all this kinda eliminated my fuel issue I thought I had wether it be water or plugged or carburater....any thoughts, please, Im stumped. I am also new to boats although good with cars so any marine specific things Ide need a better explanation of and how tos of fixing it....thanx a bunch guys

 

[Don S]

Re: MORE WOT problems on mercruiser

List of possible causes of low WOT.

Engine Won't Reach Operating RPM. Check

• Fuel condition. Type and Octane possibly old fuel
• Propeller pitch or diameter, damaged blades
• Restricted fuel pickup tube or anti siphon valve Fuel System Test
• Crankcase oil volume, high oil level can cause aerated oil and lifter collapse
• Marine growth on hull and outdrive
• Wrong gear ratio in outdrive
• Restricted carburetor air intake (clogged flame arrestor)
• Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
• Poor cylinder compression Compression Test
• Carburetor defective, or wrong type.
• Fuel pump pressure and vacuum
• Boat overloaded, improperly loaded, or improperly trimmed.
• Engine Overheating
• Engine timing and ignition system operation (Full tuneup done recently)
• Remote control cables and linkage for proper travel to open throttle plates fully.

 

[DGIFF396]

Re: MORE WOT problems on mercruiser

thank you don...i actually already printed this off of a post by gdopetactical you replted to todat around 10:30. im gonna go through the checklist but the whole fact it isnt consitant and only happens after awhile throws me

 

[Sherminator]

Re: MORE WOT problems on mercruiser

check your fuel filter, i had the exact same sypmtoms on my dads boat last year 5.0 v8, it was a clogged filter, would rev fine under no load but under load i couldnt get over 3300 rpms. the filter we changed was screwed into the carb.

 

[Haut Medoc]

Re: MORE WOT problems on mercruiser

This is a fuel problem.....
Check the filters, pick up screen in tank, anti-siphon valve, fuel tank vent......
Don gave you the tests to perform in item #3.......

My guess is you need to pull the pick up & every last drop you can out of the tank.......​

 

[DGIFF396]

Re: MORE WOT problems on mercruiser

thank you all soo sooooooooo very much, im gonna puts with it wednesday and let you know....the fact she purrs like a kitten under no load just throws me...thanx again though....will let ya all know

 

[Don S]

Re: MORE WOT problems on mercruiser

the fact she purrs like a kitten under no load just throws me...

It shouldn't throw you if you really thought about it.
Without a load at 5000 rpm, you are using less than 1 GPH (Gallons Per Hour). While getting up on plane may take 20 to 25 GPH and running at cruise speed around 10 to 15 GPH. That is a very large increase in GPH that you will never see while sitting on a trailer, and it doesn't take much to imagine a 3/8" ID tube with screen on the end down in the tank getting plugged up or a slight air leak in the fuel system to suck more air than fuel to actually cause your problem. That is the reason you do the fuel system test.
Just because you own a boat, or worked on a car, means you automatically understand how all the systems work on your boat.