Holley stumble trouble

[SSG_Karg]

Too many changes at once. View attachment 1715543421
But sometimes you just have to try things, so its all good as long as you learn something.
Eventually you'll get a sense of when big jumps are OK and when they are not. You can generally do that with field artillery and naval guns but not always with engines. Once outside of a working range fluid dynamics of the fuel and air start doing unpredictable things.


Going back to the baseline as you are is the logical move in my book too.
Then make one move from there. I'd say the .035" t-slot is a good one. And since you've done that, try 18 and or 20* and see if you can get it to idle at same rpm and vacuum without opening the t-slots.

Come back to that decision after reviewing some of what has been learned.

That's interesting! While its possible that timing may related to the stumble, removing vacuum advance from the equation did not completely resolve it. View attachment 1715543422 That's definately something to think on. I've used the Direct Connection Race distributor, which is super fast advance (no vacuum advance) and no stumbles.


I used to think something similar. Its not nuts, but its not correct. Or maybe it is nuts, but then I'd be admitting I'm crazy too.

First of all, timing's relationship with mixture density (which is more or less the same as how rich the mixture is in the combustion chamber) is tied together. In a lean running or other slow burning condition, more time is needed for the burn to build maximum pressure as the piston is going down.
View attachment 1715543423

Second, with a high overlap cam the idle vacuum is already somewhat low. If you recall the 'idle circuit' uses the manifold vacuum to move fuel from the bowl to the idle and transfer ports. So when the
both air flow and fuel flow was reduced. Opening the throttles brought in more air, but not a proportional amount of fuel.


Take the carb off, measure t-slot openings for 1/4 turn increments of the adjustment screw. Start from just touches to .045 or .050 open.

If your chicken scratch is getting impossible to reference, copy it over neatly into a spreadsheet or a fresh sheet of paper or whatever works for you.
My carb notes (almost) always goes onto a spread sheet later that day or week. I also print some stuff and copy things onto 8.5 by 11. Most now goes into file folders which I can grab easily. A few things still go into 3 ring notebook - which was my initial method or organizing.

Returning to the next steps:
Going back to the baseline as you are is the logical move in my book too.
Then make one move from there. I'd say the .035" t-slot is a good one. And since you've done that, try 18 and or 20* and see if you can get it to idle at same rpm and vacuum without opening the t-slots.

I don't know what will happen when you try a little less initial with .035. My guess is that there will not be enough air flow. That's probably why the car was running with .045" of the slots showing. Opening up the throttles let more air in.
So I would plan on it not idling as strongly with only 18 or 20* initial and .035 to .040 t-slots. In which case let more air in at idle.
One method would be to crack the secondary throttles just a little further open. Measure the throttle to base distance before you do this. Or if you have a wire gage you could measure clearance to the wall. There is no other reference for the factory setting! Then give the screw a 1/4 turn. Give it try.

A more easily adjustible method is to put a T in the PCV line. Plug the base of the t and drill an 1/8" hole in the plug. If that helps try 3/16.
(Later you can drill the throttle plates or crack the secondaries.)

See if that brings the idle speed up. It should.

Next, see what the idle mix screws are doing. They should still have an effect on the mix, especially with the t-slots further closed.
But the mixture may need more enrichment. If so, experiment with wires in the primary idle air bleeds.

Once an idle is established that is at least as strong as the baseline (800 rpm 10.5 "Hg). Then try driving it.
If the dead spot is still be there, increase the wires in the IFR.

Cheat sheet for a few IAB sizes.
View attachment 1715543418

View attachment 1715543419

View attachment 1715543420

So if the 4777 has .076 IABs, and .025" wire is placed in each one, they will be equivallent of .072 IABs.
Run the wire under the aircleaner gasket to secure it. I usually use one wire for both bleeds even though it takes longer to bend up jsut because its easier to keep track of. Two individual wires works just as well.
Typical IABs will be in the .072 to .076 range.
With the air bleeds, a jump of .002 or .004" should noticibly effect the idle through transition fuel.

