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Stumble!!

G Guest
Ok holley 750 with edlebrock high rise intake. Im getting a stumble of the line when normal acceloration. (From a stop sign) it just acts like it wants to stall but as soon as the rpms come up its ok. Same if im crusing at 30-40 and if i lean on it it stumbles then catches up but just for a split squad. I have changed my squiter out a size up so im sure im getting enough gas. Please i wana drive this thing at her full potential. Thanks all help welcomed
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panteradougpanteradougMember
http://www.summitracing.com/parts/mor-62295

You need to verify that the valve is good.

They can be partially or completely blown out.

Some have dried out diaphrams from too long a shelf life.

Apparently Holley went to a different supplier for the valves. They are using different material for the diaphrams and dry out within a year or so.

Theoretically the check valve protects it from a backfire but it doesn't protect it from defective materials.

I literally had 20 here in my Holley carb drawer.

I bought this tester. I have four left that tested good. Three were completely blown out, 13 had slow leaks through the diaphram and wouldn't hold vacuum.

Make you own assumptions. These things are crap out of the package.
ausfordausfordMember
quote:
Originally posted by Bumblebee13:
That is the next step. Its set on the first whole. Im going to move it to the second. Its supposidly the setting for higher rpm engines



There is a kit you can buy that contains 6 or more cams of different profile.

There is no rule as to a cam for higher rpm or lower rpm, they are a bit like the camshaft in the engine, different profiles have different effects on the amount of fuel the accelerator pump delivers and when in the throttle transition it is delivered.

Try them out and see which one suits your engine.
ausfordausfordMember
quote:
Originally posted by Bumblebee13:
Mine currently has the stock black one. Im gona play with that one first and go from there. Is there a color that is a happy median?


Not really, it tends to be a bit of a individual thing.

If i had to make a recommendation i would go for one thatstarts moving the pump arm immediately and for the greatest number of degrees of throttle movement and for the maximum pump arm movement (cc of fuel delivery).

From memory maybe the green cam ??
X xloMember
Just in case it's not obvious to you: It's very important to ensure your accelerator pump actuation arm screw is adjusted to start it's stroke with the slightest movement of the throttle shaft - both the primary and secondary - if a double pumper, or just primary if vacuum secondary. Without getting that right, swapping cams is hit and miss. You'll need to recheck/adjust again with every pump cam change.

Andy
ausfordausfordMember
Yeah why not ?

Also the comments of the two previous gentlemen is spot on.

Correct adjustment is of the greatest importance.

Look at the action of the cam on the accelerator pump arm and see if it's fuel delivery corresponds to the fuel requirement to eliminate the flatspot you are trying to eliminate.

If the car is in neautral and you are basically standing next to the engine, grab the carb throttle and open it as fast as you can. If you notice ANY hesitation in neutral that hesitation will be 100 times worse in gear.

Adjust things and swap cams untill you can crack open the throttle as fast as possible and no stumble occurs.


The devil is in the detail.
B Bumblebee13 (Guest)
I have 15 thousands feeler gauge sliding in and it slightly depresses the pump like the holley video says to do. Currently it has the green cam and its not the correct one. The throttle opens a 1/4 of the way before fuel starts to come out. Gona look at holleys web site and determine which one will provide a sooner delivdry of fuel
ausfordausfordMember
I must say i have never once used the Holley method of setting the accelerator pump overide spring.

Adjust the spring to provide slight pressure on the pump arm at idle so that even the slightest movement will push fuel out the shooter and then just check there is some spring travel left at wide open throttle.

That routine with the feeler gauge is confusing to most people.
Think what it needs to do and then adjust it to do that.
panteradougpanteradougMember
The clearance given by Holley between the accellerator/pump diaphram is necessary because 1)the clearance changes as the engine gets hot 2)the clearance changes when there is fuel in the pump assembly.

If you are adjusting out all clearance on the bench, that is wrong. You are limiting the amount of designed travel of the pump diaphragm. You are also reducing the amount of fuel held in the diaphragm assembly. Then you won't be getting enough shot to cover a sudden throttle opening and the engine will bog or backfire from a lean condition.

