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"stumbling" at speed

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by dyeager535, Aug 14, 2001.

  1. dyeager535

    dyeager535 1 ton status Premium Member

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    \"stumbling\" at speed

    Slightly annoying problem with my truck. Just started as far as I can tell.

    Climbing up steep hills, where I am into the throttle pretty heavy. Normally, I don't have any problems. But if I try to accelerate, the engine starts to "cut out".

    It definitely sounds/feels like a fuel problem, but not sure exactly what. It only does it under HEAVY acceleration on flat ground, and takes a good while before it starts to happen there. On hills, if I'm in second, going 35, and try to accelerate, it starts bucking, and unless I let off the gas, I continue to lose speed.

    Fuel pump I *think* is new, but I may change that. Fuel filter is new, as is the carb rebuild. I'm kind of scared that it is the tank filter that is bad, but it seems odd that it only happens when I am accelerating..you'd think the tank filter clogging the pickup would be constant.

    Definitely doesn't do this when the vehicle is not under load, so ignition doesn't seem to be a very likely culprit. Anyone else experienced this and have thoughts one way or the other? I don't want to hear "it's the tank filter" 'cause with the skid plate, its a real bear to remove!



    Dorian
    My tech/links page: <a target="_blank" href=http://www.dorianyeager.com/index2.html>http://www.dorianyeager.com/index2.html</a>
    No anti-theft measures on your truck? No pity when its stolen
     
  2. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    Re: \"stumbling\" at speed

    I had a similar problem that I swore was fuel related as well. Also a 4 speed truck and also began behaving strangley only under load. Bucking and stumbling and not as much power as it had. I replaced damn near my entire fuel system, carb , some lines, fuel pump, etc. then i replaced my vacuum advance unit which was full of fuel...that got me a minor improvement. I then tore down the distributor and found the mechanical advance mechanism was all gummed up...so I cleaned it all and again had a slight improvement. But the mian problem was still there. Finally my battery hold down broke and the battery fell into the alternator and soaked down my iggy wires...so I had to replace my 6 month old Accel Super sport wires. I found a good deal on some 8.5mm spiral core wires and lo and behold my problem was solved. The Accel's were only supplying marginal spark to the plugs, so under load it would miss...
    Once I got the new wires on it was like a new truck!

    Test the conductivity of your wires...the local parts store has a tester here. You might be surprised how bad your wires are.

    Rene

    <font color=green>Dyslexics of the world...UNTIE!</font color=green>
     
  3. wayne

    wayne 3/4 ton status

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    Re: \"stumbling\" at speed

    I had a problem like that in Donna's car. Checked everything fuel related and put on a new fuel filter. Still had the problem, so I had it in the garage and had a friend looking under the hood while I tried to put a load on it while running and it turned out to be a spark plug wire was arcing off one of the head bolts. It only did it under load like going up a hill.

    <font color=blue>Wayne<font color=blue>
    <font color=blue>Happy Trails<font color=blue>
     
  4. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    Re: \"stumbling\" at speed

    Actually I found an inexpensive inductive spark plug wire tester...it just clips on to the wire at diferent points and the strength of the light at different points along the wire shows how much drop occurs over the length of the wire.
    The Accels I tested later had almost no 'signal' at the spark plug end.

    Rene

    <font color=green>Dyslexics of the world...UNTIE!</font color=green>
     
  5. Can Can

    Can Can Pusher Man Staff Member Super Moderator

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    Re: \"stumbling\" at speed

    I had a similar problem a few years ago. Like the others above, It ended up being a dead wire.........





    <font color=red>PHOTO ALBUM-</font color=red> <a target="_blank" href=http://albums.photopoint.com/j/AlbumList?u=1289798>http://albums.photopoint.com/j/AlbumList?u=1289798</a>.......<font color=blue>TRUCK & HIKING PICS</font color=blue>
     
  6. laketex

    laketex 3/4 ton status

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    Re: \"stumbling\" at speed

    i just had that prob with my ford. I changed a fuel pump that was going out anyway, but the real problem ended up being plugs or wires, dunno I changed both and it runs like a champ now.

