[mklinger]

For you Holley experts, tell me if this is correct. When coming off idle with light throttle I get a mild bit of hesitation/stumble. If I get a bit more aggressive off idle, no issue. The carb is a Holley Street Avenger 770 with out of the box jetting, pump cams, etc.

Here's what I notice, when observing the accelerator pump and rotating the throttle slowly, there's a few degrees of rotation before the accelerator pump cam engages. Now I know there are two settings on the pump cam, hole #1 and hole #2, and I have tried them both. #1 engages sooner then #2 but in either case there is a few degrees of rotation before the pump engages.

So here's the question. Should there be a small amount of play in the pump/cam/throttle linkage or is something outa wack here. If it's outa wack, where is the adjustment? Do I change the pump cam (current cam is green).

And if you think this is not the cause of my off idle stumble, I am open to suggestions.

Thanks all.

[hal101]

yes i think you are on the right track
the pump should pump as soon as the throtel shaft moves

[Brad B. Hillebrand]

You might want to back off the idle mixture 1/4 to richen it up a little...

Brad

[WCWhitey]

I think Brad is on the right track. 1/4 turn past the highest vacuum reading will give you the best off idle/light throttle response. It may be a little stinky but that's a holley. If you have a crisp throttle when you mash it and it runs well at a light cruise you are very close.

[mklinger]

Brad, I did richen up the idle. When I set it by ear, I was better off than 1/2 turn richer than max vacuum. Still had the hesitation.

Any ideas on the linkage and the pump cam.

[kyle_blake]

Increase squirter size or try another pump cam?

I've jetted down my primary's + reduced squirter size which resulted in hesitation.

I put the carb back to stock and no more hesitation.

I then changed my pump cam, and lone behold almost identical hesitation effect!

The idle mixture screws can be 1 & 1/2 turns out which is fairly standard.

Fine tune those a bit later when everything is mint.

Here is a link I used:

http://www.mortec.com/carbtip1.htm

[Lerry: The Body Ventura..]

To me it sounds like the spring loaded adjustment screw that engages the accelerator pump arm to the lever the cam operates. You want to adjust that screw to where it barely rides the accelerator pump arm.

[mklinger]

The lack of motion appears to be solely in the relationship between the pump cam and the lever attached to the spring loaded adjustment screw. If the reaction to rotating the throttle should result in immeduate movement of the accelerator pump, then either the lever is out of adjustment ( or bent) or the pump cam does not engage the lever until after the throttle has rotated a few degrees.

So getting back to the original question, should the accelerator pump react immediatly with any rotation of the throttle or not. Unfortuneatly, I don't have access to another Holley to see if the behavior is the same or different.

Can some one take a look at theirs?

Kyle..
BTW, I have read that Holley article before, good stuff and recommended reading.

[hal101]

yes i hav ben told by many that it should

[hal101]

i hav the avenger 870 cfm and it dose

[Motor Daddy]

There should be absolutely no play in the accelerator pump linkage. Adjust the spring loaded bolt on the linkage so that there is no play.

The hesitation could be caused by many things.
First adjust the linkage to make sure there is no play.
Report back with results.

Is the carb bone stock with factory jets?

[76 Badbird]

According to the rebuild instructions for the carb you should have .015 space between the arm and the bolt. I was working on a problem similar to yours and found that I have the vacuum advance connected to the wrong port. I connected it to full vaccum under the front of the carb and not the side port next to the choke ( need to slow down on the refreshments when connecting things). I was getting way too much vacuum advance to quickly.

[Motor Daddy]

The 770 avenger is a 4160 vacuum carb, with a secondary metering block. It only has primary idle screws. It has a very small .025 squirter stock.

[Motor Daddy]

72, 75

[Lerry: The Body Ventura..]

If the lever screw connot be adjusted out far enough to engage pump arm. Then lever has been bent or it's the wrong lever. I've run into both situations with used carbs. There are two ways of adjusting the screw to pump arm. One is to open throttle fully and then adjust with feeler gauge to .015 (or little less), or adjust screw to where it just barely comes into contact with throttle unopened (this method is easier and more accurate if you do not want any play at all..

[Lerry: The Body Ventura..]

I doubt if it's the squirter. Too small/large of squirter usually shows up when full throttled. Suddenly.

[PLAY400]

Originally Posted by motor-daddy

There should be absolutely no play in the accelerator pump linkage. Adjust the spring loaded bolt on the linkage so that there is no play.

That's what I was taught. Only reason it works when you hammer it is you aren't noticing the delay cause you're pushing the spring loaded bolt all the way down FAST.

[Half-Inch Stud]

Try a Q-Jet and see if the problem goes away. HIS

[Brad B. Hillebrand]

I checked another Holley. No play at all. Instant squirter.

Brad

[Z Code 400]

Mklinger: On a Holley or even a Q-Jet, it is possible to open the throttle slowly enough that the accelerator pump does not discharge. This is especially true with the Holley Float Bowls that use a check ball as opposed the the elastomer valve.

I would like to know more about your combination (camshaft, compression, etc.) before making specific recommendations, but here are some generalizations based on many years of tuning Holleys..

