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My Mustang is missing!

Discussion in 'Engine and Drivetrain/Mechanical' started by RagTop, Jun 8, 2017.

  1. RagTop

    RagTop Old Grumpy

    It's a 625cfm Road Demon 4V.
  2. Mach1 Driver

    Mach1 Driver Active Member

    You said your tach just came back to life? If it's a stock tach, it is in the ignition circuit- the path is from ignition switch terminal C, THROUGH the tach, through the resistor wire, and to the coil. The tach could be your problem; you may want to bypass it to test. Look at the attached pdf

    Attached Files:

  3. RagTop

    RagTop Old Grumpy

    The tach is an after market cheapie that is installed on the steering column. After further experience it looks like it has not recovered. I decided to change out the old Flamethrower on the outside chance that the old one was breaking down in the heat, and I discovered that the bolt hole in the GT40P cylinder head was stripped. Couldn't get the damn thing to tighten again. That took some drilling and a helicoil kit to remount my ignition coil. I'm pleased to announce that the coil bolt now runs in the hole but the new coil did not clear up my miss. It did push it back up to 4K rpm (if you believe the tach) where I started this whole thing six months ago. I'm afraid Horseplay might be right and I'll have to install a new Ignitor, but this process of elimination is getting kind of annoying if it doesn't cure the problem. Maybe I'll run a compression check tomorrow and look at the condition of the new plugs and wires while I think about other measures. A new rotor and cap perhaps?
  4. 3175375

    3175375 Well-Known Member

    take the tach out of the equation n see what happens.

    Dizzy cap n rotor are inexpensive items to replace.
    Grabber70Mach likes this.
  5. Grabber70Mach

    Grabber70Mach Well-Known Member

    I agree eliminate the tach, I've experienced chafed wires with my first 69 would occasionally kill the car. Took awhile to track down but once found it had no more issues.
  6. RagTop

    RagTop Old Grumpy

    OK, I'm convinced! I'm heading to the garage with my 3/8" ignition wrench as I write this. I'll let you know how it comes out or if I strip anything.
  7. RagTop

    RagTop Old Grumpy

    The miss is there with and without the tach connected. I just replaced the distributor cap and rotor and the miss seems to have gotten a little worse at 3K rpm and up. This must be a fuel problem. It's hard to find anyone who can work on a car with a carb anymore.
  8. 3175375

    3175375 Well-Known Member

    I will work on it
  9. Mach1Rider

    Mach1Rider Member

    Look at the breaker plate in the dist for worn bushings or sticking/restrictive movement.
    From your description of your problem it may not be carb related.
  10. RagTop

    RagTop Old Grumpy

    I'm taking the car to a known professional Mustang expert a week from today. I had been avoiding that because his shop is an hour's drive from my home, but I suckered a buddy into driving over and bringing me back while I leave the Mustang until the problem is solved. I was talking to the mechanic last night at a cruise in, and he was opining that the distributor could be my problem. The disti that is in the car is a '68 J code unit that was custom curved to my needs back about 12 years ago. My original stock F code disti was pretty much worn out in '05 when I rebuilt the engine. I sent it to a specialist to be rebuilt and he offered me the J code unit that he had on his shelf in exchange. At the time I jumped at the offer. Maybe the springs are shot or the weights are sticky after 50K miles of driving. I hope that's the problem rather than some deeper engine mechanical issue. He said he'd run a compression test on the engine as well in case I might have a burned valve or weak valve spring. I'm just sorry that I pissed away $442 with my local mechanic because I wanted to believe he could find and fix the problem. I'll let you guys know what the on-deck hitter comes up with after his time at the plate.

    BTW, the overdrive pulley on my water pump seems to have completely cured my problems with overheating in hot weather. I was sitting waiting for a green arrow last evening in 99* ambient and the temp gauge never really moved. And I'm running without a shroud. Finally one problem solved!
    Last edited: Jun 29, 2017
  11. 3175375

    3175375 Well-Known Member

    Got my pulley n going to do some wrenching to get it installed today.

    What better way to spend Independence weekend than wrenchin'
  12. RagTop

    RagTop Old Grumpy

    So, did you go with the polished finish or the anodized black finish?
  13. 3175375

    3175375 Well-Known Member

    polished finish. I need to countersink the 4 bolt holes to get clearances squared away with the electric fan and other goodies....
  14. RagTop

    RagTop Old Grumpy

    So, once more into the breech. My Mustang is now with the doctor, a guy in my Mustang club who is pretty much the expert in all things Mustang and '69 Mustangs in particular. He pulled the plugs that I had installed about six months back and found that the gaps were all around .048. Because my heads are GT40Ps and they use the smaller long reach plug, I had used them right out of the box. He cranked them down to .035. He also ran a compression test and found that six of the eight cylinders showed 152 - 155 lbs. The rear two on the passenger side were 145 and 147 respectively. Certainly within acceptable range of variation, but the plugs coming out of those two cylinders were "blistered" per his description. He said it could be a sign of moisture in the cylinders. We both started thinking head gaskets, but my radiator is never down and there's no milkshake in the crankcase. He test drove the car with the re-gapped plugs and decided to have a look at my carb. It is a 625 cfm Road Demon and, I believe, uses the same jets as a Holley. He felt the car was showing signs of running too lean and was starving out at high speed under load. He removed the #74 jets that came in the Demon and replaced them with #76. The high speed miss began to disappear, only showing up at around 75 mph (or 3,700 rpm). He went up to #78 jets and he said he can get it all the way up to 90 mph before there is any sign of stumbling. He is installing #79 jets, replaced the safety valve (the little goody that blows out on Holleys all the time on backfires), and generally cleaned up the Demon. He's going to run another compression check and replace the plugs with new ones. He also mentioned that he was very surprised at the way my little pastel yellow gramma convertible pulled in low end acceleration, and said that he was hoping that the larger jets wouldn't erode that low end pop too severely. The jury is still out on why those two rear cylinders were slightly lower in compression than the rest. I'm taking the car to Hot August Nights in Reno, NV, which is 100 miles of up hill from where I live, and then I'll have another look at those two rear plugs when I return.
    Last edited: Jul 8, 2017
    Grabber70Mach likes this.
  15. 3175375

    3175375 Well-Known Member

    Great to hear things are clearing up!

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