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3/19/2018 9:33 AM  #1


Carb question .. Need help tuning out a surge/stumble

65 mustang with a '69 302 ( bored 0.40 over), Edelbrock Performer RPM intake dual plane.

Rebuilt the carb recently because of a leak. Went quite well. Running 6.5hg power valve, and stock jetting.
After installing the carb back on, I could tell that the car was lean, surged between 1000-1500 rpm and was getting hot at idle. My floats were just a little lower than they should have been. Got them set well, richened the engine a good bit, idle mixture screws are about 2.5 -3 turn open on either side. Car fires up very well even at dead cold. Choke still needs an adjustment but that is no problem. I will do that.

After adjusting the floats , the car is definitely running better. I can floor it, and as far as it goes quickly past that 1500-1750-1800 rpm range, the car does VERY well. and I would say for the first time, as there are no leaks in vaccum and carb, I felt the secondaries kick in, and engine hauled a** while not getting hotter than 190 and staying at 180 at idle . 

But just between 1500-1750 rpm there is that stumble. its not bad enough that the car would stop running, but the engine falls flat on its face for a milisecond, before coming back and then being very clean the rest of the range. 

I have to measure the vaccum again, but being a rpm intake, the max vaccum I see on the engine is aroud 1500 rpm, ~18psi. At idle ( 650-700 rpm) , I see about 12-13hg of vaccum.

So my question is : change the power valve to come in quicker at 7.5hg .. ? or jet up ? Or does the accelerator pump need a different cam.. or just cam placed on a different hole?

Remember the stumble/surge is noticeable only when I want to slowly accelerate, not when I want to floor it.

help me please with what I should start with

I humbly ask to refrain from "change to carb" , 600cfm is too big for a 302 talk .. if at all possible. I am very happy with this holley carb, and do not wish to change it as of now. I am seeing a lean condition and not a rich condition, I can tell because richening  made the car perform better not worse.


If it aint broke, I have'nt tried to "Fix" it yet!
 

3/19/2018 9:33 AM  #2


Re: Carb question .. Need help tuning out a surge/stumble


If it aint broke, I have'nt tried to "Fix" it yet!
     Thread Starter
 

3/19/2018 11:41 AM  #3


Re: Carb question .. Need help tuning out a surge/stumble

the answer to that is it stumbles after it accelerates. it accelterates,, stumbles then accelerated again to go on smooth past 1900 rpm or so.

I could be sitting at a red light or coasting in city .. as soon as throttle is part open (and rpm is about 1500-1700) accelerating slowly, it accelerates, stumbles for a second, and accelerates again

when float was too low, it would stumble right out the hole. now it doesnt do that


If it aint broke, I have'nt tried to "Fix" it yet!
     Thread Starter
 

3/19/2018 4:47 PM  #4


Re: Carb question .. Need help tuning out a surge/stumble

The question is whether this is a rich bog or a lean bog.  I'd advise moving the accelerator pump arm to a different hole one way or the other and see what it does.  If it gets worse go the other way and see if you can tune it out.  If it improves, but doesn't fully go away you might need to change the cam. 

 

3/19/2018 10:35 PM  #5


Re: Carb question .. Need help tuning out a surge/stumble

My advice is ALWAYS  adjust the timing (in every way) before jumping on the "poor-old-carb"! EVERYBODY wants to "adjust-the-carb" first thing.
Its amazing what a little more initial...or a little less initial...plugs...wires...distrib cap...etc. can do for surge/bog.
Jus say'in
6sal6


Get busy Liv'in or get busy Die'n....Host of the 2020 Bash at the Beach/The only Bash that got cancelled  )8
 

3/20/2018 4:45 AM  #6


Re: Carb question .. Need help tuning out a surge/stumble

6sally6 wrote:

My advice is ALWAYS  adjust the timing (in every way) before jumping on the "poor-old-carb"! EVERYBODY wants to "adjust-the-carb" first thing.
Its amazing what a little more initial...or a little less initial...plugs...wires...distrib cap...etc. can do for surge/bog.
Jus say'in
6sal6

And I'm reminded of when I thought my carb was at fault for a stumble.  I replaced it and THEN went through the timing to find that my initial  advance was off the balancer's timing scale (probably like 50-60 degrees).  I fixed that and it was a totally different car.  As usual, 6S6 drops the knowledge!

