Learn me: Holley carb tuning

We have this 66 mustang convertible in the family, and It has recently developed an issue that I can't seem to nail down. 

It idles and cruises fine, but when you dip into the throttle a bit, but not enough to kick down the trans, between about 1800-2800rpm, it loads up and stumbles like its way too rich.  If i can get it past that point, it clears out and pulls strong the rest of the way. 

I've refreshed the carb gaskets, set the floats, changed needles/seats, adjusted idle mixture, replaced the power valve, and adjusted the accel pump linkage.  The carb is a holley street avenger 670 and is running 68 mains and 70 secondary jets with a 6.5 power valve.   It ran fine at this setting previously but has now started acting up.  The motor is a FRPP 302 crate with supposedly a "mild" cam and edelbrock rpm air gap intake, hooked to an AOD trans. 

I'm thinking that the power valve may be opening up too early and loading up the motor.  It will pull 11.5-12.5" Hg at idle in park, but that drops to around 9-9.5"Hg in gear with the A/C on.  From what I understand the power valve setting is correct for a manual trans, as that would be measured at idle in neutral, but the correct power valve seems like it should be a 4.5 or 3.5 given the idle vacuum readings i'm seeing (idle, in gear, all loads on).  

I need to get this figured out, as I'm set to bring it back to my place at the end of the month, and I'd be a lot more comfortable setting out on the 5-hour drive with this licked.  

Any help/advice would be appreciated.

here's a pic of the vehicle in question:

first I thought shooter may be clogged some, that is a fairly lo vacuum reading, again try the idle jets lean them 1/4 turn see if idle comes up. you ought to get 16-18 lbs V. at 700-750rpm.also see what timing is doing during this transisition. Look for an air leak and did you change the vac. adv. port. some carbs have one above another, one is idle vacuum the other (top one) is above idle vacuum.

Holly's have an issue with ethanol gas creating a deposit in the idle air bleeds on the primary circuit. Take a look at them and clean them out. It's almost exactly what you're describing the car acts like

 It will pull 11.5-12.5" Hg at idle in park,

That does not sound right. you have a massive leak somewhere. Also I have found Holleys to be incredibly sensitive to fuel pressure as well and chasing dead spots is not possible without getting it dialed in. 

Idle bleeds would also be something I check and butterfly's closing up good and tight. 

I wouldn't go tuning on a carb that worked fine before.  Something has changed.  If you're sure that all the passages, jets and bleeds are clean then the problem is unlikely to be in the carb. 

That Idle vacuum does seem low but we know nothing about the engine and the camshaft that's in it.  You can also get low vacuum if you have the throttle blades open to keep the engine running with an ignition/timing problem.

Checking for vacuum leaks is worth doing but I'd be looking at ignition starting with a fresh set of plugs and a timing check.  If it still has points then see what they look like and check the dwell.

APEowner said:

I wouldn't go tuning on a carb that worked fine before.  Something has changed.  If you're sure that all the passages, jets and bleeds are clean then the problem is unlikely to be in the carb. 

That Idle vacuum does seem low but we know nothing about the engine and the camshaft that's in it.  You can also get low vacuum if you have the throttle blades open to keep the engine running with an ignition/timing problem.

Checking for vacuum leaks is worth doing but I'd be looking at ignition starting with a fresh set of plugs and a timing check.  If it still has points then see what they look like and check the dwell.

 

The ignition is an MSD billet distributor with an MSD coil and 6A box with the vac advance plugged.  I understand that this is not the best for economy but once i get the car back here I can futz with vac advance and the timing curves, just not enough time when I'm only at the car for a weekend at a time.  I will get a timing light and find out where it is at.  That's one thing I haven't been able to check yet.  

I checked the timed vacuum port and i'm getting zero vac there so the throttle is closed enough for that, plus the transfer port/slot is exposed what is supposed to be the correct amount.  secondaries close up tight.  The engine was previously set with the idle too high so it wasn't idling on the idle bleeders.  it would just about stay running iirc when you closed the air bleeds.  I am getting max vacuum at idle at around 1 1/2 turn out like its supposed to be.   

I don't think its a vac leak unless maybe the pcv is stuck open or something, all of the normal vac lines are hooked upt, and I changed out the plugs and fuel filter when this first started as an easy first step with no joy.  

