91 pcm stalling out

[maynards]

I am working on a 91 196 nautique and have recently installed mallory conversion kit for ignition and installed new carbureator for a no start concern. Now the engine will stall out on wide open throttle. If you ease into the throttle the engine will respond but has a flat spot on 3/4 acceleration and then will unleash power (more power on top end then ever before). I am at a loss, toy'd with carb and still unable to correct issue. Can make it change but it seems as though the engine is getting too much fuel???? Has anyone had similar issues with these conversion kits or any of these type of symptoms?

[s_kelley2000]

Have you checked your timing, especially when it is advanced?

[maynards]

thought about that, what do you know?

[s_kelley2000]

I don't have any experience with the Mallory conversion kit but on my old 91 Sport with a PCM 351 we upgraded to a Petronix electronic distributor and had similar issues until we got the timing all set. We got it dialed in using a timing light while running up to speed on the water.

The following season the module crapped out and would no longer advance but after some convincing Summit stepped up and replaced it and it has been good ever since. Summit wanted us to replace the coil but we checked it and it had proper voltage. You may want to check your coil if you get the timing set and still have issues.

[Mamigacz]

Holley fuel enrichment is done from:

-The idle circuit, which is used when the throttle blades are shut. Fuel is delivered from openings below the throttle blades. No fuel will be coming out the venturi’s at this point.
-Transition off idle circuit. These are slots (above the idle openings) that add more fuel when you first start to open the throttle blades.
-After the throttle blades pass the transition slots and the air velocity increases though the carb bores, fuel starts to come out the primary venturi’s. However, the fuel is leaned out from power valve.
-As the throttle blades open further, vacuum drops and the power valve opens allowing more fuel the go through the primary venturi’s.
-When the throttle blades get close to wide open, vacuum is very low and the two secondary blades open allowing all four venturi’s to supply fuel to the engine.

When transitioning through all these circuits, flat spots will occur without the help of the accelerator pump. The accelerator pump sends extra fuel into the carb when the blades are opening. The extra fuel helps mask transitions from one circuit to the next. With this being a new carb, some of the below checks don’t seem likely, but they are reasons for this condition:

-accelerator pump not adjusted or even functioning
-vacuum leak from the PCV valve
-vacuum leak from the carberator base gasket
-vaccum leak from any plugs in the intake manifold.
-If this is not marine carberator, are all the distributor vacuum nipples plugged?
-Is the initial timing set at 8 degrees btdc (this might not be the optimum timing for your engine but it’s a good starting point)
-Is the total mechanical advance yielding 34 degrees of total advance. I can give you a procedure if you don’t know how to do this.

Is there a backfire with the stall or flat spot?

[maynards]

Timing was off quite a bit (retarded). Still have a flat spot on acceleration and will delay when going from idle to full throttle. Engine is running too cool at approx 130 degrees and seems to be getting too much fuel. Oil smells like fuel. Still runs really good when mildly accelerating and top end is smooth. Does seem to backfire when smashing the throttle.

[Mamigacz]

I was thinking you had lean condition, but its possible there is extra fuel being dumped past the needles and seats. If you look down the carb while its running, is there gas dripping out of the venturi's? When you shut it off, does gas continue to drip out the venturi's. If this happening you should see:

-Poor idling
-hard starting
-stumbling during acceleration
-gas in the oil
-black smoke
-back firing through the exhaust

Is it idling like crap?
Does it start hard when its warm?