 

3/20/2018 9:46 AM  #7


Re: Carb question .. Need help tuning out a surge/stumble

hey.. seems like I am on the right track already! 6s6.. :D, because right before this whole thing with the carb, I went ahead and advanced the timing, to where I could hear knock in the engine on flooring it or high loads.
Then I have been backing it slowly to where I have absolutely no knocking at all on high load. (All mechanical) no vaccum advance in the picture yet. (and yes timed vaccum port is blocked)

Choke: I actually have had the opposite thing with the electrical choke, that the sucker comes off VERY quickly.. like I start it and I can see the choke opening slowly almost in the first couple seconds. and yes full 12v.! I do all adjustments with engine running at 180 -190F anyway.. so there is no question of a n unreliable result.
I am getting my idle just right now, as the idle with AC on, is a bit low, I wont mess with the carb more until that idle is just wherre I like it in gear and out of gear. (C4 tranny) .
Also, I think it still is the idle to where starting from cold (sitting overnight), it fires right away but dies quickly, and can take upto two to three tries and pumping the throttle to keep it running. I was thinking of may be increasing the choke , but only after my idle is good, and i will try again. 

SO .. the consensus seems to be a CAM related issue.I am all about taking it slow now a dayys. Patience doesnt come naturally to me, but this car and you guys have taught me patience. :D

That said I seem to have the red or the pink one (as seen in this pic https://www.summitracing.com/parts/hly-20-12?seid=srese1&cm_mmc=pla-google-_-shopping-_-srese1-_-holley&gclid=CjwKCAjw4sLVBRAlEiwASblR-63eMQSKdPBMGMLz04XKJ69p58a5YhZVvCTGPq6R8mNQ_1PVaf-7ohoC0_YQAvD_BwE) .. whatever came stock.

I will love to slowly mess with the accelerator cam to get it dialed in. that seems easy enough to me.
I will start with different holes on the one I have, and then, if that doesnt work , I will order a kit and start with that.

All this is being done on mechanically advancing timing right now. once this is done, I will introduce my last parameter of the vaccum advance, and adjust that canister to tighten or loosen as needed to avoid knocking or heating up.

Last edited by Gaba (3/20/2018 9:52 AM)


If it aint broke, I have'nt tried to "Fix" it yet!
     Thread Starter
 

3/20/2018 8:40 PM  #8


Re: Carb question .. Need help tuning out a surge/stumble

From previous experience with 600 Holley on a 302, I have typically installed a red cam and put it in hole #2.  Tighten the accel pump spring tight, but leave some give in it to avoid popping the pump diaphragm. 
Check your idle transfer slots to be sure no more than 1/16" of them show below the throttle plates.  Any more showing and you will have transition issues.  If too much shows, open the secondaries until you can close the primaries to cover more of the slot.  Carb must be removed and inverted to adjust secondaries.  WHY IS THAT???  Replace the setscrew with longer screw accessible from the top to make it easier.


Money you enjoy wasting is NOT wasted money... unless your wife finds out.
 

3/21/2018 8:54 PM  #9


Re: Carb question .. Need help tuning out a surge/stumble

Thought of something else...If you think this stumble is occurring as the secondaries open, install a stiffer spring in the secondary diaphragm.


Money you enjoy wasting is NOT wasted money... unless your wife finds out.
 

3/21/2018 10:28 PM  #10


Re: Carb question .. Need help tuning out a surge/stumble

Ok had a chance for a 40 mile round trip drive on the highway today

The idle is great now, the choke is perfect. Starting is better.

So... The only change I made today before leaving work was change the already installed orange cam on accelerator from hole 1 to hole 2

That definitely made a change :: not fixed it, but noticeable that the car would now consistently accelerate but it would have this lighter stumbling back and forth rocking feeling as I was holding the throttle barely open to coast

On the highway... Coasting at 60-65 or 70-75 whatever speed, foot off pedal, car was good

Slightly put pressure on throttle? (Not fully opening it), just slight pressure to maintain cruise, and the car would go but I could feel the rock back and forth of the engine.

Open the throttle just a bit more where it's still part throttle but it's audible, and the car would climb rpm smoothly and even accelerate quickly.

Recreated this at 20mph cruising, 30,40 , 60 and 70.. So regarldess where the rpm was the throttle response at that slight cruise amount of throttle was just feeling of rocking back and forth
Still the accelerator pump cam?

I have the assortment on the way, to try the red one if needed

Last edited by Gaba (3/21/2018 10:32 PM)


If it aint broke, I have'nt tried to "Fix" it yet!
     Thread Starter
 

3/22/2018 12:26 PM  #11


Re: Carb question .. Need help tuning out a surge/stumble

Gaba wrote:

Ok had a chance for a 40 mile round trip drive on the highway today

The idle is great now, the choke is perfect. Starting is better.

