1993 5.7 Mercruiser Low Power and Stumble at WOT

[Andclyki]

All, I am having an issue with the Mercruiser 5.7 in my 1993 Regal 233 with the alpha 1 gen 2 sterndrive. I picked up the boat end of last season and didn't get much run time on it, but I had some issues with hard starts, low power, stumbling, and stalling. I checked the timing (8deg BTDC), cylinder compression, changed the plugs, and checked the distributor. The timing was good and the cylinder compression was good. The distributor had a small amount of oil sitting in the bottom of it and the rotor/cap contacts were a little dirty. I cleaned this out and hit the rotor and cap contacts with a little sandpaper to clean off. It runs better now, but I am still struggling with lower power and what sounds like a stumble or a miss at WOT. Any thoughts on what to go after? Spark? Fuel? I linked a few videos below.

 

[alldodge]

Your vids are unavailable

Being a 93 its probably a carb, right?
Have a motor serial number?

 

[Andclyki]

Sorry about that. The videos should be fixed. It is a carb....4bbl Rochester. Engine serial is 0F010181.

 

[alldodge]

I'm thinking lack of fuel issue
So it can be fuel pump, carb settings, carb dirty, antisiphon valve might be sticking, dirty filter, fuel line restriction

Put a fuel pressure gauge inline and see if it stays up between 4 to 7 psi

If your running lean which it might be, it will destroy the motor

 

[Andclyki]

I'll hook a fuel pressure gauge on it tomorrow and see what I get. Is it 4-7 at any rpm/load or just idle?

 

[alldodge]

It needs to stay between those numbers from idle to WOT. So long as you see 4 psi at WOT your good

 

[Andclyki]

Roger that.

 

[jonny rotten]

Does it still have last years gas in the tank?

 

[Andclyki]

The gas is ~1/4 from last year (which had marine sta-bil in it) and 3/4 fresh gas from this year.

 

[Andclyki]

This morning I completed the following:

* checked anti-siphon -> it appears clean
* checked inlet to carburetor -> it appears to be clean
* installed a fuel pressure gauge -> fuel pressure is ~8-9psig through RPM range
* hooked up a vacuum gauge to the manifold ->vacuum is ~17-18 inHG

 

[jonny rotten]

Somebody smarter than me will chime in but it could just need a carb rebuild. My 4.3 with a 2 bbl carb was running like crap. A rebuild solved all my issues. Lack of power, bogging ETC.
I've had my boat for 10 years and this is the second rebuild I've done. Both times it ran like a new boat after the rebuild. Plenty of videos out there to rebuild carbs if you've never done it before like me. Both times the jets were clogged with ethanol crud. I believe the main jets are where the WOT come in and they are on the bottom of the bowl and were the most crudded up both rebuilds. (on mine anyway)

Sorta like when your weed wacker doesn't run right.....it's always the carb

 

[Lou C]

Dump out your water separating fuel filter into a mason jar & let it sit. See what the gas looks like...crud on the bottom, layer of water?
What do the spark plugs look like? What color are the deposits on the center electrode insulator? Blistered white (too lean), black (too rich) or light tan/grey (about right). On the Quadrajet the possible issues are a bad accel pump, secondary air door opening too soon, and in general clogged jets. It sounds like it’s surging. Make sure the auto choke opens all the way....

The pic of the anti siphon valve appears to show varnish build up. I don’t recognize that thing in your QJet fuel inlet and your fuel pressure seems too high.

 

[Lou C]

actually after looking closely at the vids it looks like the Weber 4bbl not the quadrajet. however the same general princupals apply.

 

[alldodge]

I second the "what do the plugs look like"?

With 9 psi (if the gauge is accurate) just maybe the carb is flooding. Did mention, hard to start, low power, stumbling

 

[Andclyki]

I second the "what do the plugs look like"?

With 9 psi (if the gauge is accurate) just maybe the carb is flooding. Did mention, hard to start, low power, stumbling

 

[Andclyki]

I assume the gauge pressure is accurate. I can double check with a second gauge. If 9psi is too high, would a regulator be needed?

 

[alldodge]

While some needle seats can take pressures above 7, the normal high is 7

Can be easy to see if the seat isn't holding. Look down the carb when its running and right after shutting off to see if you can see it dripping

Yes a regulator can be added

 

[Andclyki]

All, thanks for the troubleshooting help with this. I confirmed the fuel pressure was ~9psi and ordered a regulator. It is arriving today, and I'll get it swapped on tonight.

In other news... one comment and two observations after running this weekend...

* I put in some seafoam carb cleaner into the gas to run through. Probably will do nothing, but for $10 worth a shot.
* I pulled apart the distributor to inspect and clean while I wait for the new plugs, wires, coil, cap, and rotor to arrive. I did find some oily residue inside the distributor cap which I cleaned out. I ran the boat some after that and it did run better for about an hour then started running rough again. It was dark when I got back so I didn't get down in the bilge to pull the cap off, but I'll do that tonight too. Thoughts? Could oil be getting pushed or wicked up into the distributor cap through the shaft?
*I would note that the engine starts right up cold (after sitting for a week) but it is a hard start when it is hot after running. It is close to 15 seconds of cranking. Doing some reading, I am guessing it could be from the 9psi fuel pressure dripping gas after shutoff or the steel fuel line getting hot. Either or both could cause a flooded condition. Thoughts? Would it be worth wrapping the steel line with foam insulation to insulate from engine heat?