Boat won't go over 4,000 RPM's

[AaronWhitt82]

Well onto another problem now after I finally thought I had everything fixed and the whole mess I was having with the carb figured out.
I had my boat out Saturday for the first time this year. She started up great, idled fine and ran absolutely perfect......
I brought my new handheld GPS with me so I could dial in the speedos.
Top speed on the GPS read about 36.8 MPH @ around 3,950 RPM's on the tach.
There was only 3 people aboard the boat including myself and no added ballast.
I also have the stock 13X13 prop on the boat.
I slowed all the way to a dead stop and hammered the throttle and again it would top just under 37MPH and just under 4,000 RPM. I did this several times and it did the same thing every time.
I cannot figure out why it won't go over 4,000??? WOT should be between 4400 - 4800 RPM's

Here are a few things I could think of that might be wrong:
1. Secondaries aren't fully opening in the carb
2. Vacuum leak (I sprayed starting fluid around base gasket and it never slowed the motor)
3. Timing is off
4. Tach is off

All the motor info is in my signature

Any other input would be greatly appreciated

Aaron

 

[Bondo]

Re: Boat won't go over 4,000 RPM's

Ayuh,.....

Your list is a Good start,......

An Exhaust restriction should be added to it though......

Also,... Don has a Great List of things to Check in this case,...
I'll see if I can find it....

 

[Fireman431]

Re: Boat won't go over 4,000 RPM's

Check your prop. See what the factory recommended size and pitch are and compare it to the one on your boat. Since yours is a ski/wakeboard boat, the previous owner may have increased the pitch for more holeshot, which would decrease top end speed. Talk with a local marine service shop. Usually with a minimal deposit, they will lend out 2-3 different props to let you find out which works best for you. Just make sure you don't under prop your boat and cause your engine to over-rev looking for higher top end.

 

[Don S]

Re: Boat won't go over 4,000 RPM's

Looking for this Bondo?

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.

 

[Bondo]

Re: Boat won't go over 4,000 RPM's

Looking for this Bondo?

Ayuh,........ That's the Ticket,.........

 

[AaronWhitt82]

Re: Boat won't go over 4,000 RPM's

Don and Bondo -

Thanks for the help. I'll check everything on that list and see if I can narrow down the problem.

The prop on it now is the stock and original prop so I know that's not the problem.

I'll let you guys know what I find out

 

[HT32BSX115]

Re: Boat won't go over 4,000 RPM's

Howdy,

I'll add one more item to the list since it happened to me.

I had a similar problem (low WOT RPM) with my Wonderful high tech OMC 460 King Cobra.

It had good compression, new plugs, points, plugs, wiring, rebuilt carb, etc etc.

I even thought I had a vacuum leak and replaced the intake manifold gasket.

Once I ran out of ideas I started looking elsewhere. I removed the intake manifold once again and discovered that I could see (thru holes in the "valley") that several of the camshaft lobes were flat!

I would have never believed it. The engine only had a few hundred hours on it but that would absolutely make it run slow......it didn't affect the compression at all!

Just another possibility. I pulled the engine to completely rebuild it and the rest is outlined in my signature!!


Regards,

 

[AaronWhitt82]

Re: Boat won't go over 4,000 RPM's

Wow that sounds kind of odd. What kind of problem would cause the camshaft lobes to become flat? I've never heard of that before.
I hope thats not my problem. I guess I'll pull the carb and check it out.
I'm still thinking that it is something with the secondaries on the carb not fully opening or some sort of problem with that I hope.

Thanks for the advice though

 

[brunolund]

Re: Boat won't go over 4,000 RPM's

i've had that problem with a 350 chevy,
i,ve also seen a 460 ford running fine on 5 cylinders, and one of the good ones had a bolt stickin up from the bottom of the piston, about 1/8 inch out. you could see where it had hit the head, but the engine sounded good!!!!

 

[180shabah]

Re: Boat won't go over 4,000 RPM's

Wow that sounds kind of odd. What kind of problem would cause the camshaft lobes to become flat?

Oil starvation, wrong weight of oil, or using oil without the proper additives for a non-roller cam.

 

[HT32BSX115]

Re: Boat won't go over 4,000 RPM's

Well, I have my suspicions!

My brother routinely ran it at 5000 rpm for one thing.......the other was he put Slick 50 in it after it was rebuilt.

I wouldn't put slick 50 in my Briggs and Stratton!!!

