Holley 4160 - Mixture screws not doing anything

[MurkyAsparagus]

Hello all,

I am absolutely stumped and am hoping there’s a carburetor whisperer here on BITOG.

Currently in the process of restoring my boat - a ‘90 MasterCraft PS190. I rebuilt the carburetor, a Holly 4160, this past month. I cleaned it with an ultrasonic cleaner, blew out all the passage, etc and used a Holley OEM rebuild kit (a 703-47).

Yesterday, my good friend and I got the carb installed and started turning the boat over and it fired right up! Great, right? Well, here's the issue. It seemed to idle just fine, but when you go to adjust the mixture screws, they don't do anything. We took the carb back off, double checked the transfer slots to make sure they weren't too exposed, nothing. Then, we took off the bowls and metering plate, sprayed carb fluid throughout and blew out each of the passage. Threw it back on, again nothing. Then, we thought maybe the new power valve was blown, checked it and it was fine, went ahead and put the old one on, no difference. The rationalization behind this was that if the power valve was blown, it would be running at full rich all the time. We're at a loss here. Somehow, the carb is running on its primary circuit instead of its idle circuit and running fine. Does anyone have any ideas?

I should add that when the mixture screws are both all the way and all the way out, nothing happens. The engine should die at full lean with the screws all the way. Prior to rebuild, this occurred. The bowls have plastic, non-adjustable floats. I thought maybe the needles aren't high enough, resulting in excess fuel in each fuel bowl. But, they were in the carb before, so I don't exactly know. I was also thinking that maybe fuel is somehow flowing the the power valve check ball that I installed, but I don't exactly know how that would occur.

I suppose it could also have a vacuum leak somewhere, but I tested with starting fluid around the base and didn't find one. I will say that there is an incredibly loud vacuum sound at idle coming through the carb. It makes me think there may be too much vacuum, resulting in more fuel being pulled in.

Thanks in advance!

 

[JeffKeryk]

Check your throttle blades. If they are too far open you are bypassing the idle circuit. Common mistake. Good luck.

 

[MolaKule]

I would get a Holley rebuild kit for it in case you have a cracked gasket.

And while it is apart, I would take a magnifying glass and bright light and look for cracks.

 

[Cujet]

If the passages are all clear, no leaks and nothing else wrong with the carb, then it may be the engine requires too much throttle for the idle circuit to be fully active. Reasons could be ignition timing, leaks, load etc. Which engine do you have? 351, 350?

 

[mk378]

Turn the throttle stop down so low that it barely stays running then see if the screws change anything.

And as @Cujet said if the timing is wrong it's hard to tune up the idle. Too advanced and it will want to run fast all the time, and too retarded it will need a lot of air. Make sure the vacuum advance is connected to the advance port on the carb and not one of the constant vacuum ones. Vacuum advance is supposed to be inactive at idle to help it run slow and steady. As soon as you open the throttle a little it kicks in to give proper timing for running. Setting the base timing and tuning the idle involves disconnecting and plugging the hose to be sure that the advance is inactive.

 

[PontiacHO]

Before “tuning “ anymore, make certain the initial ignition timing is correct.
Also, check your fuel pressure because these non-adjustable floats are factory-set for optimum fuel levels at 3 to 6 psi of fuel pressure.

 

[JeffKeryk]

Turn the throttle stop down so low that it barely stays running then see if the screws change anything.

And as @Cujet said if the timing is wrong it's hard to tune up the idle. Too advanced and it will want to run fast all the time, and too retarded it will need a lot of air. Make sure the vacuum advance is connected to the advance port on the carb and not one of the constant vacuum ones. Vacuum advance is supposed to be inactive at idle to help it run slow and steady. As soon as you open the throttle a little it kicks in to give proper timing for running. Setting the base timing and tuning the idle involves disconnecting and plugging the hose to be sure that the advance is inactive.

Disagree on the distributer vacuum source; ported vacuum is for smoggers. A good starting place for ignition curve is where manifold vacuum is highest. Tune from there.

 

[The_Jeff]

You're on the right track wrt the transfer slots supposed to being squares at idle. I had this problem on my plow truck and had to turn the idle screw way down to get the slots properly square then drilled holes in the throttle blades to get enough air to run. Then, and only then, did the mix screws do anything. But that was a junky truck with probably low compression or other issues.