Tons of great information here Matt. I wish you were my neighbor!
I really appreciate everyone’s input on this. I have quite a bit of testing to do now and will work through it when I can and try my best to report back here. I have a very bad habit of changing two or three things at a time thinking that will expedite the process when in turn I’m just shooting my self in the nuts. I’ll try to contain my excitement and make one change at a time and record the results.
Bear with me here guys.....

 

[Mattax]

It would be more fun!
I bet I made around 50 changes when learning the idle transition stuff. And at least two succesful days on a chassis dyno with about 25 pulls. Top end of the frankencarb still needs be run on the track to see if the AFR stays flat from the 3000 to 6000 rpm. I have two 'normal' carbs to try but not being able to check the results of my last changes kindof kills me. No racing here yet. Hopefully I'll have it drivable in a couple weeks.

 

[SSG_Karg]

I’m still fighting this God-forsaken stumble! I’m on the verge of throwing this damn carb off the bridge with glee.
I’ve tried initial timing all the way from 22 to 10 degrees btdc with multiple curves to 30 - 38 total. Still stumbles on the transition. I’m all the way up to 75/77 jets, 28 squirter on blue cam in #1 spot. Still has the 10.5 pv in it. (Cruise vacuum is 14-15”). Tried wires in the IABs with no luck.
Would a vacuum leak around the throttle shaft make this ***** untunable? While resetting the acc pump arm I noticed gas leaking around the throttle shaft. Maybe this girl is just totally clapped out......
I don’t know what the hell to do now. See if I can borrow a known good carb from someone to try I guess.

 

[j par]

I’m still fighting this God-forsaken stumble! I’m on the verge of throwing this damn carb off the bridge with glee.
I’ve tried initial timing all the way from 22 to 10 degrees btdc with multiple curves to 30 - 38 total. Still stumbles on the transition. I’m all the way up to 75/77 jets, 28 squirter on blue cam in #1 spot. Still has the 10.5 pv in it. (Cruise vacuum is 14-15”). Tried wires in the IABs with no luck.
Would a vacuum leak around the throttle shaft make this ***** untunable? While resetting the acc pump arm I noticed gas leaking around the throttle shaft. Maybe this girl is just totally clapped out......
I don’t know what the hell to do now. See if I can borrow a known good carb from someone to try I guess.

I really like the idea of trying a known carb just to definitely rule out the carb..

 

[yellow rose]

I’m still fighting this God-forsaken stumble! I’m on the verge of throwing this damn carb off the bridge with glee.
I’ve tried initial timing all the way from 22 to 10 degrees btdc with multiple curves to 30 - 38 total. Still stumbles on the transition. I’m all the way up to 75/77 jets, 28 squirter on blue cam in #1 spot. Still has the 10.5 pv in it. (Cruise vacuum is 14-15”). Tried wires in the IABs with no luck.
Would a vacuum leak around the throttle shaft make this ***** untunable? While resetting the acc pump arm I noticed gas leaking around the throttle shaft. Maybe this girl is just totally clapped out......
I don’t know what the hell to do now. See if I can borrow a known good carb from someone to try I guess.

Make it simple for us and remind us where the stumble is, what you have for idle feed restrictions and idle air bleeds and what you have for a high speed air bleed.

That would help a bunch. I don’t want to have to reread the entire thread if I don’t have to.

 

[SSG_Karg]

Make it simple for us and remind us where the stumble is, what you have for idle feed restrictions and idle air bleeds and what you have for a high speed air bleed.

That would help a bunch. I don’t want to have to reread the entire thread if I don’t have to.

OLD Holley 650 DP. Non-adjustable air bleeds.
Stumble is in the transition from idle circuit to mains during very light throttle cruise. Like after coasting down a small hill and just starting to tickle the throttle at the bottom of the dip before rolling back in to it.

