1989 Mercruiser 5.7l Randomly slowing down?

[Chadbud]

Hey guys! I hope you all are doing well. I got an issue that’s confusing my inexperienced butt off!

I bought a 1989 Celebrity 208 SE 21’ this year in April from a local marina. The boat is in magnificent condition. It was owned by a marina mechanic who bought it new back in its year. He kept it indoors in the off season and has all maintenance/work order documentation.

The boat had been running great. The only work the marina did on it prior to me purchasing was replacing liquids, all filters, bellows, anodes, and shift cable.

During a few trips to the Allegheny River (I’m from Pittsburgh) I noticed that around half or 2/3 throttle, whether I’m towing a tube or skier, the boat would lose power. It almost felt like I ran over a big patch of seaweed and I was dragging something all of a sudden. It’s the same feeling a passenger gets in a car when they slow down quickly.

As far as my memory serves.. RPMs did not change. I could give it more gas and it would try to pick up but then slow down again. That’s when I would decide to bring it back to idle for a little before putting her back on plane. She would do great for a while and then the same thing would happen. Engine turns over and starts perfectly. No other issues were present.

After some research I figured it could be a spun prop hub or engine coupler (god I hope not!). I brought it to my mechanic who’s been in the field for 30 years and he said the shift cable (if memory serves me correctly) was not adjusted properly. He made sure to express that if that cable is not adjusted properly on this engine with a SS prop then it can cause my issue.

I was so relieved and took the boat back out to hopefully enjoy the rest of the summer taking the family boating. After two days of boating the engine was running FANTASTIC. Felt like I had more out of the hole power and I was satisfied.

However, last weekend I was pulling a skier (my dad!) behind the boat on my way back to the launch to finish the day and I felt that dreaded drag again. As soon as it happened I dropped the RPMs to cruising and things seemed fine and then it happened again so I cut the engine and pulled my father in. I brought her up on plane once more before getting to the launch ramp with no issue.


Thats where I’m at! I’m probably going to bring her back to my mechanic but I’m just really frustrated. I haven’t taken my family boating in YEARS. I just really want everyone to be happy. I’m hoping I can get some insight from this amazing community.

Thanks again! I’ll answer any questions as soon as I get them!

 

[achris]

...After some research I figured it could be a spun prop hub or engine coupler (god I hope not!). I brought it to my mechanic who’s been in the field for 30 years and he said the shift cable (if memory serves me correctly) was not adjusted properly. He made sure to express that if that cable is not adjusted properly on this engine with a SS prop then it can cause my issue.

100% export grade, BS!

The drive clutch is either in gear, or not in gear. There is no 'half way' with these systems.

I was so relieved and took the boat back out to hopefully enjoy the rest of the summer taking the family boating. After two days of boating the engine was running FANTASTIC. Felt like I had more out of the hole power and I was satisfied.

However, last weekend I was pulling a skier (my dad!) behind the boat on my way back to the launch to finish the day and I felt that dreaded drag again. As soon as it happened I dropped the RPMs to cruising and things seemed fine and then it happened again so I cut the engine and pulled my father in. I brought her up on plane once more before getting to the launch ramp with no issue.


Thats where I’m at! I’m probably going to bring her back to my mechanic but I’m just really frustrated. I haven’t taken my family boating in YEARS. I just really want everyone to be happy. I’m hoping I can get some insight from this amazing community.

Thanks again! I’ll answer any questions as soon as I get them!

Sounds like a fuel issue, and the first thing you should be checking is fuel filters, then tank venting. Change the filters, then if it happens again, remove the fuel tank filler cap, see if you hear lots of air rushing in.

Chris...

 

[Chadbud]

100% export grade, BS!

The drive clutch is either in gear, or not in gear. There is no 'half way' with these systems.



Sounds like a fuel issue, and the first thing you should be checking is fuel filters, then tank venting. Change the filters, then if it happens again, remove the fuel tank filler cap, see if you hear lots of air rushing in.

Chris...

Thank you for replying Chris! The fuel filters are brand new, they were replaced before I bought the boat. The marina I bought it from said they also replaced the shift cable too!

Okay so I need to access the fuel tank and pull the cap on it to see if there is a pressure issue, right? I’m going to try that today after work. I’ll listen for a gush of air and update you.

 

[Rick Stephens]

Sounds like a carb rebuild is in your future. The fact they put new filters on only means they were clean and new right then. If the boat was sitting very long then the fuel in the tank may be garbage. May be garbage anyway, even if relatively fresh. Any water or crud in the tank - same thing, as water in fuel turns everything to crud, will immediately clog up those brand new filters. Pull the filter, dump in a clear glass container and see what comes out.

The mechanic is so far off as to be incomprehensible. Shift cable reasoning is total baloney. Either it goes into gear or it don't. Once in gear, it is hard to get back out or it kills the motor trying to go from in gear to neutral - that's what a lower shift cable problem causes.

The gush of air would only occur when you are running on the water and the power drops. THEN you open the cap and see if it sucks air because of a plugged vent causing a vacuum.

 

[Chadbud]

Sounds like a carb rebuild is in your future. The fact they put new filters on only means they were clean and new right then. If the boat was sitting very long then the fuel in the tank may be garbage. May be garbage anyway, even if relatively fresh. Any water or crud in the tank - same thing, as water in fuel turns everything to crud, will immediately clog up those brand new filters. Pull the filter, dump in a clear glass container and see what comes out.

