I can not find this hesitation off idle

Engine built 10-05

D4TE 390 .020 bore
Speed Pro L2291 pistons
D2TEAA Heads mild port and polish
Dove roller rockers
Hooker ceramic headers
Flowmaster 40's
Crane 134042 cam
Crane lifters
Crane adj. push rods
Stock distributor
Crane XR-I ignition with Crane coil, 750-1705
Holley 4150 carb.
50cc pump
5.5 power valve
64 jets
Vacum 19" at 900 rpm idle
1" phenolic 4 hole spacer
Edlebrock Performer 390 Intake
14' K&N air filter
Timing set at 12btdc
Fuel pressure 5.0 pounds
I just have a big hesitation off idle
It starts great, less than 1 revolution
Runs great with no hesitation above 1500-1600rpm
Action taken so far

Replaced 6.5 power valve with 5.5 No change
Replaced 30cc pump with 50cc pump No change
Changed timing to 10btdc no change
Changed timing to 8btdc no change
Changed timing to 6btdc no change
The vacum advance is working per timing light and when a vacum is applied to the vacum advance
I used the original woven ground strap on the XR-I ignition and it is tight and has continuity
The dist. base plate moves freely when vacum is applied
No vacum leaks have been found in testing.
I usually keep the idle at 750 or so with the C6 in warmer weather. It has died on several occasions in the cooler weather when put in gear so I set the idle at 900 and that is fine now. It runs great like always and starts great hot or cold but it just has a pretty good hesitation off idle. This truck is just used as a daily driver. Any ideas?

 

At what rpm's does the centrifugal advance begin and is it operating smoothly and lubed? At what inches vacuum does the vacuum advance begin to advance? Fuel system ok with idle mix and transfer slot on carb vs butterfly position? On the advance one or both advancing late you'll have this stumble.

 

Thanks Beemer, The vacum advance is starting at about 800rpm. I set the timing at 900rpm with the vacum line unhooked and plugged then when I hook it back up I have 2+ degrees of timing more. It is operating smoothly and it is lubed. Fuel system is good. Idle mixture is good. I am unsure about the transfer slot on the carb versus the butterfly position. I have not adjusted anything on this. Can you tell me how to check it?

 

Vacuum starting at 800 rpm's, rpm's shouldn't effect vacuum like centrifugal, well to a point as the throttle plate is opening past the vacuum port giving a signal. With +2 degrees that tells me the primary throttle plate is opened too far on idle and your already into the transfer slots. Idle mix hard to set with little results? You must have a fairly big cam? Holley carb I hope.
You need to adjust the secondaries so that they are just cracked open to allow more air to the motor then you can close the primaries down more. This should help with your off idle hesitation as well delaying vacuum advance vacuum to the dizzy preventing advance at idle. Another way is to drill a hole in each throttle plate, if wrong you need to replace them vs secondary adjustments you can go back to should it not work.

 

Beemer, The idle mix is easy to set with final results at 19-20in. vacum. The Crane 134042 cam is small imo. 266dur.- .516lift - 1400-4800rpm. Holley 4150 carb.with vac. sec. I will look at the secondaries adjustment. I know it has never been adjusted as the carb. is but 1 year old. Thanks again. I will post my results after.

 

Fron the Holley website:
http://www.holley.com/TechService/FAQ.asp

QUESTION My vehicle has a stumble from a dead stop. What can I do to cure this?
ANSWER The most common cause of a stumble is not having an adequate accelerator pump shot. The first thing to do is to look at the discharge nozzle and make sure you are getting a good strong pump shot. If not then you need to inspect the pump diaphram for a hole or tear. You will also need to make sure that the pump passage is clear from any trash or debris. First you will need to check the adjustment on the pump. To do this you will open the throttle all the way (WOT). Push the pump arm lever down and then adjust the pump override spring to obtain .015" clearance between the pump arm and lever. If you are having a stumble and no black smoke out the tailpipe then you will need to increase the shooter size. If it stumbles and you are getting black smoke from the tailpipe then it will be nessasary to decrease the shooter size.

 

If you decide to get a new distributor, consider our D.U.I.-Davis Unified Ignition or our custom curved Duraspark distributor. Please see them at:


http://www.performancedistributors.com/forddui.htm

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I'd check into the slack in the accel linkage. Since you changed the pump and arm you might need to check it better.

If you just slowly roll the throttle, does it run fine? Then if you just pop the throttle it stumbles?...then it's accel pump.

The arm and pump should just touch each other ever so lightly, at idle. Not that it is pushing it, but just slack. (I check by just being able to click them together.....like .020 gap). Did you install the bigger cam that came with kit?
With the slightest touch of the throttle should get fuel, if not it will stumble.

Alot of people set the cam/arm by opening throttle fully and setting gap. USELESS!!!! This usually ends up with a large time lapse when going off idle. I've seen it too many times. As long as there is still spring left at WOT, then you're okay. That Holley information depends on that you have all the linkage perfectly set up. Set the linkage at idle, then go WOT and see if you have spring left. You should, then you're fine.

If I read correctly, it runs fine 1500 and up? Then it's because you have used up the slack in the linkage by pressing the throttle to get to 1500 rpm. Below that, the slack has created a dead spot.

Just my take on this scenario......

 

I finally found the problem with every ones help and would like to thank every one who posted for all the advice. Being hard headed like I am it did not dawn on me until Freightrain posted about the the slack in the acc. pump linkage. I had adjusted it to .020 but I did not take into account the slack in the acc. pump linkage to the cam at the other end of the arm. I reset it to .020 with the arm set firmly against the cam and now the hesitation is gone. I had about 1/8" to 3/16' slack to take up so as Freightrain said this is why it kicked in good about 1500rpm. I did not install the bigger cam for the acc. pump. Now that its running good I will check out the advantages of installing it. Thanks again every one