Mercruiser 3.0 bad hesitation at WOT please help

[richa311]

Ok on the lake now with my son. Boat runs great below and right up to 4000 rpm and stays at 5psi.
Boat starts to act up over 4000 rpm and fuel pressure drops to 4psi.
Is 4psi enough to feed it at wot.

 

[alldodge]

I did notice while blowing air through the carb some air is able to pass through the linkage rod that operates the butterfly valves on bottom of carb. Is this normal would it cause a vacuum leak? I don't see how that could be an airtight area since the rod needs to be able to move and rotate.

If your talking about the choke plate (ones on top) then there is no issue, if your talking about the throttle plates (ones on the bottom) then there should be no leakage. Where are you blowing air into, fuel line opening?

Boat runs great below and right up to 4000 rpm and stays at 5psi.
Boat starts to act up over 4000 rpm and fuel pressure drops to 4psi.
Is 4psi enough to feed it at wot.

Yes 4 psi is plenty of pressure. Still think it has to be fuel. This boat is new to you so someone may have changed out the main Jet(s) in the carb. If the boat came from a high altitude area or someone thought they could save some gas money they may have changed them. Did you run a wire or use a drill bit by hand to go through the center hole on the Jet? If so, you could see about getting a slightly larger Jet to try.

 

[richa311]

I did check the Jets and they are the correct 145 jets.
I don't know but really starting to wonder if it's something mechanical.
Yes the boat is new to me by the way. 1st time out with fresh fuel and the used mechanical pump it ran great until end of day when pump gave out.

The knocking I mentioned in my 1st post seems to be coming from front of motor in vicinity of timing gear cover. Could worn timing gears cause this issue or would it be at all rpms

 

[richa311]

Ok I checked the timing with a light and was advanced by 2 degrees (12 degrees) so I set it to the proper 8 degrees and filled boat with 91 octane. Boat still runs great at all rpms other than wot.

I do have another theory though. This has the ddis ignition that cuts the spark over like 5000rpm.
Could the damaged prop that I had to shave down to fix the nicks be making it spin faster. Also to note I have to give my rpm guage a tap sometimes to get it to work. Could a faulty tach cause the issue?

Just trying to think of some other possibilities

 

[alldodge]

Ok I checked the timing with a light and was advanced by 2 degrees (12 degrees) so I set it to the proper 8 degrees and filled boat with 91 octane. Boat still runs great at all rpms other than wot.

I do have another theory though. This has the ddis ignition that cuts the spark over like 5000rpm.
Could the damaged prop that I had to shave down to fix the nicks be making it spin faster. Also to note I have to give my rpm guage a tap sometimes to get it to work. Could a faulty tach cause the issue?

Just trying to think of some other possibilities

If your hitting the rev limiter then Yes that will cause the engine to loose power. So If your reaching 5K instead of 4K this could be the issue. See if you can pick up a hand held tach

 

[richa311]

I've been unsuccessful at diagnosing this issue. Boat runs fine everywhere else. You can even throttle all the way down to pull up skier or get on plane quickly without any hesitation.
The only time it has an issue is when trying to run wot across the lake.

My solution to this is I'm just gonna run it as is and not go wot. I really don't have a need to do it I guess. The boat will run approx 35 without going full throttle.

It just sucks knowing it has a small issue.

Thanks everyone for all the suggestions and if in the future I figure it out I'll be sure to post it here

 

[richa311]

I fixed this issue. Replaced the boat with an 89 21ft Scarab with mercruiser 5.7 magnum.
The 17ft just felt to cramped with gear and family loaded up. That and I didn't wanna keep worrying the engine had an underlying issue

 

[alldodge]

I fixed this issue. Replaced the boat with an 89 21ft Scarab with mercruiser 5.7 magnum.
The 17ft just felt to cramped with gear and family loaded up. That and I didn't wanna keep worrying the engine had an underlying issue

Hope it stays that way, but its a boat, things are going to happen sooner or later

 

[Fastatv]

I had an 86 Scarab I, loved that boat. How about a pic?

