Top end power loss at WOT on Mercruiser 5.7 Alpha 1997 while under load... why?

[Timboat]

Hello, I am having slight (10%) power loss at WOT and hoping for some tips to restore it.
I have a 1997 Maxum 23ft fiberglass powerboat with a Mercruiser 5.7L V8 Alpha (sterndrive) with Thunderbolt ignition (carb) with over 1000hours.
I have had the boat for 4 seasons and the engine has always been strong and been able to WOT at 4400pm and hit 37mph on a GPS with a medium load of passengers and fuel.
The beginning of this season I noticed it had lost maybe 1mph of top speed at WOT, so I systematically performed a tune-up (as I usually do each year) starting with the distributor cap and rotor, oil and fuel water separator. After these items were changed there was no change in performance. At this point I moved onto phase two of the tune-up which included putting a 16oz bottle of Sea-foam in a tank of maybe 5 gallons of gas, letting it sit for a week of light use then filling the tank all the way up with non-ethanol gas. In addition, I sprayed in the carb pretty good with carb cleaner, and replaced 7 spark plugs. Why 7? Well because 3 seasons ago when I replaced all 8 I stripped one hole pretty bad and was afraid it would only make it worse to try and remove and replace it. So I went to replace the 7 unstripped plugs and the existing plugs were fine in great condition. I used the recommended NGK non-vortec BR6FS plug with a .040 gap to replace the 7. After these last changes I used the boat for a few weekends but didn’t notice the power loss at WOT right away until I started measuring with the GPS and found that my top speed was now 33mph (4mph less) and RPM at WOT closer to 3900 (500rpm less). I also replaced the tachometer in this time frame as the old one died. I had to assume one of the 4 things I did was the culprit. The new plugs, the Sea-form, the carb got messed up somehow, or new the tach.
In an effort to fix this, I pulled each plug to verify they were all nice and tight, had the correct gap and during this effort I replaced each spark plug wire AND double verified each wire was going to the correct cylinder (I even sharpie marker labeled each wire). I also replaced the fuel filter on the carb thinking that was plugged up with gunk after the seafoam treatment, it wasn’t. I also replaced the water fuel separator, which was also fine. None of these fixed the problem. Still only 33mph top speed with light load.
The engine idles very smooth and sounds great at 3900rpm. I simply lost about 10% of the top end power and can now go only 33mph. Also I can rev up to 4400rpm while in neutral and not under load.
I was thinking it could be growth on the hull, but every year I get some light algae growth and it only slowly me down a tad bit. Plus, it’s early in the season and not much has grown.
Can I tachometer change engine performance? There was a setting on the back to choose between engine cycles and amount of cylinders?
The prop is in the same good condition from past seasons as well, no new obvious damage.
Ideas on what to check next?

Thanks!!

 

[Scott Danforth]

first you posted in an outboard forum.....

PM a mod and have it moved.

second, read the stickies on low WOT. here is the link....look at link 15

third, if you had your head repaired correctly, you shouldnt worry about the threadsert in the plug hole

4th....go to your fuel system, most likely your carb.

5th, reving in neutral is meaningless as there is no load on the motor (doesnt take any fuel like actually being under load does)

6th, seafoam does nothing. there is no magic mechanic in a can. see 4th issue above

 

[Timboat]

Hello, I am having slight (10%) power loss at WOT and hoping for some tips to restore it.
I have a 1997 Maxum 23ft fiberglass powerboat with a Mercruiser 5.7L V8 Alpha (sterndrive) with Thunderbolt ignition (carb) with over 1000hours.
I have had the boat for 4 seasons and the engine has always been strong and been able to WOT at 4400pm and hit 37mph on a GPS with a medium load of passengers and fuel.
The beginning of this season I noticed it had lost maybe 1mph of top speed at WOT, so I systematically performed a tune-up (as I usually do each year) starting with the distributor cap and rotor, oil and fuel water separator. After these items were changed there was no change in performance. At this point I moved onto phase two of the tune-up which included putting a 16oz bottle of Sea-foam in a tank of maybe 5 gallons of gas, letting it sit for a week of light use then filling the tank all the way up with non-ethanol gas. In addition, I sprayed in the carb pretty good with carb cleaner, and replaced 7 spark plugs. Why 7? Well because 3 seasons ago when I replaced all 8 I stripped one hole pretty bad and was afraid it would only make it worse to try and remove and replace it. So I went to replace the 7 unstripped plugs and the existing plugs were fine in great condition. I used the recommended NGK non-vortec BR6FS plug with a .040 gap to replace the 7. After these last changes I used the boat for a few weekends but didn’t notice the power loss at WOT right away until I started measuring with the GPS and found that my top speed was now 33mph (4mph less) and RPM at WOT closer to 3900 (500rpm less). I also replaced the tachometer in this time frame as the old one died. I had to assume one of the 4 things I did was the culprit. The new plugs, the Sea-form, the carb got messed up somehow, or new the tach.
In an effort to fix this, I pulled each plug to verify they were all nice and tight, had the correct gap and during this effort I replaced each spark plug wire AND double verified each wire was going to the correct cylinder (I even sharpie marker labeled each wire). I also replaced the fuel filter on the carb thinking that was plugged up with gunk after the seafoam treatment, it wasn’t. I also replaced the water fuel separator, which was also fine. None of these fixed the problem. Still only 33mph top speed with light load.
The engine idles very smooth and sounds great at 3900rpm. I simply lost about 10% of the top end power and can now go only 33mph. Also I can rev up to 4400rpm while in neutral and not under load.
I was thinking it could be growth on the hull, but every year I get some light algae growth and it only slowly me down a tad bit. Plus, it’s early in the season and not much has grown.
Can I tachometer change engine performance? There was a setting on the back to choose between engine cycles and amount of cylinders?
The prop is in the same good condition from past seasons as well, no new obvious damage.
Ideas on what to check next?

