351W will not rev past 4000rpm - ideas?

Removing the engine cover solves the wind issue, and makes all this diagnosis really easy.

If you don't mind my asking, where do you keep the boat?

#1 is a good idea if condition is unknown.

So is checking the initial and final timing. Set initial to dial in the final... 34 or so being ideal. 10-14 on the initial will usually runnpretty good... but depending on how much advance you’re getting, you may need to compromise on those numbers. Just don’t exceed 36 total.

On the secondaries, all those checks are fine I guess, but the first 2 things I’d do would be to check and see if the butterflies are stuck (and how they feel when moved if not) - this can be done engine off. If all seems fine then go for a ride and see if they’re getting pulled in by inspecting. This requires a helper and reasonably calm water. Operate them manually if they aren’t working or weak (not pulling in all the way). This will tell you what to focus on for the fix.

My 78 Nautique 351W had a max RPM listed on the valve cover of 4,600 if my memory is correct.
With a set of performance heads I ran that engine many times up to 5,300+.
With a 13x14 prop MPH was above 50, almost 55.   The rest of this engine was stock.
I sold it with 1,500 hours on it still running very strong. Put the new heads on at 900 Hours.
A good running 351W should give your boat a lot of pop.

MrMcD wrote: If you are on the lake, lift the engine cover and have the passenger watch the 4 bbl as you accelerate. You should hear and see the secondary open. A vacuum pot will pull up on the secondary at wide open throttle. It must be under load for this to happen.

Chris,
Unless I missed something, you haven't mentioned how you checked to see if the secondary is opening. As Mark mentions, the engine needs to be under load.

63 Skier wrote:    I'll be anxious to hear how the 16' boat is responding to the 351!

Me too!!

If the boat stumbles as mentioned at the beginning at 4,000 RPM or so it is not the 4bbl not opening.   You can run it on 2 barrells and it will run perfect at all speeds, it will just be down on power at high RPM.
Maybe as much as 25-30%.   You would not notice till you try to accelerate real hard.

You asked how to check to see if the 4bbl is opening.
If you are on the lake, lift the engine cover and have the passenger watch the 4 bbl as you accelerate. You should hear and see the secondary open.
A vacuum pot will pull up on the secondary at wide open throttle. It must be under load for this to happen.   If it does not open you can turn it open manually while accelerating on the lake to see if your power picks up like normal.

Starting to run bad at 4,000 RPM indicates other issues.
I would still start by confirming the distributor is working correctly and the advance is giving you proper timing maxing out as stated already in this forum to be 34, safe 36 on the max side.

MrMcD wrote: A couple more things to ponder. Are the 4 barrel secondaries opening? How old is the gas in that boat, with all the sitting you may be trying to run it on 2 year old gas. When you hook up the timing light verify that your timing does advance as it is supposed to. Set your timing at idle as already instructed and then increase RPM slowly while watching your timing marks, you should see the timing advance smoothly as you increase the RPM. It will stop advancing somewhere between 3,000 and 3,600 most likely but if you do this record what you see.   A dial back timing light would allow you to know exactly how much advance you have. A little rust can really mess up your distributor advance mechanism but is easy to fix.

You really should read the whole thread before giving him some more things to ponder, which have already been mentioned

Jim/Pete/Ken - great suggestions, thanks.

I think the prop is stock. I’ll check for markings next weekend.

I’ll bring my timing light up to the lake next week and will fiddle with the timing a bit. I always run 93 octane in my toys as most of them require it. I’ll also order a new cap/rotor.

I’ll also have a passenger check my secondaries and will get the carb numbers. The carb was rebuilt last summer by a friend of the previous owner, so hopefully it just needs a bit of adjustment.

Pete - you are right, I’ve only owned the boat a short time. I’ve put seven hours of run time on it so far, mostly this spring. Currently it sputters at 4000rpm and will not go any further while under load. This feels like an engine issue more than a top speed/prop issue. It comes out of the hole OK but doesn’t slam me back in the seat.

zwoobah wrote: '68 Mustang with a mid-70s wintage 351W. Holley 4160 carb and MSD blaster coil/wires. Finally got a working tach installed today. Boat pulls strong until 4000rpm and then stumbles/will not rev higher. This equates to 41mph on GPS. Diagnostic plans for next weekend: 1. New inline fuel filter. Tank definitely has some rust in it. 2. Get boat warmed up and then unplug auto choke. Choke is wired in with the 12v supply to the coil. Coil does have a resistor. 3. Maybe new dist cap/rotor? I have no maintenance history on these. Can anyone identify my distributor? Anything else I should tackle? The boat is on a lake 2.5hr away, so I need to order parts ahead of time and bring them with me.

Unplugging the choke will be the wrong thing to do because without 12 ish volts going to it, it will want to go shut and the engine will run like crap to put it in plain easy to understand english. It's normal to be wired with the 12 volt supply to the coil.

The distributor is a Prestolite with the clip down cap and the 2 wires coming out of it tells me that you have an electronic module under the cap

In no particular order I'd be figuring out what the prop dimensions are, checking the timing at idle and seeing that it advances with RPM and seeing what your total advance is.

Commander has that distributor advance spec at 6 degrees BTDC at idle of 600 -650 rpm or so and that will probably give you a total advance around 30 or so.

If you bump the initial up to 10 to 12 degrees that'll add 4 to 6 degrees to the total and improve things. Around 34 is good for total advance, 36 if you want to push it and use good gas.

Check the springs/flyweights under the advance mechanism to be sure they're not rusted and/or stuck or broken and limiting your advance.

You could also verify that the secondaries open. You have to do this under load on the water.Here's a link below that will tell you how to easily check the opening and it'll also tell you that they won't fully open with a 600 Holley on a 351

link

What prop are you spinning ?