351W will not rev past 4000rpm - ideas?

What prop are you spinning ?

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

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.

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

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.

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

Me too!!

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.

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.

#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.

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?

My 78 was a Ski Nautique and the cover may be a little smaller than your cover.
I did run it several times with the cover up as I worked out problems with a Holley 4bbl.   Finally got it dialed in with the 4bbl opening on time as needed and never had to do it again.   My lake is only 10 minutes away so taking it out for a test run was not a big deal.

I thought I'd stay out of the engine cover discussion and ask if you know how to check that the secondaries will open by hand when the engine is OFF?

I'd tell you how but you may already know. And besides 2 more people would give you the same answer right after I did that.

I'll save my breath for now.

And no need to ask anything like "why won't my engine hit 5300 rpm" like MrMcD's obviously super powerful 351 that was turning a 13X14 prop at that speed and going 50 to almost 55 mph. That tells me he had some terrible prop efficiency with 22% slip at best..

The run of the mill 240 HP 351w from PCM, Indmar etc has a max of 4400 rpm but a little higher isn'r gonna hurt it. Your engine will be happy at 4500 rpm.if it's stock

Maybe his tach read high

MrMcD wrote: I did not have GPS in 1982 and who knows if the tach or speedo was accurate .

This says quite a bit

Not doubting that it ran good, the numbers just don't work out

Did some tinkering this weekend.

Changed the fuel filter, checked and oiled the advance mechanism, checked the secondaries.

Good news - the boat now revs to 4,700rpm smoothly. This equates to 43mph. The secondaries open about 70%. I have the Pertronix ignition module - seems to be working fine.

Bad news - I have no idea what fixed it.

My timing light crapped out after two flashes so I wasn't able to check or adjust timing. Will get a new one and try again next weekend.

Prop is a 13x13 and the carb numbers are in the photo below.

There's a vacuum line coming off the carb that isn't attached to anything. Where is this supposed to go?

TRBenj wrote: 43 with a 351w in that hull? You’re missing at least 5mph... I’d start with the prop but don’t see any mention of timing numbers? Also check if that carb is marine.

What he said

Or I could tell you that the carburetor is a 600 cfm Holley Street Warrior carburetor.

And you probably know there are no streets on the water

It'll work OK, it just doesn't have marine leakage protection (and certification) which could lead to a fire/explosion