Stumble!!

There is a plague of power valve failures in Holley carbs.
I'd bet that's the issue.

Thats possible with a new carb?

If i have had back fires through the carb could that have damaged the power valve?

quote:

If i have had back fires through the carb could that have damaged the pow

Indeed, that is the primary method of power valve failure

Larry

Have you tried any different accelerator pump cams ?

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

quote:

Originally posted by Bumblebee13:
If i have had back fires through the carb could that have damaged the power valve?

Newer Holleys have a check valve to prevent this. So its possible but not probable.

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.

Ive been messing with this carb for a while. Novice mistake i guess but i noticed my pump cam is in the first hole. For engins that idle at 600 rpm. Tonight im going to move it to position 2

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.

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?

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 ??

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

Ok got the cam kit from my local race shop. Should i start with the green cam?

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.

Yes with the way it is now about half way through throttle open is when the fuel squirts out at full squirt. But the pump arm is set on the number one hole. Gona change that first

Fuel needs to squirt out immediately, that's what keeps the engine from stumbling. Adjust the arm so it squirts the moment you touch the gas pedal.

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

is this the same video?

https://www.youtube.com/watch?v=w3F3ssOb8lg

if you have 1/4 throttle movement before fuel squirts, something is Very Wrong!

the .015" feeler blade check is done at Wide Open Throttle, not at base idle. a correctly adjusted pump should be slightly 'into' the pump travel at idle to ensure that the pump will squirt immediately

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.

Read this site to learn more about carburetor tuning:
http://www.bob2000.com/carb.htm

As well as order Michael Haas' book, "Tuning Made Easy".
http://www.amazon.com/Tuning-M...l-Hass/dp/8771144625

In my case it was a very small vacuum leak. The sealing under the carb had a small crack.

#1154

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.

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.

quote:

you need the special tool for that

You can also use the scientific method of sucking on the vacuum side of the power valve, and using your tongue to plug the hole for a "gross" vacuum holding capability measurement.

Rocky

Yes that is the power valve testing equiptment i have allways used.

The most powerfull tool you can use to sort out carburettor problems is of course your biological computer, that is the one inside your scull.

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

Timming set at 15

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.

Now i just noticed my front sight holes gas is just at the very botton of the hole. It use to not be like that. What would cause that?

Fuel leaking through the crack in the diaphragm in the power valve?

Not seeing any fuel leak. When i was doing a road test it stalled out when i was going up a hill.

quote:

Originally posted by Bumblebee13:
Now i just noticed my front sight holes gas is just at the very botton of the hole. It use to not be like that. What would cause that?

What was it like ?

I agree that it is difficult to diagnose the issue w/o full info. If you are using an open spacer under the carb or an open plenum manifold, you may consider a 50cc pump and larger shooters(45). You are describing a lean condition.

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

And to add i knoticed my squiter really isnt squirting its more like dribbling fuel out

My carb is not a double pumper its secondary advance

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....

Yes sorry vacum. The carb is less than four months old. Got it from precision proformance. Any general guess why the bowl level would change?