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Help timing my truck

Discussion in 'Classic Dodge Trucks - Pre 1981' started by charm, Jul 8, 2019.

  1. charm

    charmActive Member

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    I question the accuracy of what I've done, but here's where we're at. I ran to Pep Boys, because they had it, and bought a Mr Gasket tee and pressure gauge. I ran fuel line from the tee to the windshield and installed the gauge. I don't know if those 4' of rubber fuel line tee'd off of the actual fuel gives an accurate reading or not. But, at idle, it's showing about 8 psi (way too high). When rev'd, it'll drop to about 7 (still way too high).

    Before I install a pressure regulator, do we think this reading is reasonably accurate based on the tool and the way it's run? Also, would having too much pressure cause these issues? (I suppose it would be like having to little air, so I can rationalize how it would cause the engine to stumble.)


    Polara, I'm almost at the point where I'm going to swap the carb. I spend about $150, or so, I don't remember exactly, having this carb professionally rebuilt. I hate to just junk it. But, if it'll make it go, I'm open to the idea. I'm just not there yet. I'm trying the cheaper stuff first (which is, cumulatively, costing far more than a carb).
     
  2. 7mopar

    7moparWell-Known Member

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    That would be accurate way of doing it. But what does the gauge read while driving it and when it stumbles upon itself. For a thermoquad yes the pressure is a tad out the 3-5 range. For the quad you should be ok. We still need to know what happens to fuel pressure on the road and under load. The quad is not that difficult a carb to rebuild and from the vacuum readings you have posted it should not have any vacuum leaks. It could possible have a secondary circuit problem but a simple check for blocked passages with a can of carb cleaner would verify that.
     
  3. charm

    charmActive Member

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    I gave up on that carb. I will now join the ranks of the anti-thermojunk crowd.

    I picked up a used Elderbrock 1406, bolted it down, truck runs great now.

    Still needs fine tuning. I spent all of about 22 seconds timing it by ear (I still can't time the truck with a timing mark). I haven't adjusted a single thing on the carb except to make sure the idle adjustment screws were out 2 full turns. Needs a softer throttle return spring because the gas peddle is super firm. But not bog, not backfire, nothing. I go slow, it runs decent. I put my foot in it, it runs decent.

    I bet I could get more performance out of the thermojunk, but, I just need it to work. I am not building a performance machine. It's focus is on hauling stuff and, as time goes on, I'll probably do some fun more rat rod like stuff, cool stake sides for the bed, clean up the interior a bit, basic stuff. The patina will stay. I'll keep factory wheels (although I do have a set of 16s to replace the 16.5s. Maybe replacing some of the lights on the truck just so they're visible. I may also fix windshield wipers and, if I'm feeling really ambitious, fix the washer fluid assembly.

    Immediate next step is to fine tune it and enjoy it.

    Thank you for all your help. Even though I fixed it the easy way, by replacing the carb, I really learned a lot through this process and I appreciate the help and advice.

    Next...
     
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    • 7mopar

      7moparWell-Known Member

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      The thermoquad is real particular about setup. Getting a step out of order is not good. They got a bad rap on count of bowl and float issues.
      Its the only carb I run on the carbed vehicles here.
       
    • charm

      charmActive Member

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      I think I got to the point where I intellectually understand that the Thermoquad is a good carb and capable of a pretty good amount of power, but that it takes skill and experience to tune them. I lack both of those things and I really don't have a community of grease monkeys to help me gain that experience, locally, like I did in high school. As such, I went to a carb that's built for idiots like me. Idiots that are fine giving up some power in exchange for a carb that just works right out of the box.

      If I were building a performance vehicle, and I wasn't going to invest in fuel injection, I would still consider a Thermoquad because I know what you can get out of them. But, if I were building a performance engine, it would be in a vehicle that I didn't need as a tool to help me with all sorts of projects. So, this method gets the rig on the road and back in action!
       
    • charm

      charmActive Member

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      Well, the problem wasn't the carb and I still think my lack of skills contributed to the problem, but, before I brought the truck into the shop to tune it and make it run right, I decided to run one more test. A compression test.

      Most cylinders ran 115psi on my cheap Harbor Freight set. One cylinder ran 110, one ran 120. So, 7 of the cylinders looked great! But then there was #7. Cylinder 7 was running 40psi. I checked twice to confirm that.

      So, would low compression in a cylinder allow for a pretty smooth idle but cause the back firing and stumbling I was (am) experiencing?

      What are the next steps? Now we're getting into territory I've never been in before. How do I identify bad cam/valve/etc?

      There's a good truck in there trying to get out. I need to figure out if I'm the one to find it and I could really use some advice to help me figure it all out.

      Thankfully the wife is out for the day so I haven't had to tell her the bad news...that the truck is going to require more time, effort, and money (or some gasoline and match).
       
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      • 7mopar

        7moparWell-Known Member

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        Couple of tablespoons of oil into the bad cylinder. Pull the coil wire, rotate the engine a few revolutions and test that cylinder again. If the compression comes about to nornal its rings. If not its likely a valve. With the problem with back fire I would lean more to a valve.
         
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        • 65 sporty

          65 sportyWell-Known Member

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          I would pull the valve cover also and check to see if both rocker arms are moving the same. If rocker movement looks good, take the rocker arms off and if you have the ability to, pump air into the chamber, if air comes out the exhaust, then the exhaust valve is burnt, if it comes out the carb, then the intake valve is bad. There is a hose made just to put air in, but you can use the hose from your compression tester if you remove the schrader valve first.
           
        • charm

          charmActive Member

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          I suppose rotating the engine with the valve cover off would tell if I had a flattened cam lobe?

          I think I'm comfortable with the idea of pulling the heads (both) and having them gone through if there's a valve issue. If I need to do that AND pull the cam, I'll look at a used engine. Same for if it's rings. The truck just isn't worth the bother of a full rebuild. Not sure it's worth the bother of pulling the heads, but I was dumb enough to buy this pig, I guess I'm dumb enough to keep working on it.
           
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          • charm

            charmActive Member

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            Update. Wet test points to valve train. I'll likely pull the heads and have a decent valve job done along with making sure they're flat and not cracked.

            While I'm in there, I'll replace the cast iron intake (mostly so I don't have to lift back into the engine compartment, again), the harmonic balancer, the timing chain and gears, gaskets, and, while I have the room, I'll look at cleaning up the wiring (there's what 6 wires?).

            Not excited about doing this, but in the end I 'should' end up with something reliable.
             
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