Technical Stumble when revving at idle, or in top gear when matting it.

I'll try to include as many details as I can, so maybe grab a beer.

The issue I've been chasing is a stumble when I mat it from idle in park or a cruise in top gear. it will rev a bit, stumble, then rev again strongly and smoothly. It's worse in park/neutral or in top gear cruising than in first. So it would seem to be doing this under very light/no load, and very heavy.
If I'm cruising along in top gear at 3000rpm (which is 60mph) and I punch it, I get a brief pull, a stumble where it falls flat on its face, then a good pull until I back off. No smoke, no pops. Actually fairly impressed by the top gear pull from this engine apart from the stumble.
In second the stumble is not as bad, but still there.
Punching from a dead stop is pretty lazy, but I attribute that to the cam and too tight converter. Always been like that. No stumble there.
Walking it up to 3000rpm and smashing it in first is plenty strong with no stumble.
Rolling into at cruise it is fine, no stumble.
Other than the stumble it's a top end animal for what it is. I've tried everything and it persists. I've read all sorts of advice, and done all sorts of tuning, and nothing has cured it, only changed the conditions and severity of the stumble.

Carb is a 750 Holley Street HP with vacuum secondaries. Big for a 350, but it's what Holley recommended for my max rpm of 6500 (cam's top end operating range) and the vacuum secondaries help make up for it being a little oversized.
Stock pump cam on hole #2, stock jetting, stock secondary spring. 4.5 PV due to 9" of idle vacuum. Stock accelerator pump.
Plugs look pretty good considering my cam and time spent in low rpm. Light brown color on the porcelain and strap/electrode with some soot on the ring. Don't have the plug number handy, but they're NGK V-Power one heat range colder than what Edelbrock recommends due to the compression.
Cam is a Comp XR294HR. Engine is a 10.3:1 350, (8.0:1 dynamic if that matters) Edelbrock performer rpm heads, air gap intake. Timing is 14* initial, manifold vacuum, 34* all in by 3000rpm. Any more and it diesels and labors the starter. Vacuum advance is limited to 10* via a home made limiter plate. Two medium springs in the distributor, which is a older MSD RTR from before everything was made in China, but I also use an ignition box with it (not required with the RTR).
Tried going up and down with the squirters, both cases led to worse performance.
Tried a larger accelerator pump, really hated that.
Tried every single pump cam Holley sells and they only change the severity of the stumble slightly, but don't cure it.
Secondary springs have a similar effect, the stock spring is where I feel I'm getting the best performance overall.
I have one of those tubes you put in the secondary to prevent fuel starvation under a load but haven't tried it yet.
Floats are set so fuel is seen 1/3 of the way up the sight glasses (non-removable). Changing this has no affect on the stumble.
Fuel supply to the carb is not a suspected issue, I run a 255lph EFI pump with a bypass regulator and fuel return to the tank. Pressure is 6psi. I don't know what pressure is doing under a load, but this setup will support a lot more power than I'm making.
Car does sit nose up a bit, and the engine leans to the passenger side a bit (no idea why, I've checked everything).

Like I said, it's an animal apart from the stumble. The kicker here is before my aborted attempt at installing a Sniper EFI system, it never did this. Same carb. The carb sat for maybe 4 months with no fuel in it while trying the EFI. I tried a 650 Edelbrock AVS carb and it did pretty much the same thing, which points me towards ignition. It was noticeably slower up top, but idled and cruised fantastically. Seriously, those annular boosters Edelbrock is touting are the real deal.

Anyway, if there is anyone who really knows their poop tuning Holleys and/or ignition systems in the northeastern maryland, southeastern pennsylvania, south jersey, or northern delaware areas, I'd LOVE to pay you to tune this thing. I feel I've run out of ability. I've got it pretty close to being dialed I think, I just need to figure out this stumble and maximize the combo.

