Last piece remaining - Merc 3.0 WOT Issue

[maxumized]

Ok.

1997 Merc 3.0. Only reaches about 3500rpm at WOT. Even has a prop that should be over revving the boat. I have been through everything in the WOT RPM list.

New longblock. (old engine had sub 100 compression numbers)
New fuel pump.
Rebuild carb. Even tried a new one, same results.
Cleaned and inspected fuel system tank to carb inlet.
Fuel vacuum / pressure test.
Tried two different ignitions, same results.
Verified timing multiple times.
Verified voltages under load for proper ignition feed.
Verified tach with shop tach.

In fact, the only thing not replaced at least once on this engine is the intake/exhaust manifold. Is it possible that this WOT issue is caused by some defect in the manifold itself?

It is worth mentioning that the original engine had trouble reaching WOT RPM. And when the compression read low, I figured for sure this was what was causing it. It was actually a jasper reman and supposedly replaced an original merc. So this is the boats 3rd motor (wonder if the manifold has had issues since the original motor went... possible due to overheat?)

Spoke with a mechanic friend of mine and he keeps mentioning the manifold. It's a long shot, but honestly, its the only part left that hasn't been replaced.

Any takers?

 

[totx]

Re: Last piece remaining - Merc 3.0 WOT Issue

What about the rest of the exhaust system?
Any restriction? Damaged exhaust shutter making blockage...

How is temperature on manifold and riser? Check with IR gun...

 

[Fun Times]

Re: Last piece remaining - Merc 3.0 WOT Issue

Just for the heck of it you might want to check to see if your engine serial number is with in the numbers in the service bulletins, Too see if you are doing every thing right, You never know. http://www.boatfix.com/merc/Bullet/98/98_01.pdf And http://www.boatfix.com/merc/Bullet/98/98_14.pdf

 

[futz]

Re: Last piece remaining - Merc 3.0 WOT Issue

what size boat?

 

[maxumized]

Re: Last piece remaining - Merc 3.0 WOT Issue

totx - replacing the shutter was one of the first things I did when I got the boat. Unfortuantely, it didn't fix the problem back then, and its still in good shape now. Although, it kind of feeds my suspicion of the overheat affecting the manifold to know at some point, that shutter burned up in there.

I actually have an IR gun, oddly enough. I will check, but I can hold my hand
on the manifold and exhaust elbow even after being under way for a while. Not real hot.

boat teck - plugs are right, and in normal shape. no head gasket issues. Firing order is 1-3-4-2.

futz - this is a 17' Maxum. And as I mentioned, it has a 19 pitch prop instead of the factory 21 pitch. The last owner tried to address the RPM problem with the prop. So in actuality, the RPMs are even that much lower than they should be.

 

[Howard Sterndrive]

Re: Last piece remaining - Merc 3.0 WOT Issue

outdrive gear ratio?

 

[elkhunter338]

Re: Last piece remaining - Merc 3.0 WOT Issue

I have a 21 foot starcraft, estimate boat weight is about 3800lbs, 4300lbs on the trailer axles, plus tonge weight. I run a 4 blad adjustable prop pitched at I think 16 pitch. I get 3800 rpms at sea level and about 26 mph. I run about 18-20 mph at 2800 rpms. I have the correct gear ratio for the boat, just bought a new sei outdrive this year. If I lower the prop pitch rpms increase, If I raise the prop pitch rpms decrease. with the factory 19 pitch alum 3 blade prop I get I think about 3500 rpms. If your prop is cuped I have been told this will essentially add pitch to the prop. So a 19pitch with a cup is more like a 20 or 21 pitch prop. My Engine runs good and I get full throttle and the engine has power throughout the throttle stroke. You probably have a fiberglass boat which will probably weigh more per foot than my starcraft. My boat has 2007 engine with 100 hrs on it.
So if your boat is similar in weight a 19 pitch 3 blade prop at 3500 rpms seems reasonable. Max rpms are 4300-4600 rpms. I found I lost top end speed with a lower pitch and gained top end speed with a higher pitch prop. I run my 3.0 overproped because i do not like to run the engine above 3,000rpms for hours on end. I will run 6-20 miles in a shot and run the engine at 2200 to 2800 rpms depending on weather, I rarely push it over 3,000rpms for extended periods of time.

 

[futz]

Re: Last piece remaining - Merc 3.0 WOT Issue

ok, readjust the valves. do it with the boat running, have a bunch of rags ready.

Run it, still have the problem then it's time to see if the gears in the drive are correct.

Simple to do.

 

[Schmoe]

Re: Last piece remaining - Merc 3.0 WOT Issue

OK, stupid question but I got to ask, are you adjusting the trim correctly?

 

[ron7000]

Re: Last piece remaining - Merc 3.0 WOT Issue

I have been through everything in the WOT RPM list.

New longblock.

that's awesome, that list had to be authored by someone on iboats.
I think you should add one last item to that list, a new boat


check the running surface of the hull and the drive, if it's loaded with barnacles and other fouling that will create a lot of drag preventing you from hitting top speed and normal wide open rpms.

 

[fishrdan]

Re: Last piece remaining - Merc 3.0 WOT Issue

Spoke with a mechanic friend of mine and he keeps mentioning the manifold. It's a long shot, but honestly, its the only part left that hasn't been replaced.

I doubt the manifold is causing your power problem,,, unless it's the wrong one. Or the head is the wrong one. GM 4cyl marine engines have 4 different head/manifold combinations and if you swap them around you will have power problems, blocked intake/exhaust ports. I'm not saying this is your problem, but something to check if you didn't do the engine work yourself. I have seen others post this problem before.

 

[Fun Times]

Re: Last piece remaining - Merc 3.0 WOT Issue

[Fishrdan] Varry good info right there, Good job.

 

[Fun Times]

Re: Last piece remaining - Merc 3.0 WOT Issue

that's awesome, that list had to be authored by someone on iboats.
I think you should add one last item to that list, a new boat


check the running surface of the hull and the drive, if it's loaded with barnacles and other fouling that will create a lot of drag preventing you from hitting top speed and normal wide open rpms.

I agree also try looking for any thing like a hook or rocker on the bottom of the hull using a straight edge ruler, Around the rear of the boat's bottom.

 

[maxumized]

Re: Last piece remaining - Merc 3.0 WOT Issue

Outdrive ratio is 1.94, and has been physically verified.

This boat is trailered and kept clean. Absolutely smooth hull. And 3500rpm is the best I get, fully trimmed up, just shy of losing bite in the water.

And while I know its not best to free rev in neutral, but it won't get past 3500 or so in neutral either.

 

[Howard Sterndrive]

Re: Last piece remaining - Merc 3.0 WOT Issue

And while I know its not best to free rev in neutral, but it won't get past 3500 or so in neutral either.

Then you most likely have a tachometer problem. I would verify rpm with a shop tach.

That's NOT a power problem, because revving an engine to 5000 rpm in neutral takes a whopping 5HP
Your lawn mower engine could do it.

 

[maxumized]

Re: Last piece remaining - Merc 3.0 WOT Issue

Verified a while back. And I realize it should be easy for it to hit 5000rpm... it's just not