1981 Nautique rpm issue

I have a stock 1981 Ski Nautique with the 351w that runs great but will not turn more than 3400 RPMs. Once I take the throttle past that point the engine starts to bog down. Top speed is only 34 under the circumstances. Any ideas?

Check the secondaries like Pete says. Also could be timing. We had a bog and swore it was carb related and finally found it was timing.

Timing should be set between 8-10 BTDC at idle.

However, it is more likely the timing advance mechanism is rusty or has broken springs.

You need to observe timing advance as you increase RPM. It should max out around 28 deg BTDC.

Way too much prop for that boat. Sell or trade it for a 540.

Pittgator wrote: Thanks, I thought that might be the case. Anyone have one for sale or know the best place to get one from?

Acme 540 from Delta one of our sponsors.

How did you end up with the 668?

Pittgator wrote: It was on the boat when I bought it. Not sure why. Any idea what a used 668 is worth?

The 540 mentioned by Pete is a good place to start. A good used prop in a popular size with no dings or repairs can bring $200+    Lots of props out there that sell for less than $200 though...

JQ

Yea you could do that. You could end up as gator bait too

So here is the latest. I bought the new 540 prop, installed it and did not change much of anything. Now it does about 3600 rpms and 36 mph. Once the throttle goes past 3/4 it starts to bog down and slow down. Checked the plugs and they look good. New wires as well.

Any ideas?

Sounds like starving for fuel when secondaries start to open. I would find a temporary portable 6 gallon type tank and hook to fuel pump. This would eliminate existing fuel line, restricted anti siphon valve, fuel pickup tubing, existing gasoline, and anything else in tank.   If runs ok then you have problem isolated. If not fixed, I would check fuel pump pressure.

The secondaries on my 81 and a few others I have worked started opening around 3400-3600 rpm. So if your secondaries are opening and there isn’t enough fuel this would equal a big bog and go away again when you back off a little. You would have to check pressure at this rpm though doing so at idle wouldnt do much unless it is low there too. I would expect to see 5-7 psi.

Although don’t recall ever seeing this, I suppose you could have some kind of restriction in the secondary side of carb.

Lastly, be careful with leaked gasoline when doing the above

It is very possible for a chunk of dirt or something to plug the secondary side of the carb without affecting the primary side so it could be a carb issue along with all the items already mentioned above.

Not familiar with the 540 but generally if the boat is propped correctly, WOT should get you right up to, into the max rpm range of the engine, which in your case I believe is 4400. At 4000 I'd suspect there may still be some fuel delivery issues. Don't stop with the carb cleaning - check your fuel lines, fuel pick-up and anti-siphon.   Afterward, very last thing install a new fuel filter. Don't do this until you've verified the fuel lines/ path downstream are clean and clear.