87 Mercruiser 205 V6 losing power past half throttle

[vegaschef364]

I have been around boats most of my life, but last year bought my first one so if my lingo is off a bit or don't have all the info I apologize. Bought a 87 Cobalt open bow w/a 205 V6 Mercruiser Alpha One Thunderbolt ignition w/power steering, 4.3L I think. It was in beautiful shape, the owner took very good care of it, less than 600 hrs on it. Used it for 3 weeks last year, in Missouri Lake of the Ozarks. Then garaged it, winterized it, pulled the chocks? opened up 2 large hoses to drain the water and put the battery on a tender. Gas tank half full and 95% positive I put in stabilizer. Got to the lake earlier this week, closed everything back up and put it in the water. Ran around for 20 min or so alone, then hooked up the tube for the kids. Ran fine for about 1/4 tank then it started briefly losing power, maybe from full power to 3/4 maybe 1/2 power, enough of a loss that I had to grab something to stop from falling forward. And it went back and forth for a little bit. dropped the kids off and filled it up, seemed to be fine but started happening again. First step was replace fuel filter, still had the original charcoal filter in it, so replaced it with the quicksilver paper type. Ran fine for a while on its own, hooked up the tube and started happening again. Does not seem to be missing, no odd noises when it happens, only the loss of power, and seems to only happen when I am pulling the tube, and past half throttle. The lines all look to be in great shape, replaced within the last couple years. I am no master mechanic but am pretty good with cars, can fix most things myself as long as no special equipment or hoist is needed. But new to the boat owners world so could use any advice on this issue, as well as any advice for a new boat owner. And thank you in advance for any helpful words given.

 

[Bt Doctur]

the spin on filter should be first, then fuel pump filter, then carb inlet filter

 

[dubs283]

Pic isn't super clear port side of engine but appears to have a mechanical fuel pump which oem would have the pumice filter installed factory. Is still available through Volvo iirc but merc only has paper element available these days.

Unknown through photo if engine is equipped with a spin on water sep filter, engine mounted or otherwise. Cobalt was/is a quality brand that had most premium options added but again, can't say for sure.

Based on your info the issue seems to be fuel supply related. My start point would be a fuel pump pressure and vacuum test at suspect rpm/load range. If all checks out move on to ignition/tune up

 

[achris]

Check to see it any critters have built something in the fuel tank vent hose. Sounds like a fuel starve...

Chris.....

 

[vegaschef364]

the spin on filter should be first, then fuel pump filter, then carb inlet filter

Spin on filter? Is that the fuel/water separator? I will take more pics in the morning but I am pretty sure it does not have one of those, I have replaced the pumice fuel pump filter with a quicksilver paper one, did not fix it. Where would I find the carb inlet filter? Have looked online but have not been able to find any info on that.

Check to see it any critters have built something in the fuel tank vent hose. Sounds like a fuel starve...

Chris.....

First time I have heard that one, where would that be located? I have looked online but can't find any images for it

 

[achris]

First time I have heard that one, where would that be located? I have looked online but can't find any images for it

There's 3 lines on a fuel tank. The big 'fill' hose, usually about 50mm diameter. A smaller engine feed hose, usually between 8mm and 13mm. And a small vent line, to allow air into the tank as the fuel is drawn out and air out as it's being filled, it's usually the same as the fuel feed hose, but comes from the other end of the tank.
Somewhere on the boat you'll find the vent fitting. There are many different designs and it could be ANYWHERE. It may look like any one of these...

 

[Lou C]

I think that’s a Quadrajet carb the other filter is in the fuel inlet where the fuel line enters the carb. Be careful with the fittings you need a large adjustable wrench to get it out. That filter is usually just a screen in most marine applications; GM used a paper filter there in auto applications instead of the screen type. Also the anti siphon valve on the gas tank is another cause of fuel starvation.
On this engine the large spin on filter was not standard from Merc, they had the 2 small filters, the one on the filter that you changed & the small screen in the carb inlet. My OMC version of the same engine came with the large spin on filter & the small screen in the carb inlet. You can add a spin on if you want extra filtering area; they can hold small amounts of water whereas those small pump & carb mounted filters can’t. I would for sure check & probably replace that anti siphon valve. That caused the same problem on my engine when we bought this boat 20 years ago. My mechanic found it & replaced the cheap alu one with a better quality brass one. No problems since.
Other than that the accelerator pump seals on this carb can fail and cause a similar problem. I replaced that a few years ago when I rebuilt the Quadrajet. Slightly complicated but not too bad to take apart.

 

[vegaschef364]

Ok, didn't have a chance to work on it today, also dealing with a waverunner that is not running right, not mine though, my brother in laws. His just runs like crap and dies after 5-10 seconds, thinking that is spark plugs though, they are both stored in the same garage. So been a tough week, especially with 4 teenagers wanting to play on the water, and making some attempt at working. But taking next week off. Anyways, I ended up renting a pressure tester from O Reillys. Going to test it in the morning. Pretty handy actually, they don't actually charge anything, only take a deposit and they said I could keep it for up to a week, hopefully won't need it that long, only here for another week. But again since I am new to all this any ideas on what the pressure should be? Have kind of resolved to calling a mobile mechanic on monday if I can't get it figured out, I'd rather spend the extra money than spend a whole 2 week vacation trying to fix it.

 

[Lou C]

4-6 psi

 

[nola mike]

Spin on filter? Is that the fuel/water separator?

Yeah, pour the contents into a clear container to see what you get.

Anyways, I ended up renting a pressure tester from O Reillys. Going to test it in the morning. Pretty handy actually, they don't actually charge anything, only take a deposit and they said I could keep it for up to a week, hopefully won't need it that long, only here for another week.

All of those stores rent tools, some you wouldn't think of. I've rented flaring tools, bearing drivers, balance installers, drum brake tools (!) etc. AutoZone and advance don't have a time limit on how long you can keep them.

 

[vegaschef364]

Yeah, pour the contents into a clear container to see what you get.
All of those stores rent tools, some you wouldn't think of. I've rented flaring tools, bearing drivers, balance installers, drum brake tools (!) etc. AutoZone and advance don't have a time limit on how long you can keep them.

I don't have the separator, only the small filter

 

[vegaschef364]

So what I have so far, for the anti siphon valve I am removing it from the tank and inspecting it to see if it is clogged either at the bottom screen inside the tube, or at the top on the fitting. If it is clogged, not sure if any place around here will have one in stock, if not can I clean the one I have to get me through this week, then replace it next year? Pretty sure I have located the fuel vent line, a little silver cap a little bigger than a plastic bottle cap with a hole on the bottom of it, located on the stern up towards the top. Is there a certain tool that works well to get in there and check it out? And I am guessing I would want to remove the tube from the vent and press out through the vent so I don't get anything else stuck in the tube? For the fuel inlet kind of the same thing, undo the fitting and see if there is anything clogging it up? I guess if none of those seem to be the culprit then I am hooking up the pressure gauge to check if it is the pump? And one last question, will any of these lines be under pressure with the engine off? Or just make sure I have a small bowl and some towels handy to catch any drips? Again I, and my kids really appreciate you all taking the time out of your days to help with this, definitely makes life a bit easier when there are people to reach out to when you have no idea what you are doing.