Holley didn't imagine that clearance number.


I believe that the white one is the original pump actuator.


The stumble can also be caused by too much fuel in the pump nozel.

The nozel that you want should be marked either .024 or .025. It should not have any tube extensions on it.

The tube extensions are for special application and will mis-direct the pump shot from where it is supposed to splash on the venturis.


You need a vaccum gauge to test this before you do anything.

You need to measure the engine vacuum at idle to verify that there are no engine vacuum leaks.

A blown power valve will show at this point with the engine showing anywhere from about 7 to 12 inches of vacuum at idle where it would normally be 15-17.

If you have low vacuum at idle and there are no external vacuum leaks then the power valve is blown.

You need not have a backfire to cause it to fail. It can dry out from having had fuel in it, then sat for enough time to let the fuel evaporate from the bowls.

With ethanol in the fuel like here in the US, that will cause a corrosion effect on the power valve materials and it can, and eventually will, cause the material to crack.

This is where you need to test the power valve. You need the special tool for that.

You will have two types of failures. One will be completely blown out, caused probably by a backfire, the other will show that the power valve has a slow leak in it and starts at 15 inches or so and within about 3 seconds will be at 0.

That is the corrosion failure.
ausfordausfordMember
If a power valve is blown/broken the carb will provide a very rich mixture all the time.

Your plugs will be black.

Black smoke will be emitted from the exhaust.

Use any Holley pump shooter you like with tubes or not, they all shoot at the skirt of the booster venturies, or at least they should. It is a bit acedemic anyway, as long as the pump shot goes down the carb throat there will be plenty of turbulence as it passes the throttle blades and into the manifold to vaporize the fuel.
B Bumblebee13 (Guest)
Ok played with the cams and read holleys diagrams. The green one seems to be fine. If i ease into the throttle it operates ok. When i goose it thats when the suck and hesitation occurs. I changed the 31 squirter to a 35. Any ideas. Im new to tuning a carb. Ive read on it but im clueless
ausfordausfordMember
quote:
Originally posted by Bumblebee13:
Ok played with the cams and read holleys diagrams. The green one seems to be fine. If i ease into the throttle it operates ok. When i goose it thats when the suck and hesitation occurs. I changed the 31 squirter to a 35. Any ideas. Im new to tuning a carb. Ive read on it but im clueless



It is pretty hard to give carburettor tuning advise to someone that has no clue.

Can you identify and relate to the components being refered to ?

You really need to obtain one or two of the many Holley carburettors and manifolds books available and engage in a effort of self education to become familiar with the carburettor and it's functions or advise given here will fly over your head.

Read the books, then read them again, then read them again untill you know what is on every page.

This process never stops.

After tuning Holleys for 30 years or more i still go and read books. You can always learn more.
B Bumblebee13 (Guest)
Im using a 35 squiter with the base gaske that came with my carb. I read a 1/4 gasket should be used?? Before the front sight glass was a quarter way full. Front and back sight glasses were even. Idnt know wat the hell happened. I was doing ok but it had that split second bog then rpms went up all was good. So i moved up a squiter size. After that at a low rpm it just started falling ons its butt. Switch nozzels back and now its not running like it was. 10 stepsback now
ausfordausfordMember
quote:
Originally posted by Bumblebee13:
My carb is not a double pumper its secondary advance


Secondary advance ???

Do you mean "vacuum secondary" ?

The accelerator pump shooter will have no effect on idle or slow speed steady state operation. You have 2 or more simultaneous unrelated problems that are giving you confounding symptoms.

How old is this carb ?

Do you have fuel line filters ?

Are they clean ?

Remove / clean / inspect and reset the needle and seats in both float bowls. Get the fuel level in the bowls correct again and find out why it changed....
ausfordausfordMember
quote:
Originally posted by Bumblebee13:
Fuel tank is oem. Been drained and flushed twice. I have a inline fuel filter. An aluminum one that has two filters in it and yes the flow is going in the corecy way


Is there any contaminants in the filter ?

