Top Speed Issues

[mtboatin]

Hello all. I recently purchased a 1994 Cobalt 272. This boat is equipped with a 7.4 GL PMDM and B2 props. Slowly going through things. Boat has sat close to three plus years with a full 98 gallon tank. Fuel smells fine but you know....three year old fuel. Added multiple high end fuel additives and stabilizers, a VP fuel filter, and will be slowly (literately) burning through the tank to get new fuel in. Took it out and found idle low in the 450 range. Bumped up the idle with set screw to close to the 600 mark and after a few hours found it a little low around the 500 rpm. My main question is on WOT. I know it's almost 5000k with fuel and water but only seeing 32 mph both on speedo and GPS. RPMs are at 3600 ish. I'm thinking it should be closer to 4500 and probably closer to 45 mph. Boat came from Havasu and has been in NW Montana for the last 5 plus years so there was a change in altitude back then. Can't imagine I would need to drop to B1s. Hole shot seems good again for boat of this size. Other than burning the fuel and seeing what happens with fresh and doing some carb tuning is there anything else I'm missing?

 

[alldodge]

Boat has sat close to three plus years with a full 98 gallon tank.

Took it out and found idle low in the 450 range. Bumped up the idle with set screw to close to the 600 mark and after a few hours found it a little low around the 500 rpm.

My main question is on WOT. ... Other than burning the fuel and seeing what happens with fresh and doing some carb tuning is there anything else I'm missing?

Nope, need to burn or remove all the fuel. Having to increase idle and then it gets lower again is signs deposits are probably building up. Old fuel has less BTU's so less power. Once you go thru the fuel then get new, and my guess things will get better.

Try not to add new fuel to the old stuff, you would just prolong the outcome. Adding would help, but unless your adding more the 60 gallons, I don't see a lot changing. Some what similar to running high octane in a motor not built for it, your loosing power

 

[Scott Danforth]

3 year old fuel is no longer fuel

 

[KD4UPL]

I would try to pump the fuel out and burn it up in a car, lawnmower, etc. Dilute it at least 50% with new fuel. I would think that would be easier than using up 98 gallons in a boat that wont' run very well.

 

[mtboatin]

Got it out yesterday. It is running a little better. Started easier when hot. WOT got up to about 3850 RPMs. So making a little headway. Still only seeing about 32 mile per hour with almost a full fuel tank and 8 people. Will continue to run as much fuel as possible before adding. Thinking about throwing a 2 gallon gas can in when I get close to empty that way I can really run her down low before filling with fresh non-ethanol 98 octane. It would probably rather have a lower octane but that's the only thing they offer in non-ethanol here in Montana. Also won't be filling the tank all the way this time.

 

[alldodge]

I like the gas can idea, but suggest sticking wit 87

E10 gas is fine so long as its not held more then a year

 

[QBhoy]

Get rid of the fuel. But I’d also be almost certain that all being well and original…no way that should have b2’s on it. Should have way more pitch than that. Unless something has been changed.

 

[mtboatin]

QBhoy, I would hope and think the same but if RPMs don't continue to improve with good fuel and a little tuning, B1's may be my only other option to get them higher. Hole shot is impressive the way it sits. Would love to see this boat at high 40's low 50's. Probable wishful thinking @ 6000lbs with fuel and water. Add to that people (yacht certified) and personal effects. Gets up there fast. Had ten people first outing and eight last Sunday. Throws a heck of a wake for tubing!

 

[Superc125]

Add a can of royal purple max clean, it works!!!

 

[Nextelbuddy]

i filled up full 35 gallon tank (91 non ethanol marina gas) last year august 2020 when i blew engine. literally full lol. new engine is going in now but i was scared to use 1 year old gas so i pumped it all out and put it in my friends car lol. fuel still smelled good.

I should have kept it ugh

 

[mtboatin]

Add a can of royal purple max clean, it works!!!

I bought about $150 worth of quality additives of every kind. Stable, royal purple, sea foam concentrate, techron, blue moly, gumout, etc. I mixed quite a few together keeping the overall amount per gallon fairly close. I did some research and found that this won't fix the gas that has broken down but it will help with the deposit build up and removing water. I'm anxious to get her out again and see if things are getting any better. Burned about an eighth of the tank so far according to the gauge.

 

[Texasmark]

Might have a new fuel filter handy and when replaced, cut the old one open and tell us what you found.
Too much water and too much water accumulating additive can become a problem. BTDT The additive attaches itself to the water and forms a cloudy "blob" that can make its way into your fuel line and clog it to the point that it shuts your engine down. If your engine just ups and dies on you after having been running perfectly, pull your fuel line and check it...on an Inboard-I/O I don't know of a good place. My engine was an outboard and I disconnected the fuel line at the engine, removed the connector on the input line and using the primer bulb was able to squeeze the blob out, get fresh fuel showing, reconnecting everything and was on my way.

