Off idle stumble.

[MWFan]

Just finished reassembly of fresh shortblock, heads were inspected by machinist, no issues.
440 +.030, 9.3/1 compression ratio, 452 stock heads, Mopar HP exhaust manifolds, 2 1/4" pipes, 484/284 purple shaft, Holley 750 Vac secondary, Mopar dual plane aluminum intake, 727, Dana 60 3:54/1,
Before I rebuilt it, I had blocked the vacuum advance and set the all in timing to 36 degrees. It had a small stumble but much better than it is now since I tried reconnecting the vacuum and setting initial timing to 12 degrees BTDC. I have gone up a step on the squirters but don't recall the number.
This is my 1st Mopar big block. Is there any common issue that I am overlooking?

 

[72ls5fla]

is it a hard acceleration stumble or even if you go easy on the gas ?

 

[MWFan]

is it a hard acceleration stumble or even if you go easy on the gas ?

If I depress the pedal slowly it is ok. If I "step through" to the middle or to wfo it stumbles.

 

[AR67GTX]

Have you checked the adjustment on the Holley accelerator pump link to make sure it has a slight preload?

 

[72ls5fla]

Is the carb brand new or something you have a used previously ?
You may need to change some items to work with the engine build-


On Secondaries:
do you know what spring is in the vacuum secondary pod?
if it is to light they will open to quickly -
There are about 8 or 9 different springs in a kit that Holley sells.

if the metering plate on the secondaries is to small, you will get a lean/gas starve.


If you think that the stumble comes from the primaries -

- accelerator pump:
Holley sells a higher volume capacity accelerator pump assembly.
just changing the squirters doesn't always solve things - specially if there is not enough volume of fuel being pushed out.
You can also change the cam the acc pump lever rides on. will changes timing of the arm lift and how much as well.
And I believe you can tighten or loosen the acc pump spring on the cam arm.

if not jetted correctly jetted on the primaries (something under 72-74), you will get a lean/gas starve. ethanol fuels make this worse.

lots of small things to try -

 

[dadeo]

If it only happens off idle my bet is it's dealing with the accelerator pump. Possibilities are wrong pump cam or out of adjustment as stated assuming pump is new or ok.
Andy

 

[MWFan]

When I bought the car 3 years ago the PO had an Edelbrock 600 on it. I put the 750 on and checked to see that the accelerator pump linkage was contacting the pump, but not necessarily preloaded. I'll try that.
I think the vacuum module for the secondaries has the weakest spring. I will try a step heavier and see what happens.
Thanks.

BTW, how much preload on the pump?

 

[AR67GTX]

I forget without digging out a Holley manual but the idea is to have slight preload for instant pump action, but with the throttle fully open it should still have more travel left so it can’t bottom out first.

Here is Holleys instruction. They say not to preload but to adjust to where there is light contact. That’s kind of a fine line but I guess they should know.

https://www.holley.com/blog/post/accelerator_pump_tuning_for_holley_carburetors/

 

[RJS]

When I bought the car 3 years ago the PO had an Edelbrock 600 on it. I put the 750 on and checked to see that the accelerator pump linkage was contacting the pump, but not necessarily preloaded. I'll try that.
I think the vacuum module for the secondaries has the weakest spring. I will try a step heavier and see what happens.
Thanks.

BTW, how much preload on the pump?

I don't know what this PRELOADING is all about but it's wrong!!!

With engine off have someone open throttle full and hold. Do this at the carburetor to insure full throttle!!!



You now take a feeler gauge and see if it will slide between front pump arm and the pump adjustment screw. Holley recommends .015, you just need more than 0. I use .008 because it's convenient on my valve lash feeler gauge. If you can't get a feeler in there you need to shorten the adjustment screw because you are over tasking the diaphragm and can rip it. If you have .015 or more clearance you are probably stumbling. Also it is good to take a small open end and while doing this clearance adjustment push the pump arm straight down to make sure you are getting a fully accurate reading. ( Very helpful to have 3 hands at this point).

To adjust the pump arm screw on a 30CC pump generally takes two (2) 3/8" open end wrenches.

After the pump arm adjustment have your helper with engine off floor the gas pedal and then at carburetor be sure it is actually at full throttle and or adjust cable.

Ron Sannino

 

[RemCharger]

Have the float levels been adjusted.

 

[oldbee]

All good ideas.^^^^ Whenever I rebuilt a Holley, I also pulled acc.pump all the way down (to stretch the diaphragm) while tightening the 4 pump housing screws.

 

[Nate S]

I have a very similar setup, no bog. Nothing special for the accel pump, stock cam, 25 squirters. With the wimpiest spring in the vac sec you’ll get mega-bog, especially if the little ball isn’t in there.

