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  1. #1
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    Default Engine miss over 2000 rpm

    Hello all, I was hoping someone has experienced a problem such as mine and had some advice as to what I may have missed. My 85 Capri has a Ford motorsport 302 engine with a Holley 4 barrel 580 cfm carb and Ford motorsport T-5 transmission. The distributor is original to the car and it recently started to miss or surge when I rev over 2000 rpm. Even when I rev it up stopped, it seems to be missing. The car idles fine and when I accelerate or go WOT, there are no problems. I have an O2 analyzer on the car and it runs at 12.5 to 13-1 in this condition. The car has 10000 km's since it was rebuilt and this is a new problem. To summarize what I have done so far is:
    • Changed out to a Quick Fuel 580 carb. It ran worse and had a lean condition at cruising speed.
    • Compression test of the engine and they were all within 10% of each other. (done cold to test head gasket)
    • Checked plug condition-all were tan in appearance and none were found to be bad. Replaced with new ones as a precaution.
    • Replace distributor cap and rotor. These had some slight fouling, grounding problem but no change after replacement.
    • Replaced intake gaskets. Had to replace them when glycol entered #8 piston. Had to bring it in for repair and they did not find any other problems.
    • Replaced coil. No change found.
    • Disconnected vacuum advance while driving. No change found.
    • Timing at 12 Deg BTDC. Not sure what final timing is but the timing light is steady from idle to 2500 rpm.
    • I checked every wire with the timing light and all of them seemed to be firing with no intermittent missing. I did not test the resistance of each. These are Ford Motorsport wires if anyone cares.
    • Tested intake with propane and did not find any sign of a leak.
    • Vacuum at idle is steady at 14" HG. Ford motorsport said that was exactly what it should be with this engine combination.

    To summarize, the car runs fine at ide. runs great while accelerating, and does not miss while driving WOT. It is only when the car is in high vacuum, cruising speed that is effected.

    Any experience with this and any ideas what I can test?

    Thanks for your help.

    Patrick

  2. #2
    FEP Power Member Ethyl Cat's Avatar
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    What did the vacuum gauge do when you ran it up to 2000 rpm?

    If it starts to flutter it has a valve spring problem.
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  3. #3
    FEP Power Member Ethyl Cat's Avatar
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    Just re read and saw it ran fine at WOT. Most likely not an issue
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  4. #4
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    Thanks for the response. The vacuum was steady at 20" or so at that reading so not likely a valve spring.

  5. #5
    FEP Member 8ballEinstein's Avatar
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    You disconnected the vacuum advance and had no change? I would look into this.
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  6. #6
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    Yes I disconnected the vacuum advance and noticed very little difference
    I am going to try the other carb tomorrow
    Wish me luck

  7. #7
    FEP Super Member 84StangSVT's Avatar
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    Quote Originally Posted by 8ballEinstein View Post
    You disconnected the vacuum advance and had no change? I would look into this.
    I was thinking the same thing. I would think that at 2K, unplugging the vacuum advance should have a significant effect on it. Almost sounds like it is not advancing when vacuum is hooked up.
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  8. #8

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    This is a head-scratcher for sure. Rich (12.5-13.1 is WOT territory) with the Holley, lean with the Quick Fuel.
    What's the air:fuel ratio at idle with Holley/Quick Fuel? If it weren't for the rich Holley mixture at cruise, I'd
    take a guess at too-small primary jets, a lean misfire. Holley power valve loose/leaky? This sounds like a fuel
    issue or carburetor function or malfunction situation with it showing it's face only at high vacuum and low load.

    OR, an ignition advance issue... and being that you found disconnecting the vacuum advance didn't seem to
    change the situation much.... initial's at 12-degrees BTDC, and steady from idle to 2500rpm? Do you mean that
    it remains at 12-degrees BTDC up to 2500rpm? If that's the case, the mechanical advance is big time stiff or stuck
    and not advancing. I'd be seeing what's going on down in there... and I'd be checking the vacuum advance
    diaphram by pressing it's stem in and hold a thumb/finger over the vacuum nipple to see if it is leaking or not.

    The head-scratcher being that an all-of-the-sudden thing like this with your situation makes me think of the things
    that can, through wear and tear or age, begin leaking... like the power valve diaphram (with your rich observation),
    or the vacuum advance's diaphram... with the rich and lean observations with two different carburetors would
    make me lean more toward a fuel or fueling problem. Electric fuel pump? Fuel pressure? Float levels?

    Good luck with it. Keep us posted.
    Last edited by Walking-Tall; 09-28-2015 at 08:47 AM.

  9. #9
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    I ran the quick fuel today and again, it is lean at cruising speed, about 15:1. When I ran the quick fuel last time I ran it with jets 9 sizes larger with no change in air fuel ratio. The a/f ratio at idle is 13:1 but moves around somewhat, same for both carbs. The Holley runs better at all speeds but cruise, the quick fuel is slightly better, just not that much better to warrant keeping it in place.

    Timing is at 12 degrees and increases to what I would guess is 36 degrees. It is nice and smooth on increased speed, not jerky at all.

    The fuel pump is an Edelbrock mechanical unit and the float are right in the middle. I don't know the fuel pressure but the fuel bowl settings have not changed since I rebuilt the car a few years ago.

    I phoned quick fuel and they did not answer. I messaged them and had a very poor response. Not sure what any one here has for experience with them but I am not really pleased.

