Off idle stumble

[steel1212]

Ok I worked about 4 hours last night trying to get rid of this off idle stumble. I've got it down to minor but I still notice it. Its a slight hesitation when I floor it. I've gone from 10* initial to 15* and the lightest timing springs with the red bushing and that has done the best. I'm wondering if its just my parts are tuned to run from 1500-6500 and they don't want to go from idle. Once it gets to 1k or so it runs like a scalded dog. The problem I'll run into now is that I'll have to get a retard box to use the nitrous...well actually hook the one I have up

 

[tos]

Sounds like a fueling problem to me. Have you tried tweeking your accelerator pump? Thats where I would focus and then check for any minor vacuum leaks.

 

[brianj5600]

It can be tuned out. There are some pretty good carb guys here that can help with more info.

 

[10secgoal]

Still have the edelbog ?

 

[steel1212]

No edelbog here! I have a street avenger 670. I have messed with the main squirter going up and down in size and that doesn't help any.

 

[tos]

I dont believe this is a "jetting" problem. Your accelerator pump is for those quick initial throttle movements. When you stomp on your gas your primaries cant handle that sudden need for fuel. It opens up the airway but the fuel from the primaries is sucked in buy airflow thru the venturies. That momentary burst is taken care of buy the accelerator pump. Once you quit moving the throttle then the pump does nothing until moved again. You have to get this timed right to avoid that stumble. Its not difficult to adjust. Look at the paperwork that came with the carb it should have instructions.

 

[steel1212]

I'm at work right now so I can't go look at the manuel but what kind of "tweaking" can you do to the pump?

 

[thehueypilot]

It sounds like accel pump issue. If your pump cam has a 20cc big shot (just an example) and you have a #28 squirter or smaller, nozzle size the accelator pump arm will bind in mid stroke (reason for the spring) causing a good hesitation issue. Go up in size with your squirter nozzles first before going to a bigger shot pump cam.

 

[ForceFed70]

Definately time to start playing with the accelerator pump...

 

[steel1212]

The stock squirter is a 31 and I went up to a 37, the speed shop didn't have a 35, and it bogged worse. I also went down to 28 and it didn't do any better?

 

[ForceFed70]

This is where an O2 sensor and Air/Fuel ratio guage can come in handy...

Sorry.. don't know what to suggest other than to keep playing with it.

 

[steel1212]

Could it possibly be that all my parts have a performance range of 1500-6500 and it idles at 900? I mean once the car starts moveing its never below 1500 rpms due to the 4.11s out back and its fine there. Its just if I'm stoped and floor it from idle there is that slight hesitation before it goes. Its just enough to aggrivate me! I'll never see it at the strip due to 3500 rpms launches and what not so its really not a big deal just there and I know it.

 

[ForceFed70]

Could it possibly be that all my parts have a performance range of 1500-6500 and it idles at 900? I mean once the car starts moveing its never below 1500 rpms due to the 4.11s out back and its fine there. Its just if I'm stoped and floor it from idle there is that slight hesitation before it goes. Its just enough to aggrivate me! I'll never see it at the strip due to 3500 rpms launches and what not so its really not a big deal just there and I know it.

Yes... poor idle vacum could definately be a potential cause.

The carb depends on vacum to determine how much fuel the engine needs. Hoever, you can usually get most of the stumble out by playing with the accel enrichment (accelerator pump, cam, and jet)

 

[steel1212]

"poor idle vacum" probably due to that big ol' 280H cam? I would say I have most of the stumble out. I just don't like it sitting at 15* initial that means I'll have like 43* total.

 

[ForceFed70]

"poor idle vacum" probably due to that big ol' 280H cam? I would say I have most of the stumble out. I just don't like it sitting at 15* initial that means I'll have like 43* total.

Yup that cam would do it.

15* is a little much.. but not too bad. Average is approx 13*

43 total is definately too much, but you should be able to take out some of the mechanical advace to drop it down to about 36-38* total.

 

[steel1212]

Yeah, I have the red bushing in my msd right now and I think the blue will drop it 5* for total.

Also on the nitrous side do you think I'll need to install my retard box with a 100 shot? I was told I would be fine with 10* 100 shot and 104 octane, stand alone fuel system.

 

[ForceFed70]

Your timing needs to be retarded approx 1-2* for every 50hp.

So if it doesn't ping at 13*, then yes I think 10* would be no problem.

This is the problem I'm currently fighting. For my 150 shot I back off the timing by 5*. Which makes it stumble off-idle and less power.

I'm planning on converting to fuel injection this winter and hope to use the EFI computer to also control the nitrous and automatically do the timing retard. If I wasn't planning on doing this then I would definately be putting in MSD box of something like it to automatically retard the timing. Manually changing the timing all the time is a real PITA.

 

[thehueypilot]

The stock squirter is a 31 and I went up to a 37, the speed shop didn't have a 35, and it bogged worse. I also went down to 28 and it didn't do any better?

What color accel. cam pump are you running?

 

[steel1212]

Yeah, I have the MSD retard box sitting at the garage just not plugged up. I'm thinking I'll need to wire it up to retard when the window switch ingages the nitrous from 15* to probably 10*

I'm running what ever is stock on a street avenger 670, my car is at the garage or I would go and look.

Here is a pic from summit but I can't tell from it?

 

[brianj5600]

Is the stumble if you stand on it from idle? Or just tipping into the primaries? If it is only instant wot, I would look at getting a spring kit for the secondaries. My guess is maybe opening the secondaries a little slower to allow time for the secondary main system to start flowing. How ever that may be a trade off in throttle response at any other rpm besides off idle, the fix there may be a gentle foot. If it is tip in of primaries only, you will have to work on the accel pump system. First thing I would check there is that as soon as the throttle is cracked that the discharge nozzles flow fuel. There should be no movement w/o flow.