Are there any contaminants in the small mesh screen in the inlet fitting on the carb ?

What fuel pump do you have ?

Is it functioning correctly ?

What is your fuel pressure now ?
jtpanterajtpanteraMember
The problem is not your fuel pump. The fuel dribbling out is indicative of a bad carb accel pump diaphram or clogged fuel passage between the pump and the shooter. Perhaps even the wrong or misaligned gasket on the fuel bowl or metering block blocking the passage had the carb been rebuilt. It is not difficult to check those possibilities.
ausfordausfordMember
quote:
Originally posted by JTpantera:
The problem is not your fuel pump. The fuel dribbling out is indicative of a bad carb accel pump diaphram or clogged fuel passage between the pump and the shooter. Perhaps even the wrong or misaligned gasket on the fuel bowl or metering block blocking the passage had the carb been rebuilt. It is not difficult to check those possibilities.


He has as unexplained drop in float bowl level and he has a 40 year old pump.

He needs to get that sorted out first or all the stuffing around with the carb will be a total waste of time.

You must start with the basics correct eg:

clean fuel

clean fuel tank

clean fuel filters

A fuel pump that is known to deliver reliable flow and pressure
as far as i know he cant test his in any way but it would be fair to assume that a 40 year old pump will be compromised.
They dont cost much, it's time for a new one.

Then he can check there is no crap inside the carb possibly blocking passage ways and adjust his float levels to spec with some reasonable expectation that the float levels wont mysteriously vary.

The the carb tuning fun and games can begin.
ausfordausfordMember
Yes please keep us posted.

I think it will fix one of your problems at least, but i am sure you will have further work to do.

While you are at the parts store i think you should get some gaskets for your carb because after you replace your pump i'm going to tell you to remove the float bowls from your carb and see what debris is inside.

Be prepared to do some work.
B Bumblebee13 (Guest)
Im sure. Its just weird squited just fine then went to a little drivble. I guess the pump would explain tge stalling out at an incline? This is just my luck all was good except for the little hesitation. Then the shop i took it to for an alignment dropped the car off the lift and folded the rear passenger fender and i need to get it to the body shop. Thats another story all in its self that im being lied to about. No way in hell that whe my pk brake was set it had enough
Momentum in less than 5 feet that it could roll over two 8 inch chocks and tgrough a roll up glass door. But
Magically thats what they said. Owell they are paying to paint the whole car
panteradougpanteradougMember
In order for you to keep a consistent fuel level in the bowls, you need consistent fuel pressure.

The higher the pressure, the higher the fuel level will be.

I find that even though Holley says the fuel should trickle out of the sight hole at idle, that the level is too high for going up or down a steep incline and you will get spillage through the boosters under those conditions.

I run an electric Holley pump with a Holley mechanical fuel pump with a pressure regulator. That keeps the pressure the most consistent, yet it still does vary, especially at idle.

That seems to be exactly consistent with the nature of the the pressure regulators. All seem to have about a 2psi variation in normal operation.

On a Holley, I also keep the level just at the bottom of the sight hole so it just doesn't trickle out.

Everything on a Holley is a compromise. The fuel shifts so badly under racing conditions in the bowls, that under hard braking the engine will stall.

What happens there is the fuel will slosh away from the metering block AND CAN spill out of the vents.

They are not good carbs for those reasons.
X xloMember
Deep breath.....

Your fuel level has dropped, and your accelerator pump now just dribbles?

You did RUN the motor right? Fuel for the accelerator pump is taken from the fuel bowl - if you keep squirting it by moving the throttle without the motor actually running then your carb bowl fuel level will drop on the primary side, and given enough tries, will eventually run out of fuel and the accelerator pump will just dribble.

Just to echo the thoughts of Aus Ford; I think a few nights away from the car and studying the Holley book would be time well invested. If you don't have a very good idea how the various systems work, and how they interact with one another, then you'll have a very hard time making progress - with or without the nice folks on here.