Are you sure-sure the secondaries aren't doing anything?

 

[MurkyAsparagus]

Check your throttle blades. If they are too far open you are bypassing the idle circuit. Common mistake. Good luck.

Just checked those out this evening, throttle blades are adjusted to where primaries have about an 0.020” transfer slot and secondaries are closed.

 

[MurkyAsparagus]

If the passages are all clear, no leaks and nothing else wrong with the carb, then it may be the engine requires too much throttle for the idle circuit to be fully active. Reasons could be ignition timing, leaks, load etc. Which engine do you have? 351, 350?

351W, sorry I didn’t mention that in the thread.

 

[MurkyAsparagus]

Turn the throttle stop down so low that it barely stays running then see if the screws change anything.

And as @Cujet said if the timing is wrong it's hard to tune up the idle. Too advanced and it will want to run fast all the time, and too retarded it will need a lot of air. Make sure the vacuum advance is connected to the advance port on the carb and not one of the constant vacuum ones. Vacuum advance is supposed to be inactive at idle to help it run slow and steady. As soon as you open the throttle a little it kicks in to give proper timing for running. Setting the base timing and tuning the idle involves disconnecting and plugging the hose to be sure that the advance is inactive.

Thanks for the ideas! Checked timing and it’s at 10 degrees like stock. This distributor is a mechanical advance instead of a vacuum.

I’ve moved idle all the way down to 400 with no change with mixture screws.

 

[MurkyAsparagus]

Before “tuning “ anymore, make certain the initial ignition timing is correct.
Also, check your fuel pressure because these non-adjustable floats are factory-set for optimum fuel levels at 3 to 6 psi of fuel pressure.

I’ll try to setup a gauge to check that. It’d be odd to me if it was okay last year and then this year high/low pressure but at this point anything is possible.

 

[MurkyAsparagus]

You're on the right track wrt the transfer slots supposed to being squares at idle. I had this problem on my plow truck and had to turn the idle screw way down to get the slots properly square then drilled holes in the throttle blades to get enough air to run. Then, and only then, did the mix screws do anything. But that was a junky truck with probably low compression or other issues.

Are you sure-sure the secondaries aren't doing anything?

Just checked the secondaries today and they were completely closed. The secondary screw was all the way out and the blades as closed as possible.

 

[Cujet]

You could try opening the secondaries a touch. This would reduce the opening of the primaries. 10 degrees at idle is not much advance at all. You might want to try more.

 

[MurkyAsparagus]

You could try opening the secondaries a touch. This would reduce the opening of the primaries. 10 degrees at idle is not much advance at all. You might want to try more.

I’ll give the secondaries a try. I was a bit surprised too when I bought the boat and saw its timing. Indmar, the manufacturer, has 10 degrees as stock timing.

 

[PontiacHO]

I’ll give the secondaries a try. I was a bit surprised too when I bought the boat and saw its timing. Indmar, the manufacturer, has 10 degrees as stock timing.

With vacuum advance hose unplugged?

 

[MurkyAsparagus]

With vacuum advance hose unplugged?

The advance on this engine is mechanical and not vacuum.

 

[oil is slippery]

I will say that there is an incredibly loud vacuum sound at idle coming through the carb.

Seems like a clue. May be air passing somewhere where it should not be.
10 degrees at idle is normal for that engine.

 

[MurkyAsparagus]

Update -

Double checked the timing. Then, I pulled off the bowls and blew compressed air to make sure the needles were fully seated and blocked anything from passing when the floats were fully up. Then, I tested the fuel pump pressure to make sure it wasn't too high of pressure to push past the seats. It was right at 6-6.5 psi. After, I left the secondary bowl off to make sure fuel wasn't making its way past those and running off the secondaries. This made no difference. I noticed that when the mixture screws all the way, it makes no difference but does make fluid go through the primary boosters. So, it makes me think the idle air bleeders were clogged. So, I pulled everything off, they were as clean as a whistle.


Any ideas? I'm lost here.

 

[dvblanch]

When you rebuilt the carb did you change the gasket between the throttle plate and main body? There are several different ones and possibly the idle ports or transfer slot holes could have been blocked off.