Or anything else for that matter!!! (about the only thing I'd do with it is burn it in my waste oil furnace....well maybe not....the Teflon would probably screw up the combustion chamber!!)

I am also guessing that the cam may have never been broken in properly.......It did run fine for about 10 years or so though...

Wow that sounds kind of odd. What kind of problem would cause the camshaft lobes to become flat? I've never heard of that before.
I hope thats not my problem. I guess I'll pull the carb and check it out.
I'm still thinking that it is something with the secondaries on the carb not fully opening or some sort of problem with that I hope.

Thanks for the advice though

 

[AaronWhitt82]

Re: Boat won't go over 4,000 RPM's

Hey guys I took my boat to the dealer today to have them diagnose it.
According to them they are saying that the secondaries aren't opening on the carb.
They never did water test it and as far as my knowledge goes the secondaries won't open by just running it on the muffs.
They needed to water test it and put a load on it to verify if they were opening or not.
They said "everything runs great and smooth like it should, but the only thing we are coming up with is the secondaries"

They never did check the timing either which needs to be checked as well.

I'm also wondering if its something with the ignition module or similar thats not allowing the motor to go over 4,000????

 

[Maclin]

Re: Boat won't go over 4,000 RPM's

Opening rate for the secondaries is controlled by the spring in the vacuum actuator housing. It is easy to remove but the electric choke housing has to come off first so you can get to all the mounting screws. The trickiest part is keeping track of the reddish cork gasket between the vacuum housing and the throttle body, it is used to seal the vacuum port between the housing and the throttle body. The gasket is small, and kind of "tall" for a cork gasket. It may rear or break so you need to be ready with a new one. It is in the rebuild kits.

Holley has a vacuum secondary tuning kit that comes with all the color coded springs and a paper with all their rates. It is usually around $20 bucks.

Make sure the secondary linkage is funtional. There is a linkage running from the vacuum housing to a lever on the secondary plates. On the throttle side there is a lever linked from the primary linkage to the secondary throttle plates. The lever is not directly linked as it has a curved opening that is designed to pull the secondary plate closed when the throttle is pulled back to idle.

Previously you said you thought it was a 600cfm carb but you also said your engine had 310HP. 600cfm would be a bit small for a 300hp engine, but even so should probably keep up past 4,000 rpm, but then again maybe not. If it cannot keep up then it would lean out the fuel mixture and leading to detonation and early engine failure. If it has the plain steel spring in it now then I think you have found the problem. If the carb is truly a 600cfm then I would imagine your engine would need the lightest spring, like the purple one or maybe even the "weak" yellow one (there are two yellow ones).

 

[MikDee]

Re: Boat won't go over 4,000 RPM's

Hey guys I took my boat to the dealer today to have them diagnose it.
According to them they are saying that the secondaries aren't opening on the carb.
They never did water test it and as far as my knowledge goes the secondaries won't open by just running it on the muffs.
They needed to water test it and put a load on it to verify if they were opening or not.
They said "everything runs great and smooth like it should, but the only thing we are coming up with is the secondaries"

They never did check the timing either which needs to be checked as well.

I'm also wondering if its something with the ignition module or similar thats not allowing the motor to go over 4,000????

It could be the secondaries not opening, Or, If you're not getting full ignition advance, that could be the problem. If it is a mechanical advance distributor, a common problem is, the weights, & springs, in the advance mechanism sometimes get rusted, and, or, corroded, and bind up killing power, from being around a marine environment. With newer electronic ignition this can happen also, but it is less common, and harder to diagnose.

 

[sarantis]

Re: Boat won't go over 4,000 RPM's

i consider the air secondaries very possible.In my rochester 4 bbl i put a friend driving and playing with the shaft that engages the vacuum valve with the secondaries the boat went fromm 2500 to 4500.But since you put it for fist time of the season(after much rest?)you have to take into consideration the case oof bad fuel.Personally i would connect a clear hose from the pump's output to the carb inlet and would watch if in high rps i 've full gas flow or some bubbles come up.The bottom of your boat is clear?

 

[AaronWhitt82]

Re: Boat won't go over 4,000 RPM's

Opening rate for the secondaries is controlled by the spring in the vacuum actuator housing. It is easy to remove but the electric choke housing has to come off first so you can get to all the mounting screws. The trickiest part is keeping track of the reddish cork gasket between the vacuum housing and the throttle body, it is used to seal the vacuum port between the housing and the throttle body. The gasket is small, and kind of "tall" for a cork gasket. It may rear or break so you need to be ready with a new one. It is in the rebuild kits.

