VP 4.3GS Carb Tuning Vacuum Port?

[valleyviewlawns]

I am troubleshooting a rich condition. Bad enough that it won't idle well, and fouls plugs. I just rebuilt the carb and want to tune it, but I do not know how to hook up the vacuum gauge to get vacuum. This engine only has a large port under the carb that goes to the PCV valve. I will be timing it using the shunt, although the EST timing procedure link in the DIY section goes to an Evinrude post. I think I can get it timed with my manual. Should be initial 1degree, and 18 degrees at 600rpms.

Where are people hooking up for vacuum to adjust the carb?

 

[jimmbo]

I unhook the Vacuum Hose from the Choke Pulloff, as the Choke is fully open from Heat, and hook up my Vacuum Gauge to that hose

 

[dubs283]

Marine engines don't rely on vacuum references for fuel supply/ignition advance like an automotive application

I have a plug/barb kit that I install in the intake when a vacuum reading is appropo for diagnosis

With the pcv valve hose you may stab a barb fitting into the end and attach a vacuum gauge for carburetor tuning. Looking for 17" steady at idle spec

As far as timing the shunt tool is inserted to the icm at the distributor base and b (+) voltage applied to the alligator clip. Iirc idle timing spec is 1-2 degrees btdc in service mode

 

[jimmbo]

Marine engines don't rely on vacuum references for fuel supply/ignition advance like an automotive application

I have a plug/barb kit that I install in the intake when a vacuum reading is appropo for diagnosis

With the pcv valve hose you may stab a barb fitting into the end and attach a vacuum gauge for carburetor tuning. Looking for 17" steady at idle spec

I prefer to leave the PCV in place as it is providing a mix of Air and some Hydrocarbons, which need to be taken into Account when doing the Mixture Adjustment.
Adjust the Mixture for the Highest Manifold Vacuum, not a Specific Vacuum Value.

 

[valleyviewlawns]

I unhook the Vacuum Hose from the Choke Pulloff, as the Choke is fully open from Heat, and hook up my Vacuum Gauge to that hose

I don't think I have that port. Holley 4160 with electric choke.

 

[valleyviewlawns]

Marine engines don't rely on vacuum references for fuel supply/ignition advance like an automotive application

I have a plug/barb kit that I install in the intake when a vacuum reading is appropo for diagnosis

With the pcv valve hose you may stab a barb fitting into the end and attach a vacuum gauge for carburetor tuning. Looking for 17" steady at idle spec

As far as timing the shunt tool is inserted to the icm at the distributor base and b (+) voltage applied to the alligator clip. Iirc idle timing spec is 1-2 degrees btdc in service mode

There is a brass plug on the port side, front of the intake manifold, but I think that is a cooling system drain plug. Is that right? If not, I could use that. It has a square hole in it, too small for a 3/8" drive, too big for a 1/4" drive.

 

[jimmbo]

I don't think I have that port. Holley 4160 with electric choke.

The 4160 does use an internal Vacuum Passage to get Vacuum to the Pulloff.

 

[valleyviewlawns]

I prefer to leave the PCV in place as it is providing a mix of Air and some Hydrocarbons, which need to be taken into Account when doing the Mixture Adjustment.
Adjust the Mixture for the Highest Manifold Vacuum, not a Specific Vacuum Value.

So, put a tee in that hose and use that as my vacuum source?

 

[jimmbo]

You could do that, the reading on the Gauge will be lower, but it can be used, for tuning

 

[valleyviewlawns]

You could do that, the reading on the Gauge will be lower, but it can be used, for tuning

I just don't see any other options on this engine. There are no vacuum ports on the carb or on the manifold.

 

[dubs283]

There should be a 3/4 pipe plug in the intake aft of the carburetor that when removed will give a large vacuum leak. Rig up as necessary the proper plug/barb to attach a vacuum gauge

What is it you're troubleshooting here?

 

[Scott Danforth]

I get better luck with an unlit propane torch by waving it over a carb

If you add a bit of propane at idle and the speed increases, back the idle mix screws 1/8 turn.