 

[richa311]

Sorry havent checked in for a little bit. So I had the new to me Scarab on the water last weekend and absolutely loved it. Such a nicer ride then the little bow rider. I have to admit I was enjoying the thumbs up and nice boat compliments as well.
Here is a pic of it

 

[s.hadley81]

Sounds like a case of one more footitis(I suffer from the same disease)Good luck with the scarab that's a good looking boat , it looks like we might see a post soon about a 89 glasstream that won't reach wot

 

[Fastatv]

Sorry havent checked in for a little bit. So I had the new to me Scarab on the water last weekend and absolutely loved it. Such a nicer ride then the little bow rider. I have to admit I was enjoying the thumbs up and nice boat compliments as well.
Here is a pic of it

Awesome boat. I may look for another as well. And fairly fast. This is the Scarab I, right? What was your top speed with it? Rick

 

[richa311]

It's an 89 I'm pretty sure it's known as a scarab excel 22.
It has a 19 pitch stainless prop and I was getting 54-55 out of it. Pretty sure with a 21 or 22p prop I could get 60mph.
Either way I can't complain about 54 out of 22ft boat. Much faster then the 35 from the other boat and the motor doesn't sound like it's gonna come apart at wot like the other boat. It didn't sound like it was struggling at all.

 

[Fastatv]

It's an 89 I'm pretty sure it's known as a scarab excel 22.
It has a 19 pitch stainless prop and I was getting 54-55 out of it. Pretty sure with a 21 or 22p prop I could get 60mph.
Either way I can't complain about 54 out of 22ft boat. Much faster then the 35 from the other boat and the motor doesn't sound like it's gonna come apart at wot like the other boat. It didn't sound like it was struggling at all.

Mine was the Scarab I, 21 footer with a 260 hp 350 Chevy. It would run 60 all day long....stock. What is your WOT rpms with the 19 pitch? Its been years ago but I'm fairly sure I was running a 21 pitch as well. And, does yours have the flat running pad at the stern of the boat? Just curious.

 

[richa311]

No Title

Mine was the Scarab I, 21 footer with a 260 hp 350 Chevy. It would run 60 all day long....stock. What is your WOT rpms with the 19 pitch? Its been years ago but I'm fairly sure I was running a 21 pitch as well. And, does yours have the flat running pad at the stern of the boat? Just curious.

I'm pretty sure it was around 4600 but ill have to check to be sure. It does seem like it should be closer to if not at least 5000 with the 19p prop.
Yes mine does have the large sun pad over the engine bay.

 

[richa311]

Ok I was wrong on what prop I was running. I was thinking about the 19p on the bow rider.
The scarab has a quicksilver 21p running at right around 4500 rpm.
Now I'm thinking it should be doing closer to 60 with that prop?
I did have 4 kids 3 adults, a half tank of fuel in the boat and the lake was pretty choppy though so maybe the 54 wasn't out of the ordinary for this boat under those conditions.
I'll have to gps it on a calm day with a couple people in the boat to see the speed then and also pay better attention to the tach.

 

[Fastatv]

Boats will run a little faster in light chop as apposed to very calm water, the chop provides for less water contact, less drag. I was talking about the pad under the boat....at the very end of the stern, a flat pad in which with WOT and plenty of trim, this is what the boat is riding on. Mine had this pad, just wondering if yours did. Try your GPS and see what you get. When I purchased my Scarab, my speedo indicated 52, 53. I felt it was faster that that. Ran with some other boats and they indicated 59 or 60. Come to find out that my boat had the incorrect water pick up tube ( pitot ) installed. Installed the correct one, then indicated 59, 60 like my friends boats. Later built the engine, indicated 67, 68 after the engine mods but, was turning 5200, 5300 which def was over revving the engine, So I did not do the WOT very often.

 

[richa311]

No Title

Sorry about that I wasn't sure what you meant by pad. I do have a flat section in the aft of the boat that gradually starts to round off as you go forward.
Here is a pic from the stern toward the bow

 

[Fastatv]

Yep, best I can remember, thats what mine looked like. The design of the pad is supposed to help increase the top speed...as I was told years ago. I would think you should see approx 60 mph. GPS it and let us know what your speed is. Love those Scarabs!

 

[richa311]

Thanks fastatv I'll have it out Sunday so I'll try and get a good run and gps and see where I'm at top speed and rpms.
I'll post back and let you know