    Sherman, Tx
    <font color=red>Come Awn yall...Let's go to LUCHENBACH, TEXAS!!!
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  7. dyeager535

    dyeager535 1 ton status Premium Member

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    Re: \"stumbling\" at speed

    How much does the tester run, and would any chain auto store have one? Not a bad thing to have around, easier than shocking yourself to test lol. I like the idea of it maybe being plug wires, and I will of course check that. It just doesn't seem to be likely though, my mileage is still at 15, and I'm pretty darn consistent about making sure wires are in good shape. Will check again though! Thanks for the encouragement. If only it is so simple!


    Dorian
    My tech/links page: <a target="_blank" href=http://www.dorianyeager.com/index2.html>http://www.dorianyeager.com/index2.html</a>
    No anti-theft measures on your truck? No pity when its stolen
     
  8. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    Re:

    I'm pretty sure it was less than $10...it looks like a little pen light. I found it at a parts store and thought it looked handy!

    I could not beleieve my wires were bad either...they were only 6 months old and I never would have replaced them without the battery acid finishing them off. I was truly shocked at the difference after new wires were installed!

    Rene

    <font color=green>Dyslexics of the world...UNTIE!</font color=green>
     
  9. muddin4fun

    muddin4fun 3/4 ton status

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    Re:

    Hmm...this is sorta what my camaro is doing. We've replaced the wires. Only under a load will it act up.


    No prohibiting allowed!!!!

    <a target="_blank" href=http://www.geocities.com/muddin4fun79blazer/>http://www.geocities.com/muddin4fun79blazer/</a>
     
  10. lukerz

    lukerz 1/2 ton status

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    Re: \"stumbling\" at speed

    Did you ever fix this problem?
    Luke

    83 K10 from 76 K5 & 83 C10
    Th350 Np203 355 CID
    31 X 10.5
    Plan on 2" body lift
     
  11. Re: \"stumbling\" at speed

    Have you checked vacuum lines? My truck started acting that way a few weeks back, and it turned out that a couple of vacuum lines had come loose. Vacuum advance was the problem.

    God has a hard on for Marines, because we keep Heaven chock full of fresh souls!
     
  12. Ddragggon

    Ddragggon 1/2 ton status

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    Re: \"stumbling\" at speed

    my truck ran great until it started getting cold out.. started by stumbling, and surging at high speed, then prograssivly got worse, so I checked my timing, and found out that for whatever reason, the previous owner had set the timing to almost 90 degrees BTDC. I couldn't see the line, unless I clipped it on wire #2, in which case, it was still running 20 degrees too high. so, I loosened the cap, dragged it down to 0 TDC, and worked with it from there. I'm still in process of dialing it in, ( need to advance it another 2 degrees I think for a total of 14 BTDC, and see how that runs) this explains why the stupid thing wouldn't even run on anything but premium gasoline, and why my Vac gauge wasn't ever steady...

    in my yellow suburban, the prolem did turn out to be plugged fuel filters, and fuel delivery... I think the gastank filter might be partialy plugged too, because I never have any problems with it, unless I hold the pedal to the floor for more then say 10 seconds... then it starts sputtering, and dying... I figgure the 454 is out doing the fuel delivery system in this case, although its been 6 months since I did sparkplugs/wires, so I might do them again... ( i use the crappy lifetime warranty wires from Autozone... they work beyond beautifully for about 3-5 months, then its time to take them in for new and improved ones) I know, I'm slightly crazy for swapping my wires, and plugs so often, but why not? I also change my oil every 2 month, or 2000 miles, or when it doesn't look clean &amp; Clear anymore whichever comes first...

