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  1. #1

    Default engine breaking up @1800

    My engine has started breaking up at 1800 rpms, and has progressively gotten worse. I changed the carb from 4180C to 650DP. No change. New plug wires, no change. New MSD coil, no change. New fuel pump, no change. Now the sun tach doesn't register any RPM's until it gets past 1200 RPMs. No reading (0) at idle. UGH! I am very frustrated and cannot figure out what the problem is. Please help! The car is an 83 GT. 0.30 over exploder block, GT 40 heads with upgraded springs. TFS 1 cam, stealth. The engine was running good for about 500 miles, then it started with this gremlin. Thank you,
    Mike
    79 Coupe, 357W balanced, AFR 185s, Vic Jr, Holley 750, C4 roller, 4.56 8.8 spool, 26 MT Drags, 11.00 @ 123
    83 Mustang GT, 88 GT engine, T5

  2. #2

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    What kind of ignition box do you have? MSD , factory? That might another good place to ck?

  3. #3
    Moderator wraithracing's Avatar
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    I agree, it sounds like an ignition issue. Either a power issue or something is failing in the distributor or ignition module.
    ​Trey

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  4. #4

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    I am using the duraspark module. I had it tested and its a go. Dizzy is an 85 duraspark (steel gear) I bought at advance about 5 years ago.
    79 Coupe, 357W balanced, AFR 185s, Vic Jr, Holley 750, C4 roller, 4.56 8.8 spool, 26 MT Drags, 11.00 @ 123
    83 Mustang GT, 88 GT engine, T5

  5. #5
    FEP Senior Member gt4494's Avatar
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    I've had new plugs and wires go bad when new.

    Do you have an infrared thermometer? You can see which exhaust is cold or not as hot at the rest and start chasing the problem from there.
    "If you can't explain it simply, you don't understand it well enough."
    Albert Einstein

    1984 20th Anniversary GT350
    Almost "Stock"

  6. #6

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    Thanks, I got my spray bottle and sprayed water on each exhaust port on the header. All go right to steam except #5. Water slowly evaporated. I bought a new plug. No change. Changed that wire, no change. It has to be that cylinder, Now what? Thanks for the replies!
    79 Coupe, 357W balanced, AFR 185s, Vic Jr, Holley 750, C4 roller, 4.56 8.8 spool, 26 MT Drags, 11.00 @ 123
    83 Mustang GT, 88 GT engine, T5

  7. #7

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    Quote Originally Posted by 79stang351w View Post
    Thanks, I got my spray bottle and sprayed water on each exhaust port on the header. All go right to steam except #5. Water slowly evaporated. I bought a new plug. No change. Changed that wire, no change. It has to be that cylinder, Now what? Thanks for the replies!
    Compression test

  8. #8
    FEP Senior Member gt4494's Avatar
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    don't forget to check the Dist. Cap and rotor. They can get cracks or get carbon shortscers..

    Rule out all the easy stuff first!
    "If you can't explain it simply, you don't understand it well enough."
    Albert Einstein

    1984 20th Anniversary GT350
    Almost "Stock"

  9. #9

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    compression test yielded 130 psi on #5. tested another cap and rotor. no change......Grrrr
    79 Coupe, 357W balanced, AFR 185s, Vic Jr, Holley 750, C4 roller, 4.56 8.8 spool, 26 MT Drags, 11.00 @ 123
    83 Mustang GT, 88 GT engine, T5

  10. #10

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    Quote Originally Posted by 79stang351w View Post
    compression test yielded 130 psi on #5. tested another cap and rotor. no change......Grrrr
    What are the readings for the rest of the cylinders?

    No tach reading at idle is weird.

    Base timing steady? Is that plug sparking?
    83 TC "Clone"
    85 Marquis LTS

  11. #11

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    I put in my “tested” ignition module and the tach started working again
    79 Coupe, 357W balanced, AFR 185s, Vic Jr, Holley 750, C4 roller, 4.56 8.8 spool, 26 MT Drags, 11.00 @ 123
    83 Mustang GT, 88 GT engine, T5

  12. #12

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    I would also check the connections on the multipin connector that resides by the ignition module. Going from memory and assuming 83 is very similar to my 80, its a 10 or 12 pin rectangular connector. I had some bad connections there give me trouble once (frayed, dirty and/or loose etc)

    Also going from memory I believe the green - and red + coil wires run through that multi-connector (along with oil sender and temp sender etc) before going to the round coil connector, as well as the ignition module green coil - trigger ties in and tees off to the dash for tach in that area. A bad connection somewhere like that could explain some issues and why the tach is not working consistently either.

    Hope that helps.

  13. #13
    FEP Super Member bwguardian's Avatar
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    Quote Originally Posted by at_the_junkyard View Post
    I would also check the connections on the multipin connector that resides by the ignition module. Going from memory and assuming 83 is very similar to my 80, its a 10 or 12 pin rectangular connector. I had some bad connections there give me trouble once (frayed, dirty and/or loose etc)

    Also going from memory I believe the green - and red + coil wires run through that multi-connector (along with oil sender and temp sender etc) before going to the round coil connector, as well as the ignition module green coil - trigger ties in and tees off to the dash for tach in that area. A bad connection somewhere like that could explain some issues and why the tach is not working consistently either.

    Hope that helps.
    ^^^...this, also look for burnt insulation on those wires exposing them...common problem. Wires may be grounding out touching. You can get replacement pigtails to splice in from NAPA if need be.
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  14. #14

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    UPDATE: I put my 4180C back on and it runs sooo much better than with the 650DP. I've got power again. I had voltage going to #5 and all the cylinders. The 650 must be ready for a rebuild. Anyway, the issue now is that after 1900 rpms, it stumbles and bucks until it gets to about 2500 rpms, then its running strong. So, I don't know about this issue. Any ideas? Thank you!
    Mike
    79 Coupe, 357W balanced, AFR 185s, Vic Jr, Holley 750, C4 roller, 4.56 8.8 spool, 26 MT Drags, 11.00 @ 123
    83 Mustang GT, 88 GT engine, T5

  15. #15
    FEP Senior Member
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    What did the plugs look like? Rich/Lean? If it revs to redline, then it's not an ignition problem, but a carb problem. Blown power valve comes to mind for the stumble, then clear up.

  16. #16
    FEP Senior Member 854vragtop's Avatar
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    Quote Originally Posted by 79stang351w View Post
    The 650 must be ready for a rebuild.
    Sooooo many time over the years of owning my '85 Mustang and '68 Cougars, simply rebuilding the carb (all Holley's) solved my rough running, hard starting issues. Once you've rebuilt one, it's so easy to do. Cleaning gasket material residue is the most time consuming part. A Sharpie, baggies, and masking tape to mark and bag and tag parts is the key. (BTW, this comment is really directed to those who've never rebuilt a carb).
    '85 Mustang convertible GT, 5 speed, 4V
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