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