  10. #10
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    I checked the brake booster and vacuum advance with a vacuum pump. All is well.

    Stumped.

  11. #11
    FEP Power Member Ethyl Cat's Avatar
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    Steady vacuum, no spark scatter(you should find out what total is by the way) and a good afr yet it still seems to miss.....

    I would love to see a spark waveform about now. What plugs did you install and what is the gap? Have you tried closing them up some?

    Are the primary throttle blades set correctly in relation to the transfer slot?

    When I get strange things going on I always verify that the battery is good and the charging system is working properly as well as not outputting any AC volts.

    Check voltage applied to the coil during running. If it is for some reason low it might show up as weak spark. This shows up during cruise light load conditions.

    Rotor phasing good?

    Can you run a cylinder contribution test during the condition and ID a specific cylinder or is it a random misfire?
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  12. #12
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    Thanks for the response
    I installed the same plugs that I had installed two years ago with the same gap.
    The primary throttle blades were good on the Holley and unsure about the quick fuel carb.
    Do you know what voltage should be at the coil?
    As for rotor phasing and cylinder contribution tests, pretty difficult at home.
    I did notice my old distributor seemed corroded, could that be an indicator of a rotor phasing issue?
    Patrick

  13. #13

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    Quote Originally Posted by cougman67 View Post
    I ran the quick fuel today and again, it is lean at cruising speed, about 15:1. When I ran the quick fuel last time I ran it with jets 9 sizes larger with no change in air fuel ratio. The a/f ratio at idle is 13:1 but moves around somewhat, same for both carbs. The Holley runs better at all speeds but cruise, the quick fuel is slightly better, just not that much better to warrant keeping it in place.

    Timing is at 12 degrees and increases to what I would guess is 36 degrees. It is nice and smooth on increased speed, not jerky at all.

    The fuel pump is an Edelbrock mechanical unit and the float are right in the middle. I don't know the fuel pressure but the fuel bowl settings have not changed since I rebuilt the car a few years ago.

    I phoned quick fuel and they did not answer. I messaged them and had a very poor response. Not sure what any one here has for experience with them but I am not really pleased.
    Just going to think (type, lol) out loud with these facts. This really is a brain-buster...
    So the Quick Fuel's idle/low-speed is a bit lean but close for cruise, and with the engine's good strong vacuum you're probably barely pressing the accelerator down at cruise... you'd think 9 steps rich would drop the cruise air:fuel somewhat, but maybe it's very much still operating from the idle/low-speed circuit. Both carbs a bit rich at idle would simply indicate to me that you are consistently setting idle mixture screws on each, which does have an affect on cruise mixture with your good high cruise vacuum and no doubt barely opened primaries at cruise...

    For, "process of elimination", there's two mechanical processes inside the distributor, and I think I'd be looking now in that direction, in the amount of advance each is providing, because there's maximum ignition advance at cruise (and also maximum vacuum advance at idle if vacuum advance is connected to manifold vacuum)...

    I do the following with everything I've owned, "degreeing" and marking the balancer, for 10/14/34/38-degrees:
    - measure the balancer's diameter
    - do the math of (diameter times pi (3.1415926)) divided by 360 = inches per degree
    - ie. (6.00" x 3.1415926) / 360 = 0.052" per degree
    - multiply that result by the number of degrees you want to mark, such as 36-degrees
    - ie. 0.052" x 36-degrees = 1.885"
    - precisely measure (I use a dial caliper) and mark and cut a strip of paper that length
    - transfer the cut strip of paper's length with a mark from TDC onto the balancer

    With this, you can know exactly how much each is advancing, mechanical as you've already observed it,
    and vacuum advance, also by observing it by attaching the vacuum hose, temporary to manifold vacuum
    if it's normally connected to ported vacuum... because, ignition-wise, too much of either, but more often
    vacuum advance, can cause a breaking up or mis-firing at cruise, which could be causing fuel mixture
    variations...

    Sorry about your experience with that company. It's unfortunately more seemingly common to be mistreated
    by corporations today, and twisting their arm to provide proper "customer service" is all we can do.

    Again, good luck with it. This has got me curious off the hook, as I hate 'em while happening, but love 'em for
    the experience and such as solved brain-buster problems do.

    Mike
    Last edited by Walking-Tall; 09-29-2015 at 12:19 PM.

  14. #14
    FEP Power Member Ethyl Cat's Avatar
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    Good info Mike.

    I would add to it that the OP should verify that TDC balancer is TDC #1 engine before doing any more timing measurements so that we are not chasing our tail using false information.

    We do not want to assume that low miles means still good for any part at this juncture.
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  15. #15

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    Thanks very much.

    A good idea, verifying that fundamental basic, and then proceeding to check the amounts.

    True, never assume anything.

  16. #16
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    Thanks guys. Even though the engine is new from Ford Motorsports, you can never assume it is correct or the balancer did not slip.

  17. #17
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    Update
    I installed the Holley, dropped the primary jets two sizes and disconnected the vacuum advance. Most of the surging is gone....

  18. #18

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    Quote Originally Posted by cougman67 View Post
    Update
    I installed the Holley, dropped the primary jets two sizes and disconnected the vacuum advance. Most of the surging is gone....
    Hmmm... sounds like maybe there's too much centrifugal advance then....

  19. #19
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    It has been running well since.

    I will get a timing tape and check it as soon as spring comes.

    I would check it now but it's going into hibernation next week

    Patrick

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