1988 Merc Black Max - sounds & feels like fuel starving

[C.Reese]

Hello to anyone willing to help!

All things done by me & by the 1988 Merc service manual specs
To date: compression = 125 - 128 psi (rebuilt in 2006) cleaned 50 gallon tank(vent new as well), all new ethanol proof Merc lines from tank to carbs. New Merc fuel fittings, new inline filter after new merc primer bulb, Marpac water seperation filter before bulb, Rebuilt carbs, rebuilt fuel pump, new stator and trigger because of rotten wiring, Got an old MercMaster to check out timing & sync.
I have got a solid knuckle busting 2 years of time working on this Old Skeeter Starfire in an old garage. This motor is healthy but I have finally gotten it on the water and its kicking my butt again. It will scream when it does right! Will jump on plane in an instant if it doesn't bog down. You can be running 5500 WOT and she will just lay down on you and drop rpm's while you still have the accelerator floored then snap your head back all of the sudden back wide open. She also taking off seems to bog down like its not getting the fuel it needs then kicks in and goes. It starts immediately and idles excellent after ramp start up. Well, Cold first start at ramp with 26 degree temps yesterday took forever. Enricher was clicking but acted like it wasnt putting much fuel in there.

Things I have checked per book & silently listening to y'all folks::
1. Changed to a 6 gallon separate tank sitting on deck. (Same new bulb and was firm after couple squeezes). No Change
2. Rechecked float levels - OK
3. went back through pressure checking all lines & there new and good.
4. Looked over fuel pump (rebuilt with Merc kit). Its OK
5. Currently figuring how I can check fuel pressure going down the lake. My fuel injection gauge starts at 10psipsi so it will not do any good teeing into fuel pump outgoing line?
6. Currently going to pull

 

[QBhoy]

When it’s bogging down...try holding the push key choke in and see what happens. Then get back with what happens.
All the best.

 

[racerone]

If it kicks in instantly you should look for ignition issues,----Perhaps run with a timing light to see if you are loosing spark.

 

[C.Reese]

QBhoy,

I figured I would forget to tell important information. On the last run back before loading it stalled to the point it quit two times while running about 4800 rpm. I got a friend to hit primer bulb with not really much change before it died. The other time we hit the enricher 4 - 5 pushes and she quit stalling then picked up a bit.

Racerone,

Not instantly like an intermittent or dead fire but yes I see your point. This is fast but more like when ya run the gas out in the lawn tractor. Its trying to go, stumbles,tries to go again type of thing. I checked spark from all 6 points out of switch box. Its seems good and strong per spark tester.

I will see if I can post video of it doing what I am talking about.

 

[Teamster]

I had a similar problem with mine, Chased it for a couple of years,

I Finally took it to a reputable Mercury Shop, Ended up being the CDIbox,..
Was only acting up when it was warm which made it a pita to trouble shoot.

 

[C.Reese]

Appreciate everyone's suggestions. Due to being overworked for the last month I haven't been able to get on computer or motor.

Carved out time few days ago and hooked a tee between fuel pump and carb. Ran a poly line from tee to a Mr. Gasket 15 psi Gage which I mounted on the console. Got to the lake and fired her up. The Gage was at 1/2 - 1psi at idle then as I pulled out of no wake I watched closely.

Results:

1. Bogged when I tried to hammer throttle (psi 0-1)
2. Eased into it and got on plane but did semi bogg (psi 2 @ 3000rpm)
3. After planed started bogging a little and psi back to 0-1 then boom took off like a rocket and psi jumped to 4.5 @ 5000rpm.
4. Stalled @ 5600 and sure enough Gage plummeted to 1-2 psi

I am definitely thinking that it's nice seeing the fuel psi on the fly! I rebuilt the fuel pump 2 months ago but ordered another kit today. Going to clean pulse passage out in block also. Since it did the same surge with a separate tank I know it's not a vacuum problem. Also I hooked an electric pump up and it screamed. Got to get this mechanical pump right!!!!

 

[211libwtfo]

Anti siphon valve. Have you tried running the boat on an external gas tank and see results that would eliminate one system or the other.

 

[C.Reese]

I sure did change to an external tank 211! It did seem to help a little with less boggs but it basically did the same thing. I have heard the merc bayonet fittings can pull down fuel flow so I hooked the new merc primer bulb line straight to the fuel pump. Same at lake as it was before. I have put her in / out several times to fix this. I got the rebuild kit in and will be working on it Saturday if off work..

Agitated at this but darn sure I will get it if I keep digging and getting help from y'all!!!!

 

[racerone]

The pump on the motor works off a positive pressure pulse when piston causes crankcase compression.---Also a slight negative pulse when piston is compressing the mixture in the cylinder.-----Tested the crankcase compression ?----Look closely at the check valves in the fuel pump too.

 

[C.Reese]

Racerone, yes I understand how the diaphragm works in relation to the Piston. Cylinder Compression was 125-128 across the board.

Are you referring to actually measuring the pulse of what the Piston suck & push is at the fuel pump pulse passage? Was thinking to look some more on any air entry/or restriction from the tank to the fuel pump but figured the separate 6 ga. Tank I tried eliminated any issues. Amazing how many things it can be from a fuel delivery perspective.

Thanks to everyone! I would sure like to make my April 4 day trip to Dale Hollow Reservoir here this year. Fingers crossed!!

 

[racerone]

Run with a timing light and see if you are loosing spark on one or more cylinders.

 

[Elkhunters#173]

I have the same motor and it took me a few years to figure it out. Replace stater and trigger first. I first replaced CDI's x2, coils X6, cleaned carbs a few times, checked fuel lines, tank, valve, timing. Finally put a timing light on while on the water to each plug wire and with the results it had to be the stater and trigger. Been running good for a few years now.

 

[QBhoy]

Get a friend to hold a piece of card over each carb entry when running bad. If doing that over the intake (simulated choke) makes it run better for a second...it will be a carb issue. Also tells you which one.

 

[Dukedog]

Also I hooked an electric pump up and it screamed. Got to get this mechanical pump right!!!!

pretty much answers tha question, huh... why not leave tha 'lectric pump?.. wired right it will be just as good and eliminate havin' ta pump tha bulb......

 

[Chris1956]

You do not need the inline filter, since you have the water separator. Inline filters can and do restrict the fuel flow, that you may need.

 

[Faztbullet]

What was done when rebuilt??? Big difference between rebuild and refresh. Sounds like low crankcase pressure due to worn cylinder,broken seal ring or kinked pulse hose...