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Thread: Hesitation while driving at constant speed?? 88 d50

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    Hesitation while driving at constant speed?? 88 d50

    FINALLY got my shifter problem fixed, so on to the new one. First off I don't know the history of this truck, so it could be anything. I have a mikuni carb with a new fuel filter and pcv valve. Went for my first real drive, went to get gas (first full tank I've put in this truck) and dumped in a can of seafoam. Driving down the road at a steady speed could be 35 or 65 and the truck will hesitate and it stalled a couple of times at a light. It doesn't seem to like to restart well when warm (3 cranks) but starts fine when cold. The stutter seems to get worse the warmer it gets. Kind of intermittent, pretty sure it's fuel related as sometimes it runs just fine.

    I don't have a fuel pressure gauge. Can I just crank it and measure how much fuel I'm pumping?

    Ideas on the best place to start?
    fuel pump
    carb
    vapor lock
    gas tank debris

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    Vacuum leek in the brake booster. Mine would die every time I hit the brake some what hard.
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    The vacuum line to the brake booster had slipped down a little. I pushed it up and made sure the clamp was tight, but this happens while I'm driving and not braking. Where was your vacuum leak in the booster at??

    I also found a couple of caps on my carb that were pretty sad and most likely leaking, but I've still got the stumble. I was driving it tonight and feel like it might be the carb. It only happens after it's warm, a couple of miles, and it seems to be on deceleration. So intermittently on the flat, when I hit a slight downhill, or slowing down for a stop.

    The mikuni carb is super clean and I can tell the exposed gaskets are new and soft. Maybe I still have a vacuum leak, float or other carb adjustment?? Wish I could do the Weber carb, but the wife has cut me off and I don't have enough mad money for that right now. Might have to check out the local parts yard again as I still some credit form the tranny debacle.

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    Quote Originally Posted by camoit View Post
    Vacuum leek in the brake booster. Mine would die every time I hit the brake some what hard.
    mine dies when i go over 55 mhp and go on neutral hit the brakes and just cuts off.

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    replace the booster. it has a hole in it like mine did. I could hit the brakes and it would just die... If it's full of fluid then the master is bad also. but you would have noticed that already. no brakes aaaahhhhhh ------- =)

    But as for stumble that tells me the accelerator pump may be having a problem. If it's driving down the road at 35 + or so and feels like it's running out of gas then the fuel filter is plugged.
    Also if you have the feedback carb it could be the O2 sensor screwing up.
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    This is what mine does as well. Only time it doesn't stall at stopping, is if I have the idle set high, or if I put it in "N" and keep my foot on the gas and the brake. Everyone told me it was a vacuum leak. I replaced the carb gasket, found a line with a hole it in, still same issue. Driving down the road it hesitates unless my foot is on the gas, press in the clutch, and it smooths out. Mainly at 5-50mph. I think mine has a vacuum leak at the throttle butterfly shaft. When we took the carb off, it was all black by the spring.

    Guess it is time for a new carb. I bought a new PVC (mine was clogged), no change. I also got a fuel filter, haven't replaced that yet, it is next.

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    Mine was doing this too, Panic stop I would have to side step the brakes and gas to keep running. Brake Booster? hmmm I need to check this for a leak. If it needs a Float adjustment it can also do this according to Weber. The force of the "G"s putting on brakes they said can cause float bowl to pour over edge and flood out carb. I adjusted floats on my Weber, which one float was tweeked down farther then other about 2-3mm. I have yet to try the float fix because i'm still doing list repairing.

  8. #8



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    Quote Originally Posted by cnatsu1 View Post
    The vacuum line to the brake booster had slipped down a little. I pushed it up and made sure the clamp was tight, but this happens while I'm driving and not braking. Where was your vacuum leak in the booster at??

    I also found a couple of caps on my carb that were pretty sad and most likely leaking, but I've still got the stumble. I was driving it tonight and feel like it might be the carb. It only happens after it's warm, a couple of miles, and it seems to be on deceleration. So intermittently on the flat, when I hit a slight downhill, or slowing down for a stop.

    The mikuni carb is super clean and I can tell the exposed gaskets are new and soft. Maybe I still have a vacuum leak, float or other carb adjustment?? Wish I could do the Weber carb, but the wife has cut me off and I don't have enough mad money for that right now. Might have to check out the local parts yard again as I still some credit form the tranny debacle.
    I found this tidbit on hesitation with carb. Camoit's idea seems very likely and it's an easy check. Just throttle the carb and look into barrels for a squirt from pump.
    Here is some other info about a lean setting causing the hesitation. I have the entire document which it talks about the Weber, but carb hesitation is pretty universal when it comes to diagnosing the problem.