Forgot to mention, it doesn't start stumbling right away, it starts a 1/2 second to a second after you tip into the throttle, and rolls in like maybe it could be the secondaries opening up. 

I know that the original jetting on this carb was a 65 main and 68 secondary, so its been setup quite a bit more rich than the "factory settings should be pretty close" jetting of the carb out of the box, and the car runs like a scalded dog above this rpm so if ignition was a problem i would think it would be an issue higher up. 

Here is an example of the spots it leaves on cold start, although it’s pretty much always left these iirc 

when you said 68's and 70's it sounded a bit rich, do you see the shooters working, that sounds like maybe not richining up. 

In reply to Strizzo :

Your symptoms sound very similar to what experienced with the Vette.

By chance has anyone touched the distributor recently? I found my hall-effect sensor had been moved slightly, and repositioning it cured the problem. 

Pete Gossett said:

In reply to Strizzo :

Your symptoms sound very similar to what experienced with the Vette.

By chance has anyone touched the distributor recently? I found my hall-effect sensor had been moved slightly, and repositioning it cured the problem. 

Not that I know of, that would be the sensor under the rotor right?  I did not build/setup this thing in the first place, my dad had a guy he knows do all the work (he's more of an r&r mechanic) so I don't know what has been done or how it was done.  Should it be obvious that its not in the correct place? 

still sounds like accelerator pump 

or shooter

In reply to fasted58 :

Accel pump squirts fine, and the linkage is adjusted correctly.  runs fine at first, then loads up.  if I stay in it up to 2500-2600, it will clear out and run better.  secondaries opening too soon?

In reply to Strizzo :

too small a shooter= stumble, too large shooter= bog

check secondary spring. medium (plain) or heavier should be ok for eliminating opening too soon

easiest to check secondary spring first to eliminate it

Holley stuff. Great. :)

The first thing I would do is take off the front fuel bowl and the accelerator pump squirter, and blast some carb cleaner in both directions through the hole. Ethanol gas causes this white E36 M3 to gather in the bottom of the fuel bowl (which promptly gets sucked in by the accel pump), and it caused all my Holleys and Demons to develop a lean bog right at tip-in about twice a year. 

This might help:

http://www.thompsonperformance.com/shop_products.html

Basically, the problem is probably the white E36 M3 all through the carb. 

Also, on old engines like that, you want to use full manifold vacuum for the vacuum advance, not the timed-spark port. The timed spark port was invented to retard the timing at idle, which caused the exhaust to be hotter, which helped catalytic converters convert. Set your timing to the spec with the vacuum hose unhooked and plugged, then hook it up and set the idle speed with the most load the engine will ever see. That means in gear for an automatic, high beams on, AC on. 

We used to hire guys for hose and expansion joint inside sales and would argue that the guy doesn’t need to know how to tune a Holley Carburetor to do the job.   Maybe he does?

snailmont5oh said:

 

The first thing I would do is take off the front fuel bowl and the accelerator pump squirter, and blast some carb cleaner in both directions through the hole. Ethanol gas causes this white E36 M3 to gather in the bottom of the fuel bowl (which promptly gets sucked in by the accel pump), and it caused all my Holleys and Demons to develop a lean bog right at tip-in about twice a year. 

That would make sense since it ran ok before sitting. Restricted passage could act like too small shooter.

Remove check under shooter before blasting w/ cleaner or air.

fasted58 said:

Remove check under shooter before blasting w/ cleaner or air.

And, it would only dribble out the squirter instead of hauling ass like it should. Definitely remove the check assembly, especially if you want to not search for it for an extended period of time. 

Thanks for the suggestions, I’ll take a look at the accel pump stuff for gunk and try not to lose the check piece. 

Sticky check ball on the accelerator pump?  You can probably watch the shooters when you hold the rpm up at the problem zone, and there shouldn't be anything coming out the accelerator pump shooters after the primary shot is gone.

Gasket on the power valve is split?  You said you put a new one in, right?

If it is rich enough to actually run bad, it should be smoking black like a bastard when you hold the revs up.  You are positive its not lean due to a blocked passage?  Under load, a lean condition is way more noticeable than a rich one, until it gets way too rich.  The feeling is similar when driving.

Recommended jet sizes haven't changed in the last 40 years, and Every. Single. Recommended. Jet. is at least two sizes to small.  Its the modern gas, you know.

Strizzo said:

Pete Gossett said:

In reply to Strizzo :

Your symptoms sound very similar to what experienced with the Vette.

