Need help mercury 900 90hp distributor motor

[Chevyhotrod91]

I have a 78 mercury 900 90hp outboard distributor motor, great compression rebuilt carbs fuel pump all new fuel lines primer bulb and wiring harness and new plugs and water pump, did link and sync. Runs great on muffs but on the water it has a short burst of power then nothing wont plain or get to any speed I've tried new props I know it's not a fuel problem because if I squeeze the bulb while running it runs super right and will smoke a bit , I went out one day and it was running great hole shot like no other got one plain in one second, running about 35 at 4200 rpm with a 15p 4 blade prop went home then took out next weekend and ran like crap like before starting to think it's my trigger, but it checked out good, coil checked good and each plug checked good with a gap tester. I'm at a loss any help would be awesome

 

[Chris1956]

1) What boat do you have that motor on? A prop that has too much pitch will prevent the motor from revving up.

2) The carb mixture settings usually need to be adjusted so the motor will accelerate. This is done on the water, motor warm and in gear. Have you done that? When set at 1.5 turns open, the carb mixture is usually too lean to allow acceleration.

 

[Chevyhotrod91]

It is on a 16.5 foot 1994 sunbird and yea I have done that. I got all 3 carbs set the best I could, and like i said if i give it to much fuel all it does is smoke " being to rich"

 

[Chris1956]

OK, the prop is probably OK.

Have you checked the fuel pump diaphragm for holes? If it leaks, raw fuel gets into the crankcase causing the motor to load up at idle, smoke a lot and stumble on acceleration, if it even can accelerate.

 

[Chevyhotrod91]

Yes I have checked it and I rebuilt it just in case as well I mean I could check it again

 

[emoney]

Have you tried a different fuel tank?

 

[Chevyhotrod91]

Ya I had a portable 3gallon tank I tried, and it looks like I had a typo in my post when I pump the primer bulb while it's running it runs super rich and smokes, that's why I was leaning away from a fuel problem and more towards a spark issue

 

[racerone]

I would say that if it runs ---" different "----when you operate the manual fuel pump you need to look at carburetors / fuel pump / fuel lines / fuel tank !!

 

[Chris1956]

When you rebuilt the carbs you used new gaskets, floats, inlet needle and seat, right? You set the carb float heights correctly? You cleaned the idle restrictors?

 

[Chevyhotrod91]

Yup I completely rebuilt them new everything I didnt remove the plugs but I dont think I needed to from what 8 was reading and u also soaked each carb for 24 hours roughly and yes I triple checked when setting the levels both ways

 

[emoney]

I replaced a bad fuel pump once with a new, aftermarket pump only to find out the new pump was bad as well. I would trouble shoot the fuel pump first to make sure it is, in fact, pumping gas. Sometimes, its all about a process of elimination. The bulb does get firm/hard when priming, correct? A 2 stroke fuel pump uses a pulser hose connected to the crank case to get the diaphragms working. Downstroke creates positive pressure pushing fuel into the carb, upstroke generates a vacuum that pulls the fuel from the tank. There's internal check valves inside the pump to regulate the direction of the airflow. If one goes bad, fuel pump doesn't work. I know you don't want it to be a fuel problem, but it kinda sounds like one for sure. As you know, motor only needs 3 things; fuel, air, spark, to operate. If it was idling poorly, I'd say look for a manifold leak, but you said it idles fine.

 

[Chris1956]

You might make sure the idle restrictors and main jets are clean. Each can be blown out with a can of gumout, while still in the carb.

The main jets are in the base of the carbs, under the brass hex plug. The idle restrictors are under a plug (straight blade screwdriver), next to the top of the throat.

 

[Chevyhotrod91]

Got ya, I removed the main jets when cleaning it out I believe they were .92 that sound correct

 

[Chris1956]

Sorry, I no longer have the service manual on that motor. The flood got it.

The crowleymarine site might tell you.