holley carb help,transition/stage

[scampy]

Going to start here as this is my drag only car. Can has a stumble/skip when staging or driving in pits at around 2000rpm launch is mostly good but at times stumbles of line. When staging apprx 2000 the car will have a slight miss when holding on line. When driving away from time slip both car will tend to spit/sputter untill I get higher rpm which usually don't for speed reasons. Idle is good.
So I'm thinking I want to richen the transition a bit to clean this up. Can I richen the idle mix screws to help or would that not do anything for it, it might tolerate a bit of movement in those. More so I was thinking about lowering the idle air bleed to a little smaller to do this as the carb has removeable bleeds but not sure what else thats going to change as make picking the mains slower etc or nothing but retrim the idle mix screws. This is a 750hp holley running on my 416 upper 10's heavy car. Looking to try something somewhat quick/easy without really messing up other things.
Thanks

 

[mderoy340]

It would help if you would list the pinouts of how the carb is setup, cam colors, nozzles, timing etc.

 

[Treblig]

What power valve are you running??? The wrong power valve will cause a stumble off the line and when accelerating at any speed. Changing the accelerator pump cam will also help off the line.

 

[yellow rose]

Going to start here as this is my drag only car. Can has a stumble/skip when staging or driving in pits at around 2000rpm launch is mostly good but at times stumbles of line. When staging apprx 2000 the car will have a slight miss when holding on line. When driving away from time slip both car will tend to spit/sputter untill I get higher rpm which usually don't for speed reasons. Idle is good.
So I'm thinking I want to richen the transition a bit to clean this up. Can I richen the idle mix screws to help or would that not do anything for it, it might tolerate a bit of movement in those. More so I was thinking about lowering the idle air bleed to a little smaller to do this as the carb has removeable bleeds but not sure what else thats going to change as make picking the mains slower etc or nothing but retrim the idle mix screws. This is a 750hp holley running on my 416 upper 10's heavy car. Looking to try something somewhat quick/easy without really messing up other things.
Thanks

I would doubt it’s lean. I’d bet it’s rich in transition.

If it is lean in transition, get the power valve open quicker.

I doubt that’s it though.

 

[cookietruck]

the transition slot is mostly affected by your idle feed restrictor, i would go up .002 or .004" in size and see if that cleans it up at 2000rpm steady throttle.
the stumble, make sure your accel pump linkage is tight against the arm (messing with idle speed screw can cause changes in that adjustment)...if that's good, maybe look at your pump cams next.

 

[scampy]

ok I'll try and answer what I can. No power valves front or rear. Pump setting is good Its not really at hit its under "cruise" if you will or s steady hold like staging on starting line,its like a skip. nozzles are 31 front and rear, 50cc frt brown cam, 30cc rear,don't remember right now, not pink though think it is orange.I've been up to a 37 nozzel playing with the occasion stumble off line,would come back so I've been moving down and now 31. timing is 22 idle 36 total. I just got a small pin set but have not been back into yet so these are some numbers I got with what i had at the time, some number drills.
mab .034
iab .071-.072
idle restrictor is in lower location, guess is about .032 something between .030 and .035 didn't have something inbetween on front block.
Transition slots are set equal/small square and rear plates are open some to equal out transitions when setting idle speed with screw under plate for secondarys
I've got a note that the idle restrictor in the rear block is .040 but idk if thats correct seems off?
Also car has 7-8" of vac at idle if that helps.

So we have our next race this saturday so I thought about just playing with the bleeds or mix screws to try something won't have time to really get inside and all the inside stuff is fixed orfice so no quick change. Car free revs nice and responsive,idle is nice at 900-1000 does not load up and is pretty happy, it's that steady "cruise" or stage. If I get it about 2500 or so it cleans up,I'm guessing main coming in when I leave the timeslip booth.
Thanks all

 

[yellow rose]

ok I'll try and answer what I can. No power valves front or rear. Pump setting is good Its not really at hit its under "cruise" if you will or s steady hold like staging on starting line,its like a skip. nozzles are 31 front and rear, 50cc frt brown cam, 30cc rear,don't remember right now, not pink though think it is orange.I've been up to a 37 nozzel playing with the occasion stumble off line,would come back so I've been moving down and now 31. timing is 22 idle 36 total. I just got a small pin set but have not been back into yet so these are some numbers I got with what i had at the time, some number drills.
mab .034
iab .071-.072
idle restrictor is in lower location, guess is about .032 something between .030 and .035 didn't have something inbetween on front block.
Transition slots are set equal/small square and rear plates are open some to equal out transitions when setting idle speed with screw under plate for secondarys
I've got a note that the idle restrictor in the rear block is .040 but idk if thats correct seems off?
Also car has 7-8" of vac at idle if that helps.