If you add propane and the idle drops, you are rich, and need to turn the screws in 1/8 turn

 

[Lou C]

Rich running is common on Holley carbs, one of the causes is that the throttle blades at idle are too far open, exposing too much of the idle transfer slot. That slot should be a square shape, as seen from below the primary throttle valves. At idle you want it pulling fuel only from the idle system.
A blown power valve can also cause that, although I know that the 4160 Holley carb originally used by OMC & Volvo on the 4.3 does not have a power valve. Not sure if you have that same exact model #.
I don't think a V/P 4.3 should have 18* of advance at 600 rpm!
My old OMC gets set at 6*BTDC, full centrifugal advance at 3200 rpm is 12*, for a total timing advance of 18*. Yes much lower than the V8s, not sure why but that's how it is.

Holley 0-80492 600 CFM Marine Carburetor-Aluminum

All Holley Marine Carburetors are designed to meet Coast Guard specifications. A marine carburetors fuel metering system is calibrated to compensate for unique engine loads found on boats. The J-style vent tubes direct fuel/fumes back into the carburetor for safe operation while on rough...

www.holley.com

this is the carb used on the OMC and V/P 4.3s. No power valve.

 

[dubs283]

I get better luck with an unlit propane torch by waving it over a carb

daft

 

[Lou C]

I heard of doing it that way back in the late 70s but never actually considered doing it that way. My 4.3 doesn’t really have an easily accessible manifold vacuum port so I just use the tach & get the highest rpm between the lean & rich roll points.

 

[Scott Danforth]

I heard of doing it that way back in the late 70s

I was taught the technique in the mid to late 70s lol. By the same hot rod shop owner that taught me to rebuild Holley carbs

 

[valleyviewlawns]

There should be a 3/4 pipe plug in the intake aft of the carburetor that when removed will give a large vacuum leak. Rig up as necessary the proper plug/barb to attach a vacuum gauge

What is it you're troubleshooting here?

The only plug on the intake is in front of the carb, port side. Has a square hole in it like you could put a ratchet in it, but you can't. Wrong size. I am foulin plugs, it doesn't idle, and it bogs at WOT. I just put a carb kit in it. 4bbl Holley. I'm not sure the timing is right. I will check that in the morning and then adjust the carb if I can get a vacuum source.

 

[valleyviewlawns]

Rich running is common on Holley carbs, one of the causes is that the throttle blades at idle are too far open, exposing too much of the idle transfer slot. That slot should be a square shape, as seen from below the primary throttle valves. At idle you want it pulling fuel only from the idle system.
A blown power valve can also cause that, although I know that the 4160 Holley carb originally used by OMC & Volvo on the 4.3 does not have a power valve. Not sure if you have that same exact model #.
I don't think a V/P 4.3 should have 18* of advance at 600 rpm!
My old OMC gets set at 6*BTDC, full centrifugal advance at 3200 rpm is 12*, for a total timing advance of 18*. Yes much lower than the V8s, not sure why but that's how it is.

Holley 0-80492 600 CFM Marine Carburetor-Aluminum

All Holley Marine Carburetors are designed to meet Coast Guard specifications. A marine carburetors fuel metering system is calibrated to compensate for unique engine loads found on boats. The J-style vent tubes direct fuel/fumes back into the carburetor for safe operation while on rough...

www.holley.com

this is the carb used on the OMC and V/P 4.3s. No power valve.

This carb does have a power valve. It was just replaced when I rebuilt the carb a couple days ago.

 

[Scott Danforth]

What sized Holley? 4150 or 4160?

Did you verify the idle transfer slots when you had the carb off

If your fouling plugs, you are too rich

Start at 3/4 turn out at mix screws.

Make sure the float levels are set (external adjustment)

When you had the carb apart, did you make sure the air bleeds were clear

 

[Lou C]

This carb does have a power valve. It was just replaced when I rebuilt the carb a couple days ago.

Interesting
even though the carb I linked is listed to be the one they used by Holley, maybe someone replaced it along the way with a more generic Holley carb.