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Thread: Off idle hesitation, fluctuating idle, poor Gas milage

  1. #1

    Default Off idle hesitation, fluctuating idle, poor Gas milage

    New guy here with what seems to be some common issues. I have read a lot of similar issues, but everyone has a totally different setup then mine and I am at my wits end trying to solve this. I have tried changing idle af up down, cruise af up down, IAT changes, pump squirt and Power Valve changes and nothing helps. I have waited a long time to spring for this TBI and I am feeling pretty disappointed right now. My fuel millage is just under 12MPG, the idle goes up and down when in gear, and that off idle hesitation is wearing me down to where I am about to throw my carb back on . Any help will be much appreciated. Here is my setup

    1962 Impala 2dr sports coupe

    SBC=355

    2.02 IN & 1.60 EX valves

    Cam lift= 488/495

    Dur @ 50=212/218

    Lob separation=110

    Cam setting= street/stock

    Comp=9.77

    Fuel=PWM in tank pump

    IAC=35 In gear @ Idle

    ECT=200

    IAT=119

    MAP=16.3 running @ Idle, 28.6 HG engine off

    Fuel Pressure=39

    Target Idle AF=13.8

    Target Cruise=14.6

    Target RPM @ Idle=850

    Pump Squirt=27%

    Power Valve=18%

    TH350 trans

    3:08 gears

    Weiand 8120 street warrior dual plane intake

    12" vacuum in gear/14.5" in neutral

    Using ported vacuum

    Ram horn manifolds going into true duals 2.5 exhaust with striaght through mufflers.

    I am running a standard HEI distributor set at 18 degrees Initial timing with timing control disabled.

    Learning on.

    Altitude=650' above sea level.

    Returnless fuel system

    Cruising and throttle response once past the hesitation point seems to be good, other then the terrible gas millage.

  2. #2

    Default Off idle hesitation, fluctuating idle, poor Gas milage

    Do you know what the centrifugal advance settings is for your distributor? Need to know at what RPM it starts to advance and what the total advance it applies and at what RPM. Does it also use vacuum advance? If you are not sure what is the model and part number, the info can usually be found online.



    Most motors like between 32 and 36 degrees of total advance and all in at around 2500 rpm on average. When the Atomic is controlling timing you can set these settings with the handheld.

  3. #3

    Default Off idle hesitation, fluctuating idle, poor Gas milage

    bojo wrote:

    Do you know what the centrifugal advance settings is for your distributor? Need to know at what RPM it starts to advance and what the total advance it applies and at what RPM. Does it also use vacuum advance? If you are not sure what is the model and part number, the info can usually be found online.



    Most motors like between 32 and 36 degrees of total advance and all in at around 2500 rpm on average. When the Atomic is controlling timing you can set these settings with the handheld.

    The Distributor is a cheap Skip White 6501-R vacuum and mechanical advance. I only have a generic timing light without advance, so I really can't pin those numbers down. I can have my son help me tonight using the dash tach, I might be able to at least tell you at what rpm the start and stop advance is.

  4. #4

    Default Off idle hesitation, fluctuating idle, poor Gas milage

    bojo wrote:

    Do you know what the centrifugal advance settings is for your distributor? Need to know at what RPM it starts to advance and what the total advance it applies and at what RPM. Does it also use vacuum advance? If you are not sure what is the model and part number, the info can usually be found online.



    Most motors like between 32 and 36 degrees of total advance and all in at around 2500 rpm on average. When the Atomic is controlling timing you can set these settings with the handheld.

    The Distributor is a cheap Skip White 6501-R vacuum and mechanical advance. I only have a generic timing light without advance, so I really can't pin those numbers down. I can have my son help me tonight using the dash tach, I might be able to at least tell you at what rpm the start and stop advance is.

  5. #5

    Default Off idle hesitation, fluctuating idle, poor Gas milage

    In addition to what information bojo has requested, do you happen to know what your compression is? Manual or automatic transmission? Have you adjusted the IAC at operating temp per the instructions? How many miles have you driven since installing the Atomic? When you say that it hesitates, is it like a stutter, or more like a bog and then it picks up and goes? All these things can help us direct you.

    Having your son help you find what rpm the timing stops advancing will be a great help.





    My engine is somewhat similar to yours, but I have a more aggressive cam. I run on the MILD cam setting even though my cam calls for PERFORMANCE because it didn't like the performance setting.