Andy

If you have done this, then be aware that your intake manifold now has quite a bit of raw fuel in it!
panteradougpanteradougMember
quote:
Originally posted by Marlin Jack:
...don't want to complicate the issue here, but.

Fuel bowl levels must be adjusted with the carb Level!
All 'Mustang' manifolds tilt the carb Forward to compensate for the Engine being tilted BACK, to align with the differential.
In the Pantera the Engine sits LEVEL to the Chassis, tilting all carbs FORWARD.
That is why Most of us have installed a 'Wedge' Spacer, of 5-8 degrees, under the Holleys, to tilt the carb Back to LEVEL.
also about fuel splashing out of the vents upon severe deceleration/acceleration. The racers connect both vents with a hose, in a 'U' shape, and cut a little vent in the top-side at the center.
As far as the Jets getting uncovered...That is what 'Slosh Tubes' and 'Vent whistles' are for, look it up in the book.

Last, sloshing fuel in the carb bowls is a Major reason why most drivers prefer 'Fuel Injecton'!


Good point on the carb level/angle. Novices would probably not notice that.

Yes on the fuel sloshing on the Holleys. The Weber 48 IDA's don't noticibly. The floats are set transversely vs. the Holley which are front to back.

You can run as hard as you can handle with the 48ida's and not have fuel shifting issues.

They were designed as a "racing carb". The Holley was not and as a result has all sorts of fuel delivery issues in competition due to fuel shifting in the bowls. Wink

The way the float and fuel bowl are designed in the ida, fuel can not be drawn away from the jets.

Mr.Weber was a very, very bright individual. Smiler


As far as the Holley "trick" jet and vent extensions go, the way we would verify the brakes being acceptable was to test them before a race and if the car didn't stall from the brake g-forces generated, they were not race ready?

They are just attempts to fix the unfixable with a Holley. They can somewhat help with reducing the problems but do not eliminate them.

You will have better results with a Holley at mounting it sideways like you do with dual Dominators on a "Tunnel Ram". I wonder if there are any of those on a Pantera? Marlin? What do you think? Maybe one or two? Big Grin



5 degrees on this Moroso spacer.

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B Bumblebee13 (Guest)
Well i guess you could call it ate up with the dumb a@@. Watching and reading some more i solved my fuel dribble problem. I had the squirter in upside down. I know i know been hard enough on my self haha. Replaced fuel pump and now both sight glasses are showing fuel. Rear one is a little higher guessing that y i need the angled spacer. Car is running now but it still has that slight suck hesitation. Tomorrow going to play with the cams a little. Timing now set at 12. Running smooth but still slight hesitation. Air fuel screws are 1.5 out
panteradougpanteradougMember
I have 2-1850's (600 cfm each) on my 68 GT350. It's a 347.


Marlin's 450's are 1848's. I believe he has them on 1" spacers to create plenums under each. A little unusual with 2 individual plenums and two individual primary carbs but it obviously works.

It's a little trick to make the TR work like a street manifold. Don't look up his sleeves. You don't want to know what else is up there? Big Grin



The 2x4 is a nice set up. In all honesty, I spend most of my time just trying to hold on to the steering wheel and not fall out of the car when I hit the accelerator.

The carbs are mounted backwards on mine. Seems to help with the sloshing issue?



#25's are the right shooters with no tubes. 31's should work but you should be happier with the smaller ones.

Ford put 31's in the 428's. It's a bigger mass to get going off of the line.

You need a constant 6-7 psi. An electric fuel pump pressurizing the mechanical pump is what I do.

With that you can set a consistent float level.



You should have the vent extension whistles and jet extensions. Those will help on hard acceleration.

I wouldn't challenge Marlin to a grudge match race. Those setups boogie. It's probably an honest 12 second car?


I am wondering if you have the cable installed properly on the carb? Did you test it to make sure that you are getting full opening at full pedal travel?
ausfordausfordMember
quote:
Originally posted by Bumblebee13:
It had a .30 in it from holley


Any time you have a hesitation or a bog down it means you need more fuel not less.