So... The only change I made today before leaving work was change the already installed orange cam on accelerator from hole 1 to hole 2

That definitely made a change :: not fixed it, but noticeable that the car would now consistently accelerate but it would have this lighter stumbling back and forth rocking feeling as I was holding the throttle barely open to coast

On the highway... Coasting at 60-65 or 70-75 whatever speed, foot off pedal, car was good

Slightly put pressure on throttle? (Not fully opening it), just slight pressure to maintain cruise, and the car would go but I could feel the rock back and forth of the engine.

Open the throttle just a bit more where it's still part throttle but it's audible, and the car would climb rpm smoothly and even accelerate quickly.

Recreated this at 20mph cruising, 30,40 , 60 and 70.. So regarldess where the rpm was the throttle response at that slight cruise amount of throttle was just feeling of rocking back and forth
Still the accelerator pump cam?

I have the assortment on the way, to try the red one if needed

The cams also control the duration of the squirter activity.  Some take longer to process the squirt than others.  And you can also change squirter sizes to further tune things.  There are so many variables that it doesn't take long to really screw things up!  See my article on Holley tuning and choke setup on the TIPS forum.  One article relates to fixing certain shortcomings of the Holley streetmaster carbs.
 


Money you enjoy wasting is NOT wasted money... unless your wife finds out.
 

3/22/2018 12:57 PM  #12


Re: Carb question .. Need help tuning out a surge/stumble

here is what I am thinking.. not screw up much .. but just deal with 4 simple things. .

jets. cams , PV, and secoindary springs.

I know secondary springs will be last.
Jets , PV and Cam are what I am thinking of messing with one at a time.

I will bring orange cam back to hole 1 for now.
Check the accelerator pump arm and actuator clearance (0.15") as per holley
Change to red cam , try 1 and 2 settings, see whats better. then if needed,
Go up two jet sizes. See how it does.
if it does not improve, or engine bogs like a dog.
I will go back two sizes, and then change the PV to 7.5

once all is good, test by using a stronger secondary spring to bring that in a little late , so I can keep miles per gallon with all the smiles per gallon I am already getting :D

I do know this. I have a good , reliable place to start right now. The engine is running cool with or without AC, no matter how fast or slow I drive, and no matter how much traffic i sit in . So I will know if the engine performace drops like it bogs it is too rich, or if it is starting to run hotter, its lean.
 


If it aint broke, I have'nt tried to "Fix" it yet!
     Thread Starter
 

3/22/2018 8:15 PM  #13


Re: Carb question .. Need help tuning out a surge/stumble

red351 wrote:

Jetting up only affects normal driving above idle. Your PV should be just fine. The PV uses to be called the "economy valve". The first time the engine would sneeze out the top, the valve blew out. You could then call it uncontrollable fuel injection. So we would just removed it before that happen, and automatically up jet 4 sizes' because that's about what the valve did with a little throttle was applied. Today there's a check valve built in to prevent that. A larger squirter is needed if a lot more fuel is needed fast off idle to fill large tunnels. Were not talking Cleveland's here.  Plus you may need a 50cc acc pump to follow thru. (I need two 30cc pumps to feed the red car Cleveland) The 67 barely gets by with 50. The spring is last, as said. I doubt you will need much more than a brown or plain spring. A 600cfm is never a 600 till both front and back is both FULLY open. A 302 can't move enough air, till way far up there in rpm's to open them all the way.. Plus you going need 6S6 or even bigger cam....

Nah so I was thinking like harder springs, so less secondary opening to give me better gas mileage.

I have jets here, I'll swap them out


If it aint broke, I have'nt tried to "Fix" it yet!
     Thread Starter
 

3/28/2018 9:52 AM  #14


Re: Carb question .. Need help tuning out a surge/stumble

alright

I have jet up two sizes to 93 (or 63 lol) .. the surge on cruise is almost completely gone..
Now noticeably what is left is the stumble getting off the line. It acclerates, falls, and accelerates very smoothly! and holy guacamole power! I have had some good run days with this car.. but today with 58 degrees outside, and just a little dampness on the road, the car wanted to break the tires loose everytime.. like in slow acceleration I heard the tires come loose in the back .. It was surprising!!

I think I am left with getting the accelerator pump cam just right. Looking at this diagram:
http://documents.holley.com/99-41141-100.pdf

Red one is a lower slower shot than what came with the carb (orange) .. I had changed to red , and my issue isnt resolved.

I am thinking of making a drastic change and go blue... and see how the car does on it. if nothing I will know if that is the wrong direction.


If it aint broke, I have'nt tried to "Fix" it yet!
     Thread Starter
 

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