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Thread: Bad Gas Milege, Off Idle Hesitation/Stumble

  1. #1

    Default Bad Gas Milege, Off Idle Hesitation/Stumble

    Installed atomic two years ago with no timing control, and pump outside tank, and return line. Went thru two pumps, an idle/ stalling issues, as well as performance, and fuel mileage issues. . Changed motor re-installed Atomic with timing control, and in-tank pump. Much better idle control, as well as throttle response. Fuel mileage still horrible 7-9 MPG . As well It is experiencing and off idle hesitation/stumble from closed throttle to 1/8 open. From a stop push pedal past the flat spot point it just hesitates then goes just fine. If I try to cruise thru a parking lot between these two points it will continue to sputter/ stumble until I go past his point or let of throttle.

    Hopefully I just doing something wrong on set up. Can anyone advise?



    Motor Profile Timing Control Enabled

    350 CID +.30 Idle Rpm 775

    Comp; 9.73:1 Total 3000, was 2800 No diff.

    Cam Dur. Intake 234 Idle Adv 14 Deg

    Exhaust 238 @ .50 Total Adv. 36 Deg

    112 Lobe Separation Vacuum 10 Deg Ported

    Auto Trans No Air Pump



    Initial Setup PS 25%

    Idle 675 rpm PV 15%

    CID 355 A/F Idle 14.0, Cruise 14.8, WOT 12.8

    Cam Profile "Mild" IAC Warm in Gear 0-5

    PWM No Reg Map Key on 27.3- Running 18.8

    Intake Pump


  2. #2

    Default Bad Gas Milege, Off Idle Hesitation/Stumble

    Blazer72 wrote:

    Installed atomic two years ago with no timing control, and pump outside tank, and return line. Went thru two pumps, an idle/ stalling issues, as well as performance, and fuel mileage issues. . Changed motor re-installed Atomic with timing control, and in-tank pump. Much better idle control, as well as throttle response. Fuel mileage still horrible 7-9 MPG . As well It is experiencing and off idle hesitation/stumble from closed throttle to 1/8 open. From a stop push pedal past the flat spot point it just hesitates then goes just fine. If I try to cruise thru a parking lot between these two points it will continue to sputter/ stumble until I go past his point or let of throttle.

    Hopefully I just doing something wrong on set up. Can anyone advise?



    Motor Profile Timing Control Enabled

    350 CID +.30 Idle Rpm 775

    Comp; 9.73:1 Total 3000, was 2800 No diff.

    Cam Dur. Intake 234 Idle Adv 14 Deg

    Exhaust 238 @ .50 Total Adv. 36 Deg

    112 Lobe Separation Vacuum 10 Deg Ported

    Auto Trans No Air Pump



    Initial Setup PS 25%

    Idle 675 rpm PV 15%

    CID 355 A/F Idle 14.0, Cruise 14.8, WOT 12.8

    Cam Profile "Mild" IAC Warm in Gear 0-5

    PWM No Reg Map Key on 27.3- Running 18.8

    Intake Pump



    I would change your vacuum advance to unported. It will give you a better idle, but you will probably have to reset your throttle plates to get your IAC right. Then bring your idle timing up from 14 to about 18 and see if that helps. You might end up even higher than that. Next, bring your total advance in earlier. Start at 2000 rpm and evaluate from there. Listen for detonation. I know it sounds too aggressive a timing curve, but it seems to work!
    My big block currently is set up with 24 degrees initial, 38 total all in at 1500 rpm! No pinging and it has great street manners!

    Gerry

  3. #3

    Default Bad Gas Milege, Off Idle Hesitation/Stumble



    I did try lower total timing RPM, but did not try higher initial timing, and I did try un-ported. I'll try the higher initial with lower all in RPM see if that works.

    Thanks


  4. #4

    Default Bad Gas Milege, Off Idle Hesitation/Stumble

    Well increased the initial to 18, went ported on vacuum advance, adjusted throttle plates, lowered total rpm to 2000 rpm. made a huge difference, almost no hesitation. idle still a little rough, and a little loss of mid to top end acceleration, great bottom end, but getting closer. Glad to be rid of the hesitation.

    Thanks for the intell.

  5. #5

    Default Bad Gas Milege, Off Idle Hesitation/Stumble

    Blazer72 wrote:

    Well increased the initial to 18, went ported on vacuum advance, adjusted throttle plates, lowered total rpm to 2000 rpm. made a huge difference, almost no hesitation. idle still a little rough, and a little loss of mid to top end acceleration, great bottom end, but getting closer. Glad to be rid of the hesitation.

    Thanks for the

    Ported vacuum advance was used in the early days of emission controls. Google "John Hinckley timing 101" to read all about timing. Scroll down to the ported vacuum section, read it, and then change to unported.

    So it's getting better? Then raise the initial up two degrees at a time and evaluate. Then try reducing the total rpm timing. Then, for kicks, try changing the cam type to street stock instead of mild. See how that works for your engine.

    Good luck!

    Gerry

  6. #6

    Default Bad Gas Milege, Off Idle Hesitation/Stumble

    I will try that.

    Thanks

    Rob

  7. #7

    Default Bad Gas Milege, Off Idle Hesitation/Stumble

    With a MAP reading of 27.3"Hg your local altitude should be about 2500-ft. Is this about right?

    And with a idle MAP reading of 18.8"Hg, that would be about 8.5"Hg of vacuum.

    The Performance cam setting should work better for this application along with the higher idle timing and rpm. Generally the target idle would be closer to 1000-rpm with this type of cam.

    What is the stall on the torque converter?