 

[Hysteric]

Make it simple for us and remind us where the stumble is, what you have for idle feed restrictions and idle air bleeds and what you have for a high speed air bleed.

That would help a bunch. I don’t want to have to reread the entire thread if I don’t have to.

LOL. He stated earlier that the stumble developed when he connected his vacuum advance. Measure vacuum in both states.

Have you driven the car on the transfer slot only? i.e disconnected the pump shot altogether.

 

[SSG_Karg]

He stated earlier that the stumble developed when he connected his vacuum advance. Measure vacuum in both states.

It still stumbles with the vacuum pod disconnected and plugged. I’ve tried adjusting the pod from all the way in to all the way out. Still there.

Have you driven the car on the transfer slot only? i.e disconnected the pump shot altogether.

No.

 

[yellow rose]

OLD Holley 650 DP. Non-adjustable air bleeds.
Stumble is in the transition from idle circuit to mains during very light throttle cruise. Like after coasting down a small hill and just starting to tickle the throttle at the bottom of the dip before rolling back in to it.

If that’s the case, you can open up the main air bleed .004-.006 and get it on the mains quicker.

Been awhile since I’ve been into one of those, so I have no idea what’s in them for MAB, IFR, IAB and such.

 

[Hysteric]

No.

Then try that.

 

[512Stroker]

I’m still fighting this God-forsaken stumble! I’m on the verge of throwing this damn carb off the bridge with glee.
I’ve tried initial timing all the way from 22 to 10 degrees btdc with multiple curves to 30 - 38 total. Still stumbles on the transition. I’m all the way up to 75/77 jets, 28 squirter on blue cam in #1 spot. Still has the 10.5 pv in it. (Cruise vacuum is 14-15”). Tried wires in the IABs with no luck.
Would a vacuum leak around the throttle shaft make this ***** untunable? While resetting the acc pump arm I noticed gas leaking around the throttle shaft. Maybe this girl is just totally clapped out......
I don’t know what the hell to do now. See if I can borrow a known good carb from someone to try I guess.

What is the fuel pressure?

 

[SSG_Karg]

What is the fuel pressure?

6-6.5. Holley mechanical pump. -6AN line from pump to carb.

 

[512Stroker]

6-6.5. Holley mechanical pump. -6AN line from pump to carb.

Maybe this was covered already?
What cc shot accelerator pump do you have on the primary side?
And do you know which number center squirters you have?

 

[SSG_Karg]

Maybe this was covered already?
What cc shot accelerator pump do you have on the primary side?
And do you know which number center squirters you have?

30cc pumps front/rear. 28 squirter front 31 rear.
I don’t think it has anything to do with the acc pump. If I mat the throttle it takes off, no hesitation or stumble at all. Only hiccup is during very light throttle cruise when starting to give it a hair more throttle. Just a half second hiccup then smoothes back out. If I get into the throttle quickly it doesn’t stumble. Seems like the transition is not lasting long enough until the mains can take over.

 

[512Stroker]

30cc pumps front/rear. 28 squirter front 31 rear.
I don’t think it has anything to do with the acc pump. If I mat the throttle it takes off, no hesitation or stumble at all. Only hiccup is during very light throttle cruise when starting to give it a hair more throttle. Just a half second hiccup then smoothes back out. If I get into the throttle quickly it doesn’t stumble. Seems like the transition is not lasting long enough until the mains can take over.

On my Holley set ups i always ran 50 cc shot pump with a very fast linkage cam.
I dont know what your running for initial timing but ya might try backing off the timing a bit like if your at 18* back it down to 16* this will bring down the idle rpm then you can add idle rpm with the idle screw on the linkage. That will open up the transfer slot allowing for better transition. I had to do that on my BBM stroker or it would stumble bad on the transition.
side note: I recently found out that may old trusty Mac timing light was off 3*

 

[Mattax]

Seems like the transition is not lasting long enough until the mains can take over.