 

Sounds like not enough axcelerater pump

 

I'm certainly NOT a Holley 'wizard' but..........how bout this.
It sounds like (on the web and not in person) the spring in the vacuum secondarys is coming in a tad too early BUT....you have chased that rabbit!
Maybe disable the secondarys for a test. Put it to the test just using the primaries and see what happens!
What's baffling is....it didn't do this originally.
Most carb issues can be solved by tuning the ignition!
Lumpy cams like/NEED lotsa initial ignition.
Try......lighter springs and crank in more initial LIMITING the total to 34-36 degrees.
Disconnect and plug the vacuum advance.
Do you have any support on the back of the starter? Shivel-laz need one to prevent starter damage when running lotsa timing.
IF this fix'es it and you get the 'chug-chug' diesel effect you may hafta shut it off while in gear. (we can thank this wonderful gas 'mixture' big oil sells us!) Might be a small price to pay if it cures you hesitation.
That's all I got
6sally6

 

Tried the 50CC unit. Really did not like that. It would break up badly just revving in park.

Yeah, changing the secondary spring just changes where the stumble happens. It's only maybe a one second stumble, but enough to totally make the car nose over.
I don't know that you can disable the secondaries on a vacuum setup without physically damaging something in the carb's mechanics. I could remove the fuel line and plug the bowl, but that will affect the idle with the 4 corner setup.
Total timing is at 34* now. I've tried 34* to 36* and this is where it seems to be the happiest. Vacuum advance is limited to 10*. It's balanced on a knife edge right now, any more total timing and it gets a part-throttle ping. Big cams like advance, compression does not. I Could throw more advance at it, but I would rather deal with the stumble than $10 a gallon for race fuel. The springs are a good place to start, maybe the advance curve isn't keeping up with quick throttle changes. It's more likely a timing curve issue instead of a total timing issue if it is an ignition issue.
Disconnecting the vacuum advance kills midrange power and makes it unbearable to drive. I'm talking night and day. This setup definitely needs the vacuum advance. I've never tried cranking up the initial and locking it out though.
No support on the starter. I would think even with the crap gas we have these days, going to that extreme to get it to run right is masking the real issue. I run a big cam, but it's still streetable. I run a little more compression than the typical SBC, but again it's still streetable. That's what's so annoying about this issue. It's not a crazy exotic setup, you would think a dumbass like me would be able to get it to act right.

 

Just so we're clear and everybody loddy doddy is all on the same sheet of music here, when you say 34° total timing, what you mean by that is with vacuum advance disconnected & plugged, correct. I know you know this, but.

 

I have a "cammed-up" SBF so I know they can be ........finiky...!(they can make you talk-to-yourself!)
Two other thoughts....have you tried both ported and un-ported vacuum connections for your vac. advance?
I KNOW you don't have a vacuum leak around the carb/intake!
Playing with different PCV valves can be part of the tuning process.
Since it ran well once-upon-a-time....and an Eddy carb didn't fix it....gotta be igniting/timing!!(jus gotta)

 

Might be a vaccum leak that aint real apparent till the secondaries need it? Just food for thought??

 

Go ahead and read what you wrote there^^

what happens to your vacuum when you “mat” the pedal?? Zero.

Why so dependent on vacuum advance, what’s not functioning in the mechanical?? That’s where I’d be looking.

 

Trace your steps back to this first. I'm vaguely familiar with the EFI system but what changes did you make there that could have possibly affected performance now? May be in the primary ignition system, considering an attempt to wire in a controller? When you chase a rabbit, he always circles back to where he started

Joe

 

Correct.

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I have tried both sources. Idles better on manifold vacuum definitely. I don't suspect a vacuum leak, but I haven't checked for one around the carb or intake.

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I see what you're saying. I did have the distributor out with the efi so I could run the Holley setup that the sniper could control. Maybe something happened when I was fumbling about with it during removal and reinstall.

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I'm wondering about two things. First the mechanical advance in the distributor. Have you checked the weights to ensure they're free to easily advance the distributor? It almost sounds like they might be hanging up?
Second. Is there any play at all between the accelerator pump arm and the linkage?
Lastly, I'd check the diaphragm condition on your accelerator pump. Today's gas is tough on them, and if it sat very long during the EFI change it could easily get hard and not shoot enough gas now. I'd change to a Quick Fuel blue alcohol resistant accelerator diaphragm, as they'll last forever.

