2100-2500 RPM stumble, whatsup?

[AMXguy]

 After diciding it was my Edelbrock carb causing my problems I bought a new 850 double pumper and it helped a little but I still have a problem.

  OK , what I have is a fresh 440  in a 4 spd car, stock intake, 850 double pumper. mopar electronic ignition, timing is 12 btc initial and 34 total with the vac advance not in use, 9.6:1  compression and a  cam with 110 separation ,288 duration, .485 and .507 lift.
 
 The problem, at full throttle under load from 2100-2500 RPM it fumbles and stutters a little until about 2600 then it runs hard and smooth. under no load or normal acceleration it runs great at all rpm, it's only under heavy load at those RPM's I have any problem what so ever. when I thought the problem was the Edelbrock I called their tech line and he said with a cam of that size I'll have to live with it, I didn't believe him but what do you think? I can live with this but I'd like to stop it.

[ntstlgl1970]

maybe ignition or?? seems weird that it's only in that RPM range.

It runs ok cruising at those RPM's in 4th if you accelerate slightly?

When you say fumble and stutter, is it popping or anything like that?

[MEK-Dangerfield]

maybe ignition or?? seems weird that it's only in that RPM range.

It runs ok cruising at those RPM's in 4th if you accelerate slightly?

When you say fumble and stutter, is it popping or anything like that?

I agree that it is odd you would stumble in such a narrow RPM range. Does it run fine in that range under normal driving? How do your spark plugs look? Maybe the secondary squirters, are too big or too small? 


  Mike

[AMXguy]

It's great as long as you don't nail it under 2600 rpm, you can ease into it or hammer it that makes no difference until you get past 2500 the fact changing carbs did little to change it leads me to the cam or ignition but I don't know. plugs look good, it just kind of has a flat feeling and a sputtering sound, no preignition or detonation sound to me anyway, a little like when your timing is retarded but since it's all in by 2500 I can't see that as possible.

[ntstlgl1970]

Hmm, wonder if it would be worthwhile to try a bigger squirter or different pump cam on the secondary. It almost sounds lean but tough call through a monitor....

On your ignition, is this an orange box setup, and if so, what coil are you using?

[AMXguy]

 Orange box,  MSD coil, if you ease into it and it sputters for 5 seconds to climb out of the trouble spot that couldn't have anything to do with accelerator pump problems could it?

[ntstlgl1970]

I wouldn't think so. Do you mean if you roll into wide open or just on the primary side of the carb (if you can tell)

Maybe a different power valve is in order. More than likely you have a 6.5 in there (if it is stock). What is the engine's idle vacuum reading like?

[Chryco Psycho]

so if you floor it below 2100 it doesn`t stumble , or is it already floored & stumbles in the 2100-2600 range on the way through ?

[AMXguy]

 Yes, if you floor it below about 2100 it bogs down a little as yould expect but runs fine, then as the rpm gets aroung 2100 all the way through to about 2500 it fumbles, after that it runs hard all the way to 6000 which is all I ever take it to.  if you floor it after 2600 it runs fine, it's just full throtle between 2100 and 2600 and it doesn't matter how you get there it does the same thing. normal acceleration and driving is just fine though.

[jeryst]

Sounds electrical to me. Ignition module maybe?

[AMXguy]

I thought about that, but shouldn't it stumble under moderate load as well if that were the problem?

 I had a chance to take it to town tonight and the Holley is MUCH more responsive and runs better at all speeds. the stumble is still there but not near as bad, same excact thing just less noticable.

[ntstlgl1970]

Another thought
"timing is 12 btc initial and 34 total with the vac advance not in use"
At what rpm is the timing all in by? Automatics like the advance all in pretty quick because of the torque converter if I remember right, but maybe it's coming in too fast for a 4 speed car. Can you try a stiffer advance spring on one of the centrifugal weights and see what happens?

[AMXguy]

 Good idea, I'll have a hard time finding mopar springs,  I wonder if ones for a Delco are the same? the timing is coming in pretty fast.

[ntstlgl1970]

I don't think it matters as long as it fits

[Bullitt-]

Mr Gasket 925B
Mopar Advance Spring Kit
Should be able to order from just about anyone in the parts busines
http://www.mopartsracing.com/parts/electronic.html  has a kit for $25.