4.3 mercruiser sputtering and shutting down after running over an hr

[irishfootballboy]

i have a '91 186scr monterey with a 4.3 mercruiser with alpha 1. to start out it runs ok although it does seem a little sluggish and lacking power- until this started top speed was 36-38, now 30-32. when i put the boat away for winter in 2018 it was fine. to start of the season in 2019 i decided it was time to redo the bellows. so i had the bellows done as well as replaced the trim limit switches which hadn't worked in a while. also a regular service- oil etc. first trip to lake boat was ok, but a bit weak, for about an hr and half. then starting sputtering but not shutting down. back off to neutral, idles fine, engage and go a ways, then it repeats. keeps repeating and getting a little worse each time. to the point that it will only run about 10mph and 1500rpm. anything more is sputters and will shut down if i dont go back to idle. plenty of fuel, oil pressure good, temp good. it shuts off and will fire right back up and idle fine. took to shop and compression fine and suspect water in fuel or something,replace filter and fuel. back to the lake and same thing again after an hr or so. had card rebuilt, new distributor, wires, and plugs. still same thing but a little worse now. ran fine for 1.5hr then started. got worse quicker this time. and this time it would not rev in neutral either to the point that it wouldnt idle and would shut down. it honestly seems to be starving for fuel. is there anything that could cause the fuel pump to start failing after that long running? seems like it would either be good or bad. its also odd that it cranks back up so easily after it shuts down. planning to take back to the shop but just looking for some ideas. a part of me goes back to when the bellows and switches were replaced and if there is something that could be loose or shorting that could have been affected then like a ground wire or something.

 

[alldodge]

Get it to where it has the problem then get your ear close to the gas cap and remove it and listen. If you hear air rushing in and it runs good again then you have a clogged vent line.

Remove the fuel line off the tank. The barbed fitting that the fuel line was attached to on the tank side is an anti-siphon valve. Remove the valve and see if its clean inside. Use you mouth and blow air thru both ends. The tank side to barbed fitting end should be easy, the other way should be hard.

With a compressor set at no more the 5 psi and the gas cap removed blow air back into the tank

Next need to attach a fuel pressure gauge Tee'd in between the pump and carb. Should read 3 to 7 psi

 

[irishfootballboy]

thx for the info. im not on the lake so i couldnt duplicate and try gas cap. but i did pull the anti syphon valve. blowing through from the tank side to barb side is like starting to blow up a balloon. i would consider it hard to blow through. from the barb side to tank i cant blow through at all. does this qualify as bad?

 

[alldodge]

About like a balloon, it should be easy, but the balloon is a subjective answer. Take some carb cleaner and a small screw drive or rod and push the ball inward while soaking/spraying. See if the force is easier.

The fuel pump does produce a suction but its not that high. Only other way to test is to use a pressure gauge on the fuel pump side

 

[Rick Stephens]

I think you should get a fuel pressure gauge and T it off near the carburetor. See what fuel pressures are when you have the issue. There are fittings made to T right at the carburetor, I've never had one. AD probably knows where to get it. Then all you need is a fuel pressure/vacuum gauge.

 

[alldodge]

Can make one from parts at the auto store (plastic Tee, rubber hose, fittings), but the Merc part number for the Tee is 91-18078

 

[Rick Stephens]

Can make one from parts at the auto store (plastic Tee, rubber hose, fittings), but the Merc part number for the Tee is 91-18078

That's the one. I probably spent more making mine.

 

[achris]

Put a tee fitting in the line BEFORE pump and connect a vacuum gauge. You need to see LESS than 2" of mercury. Any more than that and it means there's a restriction between tank and pump, or a blockage in the vent line.

Chris....

 

[irishfootballboy]

thx for the other comments. I checked vent and it is clear. when I got into the tank I did find water. looks to be a good bit. enough that it was pooled up right under the fuel pickup. it looks a little thick to be just water but it isn't white like the gelling from e85. I always try to use non-ethanol but i'm pretty sure some that is advertised as non really isn't. so I have the tank out and will drain and clean. since the fuel line is original from 1990 going to replace it as well since I have concerns of it not meetings todays standards and the chance of ethanol. while im at it just going to replace it all from tank to carb and put in a water separator. so, thinking I will go with an electric fuel pump. I know all about the oil pressure switch and looking for right pressure, 4-7psi? is there a particular part# anyone would recommend? and are there any other considerations like carb adjustments or anything? or just any reason in general this would not be a good idea. seems like going forward would make things cheaper and easier to work on.

 

[Rick Stephens]

Personally I think the mechanical is easiest moving forward.... however, my block isn't even drilled for a mechanical, so electric is was and is. That is fine - use good stuff. One thing I did was use the Airtex OEM pump as doing so let me get the OEM brackets and fittings and even the OEM stainless fuel line from the pump to the carburetor. A bit more spendy than building the brackets and lines yourself but sure ended up being a clean setup.

I don't remember which pressure switch I used... sorry. I'll keep looking around.