7.4L sluggish & laboring

[sodave]

I just dropped my new to me 94 chris craft crowne 282 with the 7.4L & Volvo DP outdrive in the water. The boat took a long time to plane & only wanted to stay on it if I was nearly wide open. The GPS had my top speed at around 27MPH. It has tabs that were being used. Max RPM's were around 4200.

I feel like the boat should be faster. There is a fresh set of rings on the motor & it starts easily. I understand the boat is large for a single big block, but 27 at wide open?

Is this about right for a 28' boat or should I be looking for something being wrong?

 

[Wingnutt]

Re: 7.4L sluggish & laboring

The 7.4L & Volvo DP outdrive should have more than enough power to easily get that boat up on plane and IMHO should top out around 35 mph or so. Your RPMs are about right for WOT, but something is definatly wrong!

You mentioned that the engine had a fresh ring job, was this all that was done to the engine? Or did it get a complete rebuild? If it did get a rebuild, was it properly broken in? If not, a few lobes of the cam may have some serious damage or the rings may be leaking, run wet and dry compression tests to find out. Is the engine carb'd or is it FI? If carb'd, it the carberator opening fully at WOT?

Lots of possibilities, espicially for a boat with an unknown history, but I would concentrate on the engine since that was the last thing that was known to have seen any changes.

 

[sodave]

Re: 7.4L sluggish & laboring

Thanks for the insight. It is a carb motor. I am heading down to look at it tonight after work. I am going to check spark plugs, fuel filter, & ignition timing as well. may as well start on the easy stuff.

 

[captcrankshaft]

Re: 7.4L sluggish & laboring

If it has the correct props, drive gear ratio and drive trim, I doubt the engine would be able to make 4200 rpm if it was down on power.

I'm working on a 27ft cruiser w/Merc 7.4L Bravo 3 and it won't run more than 3600rpm at WOT till its on plane then it will head straight for 4500rpm and 40mph gps.

 

[sodave]

Re: 7.4L sluggish & laboring

Really - it does take time to get to the 4,200 RPM mark. About how long does it take that 27 footer to get on plane? Ballpark me.... is it 30 seconds, more than a minute, a few minutes, 5 minutes? & When its up shouldn't I be able to pull out of the throttle some? I was hopi9ng to drop to 3,000 or lower. Not a chance of that right now.

I played with props on my old boat to control RPM's. My thinking is if max recommended WOT is 4200-4600, (& I can hit 4,200) that tells me I'm likely in the correct prop here.

 

[captcrankshaft]

Re: 7.4L sluggish & laboring

Haven't timed it. Less than a minute with 6 people on board.

I believe your 282 has 1500-2000lbs on a Fiesta Vee 266....

If everything else is operating and setup correctly, RPM is a direct function of power. If you can turn 4200rpm and your mph is down then you have drag somewhere or a problem converting power to thrust.

That said, don't believe your tach. The FV's tach was reading 3800 at 4500rpm. Can your ear tell the difference between 3900 and 4200? 3900 rpm would indicate a power problem if everything else is correct.

Or you could have two problems. An engine down on power and a prop sized (by some well intentioned soul) to crutch the engines deficiency. In this case RPM would not indicate a proper sized prop.

Don't assume anything or this could be frustrating.