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  1. #1
    FEP Super Member 84StangSVT's Avatar
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    Default Bucking Insanity At Part Throttle

    Guys,
    I'm about to my wits end trying to figure out this issue and now I'm looking for a fresh set of ideas.

    Here's a list of things that have been done to the car, which never displayed this behavior before the changes:
    1) Installation of FAST EZ-EFI system to replace Edelbrock 1406 carb. This includes fuel tank and fuel pump out of a later model Fox and vacuum actuated fuel pressure regulator set at 43 psi per manufacturers spec.
    2) New pickup coil to replace factory unit that had the insulation coming off the wiring. Tested new pickup coil before install and it OHM'd within factory specs. Cleaned and lubed distributor shaft upon install as the old coil was seized onto the shaft effectively making it not advance.
    3) Replaced all vacuum lines and block off caps. Replaced these one at a time to ensure that I did not get them crossed up.
    4) Checked distributor for play and slop. It appeared fairly tight.
    5) Closed the mechanical advance window on arm, as it was noted that the current arm had way to much advance if it actually advanced that far. Reused springs that were in the distributor.
    6) Checked vacuum advance operation to the best of my available tools. It actuated upon application of vacuum and held until release of vacuum.
    7) Rewired certain portions of the engine bay due to insulation wiring falling off. Eliminated wiring that was no longer used.
    8 ) Fixed all exhaust leaks on car. Needed to happen with the application of the O2 sensor I now need.

    I have worked a lot with the A/F ratios and accel shot values with the EFI system and I have made the driveability much better, but I still get the bucking during light roll on throttle at cruise. Engine load doesn't seem to make much difference as it does it going up hills, down hills and on flat land. During the bucking, if I hop of the throttle and roll back in, sometimes it goes away and sometimes it comes back. If I stab the throttle hard, it might cough once and then takes off like a rocket.

    I pulled the plugs yesterday to replace them with new Motorcraft plugs and the plugs looked ok for the most part. They weren't white like they were burning too lean, but they weren't really tan like I'm used to. They also weren't oily or wet.

    I set the timing awhile back and I don't have my notes with me, but I believe it is around 12 degrees initial.

    I have not changed the plug wires, which were new as of last year, Ford Racing wires. I also have not changed cap and rotor which look decent not have I changed the replaced in the past aftermarket ignition module.

    One thing to note is this car has in the past went through a smog delete so there is no air pump, canister purge or egr system. It ran ok with the carb though.

    I know this is a fairly vague description, but I'm hoping to get a fresh direction on which way to go, I have looked at this issue so much that I'm beginning to think I have overlooked something obvious.
    Brock
    1984 Mustang LX Convertible 3.8L V-6/Auto (SOLD)
    1984 Mustang GT Hatchback 5.0 V-8/5 Speed

    I'm an FEP Supporter and proud of it. Are you?

  2. #2
    FEP Super Member erratic50's Avatar
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    Default

    I know you are on fast EFI vs factory stuff, but with my factory SD setup I had wonky signaling from a throttle position sensor cause something very similar on my 86.

    My other suggestion is if any provisions for EGR are still there block them at the source with sheetmetal and gaskets and see if it makes a difference.

    Can you rig up a camera to see what advance is showing on the timing marks while it's throwing fits?


    just a few ideas.

  3. #3

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    Perhaps the new pickup coil is wired backwards? I've seen replacements that have the wire colors reversed. This would throw off the dwell and timing but also the rotor phase in relation to the cap which could cause crossfire.

    If you have an analog multimeter then there is a simple way to check the pickup polarity. Hook the negative lead of the multimeter to the orange wire and the positive lead to the purple wire. Put the meter on the lowest DC volts setting. Stick a screwdriver (or anything steel) to the magnetic pole of the pickup. The needle should jump slightly positive (to the right) when the screwdriver sticks to the pickup. When you pull the screw driver away the needle should move left (negative). You have to watch it closely because the movements will be very small. It will jump and fall back to zero. This is why you need to use an analog meter, digital ones won't react fast enough to see it.

  4. #4
    FEP Super Member 84StangSVT's Avatar
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    Quote Originally Posted by mrriggs View Post
    Perhaps the new pickup coil is wired backwards? I've seen replacements that have the wire colors reversed. This would throw off the dwell and timing but also the rotor phase in relation to the cap which could cause crossfire.

