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Thread: Need help finding misfire under load.

  1. #1

    Default Need help finding misfire under load.

    Hi all. I have a 1973 Mach 1 Mustang 302 auto. I recently purchased 600 CFM known as 1850s.

    The car starts and idles very well, yet out on the road, if I try to boot the accelerator, it just misfires and I can't get any power. If I back the throttle off and accelerate gently, then the car will accelerate and rev right through the range.

    I have replaced all electrics on the car, coil, leads, distributor etc. My feeling is that I'm not getting enough fuel in the carb, the 1850s has jets in the front fuel bowl and a plate in the secondary which is vacuum operated. I'm going to try installing some larger jets and have also purchased a jet conversion for vacuum secondary.

    Has anyone else experienced a misfire of this nature? Should I replace the front jets only, and leave the plate alone for now? Any advice would be greatly welcome. Thanks.

  2. #2
    Join Date
    Dec 2009
    Location
    Connecticut
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    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS TW 11R 205 heads, 11.8:1 comp, TFS R-Series intake, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 160A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/2800 RPM converter, M4602G aluminum driveshaft, FRPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 tires.

  3. #3

    Default

    Hi. Thanks for the information.

    On opening the carb, the front jets were .65 and the rears had a plate which has a jet value of .64.
    I purchased a number of jet sizes and fiddled around. I've currently got .73 in front & .75 in the vacuum secondary.
    So the misfire is completely gone, which is great.

    However, the car seems sluggish. Up to 3000 revs is really sluggish. I can't even make the wheels spin.
    However, past 3000 rpm, it almost feels like a turbo has kicked in and it really speeds up.
    Could it be the jet size in the primary is too large? Should in drop that back down?

    Any advice on jetting and symptoms to look out for would be great. Thanks.

  4. #4
    Join Date
    Dec 2009
    Location
    Connecticut
    Posts
    22,959

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    Quote Originally Posted by DrFaraquat View Post
    Could it be the jet size in the primary is too large?
    That's the first thing that came to my mind too.
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS TW 11R 205 heads, 11.8:1 comp, TFS R-Series intake, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 160A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/2800 RPM converter, M4602G aluminum driveshaft, FRPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 tires.

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