By chance has anyone touched the distributor recently? I found my hall-effect sensor had been moved slightly, and repositioning it cured the problem. 

Not that I know of, that would be the sensor under the rotor right?  I did not build/setup this thing in the first place, my dad had a guy he knows do all the work (he's more of an r&r mechanic) so I don't know what has been done or how it was done.  Should it be obvious that its not in the correct place? 

I have no idea how different your Ford may be than an SBC,  but on it after you removed the rotor there was a screw that fastened vertically a ground wire from the ignition module, through the hall-effect sensor heat sink. The sensor should have been pointing toward the distributor shaft, but mine was angled off a few degrees. It ran fine below and above 2k, but there was a bad miss that came in around 1900rpm and was gone by 2100rpm. 

After trying a new carb & replacing every other ignition component, I swapped in another distributor and the problem disappeared. So I started comparing both units side-by-side and that’s when I noticed the sensor was mispositioned. 

In reply to Streetwiseguy :

I'll see if i can take a look at the squirters at part throttle to see if they're dribbling past the initial shot.  Power valve is new with a new gasket.  I also blew compressed air through all of the passages in the metering block when I had it off, so there shouldn't be any blockages there. 

I can't see any black smoke out the back but its hard to tell through the plastic back window, but there is a lot of soot on the bumper above the exhaust so i'm sure there is some black smoke.  I feel like I can tell the difference between lean and fat, but I could be wrong.  the exhaust tone drops lower, misfires, shaking/stumbling, and lack of power.  I understand that it is probably a lot more likely to be lean than rich though.  Probably should just do a plug chop, but I have been hoping to avoid pulling plugs on a hot motor on the side of the road in the middle of summer. 

Pete Gossett said:

Strizzo said:

Pete Gossett said:

In reply to Strizzo :

Your symptoms sound very similar to what experienced with the Vette.

By chance has anyone touched the distributor recently? I found my hall-effect sensor had been moved slightly, and repositioning it cured the problem. 

Not that I know of, that would be the sensor under the rotor right?  I did not build/setup this thing in the first place, my dad had a guy he knows do all the work (he's more of an r&r mechanic) so I don't know what has been done or how it was done.  Should it be obvious that its not in the correct place? 

I have no idea how different your Ford may be than an SBC,  but on it after you removed the rotor there was a screw that fastened vertically a ground wire from the ignition module, through the hall-effect sensor heat sink. The sensor should have been pointing toward the distributor shaft, but mine was angled off a few degrees. It ran fine below and above 2k, but there was a bad miss that came in around 1900rpm and was gone by 2100rpm. 

After trying a new carb & replacing every other ignition component, I swapped in another distributor and the problem disappeared. So I started comparing both units side-by-side and that’s when I noticed the sensor was mispositioned. 

The MSD billet is based on a GM distributor, so I'm sure they're pretty close. 

A word to the wise. I chased a very similar stumble in a FB for two months. Kits, jets, adjusted everything I could think of, to no avail. A crusty old guy at a car show suggested a new set of plugs. I calmly explained that I had already checked and cleaned the plugs but still had the stumble. He suggested changing them anyway because it's not like they are expensive. 

A week later, I swapped the plugs, and the stumble was gone. I had a whole list of other problem I had created "adjusting" the carb, but no more off idle stumble. 

Now, all my carburetor adjustment start with a fresh set of plugs. Every time. Even when I know it's just a simple carb tweak. 

In reply to Toyman01 :

plugs and fuel filter were a couple of the first things done, I do see your point about not creating more problems adjusting things chasing a simpler problem. 

"Recently developed an issue".

Stop messing with the carb.  Replace the points and the spark plugs first above all else.

Carburetors don't suddenly develop issues.  If you have already replaced the internal gaskets, then okay, you verified that there isn't any passage crosstalk, stop messing with it, problem is ignition related.

Ran into all of this recently (as in, this week) with a K-code '65 that some prior heretic replaced the honorable 4100 Autolite with some peasant side-hung Holley.  That, over the years, decided to drool everywhere instead of putting fuel down into the engine.  After rebuilding the carb, the problem persisted, because the problem wasn't the carb but the ignition system.

(Well, not to say that dumping fuel all over the intake manifold isn't a problem, but it wasn't the source of the drivability problem.  But you first have to start with Known Bads, you know?)