Your first order of business is to verify the float level is correct. With a warm engine at idle, you should have fuel at the bottom of the sight plugs. On our 427 Ford Drag Car with a very lopey camshaft (250 @ .050 104° LSA) we used to raise the idle just enough to smooth out the lope and then make our adjustments.

Secondly, you need to verify that you do not have a vaccuum leak anywhere on the engine. This is very important to the following tuning procedure(s). The ignition timing and advance mechanism must be correctly set. The spark plugs and wires must be in good condition.

Flip the carburetor over and remove the accelerator pump . You will see a small metal 'ball' held in place by a brass strap. Take a wire gauge and measure the distance between the ball and the strap with the carburetor upside down. You should have .010" to .015" clearance. Carefully bend the strap to adjust.

Examine the lever on the accelerator pump cover for wear. The lever will be loose normally, but the roll pin should not have worn the hole in the lever out. Pull the lever back and forth on the roll pin to see how much movement you have. Some 'play' is normal. You will know if it is worn out.

Next, check the arm of the accelerator pump spring loaded lever where it rides on the nylon accelerator pump cam. You will see wear in this area on any Holley that has been running for a while. If the wear is less than 25% the thickness of the lever, you can resue it. New ones are available from a variety of suppliers.

I often polish this area to a mirror finish with a Dremel Tool and reinstall. I also use a small amount of Synthetic Chassis Grease between the pump cam and lever. This should be done periodically during 'tune ups' or other engine maintenence.

The spring loaded lever needs some clearance at wide open throttle (WOT) to prevent rupturing the accelerator pump diaphragm. Although .015" to .020" is generally thought to be an acceptable setting, virtually anything over .010" is fine. Close the throttle and check to see you have NO clearance with the throttles closed.

Once you have completed these tasks, we can move on to the next subject(s).

One of the first things you should check on a Holley when you are experiencing this kind of problem is the relation between the Idle Transfer Slots and the edge of the throttle plates at idle.

The Idle Transfer System on a Holley 'bridges the gap' between idle and enrichment. Although it works all the time, its performance is most noticeable when the accelerator pump is not working, such as very light acceleration.

Remove the carb and measure the amount of the slots that are visible underneath the throttle plates at idle. You should have .040" to .050" of the slots visible below the thottle plates with the throttle lever against the idle screw.

It is importanat not to adjust the screw once the carb has been removed from the running engine. It MUST be in exactly the same position as when it was removed.

If you have no transfer slot visible at idle, as is the case with very mild/stock engines, you will need to close the secondary throttle plates slightly. There is a Secondary Throttle Stop on all Holleys. It is a tiny screw that is visible from underneath the throttle body when the carburetor is off the car.

Closing the secondary throttle plates slightly will allow you to open the primary throttle plates more at idle, hopefully exposing the transfer slots below the throttle plates the correct amount.

The secondary throttle plates are designed to remain slightly open at idle. There are two reasons for this: (A) To prevent the plates from 'sticking' in the bore and (B) to keep the fuel supply in the rear float bowl fresh.

All Holleys have a 'Four Corner Idle System' in this respect. However, the fuel mixture is not adjustable on many Holley carburetors. It is metered through a fixed orifice.

The secondary throttles should not be open more than 1/2 turn in most cases or uneven idle will result.

If you have too much transfer slot open below the throttle plates, you can slightly open the secondary throttle plates to allow the primary throttles to be closed slightly.

In the case of an engine with a very radical camshaft (Our 427 Ford) we had to drill small holes (approximately .100" on our dual Holley 450 Mechanical Secondaries) in the primary throttle plates to admit additional air at idle and allow us to close the throttles enough to bring the transfer slots into the correct relationship with the throttle plates.

Drilling the primary throttle plates is only required with very low vaccuum and radical camshafts.

To execute this operation correctly, set the primary throttle plates to give the correct relationship between your idle transfer slots and the lower edge of the throttle plates. Install the carburetor and allow the engine to warm up and idle.

Drill a hole in the forward section of each throttle plates (start with a .060" bit) and keep enlarging the hole(s) equally until your idle speed is where you want it. Again, this is only necessary with very radical camshafts.

Next, take a vaccuum reading at the base of the carbuertor with the engine idling. If equipped with an automatic transmission, you will want the reading taken with the car in 'drive' idling against the brake.

Your power valve should be closed at idle. This means a 10.5 in/Hg power valve should not be used where manifold vaccum at idle is 9 in/Hg. You need at least 3 in/Hg differential between idle vaccuum and power valve opening point at an absolute minimum.

We use this method for power valve selection on Holleys. It has been in use in our shop for over 30 years. Except for some road-race applications, we set all Holley's up using this technique. I guarantee it works.

Take the lowest vaccuum reading and divide it in half. This is what the value of your power valve should be. If you have 12 in/Hg vaccuum at idle then you should have a 6.0 in/Hg power valve.

Many people will advise you to select a power valve that is only 1 to 2 in/Hg below your vacuum reading at idle. Although the engine will run in this condition, this method works best under all possible conditions, especially with a street-driven car and the desire for some degree of fuel economy.

I hope this information is helpful. Feel free to e-mail me if you have additional questions at: Zcode400@sbcglobal.net.

Best of luck on your project...Robert