    Depending on the information you can provide us, and based on the info we already have, I'm thinking your engine may be happier on the STREET/STOCK cam setting (even though your cam puts you in the MILD range). If you change cam settings you'll want to do a comprehensive reset and enter all your settings again. Send us the info we requested first though and we'll assist you from there.





    This is going to take some trial and error, because every engine is different. I'm sure bojo can remember assisting me with my setup and I was beyond frustrated. I threatened to throw my carb back on it more than once, but I'm glad I didn't because now mine is running great and has much better response than when it was carbed. There are some knowledgeable people on this forum who have worked through a lot of these issues first hand. Stick with it and you'll be pleased with the results.


  6. #6

    Default Off idle hesitation, fluctuating idle, poor Gas milage

    JWZ28 wrote:

    In addition to what information bojo has requested, do you happen to know what your compression is? Manual or automatic transmission? Have you adjusted the IAC at operating temp per the instructions? How many miles have you driven since installing the Atomic? When you say that it hesitates, is it like a stutter, or more like a bog and then it picks up and goes? All these things can help us direct you.



    Having your son help you find what rpm the timing stops advancing will be a great help.





    My engine is somewhat similar to yours, but I have a more aggressive cam. I run on the MILD cam setting even though my cam calls for PERFORMANCE because it didn't like the performance setting.





    Depending on the information you can provide us, and based on the info we already have, I'm thinking your engine may be happier on the STREET/STOCK cam setting (even though your cam puts you in the MILD range). If you change cam settings you'll want to do a comprehensive reset and enter all your settings again. Send us the info we requested first though and we'll assist you from there.





    This is going to take some trial and error, because every engine is different. I'm sure bojo can remember assisting me with my setup and I was beyond frustrated. I threatened to throw my carb back on it more than once, but I'm glad I didn't because now mine is running great and has much better response than when it was carbed. There are some knowledgeable people on this forum who have worked through a lot of these issues first hand. Stick with it and you'll be pleased with the results.



    Thanks JWZ28, by the way, I used to have a 1973 Z28, Loved it.

    Changing to the street stock setting was my next move, that is one thing I have not tried yet.

    Compression= 9.77

    TH350 trans

    I have adjusted the IAC per instructions, operating temp in gear, and I used a dial indicator for the initial adjustment, so they are withing 1/8 of a turn being synced together.

    I have approx 500 miles on the system so far.

    The hesitation it bogs down intermittently and it is extremely bad when cold, so much that I have to feather the throttle to keep it moving forward, and it has backfired through the TBI once during a cold take off.

    Under very heavy exceleration, it does shutter. I was starting to think that was my transmission or rear end, but the trans is rebuilt and the eaton posi unit is new. Larry

  7. #7

    Default Off idle hesitation, fluctuating idle, poor Gas milage

    Ok a "bog" to me says the pump squirt is too much, but the stutter and backfire through the throttle body says too lean of a squirt, so we'll need to look a little further. The pump squirt does seem a little high, so you could try adjusting it down around 20-25% and see if the bogging part improves at all. The stutter/backfire seems more like a timing thing to me (it was on mine). I found that my engine wanted the timing all in faster. The EFI seems to like a faster timing ramp in general. Before we move forward lets find out what your total timing is and at what rpm it stops advancing.

  8. #8

    Default Off idle hesitation, fluctuating idle, poor Gas milage

    Yeah, I was thinking its timing related. I looked at the Skip White site for specs but didn't see them. Sent them mail asking what the advance specs are so I'll post them if they respond.

  9. #9

    Default Off idle hesitation, fluctuating idle, poor Gas milage

    I am going to pick up a timing tape for the balancer in the morning so I can be a little more accurate as to total timing. I did do a reset and set the cam to street/strip and pv-25% and PS-19. The idle seemed better but it still has the same hesitation at around 1100 rpm. But that was just a short drive and no chance to learn. But throttle response and cruising all seemed fine there so I will stay with that and let you know the other numbers tomorrow once I get the timing tape. What I can tell you is that the hesitation is around 1100 rpm, and the engine starts running a bit rough at that rpm, and as soon as the timing starts advancing, around 1500 rpm is when it smooths out and picks back up.

  10. #10

    Default Off idle hesitation, fluctuating idle, poor Gas milage

    bojo wrote:
    Yeah, I was thinking its timing related. I looked at the Skip White site for specs but didn't see them. Sent them mail asking what the advance specs are so I'll post them if they respond.

    Thanks bojo!

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