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Rough idle in gear

Discussion in 'Engine and Drivetrain/Mechanical' started by Rex66, Sep 9, 2017.

  1. Rex66

    Rex66 New Member

    1966 Coupe
    302 with Holley 4bbl and Edelbrock Performer RPM intake
    points distributor
    C4 auto transmission

    As the title states, my car is idling rough when I put it in gear (Reverse, Drive, 1, 2), but idles fine in Park and Neutral. It starts up immediately every time, and it drove fine, but would struggle when stopping at a stop sign/light, or even slowing down for a sharp turn - basically any time it idled in gear.

    A little History: Car ran great until the Mallory electronics started crapping out. Car would die on the road and then wouldn't start until it cooled off. I replaced the distributor (Mallory electronic w/out vacuum advance) with a points distributor from O'Reilly's. Went ahead and replaced the coil at the same time. Ran fine for a while after that.

    Checked Timing and Idle:
    I put a timing light on it, and I noticed that after the engine warmed up, my timing light stopped working. Did a little testing, and I was getting a light when I put the inductive pickup on the coil wire or spark plug wires other than #1. I replaced all of the plug wires (and the big coil wire) and the distributor cap. Now the timing light works on #1 until I put the car in Drive - then I get no light unless I bump up the idle high enough so that it stops stumbling. Here's something weird... with the trans in Drive (brakes applied firmly), the idle is rough until I get the curb idle adjustment screw to just the right spot, then the RPMs jump up and the car idles smoothly. Timing light then starts working (on #1). Turn the screw counter-clockwise 1/8 of a turn or less, and the RPMs drop significantly and the car starts to stumble. Turn it back clockwise 1/8 of a turn or less, and it smooths out, but the RPMs in Park/Neutral are much higher.

    Checked for Vacuum Leaks: I bought a vacuum gauge and used that to set the timing. I was pulling a max of 21-22" of vacuum from the big port on the back of the carb, so I dialed it back to 20" and tightened down the distributor. I think I ended up with initial timing at around 7* BTDC. The carb's vacuum advance port has 0" of vacuum at idle, but jumps up when I rev it - that checks out. I did lots of checks for vacuum leaks - pulled all of the different lines (brake booster, transmission vacuum modulator, etc.), plugged them, and tested - problem persists. I squirted WD-40 all around the vacuum lines, base of the carb, around the intake manifold gasket, and in the carb throats. I didn't detect any vacuum leaks.

    Tested Power Valve: With the car idling in Park, I turned the idle mixture screws all the way in and the car would just about die - had to back them out quickly to keep it from dying. I'm pretty sure my power valve is working. I have performed most of my tests with the idle mixture screws at 1 1/2 turns out from closed.​

    After all of this, I set the idle in Drive high enough so that it stopped stumbling and took it for a test drive. Now it stumbles under medium acceleration. Not sure where to go from here. Could it be a bad coil? Points/condensor? Carb? Fuel delivery problem?

  2. B67FSTB

    B67FSTB The NorCal dude from Belgium

    my first thought is a vacuumleak at the intake.
    Or your advance doesn't work as it should.
    my2 eurocent.
    Rex66 likes this.
  3. Rex66

    Rex66 New Member

    Thanks for the reply! I haven't been able to detect an air leak at the intake. Manifold vacuum is currently at 21(in-Hg) at idle. If I had a leak, I don't think I could get 21(in-Hg), right? I also tried spraying WD-40 and carb cleaner around everything and didn't detect a leak that way.

    Re: the vacuum advance... I checked the carb vacuum port and it was at 0(in-Hg) during idle, but would jump up when I blipped the throttle. With the engine off, I removed the distributor cap, then unplugged the vacuum advance hose form the carb port and sucked on it. I could see the vacuum advance arm moving the base plate inside the distributor. I think it checks out too.
  4. David67

    David67 Member

    You might have crud in the carb. Has the carb been rebuilt?

    Sent from my Pixel XL using Tapatalk
    Rex66 likes this.
  5. Rex66

    Rex66 New Member

    Nope, haven't rebuilt the carb. Probably wouldn't hurt to give that a shot.
  6. Rex66

    Rex66 New Member

    I decided to check the electricals last night and this morning before rebuilding the carb, and I have found something weird (at least to me)...

    The voltage at coil (+) increases with RPMs: I ran a jumper wire from the battery (+) to the coil (+) and started the car. It was a little harder to start and it idled kinda rough. With the car running, I pulled the jumper wire, and it smoothed out immediately. I figure that jumper wire was putting about 14.6V to the coil (+), which is what I measure across the battery when the engine is running. Without the jumper wire, I measure anywhere from 10.5V to 12.6V at coil (+) when the engine is running. At cold idle (choke plate closed), it's around 12.6V. At warm idle, it's around 10.5V. When I rev the engine, the voltage increases.

    My pink resistance wire is in place and not bypassed. Shouldn't that drop the voltage to around 9V at coil (+)? I have a min of 10.5V. And shouldn't the voltage be constant? Am I burning up my coil with this higher voltage?
  7. Midlife

    Midlife Well-Known Member Staff Member Moderator

    10.5V is what is expected at the coil. Voltage will rise a bit with RPMs, as the alternator supplies a higher voltage to the battery than at idle.
    Sounds like you have a coil/distributor system that is not working as it should at idle speeds. Bad points? Condenser? Coil? moisture in the dizzy cap?
    Rex66 likes this.
  8. Rex66

    Rex66 New Member

    I was just out adjusting the points - couldn't get a feeler gauge between them (at the high spot). That's not good. I set the gap to .019" because I'm not really sure what the spec is - I see anything from .017-0.21". Anyhow, it started right up and idled nicely. Shifted into Drive, and it still idled nicely at a slightly lower RPM - very happy with that - but it bogs down when stepping on the accelerator.

    Edit: distributor cap is nice and dry. I suppose it wouldn't hurt to buy a new set of points and condenser.
    Last edited: Sep 11, 2017
  9. Rex66

    Rex66 New Member

    Problem(s) solved!! :)

    After setting the points gap, I was checking the timing and everything was running great, until I re-connected the vacuum advance - then it would bog under acceleration. I removed the vacuum advance, plugged the port on the carb, and took it for a test drive. It ran great. Found that my vacuum advance canister is adjustable, so once I got that set right, the car runs great with the vacuum advance connected.

    FWIW, my distributor is a Cardone re-man from O'Reilly's. I turned the vacuum advance screw all the way clockwise and tested: stalled under throttle. Turned it all the way counter-clockwise: no issues. Ended up setting it back in a bit from there and all is good.

    I'm not sure how this thing ever ran before with a mis-adjusted points gap and vacuum advance.

    Thanks for listening and thanks for the help!
    Last edited: Sep 14, 2017
    Sacbill likes this.
  10. Midlife

    Midlife Well-Known Member Staff Member Moderator

    These cars are awfully forgiving for being out of tune. Glad you got it fixed (and cheaply!).
    Rex66 likes this.

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