PCM 351W running super rich, barely idles, runs rough suddenly.

[Graeme]

Guys,

I have a PCM 351 Windsor, with a 4bbl 650 Holley on it. It's always run like a top, then the other day it was idling very low, not firing on all cylinders, and belching black smoke. At first I thought it was a spark problem, so I replaced the plugs. I checked the distributor, all good. All plugs arcing a spark out a good inch, so it would appear the voltage is good, and all plugs are getting spark.

So I figure it's an air/fuel mixture problem. I don't know much about carbs, and am looking for advice as to why all of the sudden something went wrong and caused the boat to run super rich. Idle speed almost gets to normal, but sputters and drops low, sometimes stalls. Exhaust is black and smells of unburnt fuel. Oil looks normal. Temps and pressures are normal.

Thinking it's something in the carb/idle air bypasses/vacuum lines. There are a bunch of plenums on this thing that I have no idea about, and what they are for. Help!

Graeme

[lively]

well for starters you would want to check all the visible areas , floats are the main reason for richness .. but with that said when has the holley been rebuilt ? .. the kits are like 50.00 bucks and they are real easy to do ... let me know

[Graeme]

I don't even know how to check the floats...these kits come with instructions?

It's not like I can't do mechanical work, it's just that everything I have ever worked on has been fuel injected...

[cadunkle]

There is a brass screw on each float bowl. With the engine running, remove it and observe fuel level. Should be right at the bottom of the hole or just below, if fuel comes out when you remove the screw the float level is too high. If it's pouring out and nothing else has changed you probably have junk in your needles and seats. On each fuel bowl there will be a large flathead screw and a 5/8" nut near the fuel inlet. Remove the screw and remove the nut, removing the needle and seat assembly. Clean with carb cleaner. Reinstall to approx the same position as before, better installed deeper than previously if in doubt. Reinstall the nut and screw.

To adjust, remove the screw from side of bowl and observe fuel level below the opening. If above, loosen the screw and tighten the nut until the fuel level is just below the hole but no fuel comes out. If below, loosen the screw and then loosen the nut until fuel level is just just below the opening.

Other things that came cause a rich condition are plugged air bleeds. Take a can of carb cleaner with the straw in the nozzle. Put the straw in each air bleed one at a time, and give a burst of spray to dislodge any obstructions. The air bleeds are the small brass plugs with small holes in them that you see when looking down the air horn. There will be 8 of them.

After doing all this adjust your idle mixture with the mixture screws on teh primary metering block. Screw in until you get a stumble/misfire/RPM drops then screw out 1/4 turn. Do this once twice per side, alternating sides. After that set your idle speed with the idle stop screw if necessary and repeat the idle mixture screw adjustment. Repeat as necessary until idle speed is where you want it, generally 700-750 RPM in neutral will get you 600-650 in gear.

[rludtke]

When the engine is ideling this way, look down the throttle bore to observe how the fuel is metering. Is it dripping? or running in a stream out of the dischrge nozzels? Dripping or streaming heavily could indicate a stuck open needle valve. A carb overhaul should fix this. Overhaul kits are not expensive. Here is a link to the Holley kit:http://holley.com/703-47.asp.

A stuck float valve will cause high float levels, which will in tur cause flooding or at least extra rich mixture and poor idle charectoristics. The problem with marine carberators, is that they do not have external float level adustments (as mentioned in the previous post), because the external float level adjustment creats a potential fuel leak source. Marine carberators are designed to not leak fuel into the bilge.

The process for adjusting marine carberator floats is availible here: http://holley.com/data/TechService/Technical/Adjusting%20Your%20Carburetor.pdf. Follow the Duracon (plastic) float instructions.

[Graeme]

Hey, thanks for the awesome info. Looks like it was the float. I took the whole primary bowl off just now, moved it and there was nothing moving inside. Shook it a bit, and a click then the float was moving. Put it back on, hooked everything back up, fired it up and it's back to normal. Definately going to to take this apart this year and clean it, and get a rebuild kit. Hopefully it last for next weekend, as I want to get some riding in!

Thanks for all the info cadunkle. At least I know how to adjust it now!

[lively]

agreed with what they said lol .. i dont have a repair manual , but holleys become simple once you have done 12 or so lol good luck

[rludtke]

I agree, the overhaul is really very simple. Tear apart, and then restore with new parts. Everything is quite intuitive. To help inspire confidence however, Holley does have assembly corrections here: http://holley.com/data/TechService/Technical/4150%204160%20Exploded%20View.pdf.

[lively]

Thanks for that ! im going to hang this on my wall ! lol never even used one just always looked at the parts and replaced ,then tuned by tach