    If you have an analog multimeter then there is a simple way to check the pickup polarity. Hook the negative lead of the multimeter to the orange wire and the positive lead to the purple wire. Put the meter on the lowest DC volts setting. Stick a screwdriver (or anything steel) to the magnetic pole of the pickup. The needle should jump slightly positive (to the right) when the screwdriver sticks to the pickup. When you pull the screw driver away the needle should move left (negative). You have to watch it closely because the movements will be very small. It will jump and fall back to zero. This is why you need to use an analog meter, digital ones won't react fast enough to see it.
    I do have an analog meter, so I will definitely check that. Thanks for the idea. This is driving me nuts!
    Brock
    1984 Mustang LX Convertible 3.8L V-6/Auto (SOLD)
    1984 Mustang GT Hatchback 5.0 V-8/5 Speed

    I'm an FEP Supporter and proud of it. Are you?

  5. #5
    FEP Super Member 84StangSVT's Avatar
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    Quote Originally Posted by erratic50 View Post
    I know you are on fast EFI vs factory stuff, but with my factory SD setup I had wonky signaling from a throttle position sensor cause something very similar on my 86.

    My other suggestion is if any provisions for EGR are still there block them at the source with sheetmetal and gaskets and see if it makes a difference.

    Can you rig up a camera to see what advance is showing on the timing marks while it's throwing fits?


    just a few ideas.
    Ive checked the TPS function and it appears to be fairly smooth and linear. Don't mean much I guess as I have already had a MAP sensor die. I will revisit it.

    Everything for the EGR is gone, including the spacer so I'm sure that is not the issue.
    Brock
    1984 Mustang LX Convertible 3.8L V-6/Auto (SOLD)
    1984 Mustang GT Hatchback 5.0 V-8/5 Speed

    I'm an FEP Supporter and proud of it. Are you?

  6. #6
    FEP Member brianj's Avatar
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    Default

    It sounds like not enough fuel at low rpm or light throttle position.
    1983 Mustang G.T. No-option stripper- I like strippers.
    5.0, GT40P heads, Comp Cams XE270HR-12 on 1.6 rockers, TFI spring kit, Weiand 174 blower, Holley 750 mechanical secondarys, Mishimoto radiator, Edelbrock street performer mechanical pump, BBK shortys, T-5 conversion, 8.8 rear, 3.73 gears, carbon fiber clutches, SS Machine lowers, Maximum Motorsport XL subframes, "B" springs.

  7. #7

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    Hey Brock,
    What's the AFR in the bucking situations? Sure sounds alot like the "stupid lean" point in carburetor idle/transition super-tuning when the too-big-for-this-combination primary idle air bleed is found... to which a slightly smaller (0.002-0.003") primary idle air bleed is then installed for slightly richer lean-best, no more bucking/surging, acceleration and MPG perfection.... keep at it
    Mike
    1986 Mustang convertible ---> BUILD THREAD
    Past Fox-chassis "four eyes":
    1983 Mercury Cougar LS
    1986 Ford Thunderbird ELAN
    1980 Capri RS Turbo

    Work in progress website ---> http://carb-rebuilds-plus.boards.net/

  8. #8
    FEP Super Member 84StangSVT's Avatar
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    Quote Originally Posted by Walking-Tall View Post
    Hey Brock,
    What's the AFR in the bucking situations? Sure sounds alot like the "stupid lean" point in carburetor idle/transition super-tuning when the too-big-for-this-combination primary idle air bleed is found... to which a slightly smaller (0.002-0.003") primary idle air bleed is then installed for slightly richer lean-best, no more bucking/surging, acceleration and MPG perfection.... keep at it
    It's hard to tell sometimes as I'm usually not watching the handheld. When I have watched it, it rapidly fluctuates between 11 and 15 but I'm scared to see what it does if I hold it in the buck zone. I'm not sure if it goes rich or lean first honestly.
    Brock
    1984 Mustang LX Convertible 3.8L V-6/Auto (SOLD)
    1984 Mustang GT Hatchback 5.0 V-8/5 Speed

    I'm an FEP Supporter and proud of it. Are you?

  9. #9
    FEP Power Member Ethyl Cat's Avatar
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    Did you build the VE table? Maybe you have a cell messed up in the table.
    BBD PERFORMANCE
    HIGH PERFORMANCE PARTS
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    1983 CRIMSON CAT OWNER

  10. #10

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    Is it popping back up the intake (lean) or out the exhaust (rich)? Or is it just misfiring? I have battled ignition misses that are best found with a spray bottle of salt water (a tablespoon in a Windex bottle full of water). And here is the key... do it at night when its dark.wet all your ignition cables and coil. If your spark is going down the outside of your wires ( which actually looks pretty neat), or going to ground or the wrong wire. Bingo bango Bob's your uncle. I have had brand new out of the box Delco wires that are no good. Everything is made in China anymore

  11. #11
    FEP Super Member 84StangSVT's Avatar
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    Quote Originally Posted by Ethyl Cat View Post
    Did you build the VE table? Maybe you have a cell messed up in the table.
    Being the FAST ez-EFI, there is very little I can do with the mapping. I can change the A/F target ratios for idle, cruse and WOT and I can change the accel fuel fom -8 to +8. Everything else is preloaded and I don't think it can be modified. It is dependent on MAP, TPS and fuel pressure settings to determine its internal "adaptive learning" mapping.
    Brock
    1984 Mustang LX Convertible 3.8L V-6/Auto (SOLD)
    1984 Mustang GT Hatchback 5.0 V-8/5 Speed

    I'm an FEP Supporter and proud of it. Are you?

