1981 formula 23 sc express will not plane

[hvywt77]

Hello, I would like some help addressing a problem with my boat. It will not plane out. I have been working on this for the last year. Here are some details, any assistance or advice anyone could provide would be helpful. Any questions please let me know…
1) Performance issue you are trying to correct: will not plane / looking for more RPM at WOT
2) Current prop manufacturer, model, aluminum or stainless as a minimum: 14”x23 pitch alum mercruiser
3) Current prop diameter and pitch (required): 14”x23 pitch
4) Wide open throttle RPM and speed with an average load (very helpful) : WOT is only 2300 speed is unknown, maybe 20mph.
5) Engine/drive make, model, year, and HP: mercruiser Chevrolet 350 rebuilt in 2012 bored .030 over, stock 274 cam, 4 bbl mercriuser 350 mag carb, holley marine fuel pump, HEI dist, new plugs, wires.
6) Boat make model, year, length and weight: 1981 Formula 23 SC Express 23ft, weight on brochures is 3900 lbs.
Last trip out the boat was closest to plane I could see the horizon while seated. I have both trim tabs all the way down. But it does not seem to have lift and will not gain any speed/rpm above 2300 rpm. I estimate speed at about 20 mph. It is barely planed out. If i trim up the bow comes up, if I trim back down no change in rpm. I took off a 16” x 14 pitch prop that was on there when I bought it. I thought it was a house boat prop and assumed my problem would be gone with a new prop…not so much. Ok. Tried to keep it short. Let me know what questions or advice you all have….

 

[MikDee]

First,
1) Make sure there's not a crossed plug wire!
2) Make sure the secondaries are opening on the 4 bbl Carb

 

[hvywt77]

yes, i have had someone else check the wires and timing for me. the secondaries are opening, when i pull throttle back out of secondary it slows slightly, when i push back to WOT i can feel it bog a little bit. I have run briefly with the spark arrestor/air filter off and had an accomplice watch the carb function. Secondaries are opening.
Here is a few more things i have checked:
-I have replaced the exhaust flaps.
- ran with an edelbrock 600cfm carb from another motor
-visually inspected engine coupler wiht bore scope. looks brand new. Previous owners brought this boat back from the dead. sort of.
thanks,
keep the questions/answers coming!!

 

[jestor68]

According to the Mercury prop selector, you should try a 14.5 X 17 Black Max(al) or the same size Vengeance(SS).

Your boat has a dry weight close to 4000 lbs.

 

[MikDee]

With that worked engine, you should be seeing 4200-4600rpm, & about 55mph! with that 23" pitch prop. Okay, your secondary's are opening up, I still say you should go over the spark plug wires yourself! Firing order clockwise on Distributor, 1-8-4-3-6-5-7-2,
Cylinder wiring from Front of engine: RT.side: 1-3-5-7, LFT.side: 2-4-6-8

An engine that suddenly can't reach full rpm, when it had before, leads me to strongly suggest a crossed plug wire, once you hear "somebody" replaced them, & the Dist cap, & rotor recently?

Don't mess with the trim tabs, you can do more harm then good on a poor running boat,,, just keep them flat with the bottom (neutral position).

Somebody put that size prop on the boat because that's what's been working good previously! Don't let an unhealthy engine make you change your prop, lets get it right!

 

[jestor68]

Who told you to put a 23 pitch prop on there?

 

[Maclin]

Did the boat ever perform correctly after the renewed engine was installed? Please provide a clearer timeline of events related to engine work and the beginning of these performance problems.

 

[MikDee]

I figure if your not having sporadic running, sporadic fuel, or sporadic electrical issues. Then the next thing to check is the timing, there's a special procedure for Thunderbolt IV, & 5, that can easily be done by the average guy. I figure someone will be along with a link

 

[MikDee]

Wait a minute! Just how Big is this boat? Is it a Cuddy cabin, or a Cabin cruiser (a hull with a bungalow on top,,,lol?). Some pix would help?
If it's the latter, then you need a prop like Jestor68 mentioned, a 17" pitch.

 

[hvywt77]

Fixed it with a 17 pitch prop! I borrowed a 14.5" x 17P three blade stainless stilleto. Got it on plane. I made the call to try the 23 pitch. I get 29 MPH gps @ 3100 RPM loaded with camping gear and ~45 gallons of fuel.

 

[hwsiii]

hvywt77, your problem is NOT fixed. Theoretically, if your tachometer is correct, you are ruining your motor by only being able to turn 3,100 RPM at wide open throttle. Your motor is lugging at all RPM thru out its range and it is using more fuel as well as probably building up carbon in the cylinders because of this. Either there is a problem with the motor or you are using the wrong test prop.

That motor should be turning between 4,400 and 4,800 RPM at wide open throttle and your gear ratio should be 1.47, please verify these numbers for me.

If your tachometer is way off then the prop slip shown is going to be a whole lot higher and you are definitely using the wrong type of prop for your application.

Please borrow another tachometer, preferably a mechanics tachometer to verify your tach is working right.

I ran some numbers for your perusal based on the information that you provided in your last posting and here they are.

 

[hwsiii]

No Title