1985 3.0L 400 stringer exhaust passages

[OMC1985]

I own an 18 ft 1985 StarCraft. 400 stringer 3.0L, 140HP. It runs strong, and planes out fine but the engine tops out at 3400 RPM. It has an electronic distributor, new plugs, wires and the compression is good. I changed to a higher pitch prop that improved the hole shot but the engine still topped out at 3400 RPM. The carb is working perfect and the plates are wide open. My question regards the exhaust flow. I noticed when I stated it the exhaust appears to be coming out around the outdrive where the sterndrive drops into the upper case when the drive is lowered, not down through the prop. If I understand the system correctly the exhaust should be going down through the sterndrive and out through the prop center hub. I am thinking that the exhaust back pressure from a plugged exhaust passage may be creating a lot of back pressure and limiting the RPM. Am I understanding the exhaust design correctly? Is there an easy way to check the exhaust passages for debris or a fault that would cause this?

 

[southkogs]

Welcome aboard: higher pitch prop should have gotten you going a little faster unless you're over pitched. Lower the pitch to get a better shot outta' the hole.

I suppose it would be possible to block the through hub exhaust, but if you run it on muffs you should be able to tell if that's happening. Besides, the Stringer can dump a buncha' exhaust between the intermediate and the upper gear case.

 

[Howard Sterndrive]

when I stated it the exhaust appears to be coming out around the outdrive where the sterndrive drops into the upper case when the drive is lowered, not down through the prop.

that is normal. There is a large grate (~13 square holes) in the anode at the bottom of your intermediate where exhaust should come out when the boat is not on plane.

I suspect your ignition timing is not advancing. If you remove the points plate from the distributor you can do a visual on the advance weights and springs.They are likely all gummed up and stuck with 30 year old oil. Lots of penetrating oil and then fresh engine oil once released will fix em.

 

[OMC1985]

Thank you for the replies. I will check the passages for any blockage. I am pulling the boat out in a few days so I will attach the muffs to verify if the exhaust is blocked in any way. Thanks for confirming the leakage around the case is normal

The ignition is an electronics version so no points anymore. The advance system is working well. I checked it and it is clean and moving smoothly. I have a digital timing light with an advance screen on it. Timing is set at 4 degrees and advances to 32 fairly quickly. I had a 17 pitch prop and bought a new 15 pitch. As stated it improved my hole shot. however the RPM did not increase at all (puzzled by that). It was the same for both props. The top end speed dropped about 3 MPH with the new prop since the RPM remained the same. I do have one question. Our marina sells premium gas only. Did this engine require premium #1 gas or did it live on regular? Since I am using premium is there any room to advance the timing a few degrees further?

 

[southkogs]

I think - as a general rule - reduction in prop diameter will help increase RPM. What diameter is the prop?

 

[Howard Sterndrive]

however the RPM did not increase at all (puzzled by that). It was the same for both props. The top end speed dropped about 3 MPH with the new prop since the RPM remained the same.

I might be suspicious of your tach. Maybe rap on it with your knuckle or hook up a shop tach for the next run.

How did you deal with your ESA when you went to electronic ignition? Did you replace stock module with a CDI unit?
The original ESA module is not compatible with electronic ignition.
Is your flame arrester clean?

any way- here's the master list for slow boats from the master (Don S.):

"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. "

 

[Scott Danforth]

I also suspect your tach. with a 15p prop I could spin my 3.0 400 stringer over 5000 RPM

a thing to check that ties in with #15 above:
the bell crank linkage to the throttle could have lost its plastic bushing or become damaged. with the motor not running, push the throttle all the way forward, then make sure the carb throttle plate is open all the way.

 

[OMC1985]

Thanks for the suggestions. If memory serves me correctly the prop is 13" diameter. I do know I bought the diameter specifically recommended for the stringer 400 in an 18 foot. I will recheck all the suggestions posted but I can offer the following.
The tach was checked with a digital timing light to verify its accuracy.
We checked the throttle plates for wide open
Boat is currently clean as it was only recently launched.
Fuel is fresh as the tank at the Marina was just filled.
Oil is exactly at the full level
Ran it with the flame arrestor and engine cover off without change. Only a few minutes.
Load is minimal gear and two people. Rated for 7 and a maximum of 1000 lbs.
Compression is on spec with one cylinder just slightly low. I will retest.
Carburetor is correct model and looks to be recently replaced.
Engine temp is at 140F
Electronic ignition was on when I got it. I will check the coil for compatibility.
I will run a test on the fuel pressure, etc.
Electric trim motor is brand new and boat is trimmed to max RPM without porpising.
I will pull the boat from the water and check the exhaust.

I am not sure about the drive ration on the gears. I will have to check them against the manual and confirm they are correct.

 

[OMC1985]

I checked everything except the drive gear ratios. I don't really want to take the gear case apart at this time. The only questionable item was the coil. The old coil appeared to be damaged inside the tower. I purchased a brand new Flamethrower with 3 ohm resistance as specified for electronic distributor. Made sure there was 12 volts to the coil. Same result. Tops out at 3400 RPM. I will check the drive ratio in the fall but I don't think that is it as the RPM does not change when the props are changed. Even if the drive gear ratio was wrong the RPM should increase/decrease with a prop pitch change. I did miss one check as I think back. I need to verify the tach is reading correctly with a digital timing light as soon as I can. I checked it once but I will check it again. Otherwise I'm stumped.

 

[OMC1985]

Well, winters gone here finally, the rivers near summer normals and the boat is finally back in the water. Spent the winter considering all the recommendations posted to my question. Slightly embarrassed now! I was pretty sure I had confirmed the reading on the tach was correct last year at the time I wrote this. I decided to go back to scratch and recheck everything. The easiest to start with was the tach reading and timing I attached a digital readout timing light to the boat and went for a run. The dash tach read 3000 RPM but the digital read 4130. Goes to show. Check things twice and save a lot of trouble. Thanks for the help last year. Problem solved. I just need to attach the correct wire or replace the tach. Possibility the previous owner wired the tach incorrectly.

 

[southkogs]

If they could all be so easy! Good news. Enjoy the boat.