So we have our next race this saturday so I thought about just playing with the bleeds or mix screws to try something won't have time to really get inside and all the inside stuff is fixed orfice so no quick change. Car free revs nice and responsive,idle is nice at 900-1000 does not load up and is pretty happy, it's that steady "cruise" or stage. If I get it about 2500 or so it cleans up,I'm guessing main coming in when I leave the timeslip booth.
Thanks all

If you are not running a power valve, you may be lean on the main jets, or maybe you need to open the MAB bleed up .004-.006 to get it on the main jet sooner.

Any pictures of your plugs?

 

[scampy]

Not right now, I can pull one and take pic maybe tomorrow but it will be after being loaded in trailer and back into garage now. Reference jets are 80 and 86 current

 

[Oldmanmopar]

You need a 4 corner idle circuit carb . Square the primaries and idle off the secondaries. Stumble should get better .

Vacuum at idle divide that in half and that is your power valve number. Any motor under 4.5 valve should have a 4 corner carb or drill holes in the venturis until it idles with the primaries squared. If your past square your into the emulsion jets and it will stumble like a dead spot when feathering the throttle

 

[scampy]

This is a 4 corner idle carb, transition slots set up as in your top picture

 

[Oldmanmopar]

This is a 4 corner idle carb, transition slots set up as in your top picture

Check the power valve 1/2 the vacuum reading is the number. Most of the carbs come with a 6.5 Once you go to a larger cam you need to go to a lower number. I am using a 3.5 and I am a little rich. I need a 2.5- 3.0

Does the carb have removable air bleeds. Smaller is more fuel larger will be less

 

[yellow rose]

Check the power valve 1/2 the vacuum reading is the number. Most of the carbs come with a 6.5 Once you go to a larger cam you need to go to a lower number. I am using a 3.5 and I am a little rich. I need a 2.5- 3.0

Does the carb have removable air bleeds. Smaller is more fuel larger will be less

OMM, setting power valve opening by 1/2 of idle vacuum is wrong. It was wrong from the beginning and it’s wrong now and it will be wrong forever.

You never EVER set power valve opening by idle vacuum.

OP, if you are going to run a power valve, then you set the opening point a couple of numbers lower than cruise vacuum.

I would suppose for a foot brake drag car if there is a chance (and it’s a slim chance) that the power valve opens while you are staged, you could open the power valve later, but you’d need to put a vacuum gauge on there and measure vacuum while you’re foot braking it.

I’m not sure how, with the engine is loaded while foot braking that the vacuum would below enough to open the power valve even if out had a 10.5 PV in there. And that assumes that opening the power valve while foot braking doesn’t need to happen. It may need to open, it may not. You just need to get a vacuum gauge on it and see what it says, and then open the power valve accordingly.

 

[scampy]

I do not run any power valves in this carb front or rear. Yes it does have removable air bleeds in the top. Thats part of my question, If I was to go smaller on the iab what else does that change,anything with the timing of the mains coming in or something anything else except richen this idle/transition circuit, or would increasing the fuel with idle mix screws also add fuel to this "cruise" 2000rpm zone or would it just be taking away from the transition circuit. obviously turning the mmix screws would have only a small margin before the idle would but a no go. Does my iab seem off would making a .002 even make a difference

 

[Treblig]

Does the engine stumble when the car jerks?? Like when you brake hard or move quickly from a stop.

 

[cookietruck]

I do not run any power valves in this carb front or rear. Yes it does have removable air bleeds in the top. Thats part of my question, If I was to go smaller on the iab what else does that change,anything with the timing of the mains coming in or something anything else except richen this idle/transition circuit, or would increasing the fuel with idle mix screws also add fuel to this "cruise" 2000rpm zone or would it just be taking away from the transition circuit. obviously turning the mmix screws would have only a small margin before the idle would but a no go. Does my iab seem off would making a .002 even make a difference

can you hear the motor get unhappy as 2000 even if it's not in gear? if so hold the motor at 2k and put a finger over your idle bleed and see if it cleans up.
you can add fuel to the transition with the idle screws to see if it clears up that way, but then your idle is going to be rich.
I would put a smaller IAB in there and see if it helps at 2k, easy enough to do...

