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Thread: Cold Start Problems and Backfire from Weber Carb

  1. #1

    Default Cold Start Problems and Backfire from Weber Carb

    85 CJ7
    Nutter Bypass http://www.off-road.com/jeep/jnutter1.htm
    TFI Upgrade http://www.jeepz.com/modules.php?op=...ile=engine_tfi
    Weber Carb
    New 258

    Problems:

    Hard to start in morning. Turns over and dies right away. If I hit gas after it turns over it dies

    Backfire / breakup from carb under heavy acceleration

    Losing manifold vacuum under heavy acceleration


    Checked the following

    Cold start issues:

    Choke is set right

    Backfire issues:

    Thought it was a timing issue - Timing OK - set to 10 BTC
    All hoses look OK
    Vacuum advance is holding vacuum

    Other info:
    Ran fine with all mods (NBP, TFI, Weber) and old motor.
    Swapped motors and have not been able to figure out the problem.

    What could be causing the problem??

    Thanks,
    -David

  2. #2
    Forum Leader John Strenk's Avatar
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    What was transfered from the old to the new motor ? Intake and exhaust? Distributor?

    It's OK to loose vacuum under hard acceleration. In fact it's normal.

    Did you happen to rev the motor up when you have to timing light set up to see if the advance is working properly.

    What is your vacuum at idle? This almost sounds like a vacuum leak.

    Did you check the compression? Just to double check their work.

    Did you check the ignition module?

  3. #3

    Default

    Transferred from old motor to new:
    Intake and Exhaust - no
    Distributor - yes

    Yes, rev'ed motor up with timing light setup, advance is working properly

    Will check vacuum at idle and post reply

    Did not check compression - will do and post reply

    Did not check ignition module - will do and post reply

    Also, losing ported vacuum under heavy acceleration - did not mention this in first post.

    Any other thoughts or things to try while I am at it?

    Thanks,
    -David

  4. #4
    Forum Leader John Strenk's Avatar
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    -=- originally posted by DavidGallagher -=-
    ......

    Also, losing ported vacuum under heavy acceleration - did not mention this in first post.

    Any other thoughts or things to try while I am at it?

    Thanks,
    -David
    This is how it's suppose to work anyway. The distributor is suppost to retard a little under heavy acceration. An engine needs a certian amount of advance depending upon RPM but further advance or retard of the timing at a particular RPM is dependent upon the load on the engine. This is easly accomplished by using manifold vacuum. High load on the engine = low vacuum = Retarded timing. Little load on engine = high vacuum = advance timing.

    Now when I was first learning about carbs and timing I was under the impression that the "ported" vacuum was pulled right at the narrowest part of the venturi. The vacuum at this point would be dependent upon the aount of air rushing thru this choke point so the faster the air was drawen thru the higher the vacuum. I actually have a Carter BBD with a vacuum port in this position but what it was connected to I don't know at this time.



    Now fast forward to the late 70's and a "ported" vacuum was picked up right above the throttle plates and there would be no vacuum at idle but would match the manifold vacuum after the throttle was opened.

    here is a picture of the "S" or "ported" vacuum pick up in a BBD. I had to hold the throtle open in order to see it but you can see the "Ring" around the bore were the throttle plate rusted.


    If you installed an 83 or later 258, I would check the electric manifold heater for vacuum leaks also. It's between the intake and exhaust manifold underneath were the carb sits.

    You know, it could be starving for gas also, Check your fuel filter and fuel pump. Somone mentioned about a leak in a weber carb that can run it dry in a few hours.

  5. #5
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    Mine did that once well not exactly but very similar and it was a bad intake manifold gasket. You could hit the gas and it would just sputter and die and made it very difficult to start. Master strenk is probably alot better at this stuff than me though. Plus he has pictures.
    1986 CJ7 Laredo
    258 I6 parts for sale PM if you need something

  6. #6
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    If the Weber ran fine on the other engine, Im going to guess a major vacuum leak somewhere. If this were a new install on the Weber, I would have other suggestions. Backfire through the carb is generally due to timing (Which youve checked) or a lean condition. The adapter plates are notorious for leaking. But they will generally raise the idle when the engine is warm, as they will not pass much air. Make darn sure the intake is sealed up tight. And do you have power brakes? That little brass piece under the carb has a small port that should be plugged if you have all the emissions pulled. One good backfire and that baby is gone. If you have absolutely no vacuum leaks, then the choke is set wrong. Someone will correct me if Im wrong but the Weber has a choke pulloff. Set the choke and start the engine and the choke will open slightly. Race the engine and it will open up more. You have got to set the choke pretty tight to get a good cold running engine. Plus there is a screw adjustment for cold idle. You might have to fiddle with that. I cant think of any more solutions right now. Hope this helps.
    Tim Comer
    '83 CJ-7, 258, 4.0 head, small cam, Weber, HEI, 2 1/2 4WD Lift, 33" Mudders

  7. #7

    Default Better late than never...

    I was hoping that sitting for the winter would have cured these problems but no such luck...

    Checked ignition module - OK

    Did not check compression yet.

    Vacuum is 20 inch Gs @ idle

    Installed vac pump onto vac advanced pumped to 15 of vac backfire problem went away and it was fairly smooth up to 3500Rs

    At idle manifold vac @ 20 in - @ throttle response, vac drops to 0 and ported vac went to 5 inches of vac

    Before vac applied, motor has major hesitation with, vac applied hesitation goes away still breaks up slightly @ top end.

    What could be causing the problem?

    Thanks,
    -David

  8. #8
    Registered derm's Avatar
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    What shape is the hose on the VAC? They are pretty skinny, maybe it is collapsing under load.
    78cj5 with 258, no more mods allowed until it is painted.....

    Before pics: www.zammikiel.com/jeep.htm

    We've lost lateral controls! Don't worry she'll hold together....You hear me baby, hold together....

  9. #9
    Forum Leader John Strenk's Avatar
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    ! Did you check/set the timing with the vacuum hose disconnected?

    Timing should always be checked with the vacuum hose disconnected otherwise you'll have problems like your describing. When I was young I had an 69 Pontiac Tempest. THe thing ran like a dog. I hated it. THe day I sold it I was checking the timing after a couple years of expperiance and discovered I had left the hose on when I set the timing. Ran great afterwards that I regreted selling it.

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