I have a 1988 ski Nautique 2001 with a 351W and a Holley 4bb. The Holley only has 2 mixture screws and does not have a float adjustment.

The boat will only start if I am at half throttle and it will only run at half throttle and runs poorly.

Right before it runs out of gas it starts running very well.

At this level of throttle it puts a ton of gas into the primary bowl.

Back story:

Very few problems for 5 years.

Kept in temperature controlled garage during last winter, drained water, laid on tongue. Before I put it up I was having bogging issues under load when pulling a skier.

When I first started it at the beginning of summer it had a ton of blue smoke and required 1/2 throttle to start and run and ran very rough. 

Work done so far:

Changed oil and filter to 10W40 from the VR racing oil to thicken gaskets and stop smoke - this helped reduce the smoke significantly

PCV valve still clicks when you shake it so it should be ok

New plugs, gapped, visually tested each plug and each is sparking

Drained and put in new new premium fuel with lead substitute and seafoam

New metal fuel line

Replaced fuel filter last year

New fuel pump

New Coil

Timing is around 10.5-11

Pulled front and back bowls off carb, cleaned up and re-gasketed, cleaned out all of the orfices carefully and sprayed with carb cleaner to make sure they were clear. 

Replaced carb accelerator pump and it is squirting well.

I can see gas flowing into the carb - of course at 1/2 throttle it is alot.

A few days ago I got it to run well at idle 700-1000 rpm but it won't anymore.

Checked oil and it is very clean.

Rear bowl seems to engage when you open it up.

Changed the electronic idle setting a lot, no changes, the carb vent never seems to close

make sure the accelerator pump is not pushed down

Reset and worked with the idle mixture screws - no improvements or declines

Any suggestions?

Your 2 mixture screws and no EXTERNAL adjustment for the floats is normal on an 88. They can be adjusted by bending a tab internally.

Did you take out the floats and needle and seat assemblies on the primary and secondary bowls as part of your cleaning? It sound kinda like you have junk keeping the needle from seating and dumping lots of extra gas into the engine.

Did you clean the primary metering block on the carburetor or the air bleeds?

I'll guess that your electronic idle setting is really the electric choke and if it won't ever close it's probably broke or you're turning the cap the wrong way. Counterclockwise will make it want to close the choke butterfly (the thing you're calling the vent)

One suggestion..............buy or download one of the many Holley books on the market these days. it'll be the best money you've spent on the boat in a while,

I'd get a paper version since you can have it right in front of you with loads of pictures and full of correct terminology.

I keep a copy in the bathroom for those times when "it takes a while".

After that maybe you can do a rebuild or decide to send it somewhere

It starts running very well right before running out of gas because at that point you're not dumping too much into the engine.

This Carb does not have a sight gauge or side screw to see the float level.

There are 2 ports that I can connect a vacuum gauge - one at the front top of the carb (no reading) and a large port at the back on the spacer between the manifold and the carb. When I connected a vacuum gauge to the rear port of the carb spacer, I got a pressure reading of 2-3.

After spraying carb cleaner around the carb spacer I did not identify any air leaks.

Spark plugs have no oil on them, but are very black which I believe indicates the rich fuel situation and fuel burns very quickly.

What would cause the 1/2 throttle requirement to get it to start and run? The carb float/needle?

The gas dumping in is probably due to poor seating between the needle and seat.

Vacuum at the spacer connection should be in the mid teens to twenty when it's idling in neutral.

The other one is probably the spot where your fuel pump overflow tube would hook up and there isn't any vacuum there. It's just a spot for the fuel to dump into the carburetor and stall the engine or make it run lousy if you have a ruptured diaphragm in the fuel pump.

So what did they send you for a pump?

If it's a Carter M6696 which shows up on their website at a price that's well.............kinda high, the connection for the tube is on the backside of the pump 180 degrees from the fitting on your old pump and would be a short stubby little fitting

Or you just have a hole there that the fitting would press in to

Or maybe it's a whole different style pump.

Here it is:

https://skidim.com/catalogsearch/result/?q=47-7267

Since I am already doing work on the carb, seems like I might as well replace the gaskets on the carb, valve cover, manifold, and valve stem seals...agree? This would eliminate most of the other options causing air leaks and the blue smoke.

I saw a few videos on how to replace the valve stem steals without removing the heads by using compressed air into the spark plug hole.

They came on mid 70's PCM engines and then PCM started using the single diaphragm Carter pumps with the sight tube later in the 70's. (unlike what the video says)

The pancake type adapter between the 2 diaphragms with the 1/8 inch pipe plug screwed into it originally had a sight bowl screwed in there with a little message printed on a plastic tab inside the bowl.

Here are a couple of pictures

An AC fuel pump all dressed up in 76 PCM blue 

and here's the message in the bowl

In your case there's nothing to tell you that the primary diaphragm has failed with that pipe plug in there.

It'll keep pumping away on the secondary diaphragm till that decides to fail at some point

Some people have put in a 1/8 NPT to 1/4 hose barb fitting and run the plastic hose from that that up to the carburetor or flame arrestor to tell them the primary diaphragm has failed