The mechanic is so far off as to be incomprehensible. Shift cable reasoning is total baloney. Either it goes into gear or it don't. Once in gear, it is hard to get back out or it kills the motor trying to go from in gear to neutral - that's what a lower shift cable problem causes.

The gush of air would only occur when you are running on the water and the power drops. THEN you open the cap and see if it sucks air because of a plugged vent causing a vacuum.

Thanks for your input!!

It sounds like it may just need a carb rebuild. Mechanic wants around 300 to rebuild and clean cath with one of those machines. Could it be possible it is a fuel pressure issue? Besides the fuel filters what other things could cause this type of problem?

Nervous to bring it back to my mechanic now haha!

 

[achris]

It is an alpha drive, yes?

 

[Chadbud]

It is an alpha drive, yes?

Yes it is the merc alpha one

 

[Rick Stephens]

Dump the fuel filter first. That'll tell you a lot without needing to spend on a carburetor rebuild.

 

[tank1949]

Most likely crap in fuel and due to setting up it may have dislodged after usage. Go purchase cheap electric fuel pump and completely recycle gas through new filter and back into tank. Hopefully it will pick up most of the crap and redeposit it into filter. Depending on tank size, you may be recycling fuel for a while. Or, drain the stuff to an acceptable container. Then, replace new filter with another new filter. Your carburetor may ALSO have an inline filter that should be replaced too. Now, here is the dreaded part. Some of the old fuel pick up tubes have check valves or filters. You should pull tubes and make sure they are clear. Also, check for insects clogging fuel air vent. Mud dobbers cause us headaches in Florida. Good luck.

 

[achris]

Need more information on your engine. 1989 5.7 could be a Weber 4bbl, Quadrajet 4bbl, or a Mercarb 2bbl.

So, let's start with an engine serial number, and which carb you have...

BTW, all of those carbs have a inlet filter.

My suspicion is a blocked vent line creating a LOW pressure in the tank. When you take the fuel cap off you'll hear air rushing IN, not out.

Chris.....

 

[Chadbud]

Need more information on your engine. 1989 5.7 could be a Weber 4bbl, Quadrajet 4bbl, or a Mercarb 2bbl.

So, let's start with an engine serial number, and which carb you have...

BTW, all of those carbs have a inlet filter.

My suspicion is a blocked vent line creating a LOW pressure in the tank. When you take the fuel cap off you'll hear air rushing IN, not out.

Chris.....

Okay I attached some pictures of what I could find guys. I’m hoping these images will reveal what kind of carb and also engine serial number. I believe it’s on the tag.

I can’t thank you guys enough for your continued help. I’m hoping to resolve this issue before the 4th for the family!

By the way I took the boat out yesterday and it ran great until towards the end of the day. (Similar to before). I took off gas cap after she was warm and didn’t notice a woosh of air in. I thought I heard a very light breeze go in but I couldn’t be sure.

I also confirmed that RPMs don’t go up and don’t really go down by more than 100 rpm when the “drag” starts and I lose speed all of a sudden.

 

[Chadbud]

By the way Chris my mechanic said I could try to loosen the gas cap 1/3 out the threads when the problem occurs and then run the engine in the water to see if the problem persists and to determine if it is a clog in the gas tank vent. Does that sound like it will work to you? Thanks again.

 

[achris]

By the way Chris my mechanic said I could try to loosen the gas cap 1/3 out the threads when the problem occurs and then run the engine in the water to see if the problem persists and to determine if it is a clog in the gas tank vent. Does that sound like it will work to you? Thanks again.

Yes.

 

[Chadbud]

Yes.

I will try this on the weekend 4th of July. That is the soonest I’ll have the boat on the water again.

If it proves to not be a venting issue. Any other ideas as to what it could be? Am I just looking at a carb rebuild at that point? Or perhaps I should have someone pressure test my fuel line?

thanks again!

 

[Fuss188]

Your pickup line on the fuel tank should have a check valve.. I would take that off and make sure it's not clogged with debris and is working properly. Also a good time to inspect your fuel lines. Also, is there a screen where the fuel line goes into the carb? If so, check that for debris as well..

 

[Chadbud]

Your pickup line on the fuel tank should have a check valve.. I would take that off and make sure it's not clogged with debris and is working properly. Also a good time to inspect your fuel lines. Also, is there a screen where the fuel line goes into the carb? If so, check that for debris as well..

That pickup line is inside the tank and leads to the bottom of it right? So I should just open the access panel on the deck and reach the line to the tank that way?

 

[Rick Stephens]

What did the fuel look like when you dumped the filter?

 

[Chadbud]

What did the fuel look like when you dumped the filter?

Update!

So I’ve pulled the gas cap half way out and sometimes all the way out and then back on when I was on the water right when the problem started. Again, same issue.. the boat loses momentum and “sounds” like it revs up and then down and then up but can’t maintain speed. I did notice the problem occurs much less often if I remain at slow cruising speed. About 15-20 mph.

After pulling the filter, fuel looked fine. Maybe small amounts of particulate? By the way since I’ve bought the boat it’s probably had about 60-70 gallons of 90 or 93 octane fuel run through it.

Again, both filters were like new.