Ok, now I need some carb tuning advice (Holley 4160 600CFM)

grumpin wrote

Dang Bill! I’m sure you’ve forgotten more about carbs than I dare say I know!

Amen, brother!  

85lebaront2 wrote

That might be true. Preston Carburetion was a Holley Warranty Center, and we also sold Rochester, Stromberg, Carter and Weber carbs. There is one book I would love to find a copy of, it was a book put out by Weber, titled "Technical Introduction to the Master Catalog" It explained in great detail how each part of the metering circuits, both idle and main along with transition circuits functioned. I haven't been able to find one anywhere.

https://www.amazon.com/Master-catalog-introduction-Edoardo-Weber/dp/B0007JV562

Nice! maybe Santa will bring me one, I think we paid less than $10 for it in the early 70s.

Seems like a good price for good information!

85lebaront2 wrote

If the throttle plates have shifted, the easiest way to check everything on a Holley is to actually remove the throttle body. With the throttle body removed, hold it up to the light so you can see if the throttle plates look even in the holes.

Thanks for all the help. Now my be the opportune time for me to switch to the Summit 500CFM carb I've been wanting...lol.

Have you seen new Holley carbs have issues like this before? This one is only about a year old, with very few miles on it...maybe 3000-4000 miles, I'm not 100% sure. Maybe the throttle body is warped from it being bolted to that plastic spacer that warped...or maybe I warped it further when I torqued it down on that warped spacer...lol. Good grief.

When I had it off, I did check the bottom with a straight edge and it looked nice and flat.

Yes, and if I had extra loose money around I would order it.

The Summit Racing carburetor you are considering is a much better design and the 500 cfm size is a better match for your 5.0 than the Holley 600 cfm carburetor you currently have.  

I wouldn't waste any more time on the Holley.

LARIAT 85 wrote

The Summit Racing carburetor you are considering is a much better design and the 500 cfm size is a better match for your 5.0 than the Holley 600 cfm carburetor you currently have.  

I wouldn't waste any more time on the Holley.

That's the direction I'm going Rick. Only problem now is that everything is starting to slow down now with engine tuning and testing. I'll probably get to it through the winter sometime.

Rembrant wrote

With the idle speed screw backed out all the way and with maybe a 1mm gap (0.040"), I can actually close the throttle a little more manually...and the idle does drop a little bit...but I don't want to force it any harder than I have been. I don't know what stops the primary throttle plates travel when they're not hitting the idle speed screw. I seem to recall there being an adjustable stop in there, only accessible from the under side of the carb...one of those things that is "factory set"...

Cory, have you tried disconnecting the throttle cable to see if the idle is lower?

1986F150Six wrote

Cory, have you tried disconnecting the throttle cable to see if the idle is lower?

David, this exact thought also popped into my head a couple days ago...so I promptly checked it and the cable is all good. There's actually a little bow in the cable because the throttle stops before the end of the cable travel. I disconnected it for a test regardless, and it didn't help.

If money were no object, I'd probably swap over to one of the aftermarket EFI kits this winter, but since that is getting into a $2000 bill, it sure makes a $500 carburetor look good...lol.

Stay tuned for more to this story...

My new O2 sensor will be here tomorrow, so I'll be testing that asap to at least see where my fuel and air is at currently.

I chased a vacuum leak [for a very long time] on my son's truck. I went through all you have described, and could not get the idle speed in the acceptable range. The smoke test [cigar ] showed in two deep puffs a leak I had missed numerous times. One of the rubber caps on a vacuum tree of the intake manifold was cracked. This cap was on an unused port facing away from my stream of carburetor cleaner and was never directly hit by the stream.

Rembrant wrote

If money were no object, I'd probably swap over to one of the aftermarket EFI kits this winter, but since that is getting into a $2000 bill, it sure makes a $500 carburetor look good...lol.

Your truck doesn't need an aftermarket EFI kit.  If you get that Summit carburetor bolted on and adjusted correctly, I think you are going to be pleasantly surprised at how well your truck can run with a carburetor.  

LARIAT 85 wrote

Your truck doesn't need an aftermarket EFI kit.  If you get that Summit carburetor bolted on and adjusted correctly, I think you are going to be pleasantly surprised at how well your truck can run with a carburetor.

I'm looking forward to it actually. What about jets and parts for the Summit carbs?...do they use the same style jets as the Holley carbs? I was thinking about buying a jet kit...with the O2 sensor I'll be able to get the thing dialed in pretty good, or at least that is my plan.

Yes, the Summit carburetor uses the same jets as any Holley carburetor.  

The 500 cfm is a better size for your truck.  As such, the jetting should already be really close out of the box and you shouldn't have to make major adjustments.  