Too much fuel almost never causes a hesitation. It takes a ton of excess fuel to cause that, so much you would notice a ton of black smoke out the exhaust and even liquid gasoline out the exhaust.

So you can be almost certain: a hesitation = a lack of fuel

So if you have a hesitation as you crack the throttle open you still need more accel pump shot.

Is the accel pump working now ? Is it shooting fuel out in a stream that hits the booster venturi ?

If it does: Have you adjusted the accel pump override spring so it is touching the small accel pump arm slightly depressing it and with no slack between the two (forget the feeler gauge routine) .
You want the pump to shoot with even the slightest movement.

Try a 35 shooter or even a 40 shooter. I used a 40 on a Holley when i was running one.

Holley carbs are made for Chevys by people with a Chevy mindset. Consequently their fuel curves are more suited to Chevs and not the more obscure (in the states) 351 Cleveland. So Holleys generally speaking don't shoot enough fuel in off the line for a 351c and so you have the off idle bog.

Ultimately i needed to make up a custom accel pump cam for the Holley i used.
ausfordausfordMember
I've seen lots of stock 351c's with 750 carbs and they have worked fine.

It is a vac secondary, so even if the throttle is stood on the engine is only going to see half the carb until the airflow goes up.

1/2 of 750 is only 375 cfm. That should not cause the off idle bog that has been described.

Though if i was going out to buy a carb for a stock motor i agree a 600 is what i'd go for.
panteradougpanteradougMember
quote:
Originally posted by Aus Ford:
I've seen lots of stock 351c's with 750 carbs and they have worked fine.

It is a vac secondary, so even if the throttle is stood on the engine is only going to see half the carb until the airflow goes up.

1/2 of 750 is only 375 cfm. That should not cause the off idle bog that has been described.

Though if i was going out to buy a carb for a stock motor i agree a 600 is what i'd go for.


Sure. I used a 4779, 750 dp for quite a while BUT the engine could use it, the car needs to be geared for it and I have obviously a different definition of "stumble".

A 600 is definitely going to be "crisper" and the 750 definitely "lazier".

There is no telling what people "expect" the engines reaction to be?

I have run the Shelby 715cfm, Ford "Boss 302" carb and the 428cj 735 carbs on a 12.0:1 cr engine with 3.70 gears and 2.32 1st gear top loader.

They were all slugs.

The 600cfm vacuum secondary R-1850 was by far the most "responsive". That IS the carb you want for your lowest 0-60 times.

The 4779 worked best on the Edelbrock torker but the combination was very "flat" until about 4,200 rpm.

Again, what "stumble" means to one person is not what it may mean to another?

The carb in question here MAY in fact be acting/reacting exactly as it should and the driver not expecting this type of a reaction?

This is very difficult to decipher from afar?
ausfordausfordMember
Marlin,

I see your logic and i feel you are quite correct.

The shooter size has more effect over fuel delivery timing rather than the total volume of fuel delivered.

As you say the 25 shooter nozzle will deliver the pump shot over a longer period of time and the 40 will deliver the same amount of fuel over a shorter time. It is the pump cam that governs the amount of fuel.

Your description of the pump spring adjustment is spot on, exactly what i have been trying to get across.
B Bumblebee13 (Guest)
So big news i put the 25 back in and noticed is wasnt squirting one of the holes was semi blocked by a piece of debris. Also swapped the green cam out with the pink one. Heaitation has got better. At high rpms and when i ease into the throttle its ok just when i stick it and at low rpms it tries to die. But when i stick it is cuts out that split second them will give it all shes got
O osofastMember
quote:
Heaitation has got better. At high rpms and when i ease into the throttle its ok just when i stick it and at low rpms it tries to die. But when i stick it is cuts out that split second them will give it all shes got

quote:
why would the secondaries even be thinking about opening right off idle?

That's why.
I also want to point out you can go to a stiffer spring if there is no external adjustment.

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