 

[mtboatin]

So the saga continues. Slowly have increased a little bit in ease of starting hot, minor increase in speed, and slight RPM increase. Now I'm seeing about 3850 RPMs and 34 mile per hour . I even checked the throttle linkage and adjusted it to make sure the car was getting Full Throttle . Ran out of fuel! Put in approximately 8 gallons and only got about 45 minutes to an hour till I ran out again and had to be towed in?!? Super thirsty. Had about half a gallon which we put in and only got about 5 minutes till it ran out of fuel again. Had to be towed in by a buddy . Each time I cycled the key and pump the throttle to Prime the carb and eventually after two or three times it would start. Went to go out the next day and put 30 gallons of fresh fuel in it and when we put it in the water and attempted starting it got no love. Just cranked. They don't allow being at the dock at all where we were so I pulled it out and spent about an hour tracing power through to the fuel pump. Long story longer I figured out after we got home and did some research fuel pump only runs in crank position and once the alternator is excited and power is supplied. What is weird is that in the parking lot I jumped the pump and put Power and ground directly to it and could actually hear the empty float bowls filling and the pump cycle off once and shut off pressure was reached. Prior to this was not getting any fuel squirting in from the accelerator pump either but did once flirt bowls were filled. This doesn't make sense. At home when we crank it over it has power to the pump. So the carbs should not have been empty unless the floats were sticking or something weird. So now that I think the fuel pump is fine we still have a no start issue started checking spark and found no power to the coil. It looks like it's supposed to be the purple wire. Similar purple wire to the back of the alternator is hot when the in crank position but not in run position. The boat does have a kill switch on it but when I grabbed the wire to check it it came loose and my hands were it goes into the body of the boat. As far as I can tell it was disconnected by the PO and was eliminated from the system. I guess there is a small chance that it could have come loose somehow looking for some information and help on how the power travels from the alternator to the coil if it indeed does. It's just a short run.

 

[alldodge]

Suggest trying to use the return key more, this is real hard to read

Using that much fuel, your plugs should be wet and fouled

 

[mtboatin]

Alldodge, you're right after looking at my book. I apologize. Was using too much brain power to make sure I had all my steps in the correct order that I wasn't making it user-friendly.All Dodge,

Traced all the wiring and found that the kill switch had a broken wire at a connector in the harness. We now have spark and she runs! Still not sure what happened at the dock with the fuel situation other than probably had a connection issue that was corrected while trying to diagnose it without knowing it.

Well I had a breakthrough. Secondaries on Holley carburetor we're stuck shut. They were so gummed up that they wouldn't move at all on their own. Cleaned them up with carb cleaner so that they will Now function.

Should help a little with RPMs and top speed. Not so much on MPG. I'll report back once I get her out again.

 

[mtboatin]

Fresh fuel, 2nd OEM fuel filter, throttle linkage slack taken up, secondaries on four barrel carb unstuck and functioning. Now seeing 4150 rpms and 37 mph with eight people.

May try new spark plugs, cap, rotor, wires in the spring. Check timing? Engine specs say 4200-4600 rpms. 4500 would make me happy. Am I missing anything?

I understand this boat is not set up to set speed records. Just trying to get her to live up to her full potential.

Crazy thirsty. I don't remember my previous 7.4 going through fuel as quick. Guessing close to 10-15 gallons per hr.

 

[alldodge]

Using 8 to 10 gal/hr doesn't sound bad. That's 4.6 to 3.7 miles per gal at WOT, but guessing you may be in the 25 mph range so still good at 3.1 to 2.5 miles per gal.

My Rinker (23ft cuddy at 4800 lbs 454) gets about 2.5 to the gal.

 

[Scott06]

May try new spark plugs, cap, rotor, wires in the spring. Check timing? Engine specs say 4200-4600 rpms. 4500 would make me happy. Am I missing anything?

Crazy thirsty. I don't remember my previous 7.4 going through fuel as quick. Guessing close to 10-15 gallons per hr.

I use 10 or so per hour skiing on my 20 ft with a 5.0 so if you are using 10-15 with a 7.4 Id think you are doing pretty well.

Yes I would go through the igintion components and set timing.

Are you at higher altitude? you will loose power ude to thinner air.

 

[mtboatin]

scott, around 3000 ft.

 

[Texasmark]

scott, around 3000 ft.

Thats high enough for adjustments to fuel air ratio. There are charts on the www to help you size correctly. I think my service manual has a chart for my 3-4 cyl 2 stroke OBs.