5.0 low wot rpm, unable to plane

[sikemd]

I have a merc 5.0 that has been running great all year. I havent ran it much the last month or so. It stays in the water and the outrive is pretty well covered in barnicles. The bottom is painted but im not sure how its looking... i went out today with 6 people and couldnt get it to plane... i dropped a few people off too see if it was weight related but even with 2 of us i hit a max rpm of 4200 which is normly closer to 6k rpm. The engine sounds fine no odd noises and idles pretty smooth... any pointers or ideas where my issue may lie? Thanks!

 

[Bt Doctur]

bottom growth will cause loss of performance

 

[Alumarine]

Are you saying it normally revs to 6000 rpm?

 

[sikemd]

Yeah, normally at full throttle it will rev to around ~5600rpm, now it won't go above 4200 and it's just not enough to get up and going. I believe it it s a 4 barrel carb is it possible the high rpm carbs are gunked? I'm highly doubtful the barnacle's are the culprit because I really don't think it could be that bad under there, it's only a 23ft boat and the bottom paint was in good shape before putting it in this year, however the outdrive is pretty rough and prop has just a few barnacles on it. I haven't hit anything and like I said the engine purrs like a kitten, she just doesn't want to go fast like ricky bobby.

 

[Bondo]

Yeah, normally at full throttle it will rev to around ~5600rpm, now it won't go above 4200 and it's just not enough to get up and going. I believe it it s a 4 barrel carb is it possible the high rpm carbs are gunked? I'm highly doubtful the barnacle's are the culprit because I really don't think it could be that bad under there, it's only a 23ft boat and the bottom paint was in good shape before putting it in this year, however the outdrive is pretty rough and prop has just a few barnacles on it. I haven't hit anything and like I said the engine purrs like a kitten, she just doesn't want to go fast like ricky bobby.

Ayuh,..... Redline on that motor is 4800 rpms at Wot,.... Yer Way under propped,....

'n you'd be surprised at how much of a Drag, sea-life is,....

Anyways, from the Wizard hisself, from Don'S Adults only section,....

Here is a list of possible causes of low WOT (Wide Open Throttle) rpm.

In no particular order

Engine Won't Reach Operating RPM. Check

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

 

[thumpar]

4200 RPM is almost to the recommended WOT RPM. 6k is way to high. The barnacles will slow you down especially if they are on the prop.

 

[DaveG55]

My 21' with a 5.0 carbed 2 bbl is on plane under 20mph and has a MAX rpm of between 4400-4800 rpm. If your wot is normally 5600 then your 800 over what is recommended for your engine. That translates to about 8" of pitch under proped. You need to correct that once you get the planeing issue resolved.

Some reasons for difficulty/not getting up on plane;
Weight, are you sure your boat hasn't taken on water weight?
Trim, are you sure your outdrive /trim tabs are trimming properly?
Dirty, grown over bottom, outdrive or rudder, don't dismiss this, pull the boat and look.
Damaged or fouled prop, aapparently you don't appreciate how badly this can affect performance.
A slipping prop hub would also keep you from planeing but would probably show a spike in rpm rather than a drop.

What I would do is pull the boat, have it weighed and compare that to what it should be, check and clean the bottom and outdrive, verify everything is working pproperly and use a proping guide to get the proper size and pitch prop for your boat. My armchair guess on a prop would be a 14"-14.5" x 17p or 19p would get you within specs.

 

[Alumarine]

I would make sure the tach was working properly.

 

[sikemd]

Appreciate the posts fellas! So... I just took it out to a shallow sandy bottom to see how dirty the hull was and it was pretty bad. We attached some putty knives to wooden polls and scraped the hull, outdrive and prop the best we could for about half an hour in choppy water. The sandy bottom became a barnacle bottom. Took it for a cruise afterward and it was still nowhere near maximum performance but there was a very noticeable increase. RPMs maxed at 4k on the way out and 5k after cleaning, top speed was 18mph on way out and 24mph on the way in. Previously I could hit ~38mph top speed (5600RPM) and around 22mph (4000RPM). I'm going to have to pull it out of the water and really get her cleaned up to get back to full performance I'm guessing... but I can at least get on plane for now. Thanks for informing me about the under propped / max RPM issue, I will look into purchasing something more "proper" (lol) before the end of the season .

 

[Bondo]

Appreciate the posts fellas! So... I just took it out to a shallow sandy bottom to see how dirty the hull was and it was pretty bad. We attached some putty knives to wooden polls and scraped the hull, outdrive and prop the best we could for about half an hour in choppy water. The sandy bottom became a barnacle bottom. Took it for a cruise afterward and it was still nowhere near maximum performance but there was a very noticeable increase. RPMs maxed at 4k on the way out and 5k after cleaning, top speed was 18mph on way out and 24mph on the way in. Previously I could hit ~38mph top speed (5600RPM) and around 22mph (4000RPM). I'm going to have to pull it out of the water and really get her cleaned up to get back to full performance I'm guessing... but I can at least get on plane for now. Thanks for informing me about the under propped / max RPM issue, I will look into purchasing something more "proper" (lol) before the end of the season .

Ayuh,.... You should not run yer motor over 5 grand, til ya reprop, then try Wot,....

4800 rpms, at full trim, light on fuel is the sweet spot yer lookin' for,..... with a clean hull of course,....