 

[MWFan]

Have the float levels been adjusted.

Yes.

 

[Crackedback]

12* with a 284/484 seems kind of light. I'd bump up the timing (18-20 range), reset idle and see if the stumble goes away.

Just don't go pounding the throttle. See if it corrects the issue, then adjust for total.

 

[dadeo]

To clarify, if it's off idle (not anything to do with secondary kicking in) focus on accelerator pump adjustment and possibly a change to a different pump cam if the adjustment doesn't cure it. If it's the secondary bog then a spring change is needed as said. I also adjust mine wide open and just under .015 but be careful that you're not compressing the little pump spring when sticking the feeler gage in there. I would also agree with more initial timing assuming no top end detonation. I had a similar off idle stumble and a pump cam change cured it completely.
Andy

 

[AR67GTX]

I don't know what this PRELOADING is all about but it's wrong!!!
Ron Sannino

OK - my error on preload - bad choice of words. I have 3 Holleys and I’ve simply adjusted them to where there is positive contact with the pump arm at closed throttle for instant response and at wide open throttle there is the .015” Holley referenced clearance or a hair less from bottoming (which I always have to look up). They respond strong off idle. Sorry.

 

[Nate S]

For those discounting the secondaries because it’s off idle, remember with an AVS set properly you can hit the throttle with your thumb IN NEUTRAL and have them open half way within a half second. With a Holley, no ball and a white spring it goes even faster. Off idle bog most certainly can be secondaries.

 

[MoparLeo]

Ok, you can set your car up for 1/4 mile which means you just floor the pedal everytime or you can adjust for a normal street drive. Need to pick what you want and don't just change jets, springs etc.. until you know what you are doing. This is how so many cars get screwed up. Carb and timing changes without direction. You need a baseline. Out of the box most carbs work good for normal driving. Why did you change the "squirters?" and you don't know what is is now ?
it is usually the guy looking for max performance in racing type driving that goes through all the trial and error to find his ideal setup. But it may not drive well on the street, but great for a drag race. No advance curve needed for full throttle driving.
Click on this link for tuning information:
http://www.mopar1.us/engine2.html

 

[RJS]

Give the car more initial as others have stated but please do this:

↑
When I bought the car 3 years ago the PO had an Edelbrock 600 on it. I put the 750 on and checked to see that the accelerator pump linkage was contacting the pump, but not necessarily preloaded. I'll try that.
I think the vacuum module for the secondaries has the weakest spring. I will try a step heavier and see what happens.
Thanks.

BTW, how much preload on the pump?
Click to expand...
I don't know what this PRELOADING is all about but it's wrong!!!

With engine off have someone open throttle full and hold. Do this at the carburetor to insure full throttle!!!



You now take a feeler gauge and see if it will slide between front pump arm and the pump adjustment screw. Holley recommends .015, you just need more than 0. I use .008 because it's convenient on my valve lash feeler gauge. If you can't get a feeler in there you need to shorten the adjustment screw because you are over tasking the diaphragm and can rip it. If you have .015 or more clearance you are probably stumbling. Also it is good to take a small open end and while doing this clearance adjustment push the pump arm straight down to make sure you are getting a fully accurate reading. ( Very helpful to have 3 hands at this point).

To adjust the pump arm screw on a 30CC pump generally takes two (2) 3/8" open end wrenches.

After the pump arm adjustment have your helper with engine off floor the gas pedal and then at carburetor be sure it is actually at full throttle and or adjust cable.

Ron Sannino

 

[MWFan]

Ok, you can set your car up for 1/4 mile which means you just floor the pedal everytime or you can adjust for a normal street drive. Need to pick what you want and don't just change jets, springs etc.. until you know what you are doing. This is how so many cars get screwed up. Carb and timing changes without direction. You need a baseline. Out of the box most carbs work good for normal driving. Why did you change the "squirters?" and you don't know what is is now ?
it is usually the guy looking for max performance in racing type driving that goes through all the trial and error to find his ideal setup. But it may not drive well on the street, but great for a drag race. No advance curve needed for full throttle driving.
http://www.mopar1.us/engine2.html

Well lets see....
I dont race.
I drive on the street and I have a large slightly modified internal combustion engine that should be quite capable of accelerating without an off idle bog.
Maybe I am screwing up my car, but if so I am doing it while requesting help from folks more knowlegeable than me.
I changed the "squirters" (accelerator pump jets) because I read somewhere that an off idle stumble could be a lack of fuel provided by the accelerator pump and larger "accelerator pump jets" (geez I hope that is the correct term) would help. I did it over 2 years ago and it's the only time in my life that I have ever done it and I recall that they were designated with a number but, silly old me, I don't remember the number and didn't feel like yanking them back out of the carb unless it is required to help my problem.
Thanks.