  8. #8

    Default Bad Gas Milege, Off Idle Hesitation/Stumble

    Actually I'm at 600 ft Southern California. I originally tried performance setting ran really poorly exhaust fumes were so bad. Ran much better with Mild setting. Tried street stock to lean on acceleration, A/F at idle was 12.6 with street stock setting o2 compensating. Currently back to mild setting seems to perform the best at mild. Have 2000 Stall converter. 1000 rpm way to high. having dieseling problems as is, installed cold plugs yesterday improved dieseling issues. 676 idle works well have no issues with it stalling what so ever at this setting. only the prolonged idle when it seems to be loading up. Even when I was running Carb with and stock converter around 700 was perfect idle speed. My vacuum in drive at idle is currently between 6"-7" hg. Before increase of initial timing from 14 to 18 Idle in drive vacuum was 4" hg. Currently I have made some adjustments, total rpm to 2400, A/F kept the same, total timing 36, It seems that the biggest improvement made was the non ported setting as far as the hesitation concerned. Its 99% improved still when just off idle trying to maintain a slow speed thru parking lot it surges until I push pedal past the flat spot, similar to a bad spot in a throttle position sensor. That's it on the performance. The fuel economy is my biggest concern, i've been trying to keep track of mileage its right around 5-7 MPG This is around town mainly not a lot on freeways but some. That's all I can think of now.


  9. #9

    Default Bad Gas Milege, Off Idle Hesitation/Stumble

    Blazer72 wrote:

    Actually I'm at 600 ft Southern California. I originally tried performance setting ran really poorly exhaust fumes were so bad. Ran much better with Mild setting. Tried street stock to lean on acceleration, A/F at idle was 12.6 with street stock setting o2 compensating. Currently back to mild setting seems to perform the best at mild. Have 2000 Stall converter. 1000 rpm way to high. having dieseling problems as is, installed cold plugs yesterday improved dieseling issues. 676 idle works well have no issues with it stalling what so ever at this setting. only the prolonged idle when it seems to be loading up. Even when I was running Carb with and stock converter around 700 was perfect idle speed. My vacuum in drive at idle is currently between 6"-7" hg. Before increase of initial timing from 14 to 18 Idle in drive vacuum was 4" hg. Currently I have made some adjustments, total rpm to 2400, A/F kept the same, total timing 36, It seems that the biggest improvement made was the non ported setting as far as the hesitation concerned. Its 99% improved still when just off idle trying to maintain a slow speed thru parking lot it surges until I push pedal past the flat spot, similar to a bad spot in a throttle position sensor. That's it on the performance. The fuel economy is my biggest concern, i've been trying to keep track of mileage its right around 5-7 MPG This is around town mainly not a lot on freeways but some. That's all I can think of now.



    The non ported vacuum was probably not what helped. Non ported gives you vacuum advance at idle, which makes the engine idle cooler, and with less fuel, so that you can close the throttle plates more. Anytime the throttle is opened a bit, there is no difference between ported and non ported vacuum. Only difference is at idle. I would try increasing the idle timing from 18 two degrees at a time and evaluate. You might end up at 24 degrees like I am, and I might go even higher.



    You might try adding more vacuum advance, like 12-14 degrees. It might help your fuel mileage at cruise.

    I also had a problem with run on after turning off the key. Higher initial timing seemed to help. I also found that after many adjustments of the throttle plates, they were not even, and the rear plates were actually closed. Fuel would get trapped on top of the rear plates and cause the run on. So what I did was get the plates opened the same amount with a dial indicator and then I marked both screws at the same spot(3 o'clock in my case). Now I can adjust all I want, and as long as the marks are on the same clock position, the throttle plates are open evenly.



    Gerry

  10. #10

    Default Bad Gas Milege, Off Idle Hesitation/Stumble

    I think MSD has pointed you in the right direction here. That cam will want more idle RPM. Since you have such a tight converter, this might not be possible.

    There are pro's and con's to ported vs unported vacuum advance. Personally I say if it is a car that is driven more around town than on a highway, always use ported as it typically will assist in on again, off again throttle conditions. YMMV

    I would concentrate on timing at this point. Since we are mostly talking about the condition from idle to off idle but not WOT, I would suggest we "play" with the initial timing and initial RPM. With only a 2000 stall (*rated*) I would say not to push idle much above 800 but I would try to get closer to this number than the 675 you are at now. After changing target idle I always go back and readjust the IAC since timing and idle will effect the idle control circuit. Initial timing should be at 900 or so. I personally do not believe that the timing on a SBC will want to be as high as Gerry suggests, most small blocks like it around the 18 degree initial with a cam, some slightly higher, some lower but that is typically a good start. If your cam revs quickly it will probably like a fast timing ramp as well. Bringing it all in around 26-2800 RPM if not sooner, but that shouldn't effect the off idle so that can be done later once this flat spot is figured out.

    Assuming that these changes made no difference the only other fault would be fuel. With the performance setting for the cam, it typically will increase fuel delivery to satisfy a longer duration, with the lesser profiles it will lessen delivery curve to you should start to fall flat on fuel. I would increase pump squirt a bit, perhaps to 30 and see if there is a difference in this flat spot. A few points in either direction as you go through some tests will show you if it needs more or less.

    I know you said that you have tried all of the profiles but did you do a full system reset in between each? Without this the previously learned settings and automated tweaks are still in place which will hinder your ability to evaluate the new profile correctly.

    Your IAC of 0-5 in gear may be fine, but compensating for that tight converter may require a bit more air so the blades may want to be opened a slight bit more.

    I would start with a full reset, go back to the mild setting with some of the tweaks suggested and see if you can't work it out. If not, do another full reset and change to performance and try again.

    My gut tells me that you are fighting the load of that big blazer too early and that mouse wants more time to produce the torque it needs to get it moving.

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