Help with slow boat only 3000 rpm at WOT

[ibrnrbr]

I have a 78 Bayliner 25.5ft with a 350/280 penta. The boat runs smooth but wont get over about 3000rpm at WOT. This only pushes the boat 15-17mph (gps) and wont plane out. ive had problems with the boat since i bought it but one day it was doing over 30 on gps. It has new; electronic distributor and coil (IS advancing), new batteries and all cables(Is charging and alternator tested good), carb rebuilt, new water seperator/filter, new manifolds and risers, fresh bottom paint, and 148-150 compression on all cylinders. WHAT AM I FORGETTING? maybe wrong prop? but it did do over 30 before and im assuming the previous owner wouldnt have started troubleshooting by replacing prop. Please help. i dont know how fast this old boat should go but im hoping faster than 15. Thanks.

 

[sqbtr]

Re: Help with slow boat only 3000 rpm at WOT

Re-check your firing order

18436572

Just a thought

 

[Don S]

Re: Help with slow boat only 3000 rpm at WOT

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.

 

[oops!]

Re: Help with slow boat only 3000 rpm at WOT

the answer will probably be in what don gave you....

but here is my GUESS....if the throttle will hit 4500 in neutral and seems strong....you might be way over propped....the numbers are on the prop....see what pitch it is ....and post that in the prop section for conformation.

 

[Don S]

Re: Help with slow boat only 3000 rpm at WOT

Revving to 4500 rpm in neutral is a NON-Test. You could pull all the spark plug wires off of one side and still hit 9000 rpm without a problem.

You can't loose that much rpm with props alone. Changing pitch 1" will usually change the rpm 100 to 150 rpm.
Assuming the boat has a 19 pitch prop installed now, he would have to drop to 9 pitch prop. That doesn't even exit.

 

[Eric 3.7LX Alpha 1]

Re: Help with slow boat only 3000 rpm at WOT

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.

Thats a great check list, unforunately, it's long. I would personally print it out, and start at the top working my way down checking off that list until you find the problem. U have the service manual for that engine/drive?
Keep us posted,
good luck.

 

[ibrnrbr]

Re: Help with slow boat only 3000 rpm at WOT

ok so other then the carb itself being bad everything checks out. I havent done the fuel system tests yet either. i put a new anti siphon valve and the engine will run off the fuel it has in lines, die, and not restart. when i pull off hose there is pressure released as if the fuel pump cant move the ball to let fuel up. any ideas?

 

[ibrnrbr]

Re: Help with slow boat only 3000 rpm at WOT

All fuel tests passed but now I'm also finding something that may be an issue. I have an electronic distributor and when I remove timing shunt after setting base timing to 8* it jumps to 20*??? Anyone want to buy a boat haha

 

[tschmidty]

Re: Help with slow boat only 3000 rpm at WOT

Set the base timing and don't worry too hard about the timing when you remove the shunt, although hopefully someone will post more definitvely. Did you double check the firing order per the first post? I'd also check the coil since a bad coil can have trouble keeping up spark at higher rpms. Since that would be a non stock distributor, I would reiterate to check the spark order and make sure you are getting the right voltage at the coil. Was originally a resistor wire (presuming cause it's a '78), did it get installed properly with full 12v going to the coil? How are the plugs and wires?

And you say all fuel test have passed? What was the deal with the carb being "Bad"? Are the secondaries opening all the way at full throttle? Going to be a quadrajet with vacumn secondaries I am guessing and they can be a little tricky to get to open fully.

 

[ibrnrbr]

Re: Help with slow boat only 3000 rpm at WOT

Firing order is correct. Coil was new along with wires as pakage when distributor was purchased. I did cut out resistor thing and run 12v. I also put cyl 1 to TDC just to make sure timing marks lined up because I'm out of ideas. As for the carb- its a 2 barrel carter so no secondaries to worry about. Any other suggestions would be great. Im not hesitant to try ANYTHING on this boat- just make it work.

 

[bigdan1]

Re: Help with slow boat only 3000 rpm at WOT

had this problem in a car ...check if fuel cable on carburator goes full travel

your carburator might not go wot ....takes 10 seconds to check (when engine stopped)

 

[ibrnrbr]

Re: Help with slow boat only 3000 rpm at WOT

yes the carb travels all the way open when throttle pushed