Good news! My new replacement O2 sensor kit arrived today and it works! lol.

Now, I need some carb tuning advice. Keep in mind this is kind of a moot topic right now as I would like to swap out this Holley carb for a 500CFM Summit, but I still need to understand what is going on here.

I may very well still have a vacuum leak, but I probably won't have decent time to trouble shoot it any further until next week when I'm on vacation.

Ok, so I got the truck warmed up and took it for a drive. The AFR is not that bad, but it was definitely too lean at idle. I adjusted the mixture screws up to 1.5 turns, and the engine now has a warm idle of around 14:1 plus or minus a half a point. That part seems OK. This is where I need help in understanding what is going on inside this carb (and I will go read my Holley books again tonight too!).

Cruising down the road a very light throttle...the AFR is OK, basically about the same as when it is at idle. Usually between 13.5:1 and 14.5:1 I'll say. If I start to apply any light pressure to the throttle while there is a load on the engine, it immediately goes lean for a second and will then settle back down a bit into the 14.5:1 to 15:1 range or there abouts.

Not sure if I am describing this well enough...

If I run the truck through the gears, and increase the RPM's slowly with light throttle pressure, the AFR will stay pretty good. If I put a little extra pressure on the pedal, it will go lean...sometimes on briefly, but enough to stutter and buck a bit. If I push the pedal even further, the AFR comes back to normal again.

WOT AFR is fine. It's just that transition from very light throttle with no load to a light throttle with a load on it, and it goes lean and runs like poop. Above 2000 RPM it seems fine, and in the lower gears it seems fine...but cruising along in 4th gear at around 1800 RPM, it doesn't like it there and will stumble and miss until I push it up above 2000 RPM.

Am I making any sense at all?

What adjustments do I need to make to help with that lean transition? It seems like idle and light throttle low load is OK...and WOT is OK, but everything in between is bad...lol.

Light throttle "cruise" should be lean...

Sounds like your accelerator pump is not hitting soon enough, but no one drives around creeping up the revs while glued to their afr meter.
Unless you open the throttle enough to push that lever you're going to have a tiny dribble that would make grandpa proud.

There is nothing going to make up for that.
Even an AFB/AVS is not going to raise the needles until it has a noticeable change in vacuum.
(and they can be set up very sensitive)

Put a vacuum meter next to your afr and watch the two dance together.

ArdWrknTrk wrote

Light throttle "cruise" should be lean...

Yes, but not so lean that it bucks and stumbles, right?

I know that part of my problem is that with the 5spd and 3.08 gears, and having to drive 10 miles at 45 mph just to get to the highway causes me to run at pretty low RPM's....low speed, light load, low throttle, etc. The stretch between 1500 RPM and 2000 RPM on this engine has never run quite right. Maybe it's that stupid cam I picked...I dunno.

I guess if any carb is going to run like that, then I won't have much choice but to go with EFI.

In any case, I'll keep hunting for a vacuum leak as well.

I'm not "the authority".. that would be Bill.  🙏

But if you have a cammed 302 with a 600 cfm carb and are loping along at 45 in fifth with 3.08 gears I can't see it being much better.

You spec'd the engine.....
What did you think you were going to get?

Either grab it by the stones and squeeze tight or stay home.

ArdWrknTrk wrote

But if you have a cammed 302 with a 600 cfm carb and are loping along at 45 in fifth with 3.08 gears I can't see it being much better.

You spec'd the engine.....
What did you think you were going to get?

Either grab it by the stones and squeeze tight or stay home.

Haha...no, there's something wrong with it. If you could see it and/or feel it, you'd understand.

It's no big deal. I'll figure it out eventually. I might be slow, but I'll get there.

Yes, there is something wrong.  And I'm going to suggest some things, but if there is a vacuum leak then you need to fix it before you "fix" it by changing something else.


You say the AFR is "OK", but that's really a moving target.  At idle you should be able to go so rich with the AFR that the idle slows down.  If you can't then that is proving there's something wrong, like a vacuum leak.

At cruise it should be around 14.7:1.  And as you slowly open the throttle it will go more and more lean until the power valve opens, which should bring it back to around 14:1.  And I agree with Jim - put a vacuum gauge on it so you can see at what vacuum level the power valve opens.

If there really isn't a vacuum leak you probably need to rejet slightly richer.  And/or change the power valve to one that opens slightly sooner.  For instance, if your PV opens at 6.5" try one that opens at 7.5".  But, before you do that watch the vacuum gauge to determine where in the vacuum range the problem happens.

At WOT you should have an AFR of around 12:1.

Regardless, I think getting the AFR correct will have the engine running smoothly in all situations.  And a carb can do that - but you have to be the feedback loop.  Run the test and modify.  Run the test again and modify.  Etc.