If the throttle is low enough, then this could be basically true. Lasting long enough would only be if increasing the throttle. But lets say you start to give it a hair more throttle and hold it. Is the delay the same or does it continue?
Subtle difference. If its a delay, then it might be related to the initial transition slot relationship.
If it continues, it seems to me more likely its the idle system choking out of fuel at the top of the transition. Also could be a goofy block with a kill bleed in the dogleg or something like that.
Anyway, what I'm thinking is that if restricting the IAB didn't make it richer, then at that throttle position and vacuum, the IFR is restricting any additional flow.
Whether the correction should be done there, or it would be worth trying to ge the mains to start earlier is hard to say.
This is where a WOT 3rd gear run to max Hp with a WBO2 logger can be useful. it would indicate whether an increase in MAB as YR suggested might be needed for the top end. And if it did, then I would try that first. If not, then I might try the larger primary IFR first, depending on what we find in the block.

 

[SSG_Karg]

I dont know what your running for initial timing but ya might try backing off the timing a bit like if your at 18* back it down to 16* this will bring down the idle rpm then you can add idle rpm with the idle screw on the linkage.

I’ve tried initial timing settings all the way from 10* to 22* and everywhere in between while recurving the distributor to achieve settings of 30* to 38* total.
The stumble seems to occur just as I start to give it a little more throttle during cruise. I have a vacuum gauge in the car and the stumble occurs at about 14” which is just about where steady cruise vacuum is -15”.

 

[mderoy340]

30cc pumps front/rear. 28 squirter front 31 rear.
I don’t think it has anything to do with the acc pump. If I mat the throttle it takes off, no hesitation or stumble at all. Only hiccup is during very light throttle cruise when starting to give it a hair more throttle. Just a half second hiccup then smoothes back out. If I get into the throttle quickly it doesn’t stumble. Seems like the transition is not lasting long enough until the mains can take over.

You need to buy a 61-80 drill bit set and find out what the IFR, IAB, and MAB sizes are. Without knowing you are just guessing. Someone may have previously gotten that carb out of whack. In the 2-3k rpm range the carb is going back and forth between the idle/transition circuit and primary depending on gear and load. To extend and richen the transition circuit a larger IFR and smaller IAB is needed. As an example if you currently have .031 IFR/.070 IAB you would fatten the circuit to .033 IFR/ .067 IAB. You only have to do this on the primary block.

 

[Mattax]

To extend and richen the transition circuit a larger IFR and smaller IAB is needed. As an example if you currently have .031 IFR/.070 IAB you would fatten the circuit to .033 IFR/ .067 IAB. You only have to do this on the primary block.

Because the inter-relationships are sensitive my general recommendation is that if its needs to be extended, do not richen.
Since the IFR controls fuel, and only fuel, increasing the IFR will, for a given pressure difference supply more fuel for the same air flow
and will continue to supply fuel at an airflow where it previously choked.
To keep the same air to fuel ratio but extend the flow to a larger throttle opening, increase both the IFR and the IAB. (edited: completed sentence)
In this instance I would suggest first changing the IFR. That alone will richen it.
If the AFR was good before, then likely increasing IFR will need to be matched with increasing IAB.

I have a vacuum gauge in the car and the stumble occurs at about 14” which is just about where steady cruise vacuum is -15”.

Choking happens to occur around 14"Hg. Whether this is why your carb is running out of fuel there or not or just chance, I don't know.
But this phenomenon is why tuning a idle system on an engine with low vacuum at idle is a bit more of a challenge than one with high vacuum at idle.

By choking I mean that increasing pressure difference doesn't result in any additional increase in mass flow of the fluid.

