Holley/Demon Carb stumble

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Maybe just maybe its not the carbs maybe the converter is too tight for the cam and nothing you do is gonna help other than changing the converter or the cam.
I have seen this before and while carb tuning is good and interesting its hard to fix a problem that isnt there.
 
My gut is to try slowing the advance so timing from 2800 might be 30 - 32 and 38* is above stall.
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I’d actually go the other way.
I’d lock out the distributor.

I do agree with trying the small carb at the track to see how the car behaves with it on there, with the new converter and tires.
But that could be a couple of runs, then swap it out at the track type deal if it worked “as advertised”.

Still wondering why the decision was made to go with the tighter converter and taller tires.
 
Look like i'm going to have to race the 600, just for plain and simple DATA>!
 
If i had a adj msd that i could retard the timing, under start mod, a lock out might work, but 38 degrees on the starter is hard on my started.
 
PRH said:
Still wondering why the decision was made to go with the tighter converter and taller tires.
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Old converter stalled at 3000 rpm(stock 11" converter tweaked to get 3000)........new converter,9" stalls at 4200 rpm and it does! However! at 2200 it takes more throttle then the 3000 rpm converter did(the rpm that i leave the line at). ten feet out of the hole it's a 4200 rpm. its at the initial hit that it stumbles. And when i say stumble is anything from a micro stumble to a full on stumble that i have to take my foot of the floor to keep it from dying.
 
Okay..... yeh, now I remember.

I’ll say it again....... a little finesse on the gas pedal might do wonders here.
 
PRH said:
Okay..... yeh, now I remember.

I’ll say it again....... a little finesse on the gas pedal might do wonders here.
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Go to the 4:30 mark of this video and you will see and hear the stumble
 
Again, that sounds like a late opening power valve. You get a giant hole in the fuel curve and nothing will cover that up.

I also had a carb that had annular boosters that I could never get the hole in the fuel curve fixed.

I finally junked it. Most carb guys wouldn't even take it on.

FWIW, if it's not a PV issue (either opening timing or power valve channel restriction or both) it could also be a booster issue. It could be getting on the booster too late. And you are killing it with a long pump shot. Or, it could be getting on the booster too late. And no pump shot will fix it.
 
Cudafever said:
I'm just mad as @#$%@#$^^@ at myself!
Get home from work, wife has honey does, final get back to my car. Go to shut my hood and my new Super Sucker spacer is just a little bit higher than the spacer I was using before and the air cleaner/carb stud was hitting my hood. pull it off and get my hacksaw out and proceed to shorten it. well won't go threw all the details of my stupidity!!!!! but the last push of the hacksaw was threw my KNUCKLE!!!!!!
Just missed my tendent(thank heavens!) and 7 stitches later, as well as a tetanus shot.
I'm back home.:BangHead::BangHead::BangHead:
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Good nightmare I'm only on page 14 and they have killed you and your wallet ! I hope somewhere in the next couple of pages of reading I catch up and you're still alive and your car hasn't burnt down!!
 
yellow rose said:
Again, that sounds like a late opening power valve. You get a giant hole in the fuel curve and nothing will cover that up.

I also had a carb that had annular boosters that I could never get the hole in the fuel curve fixed.

I finally junked it. Most carb guys wouldn't even take it on.

FWIW, if it's not a PV issue (either opening timing or power valve channel restriction or both) it could also be a booster issue. It could be getting on the booster too late. And you are killing it with a long pump shot. Or, it could be getting on the booster too late. And no pump shot will fix it.
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6 inch vac in gear. 4.5 power valve.
 
Wow 10.5??????????????
Well i sure don't remember you or anyone else recommending a power valve that that stays open from Idle to top rpm.
Real confused there for sure.
 
Cudafever said:
Wow 10.5??????????????
Well i sure don't remember you or anyone else recommending a power valve that that stays open from Idle to top rpm.
Real confused there for sure.
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Ok, I'll type the short version.

Power valve opening has ZERO to do with idle. Nothing. Zip. Nada. And it doesn't matter who says it does, because it's just wrong.

