This happens to be the biggest group of gearheads I know...

So I need a little advise. I have a direct drive ski boat. Basically a chevy 350 with a Holley 4 barrel on it.

I took it out today to test it and it went poorly.

I dewinterized it a month or so ago with no real problems. Although I did have to raise the idle mildly to prevent stalling. I figured this was old fuel.

Added ½ a tank of fresh fuel on the way to the lake. Started without too much effort but acted like the idle was weak. Headed out and tried to punch it and it fell flat on it face and bogging. Eventually stalled. Tried that a couple of times. I could gently get it up on plane 2-3k rpm but even then if you punched it, it fell flat

Ideas for how to go about diagnosing, from the gearheads??

How much was in the tank when you added the 1/2 tank?  Might be just bad gas.

But the falling flat when you punch it sounds like a bad accelerator pump.  The diaphragms crack, I think.  Vague old memories.

And, don't overlook the old mouse nest in the air box.  They can work fast.

 

Airbox on the boat has a flame arrester type filter on top of the carb. so pretty rodent proof

I didn't figure it was fuel pump unless the bowls are really empty.

Accelerator pump could be a possibility but you would think it would bog and then pick itself up. Just transition badly but it doesn't do that.

One of my suspicions is trash in the carbs. Possibly stopping up the secondaries. So when you punch it; it goes really lean and falls apart...

I would suspect gas too if it just sort of ran crappily but this is really bad.

It's a small block chevy with a Holley.  Not a whole lot to inspect.  The bowls have a sight glass, right?  Should answer the bowl fill question.  You can look down the barrels when you work the throttle for the pump.  The bowls come off pretty easily for draining and inspection.  That's why they're a popular carb, I think.  Easy to work on.

I'd start with making sure you've purged all old gas (just because it can't hurt), then look at the accelerator pump per Zed Head's advice.  Check to make sure you're getting good squirts in both primaries when you pull on the accelerator rod.  Even with a good diaphragm, I find that the passenger side accelerator pump orifice in my 4bbl Holley sometimes gets plugged, especially after sitting all winter (and I run Stabil every fall).  I've had to clean out the orifice with a needle in the past when I couldn't get it to flush out on its own.

I'm no Holly expert, but if you can nurse it to mid-RPM's, but it still dies when you punch it, I would suspect crud in the mains.

I would agree with your expectation if it was an accelerator pump issue. Even with no accelerator pump at all, you should be able to nurse it to WOT and full load. And even if you didn't nurse it, I think it would bog bad, then catch and scream.

I think your mains are plugged and you're just running on the idle jets.

Edited - getting too grammatical.

Edited July 16, 20213 yr by Zed Head

I will have to look. I dont think I have float bowl sight glasses

When you winterized it last year, did you drain the bowls or was there fuel in there over the winter?

My armchair mechanics opinion is that it's just a "simple matter"* of taking the carb off and giving it a good deep cleaning. 

 

* Proof is left to the student.  

Usually I pull the power to the fuel pump and run it dry. I don't remember if I did that last year, as my memory doesn't go back that far! 😉

Are the revs going up as it falls flat? Wondering if the prop is slipping on the shaft from worn out hub splines. Another thing, is it going into gear completely? Mine would "spin out" if the gear shift drive cable was out of adjustment.

No, The revs drop fast and then chugs at 7 or 8 rpm before dying.

I believe it is going completely into gear