    good luck with it,

    -Rich

    My truck isn't ugly, you have a skewed sense of beauty

    <font color=blue>2007 Ford XLS++ intrepid Ram Superduty 7000 R-type's suck</font color=blue>
     
  13. dyeager535

    dyeager535 1 ton status Premium Member

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    Re:

    Nope, never did. Took it on a trip to central Oregon, and it started doing it going up a mountain pass. By this time I had already replaced the fuel pump (free, and the one on there was unknown condition) and thought I had solved the problem

    Anyway, ALMOST reach the summit, and it starts to buck in 3rd(ok, its fourth, but shifter says 3rd)..I'm going 60..(no traffic thank goodness)...I let off gas, which usually solves problem....no dice, still bucking, dropping to about 45mph...shift to 2nd, still bucking, losing more speed....finally end up in 1st, 5mph up hill, still bucking...it's gonna stall if I don't floor it, so I floor it,and it dies. Just utterly dies right there in the uphill lane. Theres a gravel turnout just down the hill aways, so I coast backwards downhill, and back onto the turnoff. Try cranking engine for a bit, nothing. Doesn't even TRY to start. Hit the steering wheel a few times (note to self:get steering wheel without breakable plastic center cap) and pop the hood. Thinking ignition. BUT seeing that theres nothing I can do about that, I pop the air cleaner, look down the primarys, and hit the accelerator pump. Whaddya know, absolutely no gas in the carb. Wait a bit, crank it over some more, no start, obviously fuel related.

    Thank GOD for the Sm465, as where I pulled off, there was no way that truck was getting back on the road without mechanical help. Put tranny in low, and use the starter to pull me back on the road, pointed downhill towards town. Pick up speed, drop it in 2nd, coast downhill a mile or two while it turns over, finally just fires right back up under compression. Chalk up two more for a manual tranny over an automatic. Both got me back on the road, and allowed me to start it by forcing the fuel pump to work. Can't do either with an auto trans : )

    I say what the hell, no stores are open now anyway, so I turn around, and try the pass again. That was almost two months ago, and I have not had one SINGLE problem since that one uphill climb. Although I can't be certain it was THIS carb (I rebuilt a couple at the same time) I remember on one that the float was a touch sticky on it (when I was putting it together) , and remember that it would sometimes stick at what would be slightly above "full" for the float bowl. So I made some adjustments to try and alleviate the problem, which I thought I had.

    So what I assume happened this last time, was the float stuck in the "full" position, and finally fuel pressure freed it. The other times I'd attribute to the same. Thinking about it right now, I realize that the problem ALWAYS happens with the nose uphill, and usually at pretty good engine RPM's...high fuel flow into the float bowl, and with the fuel "sloshing" to the back of the float bowl, the float WOULD be as high as it can go, forcing the needle into the seat. When you set float height, you are supposed to use LIGHT pressure to push the needle into the seat. I noticed that if I exceeded "light" by a hair, the float would stick. So the fuel level more than likely pushes the float into the same situation when going uphill, but a combination of factors (length of hill, grade, engine speed) makes the problem intermittent.

    Since it isn't giving me consistent problems, I'm not going to mess with it. I'm working on another motor which I am buying a TPI setup for as we speak, so no more carb. This is a 1974 carb, so not only are there no vacuum hoses (compared to newer ones) it is old. It's a physical problem with the casting though I believe, I do recall trying two different floats in two different carbs, and both floats had problems only in this carb.

    Distributor problems aren't that bad of a shot either, since this one was sitting on the bench of some truck when I bought it, unknown application, but there was no fuel in the float bowl, it was obvious no fuel was the problem : ) And since the TPI distributor will replace it as well, not going to spend time on any of them, unless it becomes persistant!

    Dorian
    My tech/links page: <a target="_blank" href=http://www.dorianyeager.com/index2.html>www.dorianyeager.com/index2.html</a>
    Why insist on counting when the ring gear has the tooth counts stamped in?
     

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