Launched it today. question about prop

[footbrake]

Finally launched it today and it's wonderful. question is, I could only get 3000 rpm w.o.t. out of it. Motor ran smooth,good oil pressure, good temp. My marina says could be the wrong prop on it, because it should be able to do 4000-4500 rpm. It's a 2.5,120 hp OMC. Any input on this? Thanks I have some pics of her too. View attachment 194899

 

[Bulbash]

Re: Launched it today. question about prop

What prop do you have right now? Did you do the tune up fuel filter, spark plugs, ignition?

 

[Don S]

Re: Launched it today. question about prop

Not a prop problem to be that many rpm off.

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.

 

[footbrake]

Re: Launched it today. question about prop

Don't know about the prop, but put new plugs,wires,points condenser, cap, rotor, the works and had a reliable marine guy do it. I guess I'll have to have the marina pull off the prop to see what it is. The boat came with an extra prop also

 

[Don S]

Re: Launched it today. question about prop

The PROP IS NOT YOUR PROBLEM

May be the answer you wanted to hear, but it's not causing a 1000 to 1500 rpm drop.

 

[bonzoscott]

Re: Launched it today. question about prop

Important is history. Did it ever run good (for you). Did you just acquire the boat? The first thing I would check in Don's list is:
#15 Remote control cables and linkage for proper travel to open throttle plates fully. Motor off, move throttle as far as it will go, push carb choke butterfly open and look down in the carb to see if throttle plates fully opening.

 

[footbrake]

Re: Launched it today. question about prop

Yes I just acquired the the boat. This was the first time out. I will check the throttle plates next time at the boat. thanks

 

[Idlespeedonly]

Re: Launched it today. question about prop

Check to make sure the tach is correct.

 

[Don S]

Re: Launched it today. question about prop

Yes I just acquired the the boat. This was the first time out. I will check the throttle plates next time at the boat. thanks

Defective tach, or wrong gear ratio in the drive would be the first two things I looked at.
Someone probably replaced the drive with a V8 ratio.

 

[footbrake]

Re: Launched it today. question about prop

ok, checked the throttle, it's opening fully. Got the prop off and it's 13 3/4 x 15. The other prop I'll have to look up. It's a Michigan prop and it's stamped J 8 0.

 

[Don S]

Re: Launched it today. question about prop

ok, checked the throttle, it's opening fully. Got the prop off and it's 13 3/4 x 15. The other prop i'll have to look up. It's a michigan prop and it's stamped j 8 0.

It is NOT A PROP PROBLEM


Since you are bound and determined that you have a prop problem, let me just move this over to the prop forum,

 

[Bulbash]

Re: Launched it today. question about prop

It is NOT A PROP PROBLEM

100% Agree

 

[Bulbash]

Re: Launched it today. question about prop

Wrong gear ratio in outdrive

how would you check if thats the case?

 

[jakedaawg]

Re: Launched it today. question about prop

Did you have the boat trimed out when you only reached 3000 rpm? BY "trimmed out" I mean did you use the little tilt button to raise the outdrive slightly which results in the bow lifting out of the water. Speed and engine RPM both increase when done properly. How many people did you have in the boat for the test drive? The amount of excess weight and where it is stowed will also affect RPM. I would not expect the wrong prop unless there wasn't a prop on it when you bought it or it appears brand new.

 

[jakedaawg]

Re: Launched it today. question about prop

After looking at the pictures I believe you have an older vessel. I think that if you didnt have a boat load of people, and did have the outdrive trimmed properly, I would first take a look at the floatation under the floor area. This foam could be the old open cell foam and therefore be waterlogged even if it has been out of the water for years.

 

[steelespike]

Re: Launched it today. question about prop

Your gps speed at 3000 could help evaluate things better.
I agree with Don The prop isn't the problem In fact it may be an indication that something else is going on.
I think a healthy setup could run a 17" prop for good overall performance and about 28 mph gps wot.
Might even do a 19" for about 32 mph but a little struggle for hole shot and added weight.

 

[footbrake]

Re: Launched it today. question about prop

By" trimmed out" did you mean on plane? I thought the tilt button was just to lower the outdrive into the water from the raised position. sorry but this is my first boat and had the outdrive fully lowered. I didn't know you were supposed to trim the boat with the trim switch on the dash. OMG I've probably waisted a lot of peoples time on here and I apologize for it. I had just myself and my friend that's had a boat for 35 years and when he asked if I could trim it I thought he meant by the trim tabs which I don't have, and said ''no I can't trim it". To make things even worse, I don't think I had the throttle wide open . When we went out and it wouldn't go past 3000 rpm the shift lever was like at 10 o'clock. There was a detent in the travel and I thought that's where it ended. When we checked to see if full throttle was opening the carb all the way. we pushed it further to like 8 o'clock to fully open the throttle. I bet it was never fully opened on the water and to compound things was never trimmed. Wow

 

[jestor68]

Re: Launched it today. question about prop

Your OMC Stringer stern drive has only tilt on the outdrive. Do not try to move that while running; you will damage the tilt mechanism. :faint2:

If it has TRIM, that would mounted on the front of the engine(at the engine mount) and would be controlled with a separate switch. Even without electric trim, you can manually adjust the drive angle by turning the adjusting lever at the front engine mount in order to set the drive angle to something near a neutral trim angle.

It sure sounds like you never opened the throttle all the way.

 

[steelespike]

Re: Launched it today. question about prop

No problem, every new post is a learning experience.
I couldn't tell it was a "stringer" or did I know their idiosyncrasies.
I'm sure we would all like to know your final results.

 

[footbrake]

Re: Launched it today. question about prop

Yes jestor68 the stringer 400 does not have a trim switch but can be set manually like you explained by raising or lowering the front of the engine . My marine mechanic noticed the front of the engine was unusually low which would tilt the outdrive up. He adjusted the front of the engine about an inch higher. We're gonna try this combination this weekend.