 

Let's see, during the efi install and then efi removal/carb reinstall I removed and reinstalled the msd distributor. I bypassed the ignition box and reconnected it. I changed from a mechanical fuel pump setup with no return to a full efi style system that I retained since I liked it so much. I also fiddled with wiring behind the dash which is a rat's nest (I have a complete harness waiting to go in the car along with some other things to make it nhra legal so I can do some nostalgia drag events locally). The carb sat dry for about 4 months while fighting with the efi. I also changed to a resistor style spark plug and 10mm moroso wires.

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I haven't checked the operation of the mechanical advance since reinstalling this distributor, but it was operating smoothly last time I checked. It has been a while, I'll have to check that. I have never tuned the curve with the stop bushings or springs however, just limited vacuum advance with a plate.
Pump actuation is dead on when you open the throttle. It's a newer street hp carb so it's got all that corn gas safe stuff in it. I had it off recently and checked it for tears since that was where my thinking went as well. It appeared to be perfect.

 

But having the same problem with Eddy AVS....
Have you done a voltage check from the battery to the plugs through the entire ignition system? First instinct is always lean carb response but most times it's ignition. I've looked at EFI once or twice but I always shy away from it. I like to keep it simple as possible. MSD ignition is as far as I'll go on electronics. Good luck!

Joe

 

I did a lot of continuity and voltage tests as part of the efi troubleshooting. Everything is good to and from the coil, to and from the ignition box, and all the plug wires resistance was within spec. Spark kernel is nice and fat at the plugs.


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What plug gap are you running?

 

Worth a look!!
6sally6

 

It certainly sounds like a lean stumble. However, it would be nice to look at the ignition system on a scope and eliminate it as a source of the problem. What is confusing is that it didn't appear to change when you did things to richen it up on initial acceleration. Have you talked to Holley? They helped me out on a very light acceleration surge.

If you have access to carburetor parts, you could disable the vacuum secondary by replacing the actuator cap with one that you drilled a hole in. The actuator uses venturi vacuum so it wouldn't affect the mixture.

 

Transmission ?, rear ratio ?, tire size ? , get the timing all in by 2000-2200 , when you push down on the gas pedal the vacuum advance goes away , tuning for performance doesn't usually entail fucking with a vacuum advance , its not part of the equation . you can try running up to 30° initial , just limit the mechanical advance accordingly .

 

Did this modifed distributor ever work?

 

As one other suggested take out the back barrel link and try it as a 2 barrel. Make every drive test the same.

 

This may sound like a long reach, but have you checked the powervalve to see if it's working? Could it possibly be stuck from sitting?

 

.045.


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I posted on their forum a while back about it. Got no responses in 24 hours so got annoyed and deleted it.

I do have a spare vacuum secondary assembly for this carb I could try that on.


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Trans is a th350 that’s been built for performance (600 lb/ft rated).
3.70 gear, 29” tall tire.

Checked operation of the mechanical advance, it seems to be tip top. Noticed I had 2 silver springs in it, I must have been futzing with it at some point and forgot (or was a few beers in).

Also noticed my floats were set higher that I recall setting them. About 2/3 up the sight glasses. Backed both down to 1/3 of the way up. It actually helped a fair bit with the top gear stumble, but didn’t cure it.


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Not sure what you mean. I’ve been running this distributor for 16 years or so. I haven’t gotten to tuning the curve yet if that’s what you mean.


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It’s gotta be, the car is pretty strong for what it is. Feels as strong as it ever has. I know feels and is are two different things though.


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Did some adjustments today. Bumped my base timing to 16* (big cam) allowing me to back the curb speed screw off a bit. This also allowed me to lean the idle mix out a little. It’s more responsive down low now and not as fat while still maintaining a decent idle. Stumble is noticeably reduced in park and when matting it from a roll. Idle is only going to get so good, I really need a looser converter.
Swapped to a white pump cam on hole 2 on the throttle arm and the cam. This moved the stumble way up the rpm range. Haven’t wrapped my head fully around what that means yet in relation to the operation of the vacuum secondaries, but I think the white cam with a softer secondary spring might be the ticket.



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