    8. The last thing I do after this is make sure that there is little to no off-idle hesitation which would indicate a lean mixture. The accel pump in the Weber should prevent that, but sometimes, it’s possible that your final mixture setting is on the lean side and so you might get some hesitation. If you do, turn the mixture screw out just a hair, like 1/16th of a turn and then run the engine at 2000rpm for about 10 seconds and then let it idle. Now snap the throttle open quickly. Hesitation? No, great, you're done. Still some hesitation? Make another 1/16th turn out and repeat.
    9. Now when you're doing this, it’s important that you are able to tell if you are hesitating due to a lean mixture, or stumbling due to an overly rich mixture. There are several clues that will help you out here. The first is if its leaning out, you may get some backfiring through the carb. The second is if its over-rich, you'll see lots of black smoke when you snap the throttle(s) open. The third is if turning the idle mixture screw out a sixteenth of a turn makes it worse, then its over-rich. Ultimately, what you >want to see when you snap the throttle open is a hint of black smoke out of the tailpipe. No smoke, you're too lean. Just a little smoke is what you want. Now, if you've got a nice, perhaps *slightly* burbly idle around 900rpm, and an engine that snaps enthusiastically off idle, you're done! The main key to remember is that you want your idle screw having to move the throttle as little as possible, since, again, if you have to open the throttle very far, it will uncover the transition ports, and that suggests that your idle jet is too lean.
    One other consideration is that if you have the throttle just about closed and the mixture adjusting screw pretty far in, then your idle jet may be too rich.
    Another note: I keep saying that you want the throttle as closed as possible, but in actuality, having the idle speed screw turned in about a half to one turn (after making contact with the bellcrank) is about right. Any more than that and your idle jet are too lean, any less than that and your idle jet maybe too rich. Your plugs can also show whats going on inside the chambers. In my experience the trucks run a little rich anyways, but it could go either way.

  9. #9

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    Hmmm, should be able to disconnect and cap off the vacuum line going to the brake booster to check and see if this is the problem?? I'm assuming you should be able to still brake but it will be reduced braking. Not sure.

    Thanks for the carb info bradmph. I haven't worked on a carb for a LONG time. Will try it out today. No carbs at all at the local JY.

  10. #10



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    Yes it will still stop. Just be ready to PUSH HARD,,,,,,,, Real HARD...
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    I replaced my booster a few months back :/ and it still cuts off

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    Quote Originally Posted by flbadrian View Post
    I replaced my booster a few months back :/ and it still cuts off
    Have you done the fuel filter? Also check to see that the accelerator pump is pumping the full stroke and the squirter's aren't plugged up on one side.
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    Quote Originally Posted by camoit View Post
    Have you done the fuel filter? Also check to see that the accelerator pump is pumping the full stroke and the squirter's aren't plugged up on one side.
    Yes I have changed the fuel filter... I have no clue what a accelerator pump is? Sorry

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    With the engine off and you look into the carb you will see a squirt of fuel when you turn the throttle.
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    Quote Originally Posted by camoit View Post
    With the engine off and you look into the carb you will see a squirt of fuel when you turn the throttle.
    Okay I'm going to do this later and hit you back up. I'm suppose see gas go in there right?

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    Yep use a flash light to see in there. Also in the manual section under over 216 manuals> look for the montero manual.It covers your truck and will help you understand how the carb works.
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    Quote Originally Posted by camoit View Post
    Yep use a flash light to see in there. Also in the manual section under over 216 manuals> look for the montero manual.It covers your truck and will help you understand how the carb works.
    alright i looked in to my carb and i seen the gas squirt, it squirted a good stream too alot of pressure ... what do i check for now?

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    What it your ignition timing set at? Also you might want to run a can of cleaner through the fuel system.
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    Quote Originally Posted by camoit View Post
    What it your ignition timing set at? Also you might want to run a can of cleaner through the fuel system.
    It's at 5. Like how it says under the hood, and I already did that 2 weeks ago ran some JB in it...

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    Try 8 BTDC and see how that goes.
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    Quote Originally Posted by camoit View Post
    Try 8 BTDC and see how that goes.
    The timing right?

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    seafoam it? good cleaner that might help while your diagnosing. sorry just saw your past post on cleaners. Try taking your distributor cap off and grab a hold of that rotor and turn it. See if it snaps back quickly or just sticks. The advance on mine had worn out springs and would just lay there after I turned the rotor imitating an advancing of it.

  23. #23

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    Quote Originally Posted by BradMph View Post
    seafoam it? good cleaner that might help while your diagnosing. sorry just saw your past post on cleaners. Try taking your distributor cap off and grab a hold of that rotor and turn it. See if it snaps back quickly or just sticks. The advance on mine had worn out springs and would just lay there after I turned the rotor imitating an advancing of it.
    The rotor don't move its kind hard, like theirs a little movement but won't do a 360 turn.

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    yes on the timing. 8 deg. As for the test bradmph was talking about the rotor should move slightly advanced when turned by hand. about 3 deg then move it's self back. Just a tiny amount of free movement thats all. It should move thats he is after.
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    you could try disconnecting the vacuum ine from the egr and plugging it. This will eliminate the egr as a suspect if nothing changes.

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