WOT issues - stumped

[mike220ds]

This has been a nagging issue since I have owned this boat. Here is the background: I purchased this boat used five years ago. It is a 1998 stingray 220DS (deck boat) with a 1997 5.7LX Mercruiser Alpha (1.47 outdrive ratio). The shortblock was replaced prior to my purchasing it due to improper winterization. The boat came with a 19 3-blade standard prop which would give me about 4000 rpms. I have tried numerous props and have settled on a quicksilver 4-blade 16 pitch which gives me 4200 rpms. These numbers are with ? tank of gas and just me in the boat. The bottom is painted.

The engine runs fantastic except it lacks power on the upper end it appears. I typically run with 6-8 people in the boat and it has no problems pulling a slalom skier up. But it seems to start loosing power around 3800rpms. Here is what I have checked/done:

1. Full tuneup including replacing coil and wires. Confirmed wire routing 1000 times...
2. checked timing at idle and up to 3000 rpms ? I am getting full-advance.
3. checked to ensure secondaries are opening up while under load ? this was kinda scary.
4. confirmed tach with shop tach. Speed numbers also match what the rpms are.

I never run at WOT, so this is not a critical issue yet it is bugging me. This boat should be running closer to 4800rpms with this prop setup. I may rebuild the carb in the offseason ? it is the last thing I can think of. This engine is a bit of a mystery to me as well ? it is the last year with the 4bbl carb and pre vortec. I have no idea if it is a roller cam or not. It does have an electric fuel pump.

Thoughts?

 

[mike220ds]

Re: WOT issues - stumped

Thanks for the response. I replaced my manifolds and risers this spring and could feel the flappers were intact. Now those may not be the original flappers...but I did have a full outdrive service last spring including bellows replacement. Would flapper pieces have shown up then??

I am currently running 30WT oil. Temp is a solid 160.

The fuel system has got me curious and I have not done any pressure test. I did do a WOT run with the gas cap off with no difference. Supposedly, that is a test for a problem anti-syphon valve..but I am not sure.

The carb will leak out the fuel in the bowls after a couple of days, so I am suspect of that. In other words I really have to crank/prime the carb before it will start if left for over 5 days.

 

[mike220ds]

Re: WOT issues - stumped

Bubba - again thanks for the response. This sounds like an easy inexpensive fix that I should just do. I am fairly ignorant on this part (I'd be more comfortable rebuilding the carb). Where is the anti-syphone valve?

 

[rbh]

Re: WOT issues - stumped

good morning-
I know with our boat last year we could not go over 4000 rpm without knocking and pinging and it turned out that the peaple we bought it from initially base timed the motor to 16 degrees as soon as we set it back to 8 degrees, what a difference.
rob

 

[wca_tim]

Re: WOT issues - stumped

The next thing I would try is a cheap ca. $20 fuel pressure gauge right before the carb... (if you have a way for someone to watch it as you run faster.

Could be anythin gtom a filter, tank screen, antisiphon valve, etc... if the pressur is low.

Note that if you're leaning it out at higher rpms, it is kind of a critical issue - that's a good way to trash a motor in a hurry.

i would NOT run thinner oil! That engine should be just fine on 30 wt...

antisiphon valve is a ball check valve to make sure that you can't keep siphoning gas out of the tank in the event of a fuel line rupture (think accident, or the like and filling the bilge up with gasoline). If it or the tank vent are clogged, this can reduce the availability of fuel to the fuel pump.

In addition, depending on the set-up you have the pick up tub that goes into the tank has a filter on the end of it and can get clogged. If the line itself is rubber with no spring inside it it can get fatigued and be sucked part or all the way closed under higher fuel pump rates...

The best way to either confirm or eliminate a fuel supply issue is to check fuel pressure right next to the carb under the conditions you're seeing a problem...

IMHO

 

[mike220ds]

Re: WOT issues - stumped

One of the reason I have avoided putting a fuel pressure guage on is that my fuel line from the pump to the carb is steel - and there does not look to be any play in that setup to plug in a guage.

FYI - at higher rpms I don't any pinging or detonation -but it is possible I just can't hear it. My timing is at 10 BTDC and full advance is in by 3000ish rpms. I did this according to the Tbolt V manual.