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Thread: Bucking when coming back in throttle....thoughts

  1. #1

    Default Bucking when coming back in throttle....thoughts

    1974 gas series III. Had total bare frame up five years ago but set until i got it about one years ago. Dealing with fuel new stock pump and lines. I had replaced all filters and soaked and rebuilt Weber as i saw lots of junk in first filter. I have run several tanks of fuel through it but lately it is really bucking at times when i come back easy in throttle. Will take wide open throttle like up hill in 4th but it is that easy going from no load to slight throttle input. I noticed today that last filter at carb looked almost dry after shut off and only a little bit of fuel at idle. I have not pulled the pickup tube nor had bowl off the mechanical pump which along with almost everything was new when assembled. All filters look clean now but as i said have never looked in tank. Would one suspect pickup and gum on screen and or screen in mechanical pump as some first things to check? Thank you.

  2. #2

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    Seems to me that if it'll go uphill in 4th w/o symptoms of fuel starvation than it'd be a waste of time to pull the fuel pickup or the screen in the mechanical pump. Weber carbs are sorta bad about the idle jet getting clogged up, so I'd be more suspicious that the fault lies with the carb.

  3. #3
    Join Date
    Apr 2008
    Location
    The Granite State (NH)
    Posts
    3,458

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    I'd suggest that it might be fuel-related. In fact, it might not even be engine-related.

    A similar thing happened to me...under power, the truck seemed to go just fine, but under light throttle, at the cusp between coasting and the engine powering the driveline, the truck would buck pretty alarmingly.

    It turned out that the circlip that positions the rear output shaft gear in the transfer case (item #78 in these illustrations here) had broken and fallen into the sump, and was allowing the rear output gear to wander up and down the shaft. The gear would wedge solidly on the shaft when under load, but would slide up and down the shaft a little bit as the driveline alternated between being driven by the engine and being driven by the wheels during coasting. This small movement of the gear was enough to cause a rather startling bucking.

    Not saying this is your issue, but certainly worth looking into any fore/aft slop in the gearbox and transfer case, including making sure the castellated nut (item # 37) on the back of the transmission mainshaft is tight and properly saftied.
    --Mark

    1973 SIII 109 RHD 2.5NA Diesel

    0-54mph in just under 11.5 minutes
    (9.7 minutes now that she's a 3-door).

  4. #4

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    Thanks for reply.....Here is what i have done so far. Soaked and rebuilt Weber again two days ago,tried new coil,pulled feed tube in tank and saw that all was unrestricted,checked all in line filters,replaced spark plugs. Got on road and same thing again. Starting with cold engine it would run perfect for about five minutes. Change through gears into forth and bucking started again and limped home in first gear. Replaced carb with a known good one off running Rover and back on road and same thing again. I forgot to add also a new distributor and wires. I put NGK plugs in it but am going to get a set of Champion RN12YC and try. This thing on cold engine starts instantly and sounds perfect, go through gears and think this is running so good.....then the misfire,bucking starts. Never total loss and stop running but goes to hell. Mechanical issues ref. trans, transfer are not involved as i just put overdrive on and all new ujoints and all was good with normal drive train slack. I also have a volt meter installed and needle does NOT drop when this bucking starts. Thinking to put electric fuel pump on and try this although the mechanical pump is new also. I hate to chase parts this is sure trying to be hard to find. Any more help sure welcome.

  5. #5
    Join Date
    Apr 2008
    Location
    The Granite State (NH)
    Posts
    3,458

    Default

    There've been lots of complaints with brand new fuel pumps that were bad right out of the box or shortly thereafter in the last few years, though if you are getting adequate fuel delivery under full load, but not at partial load, I doubt fuel pump issues.

    Have you checked for a vacuum leak once the engine is hot? Sometimes stuff will be straight and tight when the engine is cold, but once hot and expanded, gaps will open up, allowing air in where air shouldn't be coming in. Carb base to intake manifold, intake manifold to engine block, etc. Especially on whichever carb is famously prone to warpage of its base (I don't have a carb, so I didn't keep track of which model carb has the warpage issue).

    Vacuum leakage doesn't matter as much when you have huge volumes of air and huge volumes of fuel being used under heavy load, but when only a little air and a little fuel is being used under light load, a vacuum leak will have a much greater impact on the fuel to air ratio.

    Does adjusting the choke (reducing the amount of air coming in) when it's acting up help any?
    --Mark

    1973 SIII 109 RHD 2.5NA Diesel

    0-54mph in just under 11.5 minutes
    (9.7 minutes now that she's a 3-door).

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