Thanks!!

 

[Grub54891]

You have to fix/change the last plug. Then you can verify that is not causing an issue. losing one cylinder will cause less power.

 

[GA_Boater]

Moved to Mercruiser. Thanks, Scott.

 

[Bondo]

4th....go to your fuel system, most likely your carb.

Ayuh,..... Agreed,.....

 

[Rick Stephens]

Spraying carb cleaner on your carburetor is not a useful undertaking. If it is crudded up (sure sounds like it) then it needs rebuilding and all the ports proven clean and open. Same with Seafoam. No such thing as a mechanic in a can. There is something to be said about putting injector cleaner in an injected motor, personally I use ATF instead, but Seafoam in your tank didn't do anything.

Tachometers are designed to work on motors with different number of cylinders.

 

[achris]

I didn't see a compression test in that long list of things done. That would have been my FIRST check....

Chris.....

 

[Timboat]

Thanks everyone for the tips. Looks like my next steps are either to repair/replace the MerCarb AND/OR to verify compression in all 8 cylinders along with proper spark plug health and fitting in the partially stripped hole.

Although the compression check is relatively easy and quick I am nervous about taking the spark plugs out again as putting them back in my further strip the threads on the engine block even more. I already have one sleeve that fit so nicely in the stripped hole and don't want to monkey with a good thing.
In the other camp, I can get a re-manufactured Sierra Mercarb 2 barrel online for $350 that looks to be a relatively simple swap out with only one adjuster screw for tuning.

Any further thoughts one which route to head down first? taking in account the "low" cost of the carb replacement and the risk or further spark plug hole stripping damage...

thanks!! (i always learn so much on this forum)

 

[Bondo]

Although the compression check is relatively easy and quick I am nervous about taking the spark plugs out again as putting them back in my further strip the threads on the engine block even more.

Ayuh,.... Don't cross-thread 'em, or over-torque the 'ell out of 'em, 'n it ain't a problem,....

 

[Timboat]

I installed a remanufactured mercarb 2 barrel from sierra, it was exactly what was in the engine before and fit and installed with ease. The boat idles nicely and it started right up, even the next day after installing it.
...but it did not solve my low WOT problem. In fact the problem is now worse. During my speed test I only got to 3500rmp and 30mph (down from 37 a few weeks afo). So it's another 10 Percent worse after the carb switch out. Might I need to adjust or replace the throttle cable allow for more fuel to enter the carb at wot?

Its strange that after the carb swap it got worse, which makes me think it is carb/throttle related.

thanks

 

[devildogae]

I installed a remanufactured mercarb 2 barrel from sierra, it was exactly what was in the engine before and fit and installed with ease. The boat idles nicely and it started right up, even the next day after installing it.
...but it did not solve my low WOT problem. In fact the problem is now worse. During my speed test I only got to 3500rmp and 30mph (down from 37 a few weeks afo). So it's another 10 Percent worse after the carb switch out. Might I need to adjust or replace the throttle cable allow for more fuel to enter the carb at wot?

Its strange that after the carb swap it got worse, which makes me think it is carb/throttle related.

thanks

I would think putting the new carb on, it's not tuned, i.e. fuel air mixture set correctly for best performance. Might want to look into that. Also a compression test would be easy and very informative. Post back with results

 

[porscheguy]

Is every mercarb jetted/calibrated the same for every application?

 

[Scott06]

Mercarbs have different jets and Venturi clusters depdning on application. I'm not sure if Venturi is too much different but I think it has different holes for accelerator pump nozzles.

For OP did you check you are getting the full ignition advance curve not exactly sure what yours should be but like 26-28 degrees by 3000 rpm or so, also did the boat get water in the floatations foam that could be adding weight ?

 

[Timboat]

Last week I checked the compression and had 150-170psi in all 8 the cylinders and got all 8 plugs back in nice and snug. So I am ruled out compression being the issue. I then continued down this LONG list of possible low WOT issues and revisited the carburetor being the culprit. I had heard the re manufactured sierra carbs (rochester 2bbl) are often time sitting on shelfs in the warehouse for years and dry up and or simply aren't tuned right, and I had just installed on of these and experienced EVEN worse WOT performance. So, I bought a rebuild kit for the old original carb and gave it a chem bath. After about 12 hours of rebuilding, installing, configuring and testing I am proud to report 35mph on GPS speed test today! almost back to full speed!

I believe the problem to be the old carbs condition and the manufactured one was just a piece of junk.
The old carb had heavy deposits on 3 jets (screws with holes in them) on the long stem that moves up and down to depress the jet was fairly sticky (See pic). I could see that affecting acceleration and performance.

Are there any other particular parts of the carb that can cause low WOT?
I actually didn't replace the accelerator pump as it looked fine. But did adjust the float and cleaned the venturi and then sent compressed air though it. There are about a million places for gunk to get stuck in that.

This entire problem started after I put Sea-Foam in the fuel tank and sprayed some carb cleaner down the thottle body.
I would NOT recommend doing either of those to try and help performance!

thanks again for the tips!

 

[Scott06]

Absolutely the two main jets plugged and the power valve opening plunger sticking would limit wot performance. Is your ignition system up to snuff? Cap rotor and plug wires? Have you checked timing and are you getting full advance?