 

[mderoy340]

I do not run any power valves in this carb front or rear. Yes it does have removable air bleeds in the top. Thats part of my question, If I was to go smaller on the iab what else does that change,anything with the timing of the mains coming in or something anything else except richen this idle/transition circuit, or would increasing the fuel with idle mix screws also add fuel to this "cruise" 2000rpm zone or would it just be taking away from the transition circuit. obviously turning the mmix screws would have only a small margin before the idle would but a no go. Does my iab seem off would making a .002 even make a difference

A smaller IAB will richen the upper rpm portion of the idle circuit ie 2000rpm. Your car cleans up at 2500 due to the primary being all the way in. Put .067 IABs in at least the front and readjust idle mixture for best idle. On a race car I set idle mixture to the fat side since you really don't idle much and the motor is run much colder than a street car.
The IAB controls the air allowed in the idle well, the IFR the amount of fuel, the idle mixture screw controls how much of the idle air/fuel mixture goes to the idle and transition slot.

 

[scampy]

A smaller IAB will richen the upper rpm portion of the idle circuit ie 2000rpm. Your car cleans up at 2500 due to the primary being all the way in. Put .067 IABs in at least the front and readjust idle mixture for best idle. On a race car I set idle mixture to the fat side since you really don't idle much and the motor is run much colder than a street car.
The IAB controls the air allowed in the idle well, the IFR the amount of fuel, the idle mixture screw controls how much of the idle air/fuel mixture goes to the idle and transition slot.

This is kinda what I had in mind, drop the iab down, your thinking that much from apprx .072 to .067, of course idk is that even alot for iab. Will this change anything timing wise with the mains coming in? or just swap those iab in front only and retrim the idle mix screws. No issue with having different iabs front to rear?

 

[mderoy340]

This is kinda what I had in mind, drop the iab down, your thinking that much from apprx .072 to .067, of course idk is that even alot for iab. Will this change anything timing wise with the mains coming in? or just swap those iab in front only and retrim the idle mix screws. No issue with having different iabs front to rear?

Changing the IABs will not change the primary circuit timing coming in. Going smaller on the IAB will extend and enrich the idle circuit.
When you change the IAB size you need to readjust the idle mixture screws.
I suggested changing the front only since you did not know the size of the rear IFR. The car is running mainly on the front idle circuit on the return road, the rear idle is supplying some fuel but most is coming through the front transition slot.
On my Proform 750DP I run .035 IFR bottom position, .067 IAB, at 3/4 turn all 4 corners, .020 transition slots and it works great for me at sea level.
An easy way to check if the Idle circuit is good is to slowly rev the rpms up in neutral until till the mains come in. If it starts to misfire you are to lean.

 

[Mattax]

I ahve only a few things to add, but they may be helpful.

idle restrictor is in lower location, guess is about .032 something between .030 and .035 didn't have something inbetween on front block.

On the idle restrictor, the difference between .031 and .034 is pretty big.

Transition slots are set equal/small square and rear plates are open some to equal out transitions when setting idle speed with screw under plate for secondarys

That's a starting point. It's perfectly OK to adjust slightly.

I've got a note that the idle restrictor in the rear block is .040 but idk if thats correct seems off?

Often they are larger. It depends on how the carb details. I'm assuming this is not set up with 1:1 linkage.
With the rear plates "open some" are ther secondary transition slots exposed?

most is coming through the front transition slot.

I agree unless the secondary t-slots on this carb go low enough to be exposed. That's why I thought it was worth asking.
Perhaps its also worth asking how many turns out the secondary idle mix screws are?

Also car has 7-8" of vac at idle if that helps.

makes it candidate to using 4 corner idle, which you are.

Car free revs nice and responsive,idle is nice at 900-1000 does not load up and is pretty happy,

But this is in neutral. That's a good start.
The real test is with a load on it.

I do not run any power valves in this carb front or rear.