Holley has a vacuum secondary tuning kit that comes with all the color coded springs and a paper with all their rates. It is usually around $20 bucks.

Make sure the secondary linkage is funtional. There is a linkage running from the vacuum housing to a lever on the secondary plates. On the throttle side there is a lever linked from the primary linkage to the secondary throttle plates. The lever is not directly linked as it has a curved opening that is designed to pull the secondary plate closed when the throttle is pulled back to idle.

Previously you said you thought it was a 600cfm carb but you also said your engine had 310HP. 600cfm would be a bit small for a 300hp engine, but even so should probably keep up past 4,000 rpm, but then again maybe not. If it cannot keep up then it would lean out the fuel mixture and leading to detonation and early engine failure. If it has the plain steel spring in it now then I think you have found the problem. If the carb is truly a 600cfm then I would imagine your engine would need the lightest spring, like the purple one or maybe even the "weak" yellow one (there are two yellow ones).

Maclin -

Thanks alot for the help.
I have already removed the secondary vacuum actuary housing and checked everything out.
The diaphragm is still brand new and the cork is still brand new from the rebuild last year. No tears or anything missing.
It also has a red spring in the housing, but I'm not sure if that's a heavier spring or lighter?
I also shot some carb cleaner through the vacuum port and some compressed air to make sure it wasn't clogged with anything and it was fine.
Secondary linkage is not bound and moves freely as well. I understand the concept of how all that works.

As far as the CFM goes I thought it was 600, but then again maybe not. I still cannot seem to find out all the specs on the carb because Holley still says they have no listing for PN 75021 which is listed on the airhorn.

I'll go ahead and put the lightest spring in the secondary housing and see what that does for me.

Also it is not backfiring, sputtering, bogging or doing anything of that sort and runs smooth.

Thanks

 

[AaronWhitt82]

Re: Boat won't go over 4,000 RPM's

i consider the air secondaries very possible.In my rochester 4 bbl i put a friend driving and playing with the shaft that engages the vacuum valve with the secondaries the boat went fromm 2500 to 4500.But since you put it for fist time of the season(after much rest?)you have to take into consideration the case oof bad fuel.Personally i would connect a clear hose from the pump's output to the carb inlet and would watch if in high rps i 've full gas flow or some bubbles come up.The bottom of your boat is clear?

Sarantis -

I know it doesn't have bad fuel because I drained it before Winter storage and just filled it with 93 octance and 2 pints of Sea Foam so I know the fuel is fine.

 

[Maclin]

Re: Boat won't go over 4,000 RPM's

I think I have a spring kit around, I will try to find the rating in relation to the others for that spring.

 

[AaronWhitt82]

Re: Boat won't go over 4,000 RPM's

Maclin -

I just looked on Summit Racing and they don't even list a Red Spring in the kit for the Secondaries.

Here is what they list:

White (quickest)
Short Yellow
Tall Yellow
Purple
Plain
Brown
Black (slowest)

I guess I'll start with the lightest and tune it from there to see when and if they open.

Thanks again,
Aaron

 

[Maclin]

Re: Boat won't go over 4,000 RPM's

The red is probably really purple, they are just barely tinted at the top.....

White - Lightest
Yellow (Short Spring)
Yellow
Purple
Plain (Steel grey)
Brown
Black - Heaviest



In the automotive world the plain jain 750cfm vacuum secondary Holley comes with the plain steel spring. Most slightly hot-rodded engines I worked on liked the purple or yellow or short yellow. The white was almost always too much.

By contrast when I looked up the listing# on the Marine 735cfm Holley on my 5.7 engine in the boat it called out the white spring. Marine applications need to run richer, and the secondaries need to open up and run fuel thru them to keep the gas in the secondary bowl as fresh as the primaries. I did change mine to the purple because I could tell it was opening way too soon and way too much. You will know by the sound, when it is too far open then when you back off there is a very pronounced whistle/sucking sound that changes pitch as the secondary plates close. My carb is way quieter now and has about 200 more rpm, but I had a different situation than yours as my carb is definitely 735cfm.

If your carb is really a 600 cfm then you would need one of the yellows or even the white to feed a good running 5.7. If it is larger than 600 then I would think back off a couple from there. (I am guessing yours has the purple in it now.)

Holley makes a quick change spring kit that replaces the stock housing. I had one or two of those thru the years, and that is the ticket. You can change springs by removing a lid with two screws, very slick. Jegs and all the hot rod houses have stuff like that, as well as the spring kits.