violent vibration over 2000 rpm

'94 Ski Nautique, 351W with throttle body injection, idles well, runs smoothly up to about 2000 rpm and then proceeds to vibrate violently, almost like it's running on only 1/2 of the V-8, as you increase the speed. When I give it full throttle it accelerates quickly and smoothly for a few seconds and then dies back to a slower speed. I just changed all of the spark plugs. There's no water apparent in the oil. Oil pressure and water temp seem normal. I don't believe the prop or shaft are bent which would be evident at idle to 2000 rpm or on full acceleration.

The next thing I'm going to check is if all four injectors are spraying fuel as the throttle is applied. Like I said, it feels like only 1/2 of the engine is running when it starts to vibrate violently.

Might it be contaminated fuel as my daughter had just installed ten gallons?

If plug wires were crossed wouldn't it run poorly through out the rpm range?

Check the distributor cap for moisture and corrosion

Since you have a 94 with TBI, it could be the ProTec ignition module finally going bad. If that's the case, your best bet is going to be converting it back to a carb with a conventional distributor and coil as ProTec replacement parts are nonexistant. I got rid of the ProTec on my 94 a few years ago and it's never run better. Lots of other members have done the same swap with great success.

Bob,
Does it run rough with no load as well as under load?

halfnelly wrote: Since you have a 94 with TBI, it could be the ProTec ignition module finally going bad. .

I agree with Brian that the Pro Tech would be suspect. I feel you may get some indication of an ignition issue by using a timing light above the 2000 when it starts to run rough.

desertskier wrote: Looks like you have several threads going. Is the red dash light still on? .

Bob,
Here's your thread on the warning light
I suggest keeping like problems together in one thread since they may actually be the same problem.

Also, the protec module needs good battery voltage at the input or it won't function correctly. Check the voltage at the 12.5A breaker on the back of the engine. It should be close to battery voltage. If it's low then you probably have some IR voltage drop through the dash wiring.

making it easier to finish the thread.

1xsculler wrote: '94 SN test run = total success. I disconnected both water temp senders and both oil pressure senders and the motor never ran better and zero vibration or cutout at 2800 rpm. So, I reconnected them one at a time and isolated the problem to the smaller of the oil pressure senders which I hadn't noticed had also sprung a little leak. It was obviously putting the computer into Limp Mode. You guys saved me from converting to a carb and distributor for now at least. Thank you very much!