There might be a benefit to running a front PV.
IF there is a situation where the mains are starting to contribute fuel other than wide open throttle, a PV should keep it leaner, cleaner and stronger.
I'll leave that discussion to Mderoy and others

Thats part of my question, If I was to go smaller on the iab what else does that change, anything with the timing of the mains coming in or something anything else except richen this idle/transition circuit, or would increasing the fuel with idle mix screws also add fuel to this "cruise" 2000rpm zone or would it just be taking away from the transition circuit. obviously turning the mix screws would have only a small margin before the idle would but a no go. Does my iab seem off would making a .002 even make a difference

Going smaller on IAB will enrich the idle and transition. Opening the idle mix screws may rob the transition slightly but its not guarenteed to go that way.
The reason is transitions slot itself is a variable air bleed and restriction to the fuel in the idle well that is partially mixed with tiny air bubbles.
So its a question of whether the fuel-air in the well is restricted by the t-slot opening when the throttle plate is near the top. If so, reducing restriction to the idle port will allow slightly more fuel.
IAB changes are less sensitive than IFR changes, but dropping just the primaries by .002 is reasonable. This is fine adjustment, not a radical retune.

Going smaller on the IAB will extend and enrich the idle circuit.

My experience has been a little different on this point.
I've gone smaller on IAB to successfully enricher a lean transition, but did not observe it to carry it higher in the rpms.
For that (extending into higher rpms) it seems a larger IFR is generally what is needed.
Most of the time when I've gone smaller on IAB I have had to retrim the mix screws, and sometimes slightly adjust the thottle position and the mix screws.

 

[scampy]

OK guys my bleeds came, turns out the ones in the carb are .075, got the extra eyes out and can read the number so I only changed it down to a .069. Idle mix screws I barley touched. I did this on the primary only. Rear iab is .075 as well did not touch anything there. I'll know a little bit more tomorrow when we hit the track.I thought about still trying the .067 but was concerned when I saw .075 in there now. It seems the idle maybe up apprx 100rpm from what it was but will see. Ran out of time to play more,golf cart wouldn't run when I went to load,thats another story. I assume no issue with going down more on the iab or should I look at also changing the iab in the rear?

 

[scampy]

I ahve only a few things to add, but they may be helpful.


On the idle restrictor, the difference between .031 and .034 is pretty big.
That's a starting point. It's perfectly OK to adjust slightly.


Often they are larger. It depends on how the carb details. I'm assuming this is not set up with 1:1 linkage.
With the rear plates "open some" are ther secondary transition slots exposed?

I agree unless the secondary t-slots on this carb go low enough to be exposed. That's why I thought it was worth asking.
Perhaps its also worth asking how many turns out the secondary idle mix screws are?


makes it candidate to using 4 corner idle, which you are.


But this is in neutral. That's a good start.
The real test is with a load on it.


There might be a benefit to running a front PV.
IF there is a situation where the mains are starting to contribute fuel other than wide open throttle, a PV should keep it leaner, cleaner and stronger.
I'll leave that discussion to Mderoy and others


Going smaller on IAB will enrich the idle and transition. Opening the idle mix screws may rob the transition slightly but its not guarenteed to go that way.
The reason is transitions slot itself is a variable air bleed and restriction to the fuel in the idle well that is partially mixed with tiny air bubbles.
So its a question of whether the fuel-air in the well is restricted by the t-slot opening when the throttle plate is near the top. If so, reducing restriction to the idle port will allow slightly more fuel.
IAB changes are less sensitive than IFR changes, but dropping just the primaries by .002 is reasonable. This is fine adjustment, not a radical retune.


My experience has been a little different on this point.
I've gone smaller on IAB to successfully enricher a lean transition, but did not observe it to carry it higher in the rpms.
For that (extending into higher rpms) it seems a larger IFR is generally what is needed.
Most of the time when I've gone smaller on IAB I have had to retrim the mix screws, and sometimes slightly adjust the thottle position and the mix screws.

I have the 2ndary tslots exposed just slightly like the front and is a progressive linkage

 

[Mattax]

.075 to .069 in IAB is a good sized jump. Enough that if that's going to effect the problem condition it should be noticible.

As far as the secondary t-slot exposure, someone here may know the HPs and see this.
If not next time the carb is off the engine, see if the secondary t-slots are shorter or located a little higher up in the throttle bores than the fronts.
If so, then opening the secondaries so the t-slot exposure is the same as the primaries means the secondaries are open more than the primaries.
That might be OK or might be something that needs exploring at some point. By exploring I mean reducing the secondary idle position a hair and opening the primaries a hair.

Have fun at the track.

 

[justinp61]

Go here racingfuelsystems - Holley.

 

[Mattax]

should I look at also changing the iab in the rear

progressive linkage

Bringing the engine to 2000 rpm in staging even with an automatic I doubt the linkage has moved the secondary throttles open at all.
If you want to test the effect, make it seperate test. It may impact what happens as you leave the line. I haven't experimented with that myself so can't offer a first hand perspective on it.