Rough running 351 windsor under load at high rpm

My recently rebuilt 351 windsor on my 1977 searay is running real rough under load at 3000 plus rpm. Ive replaced spark plugs, changed fuel filter.runs beautifully at the dock but under load no dice. Ive got a 2 barrel rochester and the choke is working fine. Im completely baffled with this... Does anyone have any ideas? Its been suggestef that my timing may be off?

Re: Rough running 351 windsor under load at high rpm

It's possible you have a couple of plug wires crossed. Carefully double check them all.

Yes, it's possible the timing is off. I believe you have timing marks on both sides of the TDC mark, be sure you have the timing set on the BTDC side, and not the ATDC side of the marks.

If the above is ok, you might want to do a compression check.

Re: Rough running 351 windsor under load at high rpm

Yup, I'd verify plug wire order and base timing first (10-degrees BTDC). Don't forget to check timing advance, too, should be around 32-36 degrees total advance.

Re: Rough running 351 windsor under load at high rpm

Hgruetzner said:

My recently rebuilt 351 windsor on my 1977 searay is running real rough under load at 3000 plus rpm. Ive replaced spark plugs, changed fuel filter.runs beautifully at the dock but under load no dice. Ive got a 2 barrel rochester and the choke is working fine. Im completely baffled with this... Does anyone have any ideas? Its been suggestef that my timing may be off?

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Ayuh,.... Welcome Aboard,.... Was the carb rebuilt too,..?? Was there crud in the filter that ya changed,..??

It sounds like it's goin' Lean, under load...

Re: Rough running 351 windsor under load at high rpm

Bubba1235 said:

It is SOOOO easy to get the plug wires crossed on the 351 because they will still run fine without a load at idle. Double, triple check it before doing anything else. (Yup, been there and done that.)

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That sounds like a :facepalm: moment...

Re: Rough running 351 windsor under load at high rpm

Bubba1235 said:

Would you beleive two days of scratching my head,,, and lots of, umm, colorful language.

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Huh... That's nothing. I spent two years chasing down and fixing an oil leak... :facepalm:

Colourful language, had that to spare....

Chris........

Re: Rough running 351 windsor under load at high rpm

Bubba1235 said:

Would you beleive two days of scratching my head,,, and lots of, umm, colorful language.

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My education came after 5 hours of head scratching while working on a 302 (888).
Idled perfect, sounded smooth as silk, for around 3 minutes then pop and die. I played with the carb adjustment (several times), checked the plug wires and firing order, reset the dwell, re-checked the timing. Run some more and it would pop and die again. After the 3rd time checking the firing order, I found 2 crossed that I didn't see before.

Another customer brought his boat in with the problem the OP has. First thing I did was check the wires (he tuned the boat up). While checking the firing order he told me he check them twice and that wasn't the problem and not to waste my time and his money looking there.
Found the crossed wires, moved them and sent him on his way (no charge) to do the on the water test. Came back smiling a couple hours later and handed me two $20 bills.
He came back quite often for repairs and service.

Re: Rough running 351 windsor under load at high rpm

No one else seemed to mention it so I will. Check the points for both gap and corrosion. 75% of all carb problems are electrical!

Re: Rough running 351 windsor under load at high rpm

Bubba said:

At least I am in good company! I think what messed me up is that like yours, my engine ran great at idle and that kept saying it couldn't be the firing order in my mental diagnostic flow chart.

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Ford V8s are really good at that. MANY years ago my parents had a Ford Fairlane, with a 302 in it. It lost all oil pressure but still ran fine. Plenty of power and no sounds like it was having any problems... Long story short, the turkey who replaced the spark plugs must have dropped one and cracked it. Somehow it misfired just as the intake valve was opening and broke the rocker bolt. That allowed the lifter to pop up out of its socket. That's where the oil pressure went, but it also meant it was running on 7. We only found the problem because we did a compression test which showed a 'dead' lung. Found the broken rocker bolt, misplaced pushrod and the lifter out of its socket in the process of getting things off the engine to investigate the low compression.

Just thought I'd share that

Chris.........