There may be (might) a case when an open power valve can affect idle. Mattax can explain it better, but the short of it is if the main jet is small enough and the idle bleed is big enough than maybe PV opening may affect idle. I could have that wrong. If I do, Mattax can correct it for me.

That being said, we do not chose power valve opening by idle vacuum. Ever. If it's a street car, you use cruise vacuum to pick PV opening. If it's a race car, put it on the chip, or flash the converter and set the PV opening just under whatever the vacuum says. And yes, I'm a fan of using a PV on a race car.

In my case, I cruise at 15 inches. I'd love to have a 12.5 PV but I can't find one. So I have to use a 10.5 until I do something. I'm going to buy a PV tester and then I'm going to play with springs and spring installed height to make the PV open where I want it.

So think about how the PV works. It's a system that allows you to use a smaller main jet for cruise and then the PV opens under load (low vacuum) and adds fuel.

The later that PV opens, the harder it is for the booster/accelerator pump to cover up the delay in fuel delivery. Even if you are on the booster, without the extra fuel of the PV, you get a big lean spot.

I think it's worth mentioning here that when you do as you are doing in your video, every time you smack the throttle like that, any fuel in the intake system that was vaporized turns back to solid fuel. Almost at the touch of the throttle. Solid fuel doesn't burn. It also almost never gets back to a vapor. That means it becomes wall flow and it just runs right down the manifold walls as a liquid and essentially runs right through the chamber where it stratifies and ends up out the exhaust pipe.

So anything you can do to help getting that wall flow down to a minimum helps with power.

That's the short version. Mattax and PRH can add somethings I'm forgetting or don't know.

One of the biggest tune up sins with a Holley is incorrect PV opening. It's been wrong forever. And Holley is STILL teaching people the incorrect way to select power valve opening.
 
Yup! You made me open up my new holley book and look up the pv setting.......same as the old one PV number below Idle vac.
I would really like some more to chime in on this.
The Demon carb cam with 4.5 from the factory.
 
Cudafever said:
Yup! You made me open up my new holley book and look up the pv setting.......same as the old one PV number below Idle vac.
I would really like some more to chime in on this.
The Demon carb cam with 4.5 from the factory.
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And Barry Grant was wrong. I can't explain why people refuse to learn. But it happens. Hell, there are people who think a spark plug is lean if it isn't brown. Or a power valve can affect idle. There is a million things that aren't true that are treated as true. Bob weight is another. It never stops.

But I'm right about PV timing. Mark Whitener has a really good video on YouTube where he PROVES an open PV at idle doesn't do a thing. Yet, people still argue that it does because it's in a book.
 
BTW, if you go to the fuel and air systems forum, you'll see a thread started by 12many. It's called the definitive power valve opening or something. It's at or near the top of the forum.

12 did some really good testing on PV openings and his A/F ratio and jetting. A couple of other guys have also added some good info in there too.

It's worth the read.
 
I think @yellow rose got it covered.
The only explanation I've seen for how we got to race cars using low manifold vacuum enrichment came is from Tuner.
I'll dig it up and put it on the Power Valve thread.
But the short of it wasrelated to a lack of power valves that could survive the new fuel formulations.
More radical engines generally needed to richer earlier in the throttle. The original 3310s for the 396 Chevy had 10.5 and 8.5 PV openings. The Chrysler FSMs I've looked at show 8.5 PV for the Holley 4160s where they were used.
An engine that has good part throttle fuel distribution and efficiency will not need enrichment until much further in the throttle, maybe 6.5 PV or even 5.5 PV.
 
I would really like some more to chime in on this.
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Years ago I reached the point where I got tired of arguing with people about it.

The PV rating has nothing to do with the idle vacuum.

The PV’s purpose is main circuit enrichment at high(er) engine loads.

What does that have to do with the idle circuit, or idle vacuum..... or the engine idling at all?

I highly doubt it is the source of your bog problem though, however it is a cheap and easy test.
Just put a PV plug in the primary side and jet up accordingly.