  12. #12
    FEP Super Member 84StangSVT's Avatar
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    Quote Originally Posted by 2nd chance cobra View Post
    Is it popping back up the intake (lean) or out the exhaust (rich)? Or is it just misfiring? I have battled ignition misses that are best found with a spray bottle of salt water (a tablespoon in a Windex bottle full of water). And here is the key... do it at night when its dark.wet all your ignition cables and coil. If your spark is going down the outside of your wires ( which actually looks pretty neat), or going to ground or the wrong wire. Bingo bango Bob's your uncle. I have had brand new out of the box Delco wires that are no good. Everything is made in China anymore
    Good question. It sounds like a mixture of both honestly. It only seems to do it at part throttle, under load. Idle is good, basic cruise seems good and I can punch it and it is good. When its having its episodes, if I hop off the throttle or punch it, it clears up and runs good.
    Brock
    1984 Mustang LX Convertible 3.8L V-6/Auto (SOLD)
    1984 Mustang GT Hatchback 5.0 V-8/5 Speed

    I'm an FEP Supporter and proud of it. Are you?

  13. #13
    FEP Super Member 84StangSVT's Avatar
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    Quote Originally Posted by Walking-Tall View Post
    Hey Brock,
    What's the AFR in the bucking situations? Sure sounds alot like the "stupid lean" point in carburetor idle/transition super-tuning when the too-big-for-this-combination primary idle air bleed is found... to which a slightly smaller (0.002-0.003") primary idle air bleed is then installed for slightly richer lean-best, no more bucking/surging, acceleration and MPG perfection.... keep at it
    I retested this last night and here's what the A/F did. I intentionally found the spot where she likes to buck which yesterday was about 2000 rpm, about 1/2 throttle and under load in 3rd gear or higher. When this happened, I was watching the handheld and held it in the buck zone, longer than I really wanted. From right before the buck the A/F ratio rapidly dropped from 13.7 to 11.2 and held. It appeared the buck really started around the 12.6 area.
    Brock
    1984 Mustang LX Convertible 3.8L V-6/Auto (SOLD)
    1984 Mustang GT Hatchback 5.0 V-8/5 Speed

    I'm an FEP Supporter and proud of it. Are you?

  14. #14

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    Try the salt water. its cheap and easy

  15. #15
    FEP Super Member 84StangSVT's Avatar
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    Quote Originally Posted by 2nd chance cobra View Post
    Try the salt water. its cheap and easy
    I will try that this evening.
    Brock
    1984 Mustang LX Convertible 3.8L V-6/Auto (SOLD)
    1984 Mustang GT Hatchback 5.0 V-8/5 Speed

    I'm an FEP Supporter and proud of it. Are you?

  16. #16
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    Every now and then, simply changing plug gap can be a fix. Maybe try and increase it a bit. If you get no change, put it back to where it was. Easy and free!
    '85 GT

  17. #17
    FEP Super Member 84StangSVT's Avatar
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    Dumb question. Is it possible that the vacuum advance system is not working properly/smoothly causing the issue? There is so much conflicting information on whether to run it on ported vs manifold vacuum it's ridiculous. Maybe I should try it on a different vacuum source?
    Last edited by 84StangSVT; 08-16-2017 at 11:58 AM.
    Brock
    1984 Mustang LX Convertible 3.8L V-6/Auto (SOLD)
    1984 Mustang GT Hatchback 5.0 V-8/5 Speed

    I'm an FEP Supporter and proud of it. Are you?

  18. #18
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    Quote Originally Posted by 84StangSVT View Post
    Maybe I should try it on a different vacuum source?
    Also, you could try things out with it disconnected.

    Do they even recommend using vac advance with that system?
    '85 GT

  19. #19

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    Quote Originally Posted by 84StangSVT View Post
    Dumb question. Is it possible that the vacuum advance system is not working properly/smoothly causing the issue? There is so much conflicting information on whether to run it on ported vs manifold vacuum it's ridiculous. Maybe I should try it on a different vacuum source?
    The vacuum source has little to do with how it runs going down the road. The only time the advance differs between ported and manifold vacuum is when the throttle is closed.

