Which carburetor adjustment procedure (fuel - air mixture) is best?

[KM7]

Adjusting the fuel/air mixture on a 1992 5.8 L with a Holly 4 bbl

There are 2 different methods:

1) Per an original Shop Manual (615 pages)

Idle speed screw just touching but not moving the cam

Mixtures screws seated then back out 7/8 (for my engine)

Start engine and warm up

Disconnect throttle with engine running (it would be hard to start with no throttle to pump or to raise the idle)

Shift into fwd gear

Turn the mixture screws IN until you get an rpm drop (this is called a “lean roll”)

Then turn them OUT until you get the RPM in the manual (550-650)

(This assumes that you will be at an idle Below the specs when you get the lean drop. What if you are already above it?)

While you are turning the screws back out, If the idle begins to drop due to the mixture too rich, turn them to get it running smoothly

Recheck the idle speed and use the idle speed screw to adjust

(But we just set the idle using the mixture screws, we have not used the idle stop screw to set the idle at all

In the first step the idle speed screw was set to not be affecting the idle)

OR

2) Per a Clymer manual for 86 – 93 boats

Start and warm up engine

Use the idle speed screw to adjust the rpm to the specification (this is different from above)

Adjust idle mixture to obtain the highest rpm (no lean roll method is used)

Readjust the idle speed screw to obtain rpm per the manual


Which is best and why?

Last time I used the Clymer procedure and I think I'm running a bit rich. Take a look at the pictures of the spark plugs

 

[Scott Danforth]

Use the clymer manual as toilet paper, as that is all its good for.

When was the last time you rebuilt the carb?

If you have had a backfire, you may have blown the power valve. That would cause it to run rich

 

[KM7]

Carb was just rebuilt at the end of last summer. Is there a way to test the power valve?

 

[Scott Danforth]

If you had a backfire, and now you are running rich, the power valve has ruptured.

 

[KM7]

I'm not sure about the backfire. It may actually never have had one. Did you look at the pictures of the plugs? Do you agree that they look like I'm running maybe a little rich?

If it is a little rich, that could be because of how I set the mixture screws. The procedure I have been using is the Clymer method that does not use the lean roll. Using it, the mixture screws are at 1 1/4 out. That is a bit more that the 7/8 the book says and may be rich.

I plan on doing it again using the 1st method from the original shop manual. I'll see if that results in a leaner setting.

The condition of the plugs may also be from running at idle a lot lately. I have been doing a lot of work on it and that frequently has me at idle. I have heard that can result in plugs getting a little black from running rich.

I would sure like to be able to check the condition of the power valve before I pay for another rebuild. There must be a way

 

[Lou C]

Your plugs really don't look too rich to me, esp if the engine was idling. You might want to go over to holley.com and do some reading on how to set up the idle mixture. On a Holley there is also the issue of the idle transfer slots, these should not be exposed too much or then you are starting to bring in the main system instead of just the idle system and can get a very rich idle. This has to do with the angle of the primary throttle plates and can't really be checked unless you have the carb off the engine. I tried a Holley 4160 that was specifically for my 4.3 with no power valve and bigger main jets and it ran rich no matter what I did. In frustration I just rebuilt my 30 year old Quadrajet and it ran much better. Must have been something else wrong with the Holley even though it was brand new.

 

[Lou C]

sorry pix don't seem to work on this site anymore, try google chrome instead of IE...
the part of the slot that is exposed, should look like a square instead of a slot....

 

[GA_Boater]

sorry pix don't seem to work on this site anymore, try google chrome instead of IE...
the part of the slot that is exposed, should look like a square instead of a slot....

This pic you just posted?

 

[GA_Boater]

This pic you just posted?

Clicking on your pic it's much larger.

 

[KM7]

@ Lou Thanks I'll check out the holley site.

You said: "....idle transfer slots, these should not be exposed too much or then you are starting to bring in the main system instead of just the idle system and can get a very rich idle. This has to do with the angle of the primary throttle plates and can't really be checked unless you have the carb off the engine."

Based on what you said about the look of the plugs, you think I'm not running way too rich, correct? Would that mean that the angle of the primary throttle plates are OK?

Exactly what do I look for if I do take the carb off? That is not hard to do. I'll check for instructions on the Holly site but if you have another suggestion or a link I'd like it please.

What about the 2 idle & mixture setting procedures above? Which do you prefer?

The first procedure using the Lean roll seams the most thorough. A slightly modified version of that has you turn out after the lean roll till you get a drop due to too rich then back in to get back in to get the smoothest idle.

Then set the speed with the idle speed screw. Of course that only works if you need to turn the idle UP.

Thoughts?

 

[jimmbo]

I just use a Vacuum Gauge and adjust the mixture for highest vacuum.
As for the Throttle Plate to Transfer Slot positioning. If the stock carb is used, and the engine is in good shape and Timing is correct, just set the idle speed to spec. There is some Wiggle Room. The Transfer Slot/Throttle Plate Position becomes an issue when setting up a carb not intended for that engine size or operating Parameters. Holley 4 barrels allow the Secondary Throttle Valve to be adjusted, to provide more Idle Air Flow, so the Primary Plates can be set closer to optimum.

 

[KM7]

Lou, Is this what you are talking about re the idle transfer slots:

http://forums.y-blocksforever.com/Topic131647.aspx

 

[KM7]

jimmbo where can you connect a vacuum gauge on this carb, 4160 ?

 

[Lou C]

Lou, Is this what you are talking about re the idle transfer slots:

http://forums.y-blocksforever.com/Topic131647.aspx

Yes it is. Your plugs don’t look too rich to me esp if that’s from running at idle. Leave it the way it is and run it in the water then check plugs again.

 

[jimmbo]

Yes it is. Your plugs don’t look too rich to me esp if that’s from running at idle. Leave it the way it is and run it in the water then check plugs again.

I put a Tee fitting in the Choke Pull Off Hose, Any Manifold Vacuum port would work. Most Hollies have at least one.

 

[KM7]

@ Lou - Thanks
@ Jimmbo - I'll try that, Maybe a very long vacuum hose so I can watch while I crank and or work the throttle. Hmm a remote starter switch would be nice as well.

 

[KM7]

The Idle transfer slots look good, very square. I do not see a place to connect a vacuum gage. Do you?

 

[KM7]

Is there a way to test the choke? It is electric. When does the plunger in the 3rd picture move? Can I connect and disconnect the wires to it to see what it does?

 

[Redrig]

My Holley 2bl didn't have anywhere to connect a vac gauge either .

I ended up hooking it into a plug that was on a "runner" on the intake manifold , worked great as that is of course full manifold vacuum.

but I didn't actually use vacuum to dial in the carb. I just used the process detailed in the stickies here , worked great

 

[KM7]

The port I thought would work, was no good. It is the one in the 5th picture above, the one that shows the linkage. It's the hole at the top. There was no vacuum there. I have tried the OMC fuel air mixture per the first procedure but can't get it to idle below around 800 in gear (1000 in neutral)

I'm checking everything I can think of. So what can I use to check vacuum ?

Please include a picture if you have a suggestion.