It's one of these situations

 

[Mattax]

What mderoy340 suggested about buying a set of small drill bits is right on.
Once we look at the block, and especially with measurements, we probably can get a sense of what is going on and if there is something wacky.
Here's an example of the 'wacky' although a surprising number of metering blocks were done like this.
Holley Jet size

 

[512Stroker]

I’ve tried initial timing settings all the way from 10* to 22* and everywhere in between while recurving the distributor to achieve settings of 30* to 38* total.
The stumble seems to occur just as I start to give it a little more throttle during cruise. I have a vacuum gauge in the car and the stumble occurs at about 14” which is just about where steady cruise vacuum is -15”.

I guess I dont understand why you have this issue with that much vacuum?
I only had stumble issue with engine that had half the vacuum you have.

 

[yellow rose]

30cc pumps front/rear. 28 squirter front 31 rear.
I don’t think it has anything to do with the acc pump. If I mat the throttle it takes off, no hesitation or stumble at all. Only hiccup is during very light throttle cruise when starting to give it a hair more throttle. Just a half second hiccup then smoothes back out. If I get into the throttle quickly it doesn’t stumble. Seems like the transition is not lasting long enough until the mains can take over.

PUZACTLY (as RRR says) is what I think the issue is. Drill put the main air bleed .004-.006 thou and go test.

The issue is IF you go to big, it’s harder to go back. That’s why screw in brass is so bitchin’ (what is that??? a right out of the late 1980’s saying??? yes I’m old) because if you have changeable brass it’s cheap to replace.

If you do go too big, you can buy replacement OE brass bleeds from BLP with the correct hole in them.

You’ve got to get it on the booster quicker, and when you do, you’ll probably find you’ll need T slot restricters, which is an easy modification.

Get it one the booster quicker and see what happens.

BTW, I was talking to FABO member lead69 last night and this very topic came up. We both believe that Holley sends out all these “universal” “performance” carbs so nasty pig rich that you can get away with a late opening power valve, which when you get the power valve opening at the right time, you are now even more pig rich than you were before.

So there was some method to the madness of how these carbs came out of the box. It’s insane, but that’s besides the point.

Once you start correcting the idle, transition and cruise circuits by leaning them out and cleaning them up, you end up with the booster being delayed because it’s no longer fat. You had excess fuel there when the car was pig rich and now you don’t, so it makes it seem like you’re getting on the booster too late, but booster timing was retarded because it could be. Now it can’t. Also you need to remember that the engine mostly runs off the fuel puddle in the bottom of the intake...I hope I’m getting that correct but I’m pretty sure I am.

I hope this makes sense. I know I say it better than I type it, and I don’t say it very well.

 

[mderoy340]

.004-.006 is a huge MAB change especially when the OP has no idea what the MAB is now. IMO you fix the idle transition first, then move to primary.

 

[yellow rose]

.004-.006 is a huge MAB change especially when the OP has no idea what the MAB is now. IMO you fix the idle transition first, then move to primary.

I get that. But the OP has been bunting. It’s time to swing for the fence and get the thing to lose the stumble.

His work won’t be done any time soon. Once he gets the stumble out, it will be pig rich again on the T slots. So he’ll have to clean that up.

It’s quick the pin the MAB. And spending all day drilling .002 at a time is wasting time when he’s been messing with this thing for awhile now.


I still believe Holley (and everyone else cloning these carbs) delayed the mains because they are so pig fat everywhere. If they brought the booster in any sooner, it would be all the worse.


Just my .02 based on what I’d do.

Edit: I forgot to mention that when it gets on the booster quicker it will most likely be on the fat side on the main jet. But he needs to get the stumble out, the go back and work on idle, transition, cruise and the WOT.

 

[SSG_Karg]

Pin drills are on the way. Gonna yank the carb again tonight and break it down for another inspection and cleaning. Will measure all bleeds and report back with results and some pics of metering block.
I’m beginning to wonder if ordering a Proform main body isn’t the way to go here. Could be money well spent if I have to go down the rabbit hole of drilling or filling these factory holes.