My default PV is a 6.5, and almost never use anything lower......... even when the cams are way way bigger than yours.
Even a motor with an .825 lift cam that’s in the mid-280’s @.050 pulls reasonable vacuum when you’re tooling up the return road at 2500rpm........ so the PV would be closed....... helping to keep the plugs cleaner.
 
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PRH said:
I’ll say it again....... a little finesse on the gas pedal might do wonders here.
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This is also a cheap and easy test.
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You guys tell me if I'm wrong. (I'm almost always dealing with traction limited car so finess off the line is always needed. Traction here seems really good.)
With the new higher stall converter (and taller tires), isn't it worth experimenting with different launch rpms for best 60' ?
And while higher rpm should launch harder, would it be also woth trying some lower rpm launches to see what if any effect it has?
 
When my friends GTO was still a bracket car he ran a ported 3310 I did for him.

He was bracket racing and wanted it to be as fool proof as possible.
He had a 4800 converter, and you could pretty much do whatever you wanted for a launch technique and it would be 100% stumble free.
He got to where he was basically “stomping” on the gas pedal.

We were playing around looking for some ET one day, so I put my ported annular 850 on it.
I had it working just how I wanted on my car, so I wasn’t going mess around with it much.
With him driving it, it had a very slight bog on the line..... and ended up ET’ing the same as with his carb, but was up about 1.5-2mph.

I drive it with a little lighter touch on the starting line....... which proved to be worth a solid tenth...... and it still ran the same speed he did.
I even instructed him how to do it...... he just couldn’t get the knack of it...... well, at least with only a few test passes he couldn’t.
I just drove it the same way I always did........ no problemo.

Basically, he stomped on it....... I quickly depressed the gas pedal.
 
Another story......
20 some-odd years ago my Dad was bracket racing his 79 ‘Doba which had a 13:1 360 in it.
He’d been running the same Holley 650DP on it for years.

At about that same time I had taken a 650 main body, knocked the boosters out, machined the venturi to a larger diameter, then did some porting and blending on it, installed downleg boosters, modified the metering blocks, and started playing with it on the dyno.
On the flow bench it flowed about 825cfm(calculated wet).
I got it working pretty well, and ended up selling it to a guy with a Dirt modified after it crushed the carb he had been using on the dyno.

So, my dad wants to go quicker, but isn’t up for buying a bigger carb. He knew about my 650 experiment and was curious if it would work for him.
He also likes doing his own stuff.
So, the decision is made to lightly port the 650 without replacing the boosters.
I do one hole, he “copies” it to the other 3.

We dyno it....... the signal at the booster is now a lot weaker, so it requires a bunch of jet be added.
This makes the part throttle rather rich, but the throttle response is killer.

We bring the car to the track...... the initial hit is fantastic, then some hesitation, then it goes.
The pump shot lasts way to long.

I install the shortest pump cams, rotated way around on the shaft so about 1/2 the lift is already used up with the throttle closed, put some 25 squirters in it...... its perfect.
Totally bog free.
It needed almost no shot because the main circuits came in almost instantly.

One little tidbit....... my Dad had messed with squirters and cams for several track outings and couldn’t get it to work all that well.
Like I said, he liked doing his own stuff...... but after a bunch of trial and error, he was thinking he was probably going to need a different carb.
He’d make a pass, try changing something, male another pass, etc.
A few times he asked for some guidance, but it seemed like he was determined to figure it out.
Finally, he decided maybe I should look at it.
So, based on his feedback I’m making adjustments to no avail.
I finally make a pass in it myself....... and can tell right away what’s happening.
That’s when I set it up like I mentioned above...... which worked fine.

Like I said earlier in this thread....... for me, a lot of the tuning is tactile........ especially the starting line stuff.
I need to “feel” what it’s doing.

As for my modded 650 experiment, it’s much easier to get the same kind of results from your 650 these days.
Just buy a $150 carb body and a primary metering block.
 
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Thank you all for not giving up on me and letting me think, dig and learn. This was all mentioned before but i just didn't get it!!!!!
 
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