    If the vacuum advance is over advancing then it could cause it to run poorly at part throttle. Does the bucking stop if you disconnect the vacuum advance?

    The vacuum advance does alter the rotor phase in relation to the cap. If you have a rotor phase issue then adding vacuum advance could exacerbate it. You may "solve" the problem by reducing the vacuum advance but that's only fixing the symptom, not the cause.

  20. #20
    FEP Super Member erratic50's Avatar
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    Where did your ignition parts come from? Refresh my memory - what exactly are you running? Any way to test each part independently of one another?

    My vote is rig a camera and a timing light and see where advance is relative to the marks when she's unhappy. Perfect use case for a chassis dyno -- unfortunately.

  21. #21
    FEP Super Member erratic50's Avatar
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    Does the fast use a factory style MAP sensor? You running with or without vacuum line installed? What does fast call for?

  22. #22

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    I am with erractic on this one. You will definitely save time and POSSIBLY money by strapping this thing down on a dyno and seeing what the timing actually IS as opposed to what it is set at. They aren't just for a high HP race engine. Balancer delamination, timing chain stretch or jumped a tooth can dramatically throw things out of whack. That is the only way to check the actual timing that I know of unless you have a friend willing to ride on the fender going down the road.
    Ford Racing wires are without a doubt the worst plug wires in the known universe IMO. Good luck. The solution IS out there, we just gotta pin it down.
    Last edited by homer302; 08-17-2017 at 10:30 AM.

  23. #23

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    Quote Originally Posted by 84StangSVT View Post
    I retested this last night and here's what the A/F did. I intentionally found the spot where she likes to buck which yesterday was about 2000 rpm, about 1/2 throttle and under load in 3rd gear or higher. When this happened, I was watching the handheld and held it in the buck zone, longer than I really wanted. From right before the buck the A/F ratio rapidly dropped from 13.7 to 11.2 and held. It appeared the buck really started around the 12.6 area.
    This isn't making alot of sense. For all intents and purposes, 1/2 throttle might as well be full throttle, AFR-requirement-wise. 12.5:1 in that scenario should cut the mustard. That's well past the idle or transition circuits, or even the equivalent to an accelerator pump's fuel contribution. 11.2:1 shouldn't usually cause rich misfiring... maybe... I say look at how much mechanical advance is happening at 2000rpm, and also how much the vacuum advance is adding then. Opening the throttle up to 2000rpm in Neutral should closely mimic your scenario, so with a timing light connected, see where the timing is at, both without and with the vacuum advance connected...
    Mike
    1986 Mustang convertible ---> BUILD THREAD
    Past Fox-chassis "four eyes":
    1983 Mercury Cougar LS
    1986 Ford Thunderbird ELAN
    1980 Capri RS Turbo

    Work in progress website ---> http://carb-rebuilds-plus.boards.net/

  24. #24
    FEP Super Member 84StangSVT's Avatar
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    Ok, here's what I saw this a.m........sorry, I have been tied up getting the oldest boy off to college.

    Idle at 700, car is showing 17" hg on manifold vacuum, port vacuum 0"
    Unhook advance and set base at 10 degrees. Hook up vacuum to port and no change. Hook up to manifold vacuum and it takes it up to 22 degrees at idle.
    With the advance hooked up and applying rpm to 2000, it goes to 36+ as it goes to the end of the factory scale on the balancer. With vacuum disconnected, it goes to 22 degrees at 2000.

    Anything higher in the rpm scale with the advance unhooked does not show any difference in timing. Up to 4500 rpm, it still shows 22 or so degrees of timing.
    Brock
    1984 Mustang LX Convertible 3.8L V-6/Auto (SOLD)
    1984 Mustang GT Hatchback 5.0 V-8/5 Speed

    I'm an FEP Supporter and proud of it. Are you?

  25. #25
    FEP Super Member erratic50's Avatar
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    Generally 36 - 38 degrees total advance about right. Factors like swirl do a lot to influence what is sufficient and what is too much.

    i would retard base until your overall timing advance is sane. Then you know what to play with from there. Limit max then bump up base, rinse repeat until you get to decent initial advance and total advance numbers. You might have to play with weights and springs or add a small canister (to even out pressure changes) or a valve (to flow restrict) on the advance line just to make it happy in transitions.

    seems like the timing is changing faster than the EFI is able to react.

    You could always declare this dizzy faulty and try something else too.
    Last edited by erratic50; 08-18-2017 at 11:36 AM.

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