weird off-idle hesitation

[Marx3]

stock 170 hp 8.2:1CR chevy 350.
Qjet 17059276
All airbleeds .070
71 jets, 44 rods, tip-in perfect.
.036 tubes and .050 DCR.
acc.discharge .027
Car ran perfectly. Perfect response, no hesitation anywhere.
Then I put on better iron heads ( should be hitting about 280-300 hp at the crank ), a 218/224@050 flat tappet cam, an edelbrock dual plane and built a 2.5" dual exhaust.
CR is 10:1
Car still had great response everywhere.
Timing was mid to high 30´s ( cant remember exact number ), so I backed it down a bit because it would ping from the the higher CR.
Backed it down about 5 degrees to low 30's.
The timing is now 15 initial, 32 total plus 14 degrees vacuum.
Now that I have backed the timing down about 5 degrees, the engine has developed an annoying off-idle stumble.
I dont understand ?
I have been runnign proted vacuum and tried full, just to see if it would help. It didn't.
Figured the engine might needed a bigger shot from the acc. pump so I upped the discharge holes from .027 to .031. Didn't change a thing.
I can always put on another airhorn, if I need the discharge to get smaller.

[77cruiser]

Can you shorten your advance curve so you can put your initial back up?

[Marx3]

Yeah I guess I could easily put in the stock advance center piece and weights, just to try if this would solve the problem. That should put my curve down around 10-12 degrees again.
I prefer it up around 15-18 degress though.
I just dont get why I get a noticable hesitation, just because I back the timing down from 20 to 15 initial.

[Cliff Ruggles]

The accl pump will only effect off idle or transition if you are getting into the throttle quickly, or going to heavy/full throttle.

A hesitation caused by reducing the timing tells us in most cases that the mixture is pretty lean and requires a lot of timing to burn it effectively.

You can at this point add timing back in or fuel, or a little of both to correct the issue.

I'd start by raising the APT screw, to richen it up some in that range.....Cliff

[Marx3]

re-checked timing today. It turned out that was a bit more retarded, than I thought.
It was at about 5 BTDC initial.
Got it up to around 15, like it should. That helped a bit.
Then I re-checked pirmary calibration and it WAS a bit lean... ( did both the tip-in and put in a wideband too )
I was at about 16:1 at cruise.
backed the APT out a bout 1 full turn. Tip-in was better and the wideband now said 14.5-14.8:1
Ideal. Car feels a bit stronger and has much less of a stumble / hesitation.

The hesitation is noticable on of-idle take off.
The stumble is not really there, once the car is moving.

I have a feeling that the 214/224@050 cam ( 112 lsa ) is struggling a bit with the 2.41 rear end and the stock converter?

Do you think .031 discharge could be causing this? I know .031 is propably a couple of sizes larger than needed, but do you think it will cause a stumble?

[Marx3]

found an air horn with .026 discharge holes and installed it. This seemed to help some. The off-idle hsesitation is still there. I tried different settings for the APT... turning it down for a leaner jetting didn't really hange the hesitation. Raising the APT for a richer jetting didn't either.
I backed up the APT adjustment with a wideband ( mostly because I wanted to see if I could log the data and maybe spot a lean or rich situation when accelarating ). at 17:1 car runs fine but hesitates, at 14.8-15:1 the car runs a bit stronger, bit still hesitates. At 14-14.2:1 the car runs fine, but still hesitates.

What the heck?
What do you think about a 214/224@050 cam against a 2.41 gearing? potential trouble?

[Jeffs68]

If the hesitation is right OFF idle, can you monitor what the A/F is doing at that moment? Might be a longshot, but maybe you can tell if it's a lean or rich problem.

Maybe the secondary air flaps are coming in too quick???

[77cruiser]

Open up the DCR a few thou.

[Marx3]

I haven't been able to log my runs, but it seems that the AFR takes a quick dive from 15:1 to around 12:1, when touching the accelerator. The secondaries are adjusted real well. Loosened them to the point of bogging and backed up a little. They are at about 3/4 of a turn. (3800 lbs car with 2.41 rear )
The engine performs excellent in every area, except for this off-idle hesitation.
I don't get it... Discharge is at .025-.026... Not exactly huge...
Why open up the DCR?
I am completely open to trying anything, but why?
 :-)

[77cruiser]

Thought you had a lean hesitation, but I doesn't look to be.
Is the accel. pump giving a shot as soon as you touch the throttle?

[Marx3]

It feels exactly like a lean stumble, but it appears it aint :-) Yes, the pump shoots simultaneous with the throttle moving. Nothing wrong anywere it seems.

[77cruiser]

Have you tried taking the vacuum advance out of the equation? Just unplugging it just to see what happens.

[Marx3]

Yup, I ran ported before and it makes no difference if it I shooed up to full or ported

[Cliff Ruggles]

I've ran into this problem a couple of times with "low" compression engines and bigger than stock cams.  You get a very momentary hesitation from going rich right off idle.

Haven't seen this with higher compression.  You may simply need to limit fuel to the mixture screws and transfer slots slightly.

I'd also hook up a vacuum gauge and see where the vacuum is at the same moment you get the "stumble".  The power piston may be coming up if the spring under it is too strong.....Cliff

[Marx3]

Hmm I haven't given the pp spring any thought. I'll try and mess with it.
I'll report back.