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Thread: Series IIA 109 Rough Idle Misfire Vacuum Leak Troubleshooting

  1. #1

    Default Series IIA 109 Rough Idle Misfire Vacuum Leak Troubleshooting

    Hello Rover Community, I'm hoping you can help me out. I am new to the series world, and learning as I go. Please see the video below for current running condition, followed by an explanation.

    https://youtube.com/shorts/-w1G8o39K5E?feature=share

    I've been fostering a IIA 109 pickup for my friend Bill Simone's family since his passing 10 years ago. I get it out of the garage every now and then to give it some exercise. For some time now, the truck has been running rich at idle. Fuel mixture screw on the Zenith carburetor changed nothing. Recently, I've been motivated to try and resolve the problem to get this truck running as well as it did 10 years ago. I started with a new carburetor to try and resolve the issue. No change. Idle mixture screw makes no change to how the truck runs. I went through and set valve tappet clearances, and then double checked. I started to suspect ignition problems. The truck is fitted with a rebuilt Lucas 25D from Advanced Distributors with electronic conversation, and a Crane Cams Fireball XR700 Ignition module. I replaced the coil, wires, plugs, cap and rotor arm. Plugs get fouled up pretty quickly because of how rich the engine is running. I've been cleaning them up and checking gap every now and then during this diagnostics journey. Based on the workshop manual, since I'm running 90 octane ethanol free fuel, base timing is supposed to be set to TDC with vacuum advance disconnected, which is where I currently have it set. Compression test shows 120psi in each cylinder. Yesterday, during a test drive, engine speed was increasing while applying the brake peddle. I started to suspect a vacuum leak. This morning, I hooked up a vacuum gauge directly to the intake manifold. Very low and erratic vacuum. I sprayed carb cleaner around the intake manifold in hopes I would find a failing intake gasket. I couldn't get a jump out of the RPMs.

    I'm struggling to determine next steps for troubleshooting. Any help that can be offered would be greatly appreciated.

  2. #2
    Join Date
    Jul 2020
    Location
    UK
    Posts
    180

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    That sounds as though its running on full choke. Check that the choke isnt incorrectly set. Also its a long shot, but try running without the air cleaner hose connected to the carb. the hose can collapse inside also check the air cleaner is not too full of oil or that water hasn't got in & raised the level or other blockage

  3. #3

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    First of all is that 'ZENITH' carb a REAL ZENITH or the knockoff. [Real 1s have the name cast into the side of the float chamber.]

  4. #4

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    Quote Originally Posted by charles1943 View Post
    First of all is that 'ZENITH' carb a REAL ZENITH or the knockoff. [Real 1s have the name cast into the side of the float chamber.]
    Very much a knockoff. Ordered from Atlantic British.

  5. #5
    Join Date
    Apr 2021
    Location
    SF Bay Area
    Posts
    204

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    Sounds like you checked all the right places.

    Do you have a brake booster/servo? That runs on engine vacuum, and it might have a leak if you noticed some change in idle with brakes depressed. Try the carb cleaner trick around there...

    Also, my S3 likes it at -3 BTDC. It just runs better...higher idle RPM when fully warmed it at that setting.

    Check your choke linkages and such. They're cheap and tinny...you can bend them. Might have been dropped at the factory. You didn't mention a choke.

    Spark plug wires? Sounds like you're not getting a good burn. Good luck.
    ...┌───────┬──,,
    ...|______OD__|__\\_____
    ...d ..__ ......... |... | ..__....p
    └/ | \────┴──┴/ | \─┘≡
    ..../..@........................@

    1973 Series 3, 109

  6. #6

    Default

    Quote Originally Posted by vlad_d View Post
    Sounds like you checked all the right places.

    Do you have a brake booster/servo? That runs on engine vacuum, and it might have a leak if you noticed some change in idle with brakes depressed. Try the carb cleaner trick around there...

    Also, my S3 likes it at -3 BTDC. It just runs better...higher idle RPM when fully warmed it at that setting.

    Check your choke linkages and such. They're cheap and tinny...you can bend them. Might have been dropped at the factory. You didn't mention a choke.

    Spark plug wires? Sounds like you're not getting a good burn. Good luck.
    I do have a brake booster. I was suspect of a leak. Applied vacuum to to the brake booster, and it held just fine. I have been testing the engine with the vacuum supply line disconnected from the brake booster and intake manifold, and have the vacuum gauge in place of it.

    Since replacing the carburetor, I haven't even had the choke cable hooked up. I don't believe the choke is a factor at this point. Choke plate is straight up and down without the cable attached.

    As part of my troubleshooting process, I replaced wires.

  7. #7
    Join Date
    Dec 2010
    Posts
    269

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    vlad and p80 covered it all for you. I really thot it'd be the choke flapper.
    you could have a looksee at the sediment bowl, see if there's a bunch of air going thru it?
    a longer shot: someone missed a tooth re-installing the distributor?
    since you have a new carb and the new fangled after market ignition i'd start there. (with the new stuff that got installed)

  8. #8
    Join Date
    Dec 2010
    Posts
    269

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    keep us posted on this please? also: just how old is the gas in the tank? got any preservatives in there?

  9. #9

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    Quote Originally Posted by jimrr View Post
    vlad and p80 covered it all for you. I really thot it'd be the choke flapper.
    you could have a looksee at the sediment bowl, see if there's a bunch of air going thru it?
    a longer shot: someone missed a tooth re-installing the distributor?
    since you have a new carb and the new fangled after market ignition i'd start there. (with the new stuff that got installed)
    My plan at the moment to is triple check the valve clearance, double check the carb for float height, advance the timing, and see how things run at the point. Advance Distributors custom built this ignition setup. They have advised me to aim for 15 BTDC. I performed a quick compression test last night to document where things currently sit, and how the truck was set up during that video.

    Note, engine was not at operating temperatures for compression test.

    Cylinder 1 137 dry

    Cylinder 2 140 dry

    Cylinder 3 127 dry

    Cylinder 4 133 dry

    The fuel that is in the tank at the moment is almost entirely fresh. I filled the tank with 90 ethanol free during a test drive this past weekend.

  10. #10
    Join Date
    Dec 2010
    Posts
    269

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    these older engines will practically run on anything a match burns. what's got this running bad will likely be most obvious.
    my original engined series 3 compression is probably under 100 on any cylinder and looking at your exhaust smoke i'm inclined to concentrate on fuel mixture. let us know.

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