WOT problem

[dgopetactical]

1988 mercruiser 5.7 alpha 1

my normal wot was around 4500 and a speed of 45mph loaded. The last couple of trips she just feels different. My wot is now around 3000 speed of 30 and feels boggy..
When I back the throttle back a inch or two the rpm's jumps to around 4000-4100 with a speed of 35 mph and it sounds and feels alot better.

The spark plugs and motor/outdrive oils were changed (gap of .035). a new exact replacement prop was installed 14x19(michigan match). And the fuel water seperator was replaced. this was all done prior to the power loss(a few weeks ago) the boat ran good for a half of a day and then started the power loss.. No water in the fuel that I can see. It does not back fire or sputter just a feeling of lost power. The only "minior change" to anything is this new prop is cupped the old one was not. I canged the water seperator again and this did not help.
Please help!
Thank you,
Dgope

 

[Don S]

Re: WOT problem

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
• Remote control cables and linkage for proper travel to open throttle plates fully.

 

[dgopetactical]

Re: WOT problem

Thanks Don, I will start with the stuff on your list i can do myself. Just checking to see if anybody had similar problems to make the list shorter.
thanks,
dgope.

 

[Haut Medoc]

Re: WOT problem

I would check items 3, 7, 8, 9.....​

 

[dgopetactical]

Re: WOT problem

thank you. I do not have the means to vacume test my fuel system. but I will try to work my way up to that point.

 

[Don S]

Re: WOT problem

Either get a gauge, or do like a lot of other people do. Spend the rest of the summer and replace a lot of good parts in the hopes you chance on the cure.

 

[Haut Medoc]

Re: WOT problem

The gauge is cheap, you really need to do those tests.....​

 

[tmh]

Re: WOT problem

Just curious because I am still working thru Don's list myself - how long would it take an average DIYer to do all these things? I'm "well below average" on repairs but am curious if some items on the list are better left to the mechanics unless one is "well above average". Being a novice, i have no feel for the level of difficulty and time so some of these things are a bit intimidating to me, as i am sure they are to other non-wrenchers.

Thanks.

 

[John_S]

Re: WOT problem

A 1500rpm loss is not a cupped prop problem.

I'd check the spark plug boots to make sure they are tight and make sure you didn't crosswire any. Only because it seems to be sudden issue, and thats were what you had your hands last.

Then I'd check base and advance timing. Good, move the the fuel related items in the list. And then the rest of the list.


tmh, I consider myself an average backyard mech. It's been quite some time since I f'ed something up to cost some good $s or large redo time. Nothing on that list scares me. I wouldn't worry about the time to do each one. Being careful and thural, is more important. Read, and follow the mfg service manuals. Ask here when not sure.

 

[dgopetactical]

Re: WOT problem

I found the problem. This boat was purchased in may and apparently the owner riged a homemade oil dip stick that was not accuarate or ment for that engine it had dots on it and I thought everthing was fine. To make a long story short I changed the oil and the above problem started to accure. When I filled the oil I went by the dipstick not by the quart. (pumped out of a 50 gal drum) since the dip stick was not correct I put 1/2 gallon more oil in than I should have. I have got the correct dip stick now and I gained back my 1,500 rpm and doing fine at 4,500 wot.

 

[rmurray]

Re: WOT problem

I have the same motor and I am wondering what the recommended RPMs should be at WOT for this motor? I want to make sure I am running the correct prop size. RPMs max out just over 4000 at a speed of about 45MPH (21' bowrider). I think I should be running a little higher than that. My motor is actually the 1987 Mercruiser 260HP Alpha One. Thanks for any help!

 

[Don S]

Re: WOT problem

Start a new thread with your question. Even if it's a similar problem. It really confuses the replies when trying to answer two questions in the same thread.