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Would 90psi on all cylinder for a 4cyl cause it not to start?

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  • #1

    Would 90psi on all cylinder for a 4cyl cause it not to start?

    Im working on a 78 Datsun with a l20b 4 cyl that wont start. New carb, fuel pump and coil. A friend of mine bought this truck from a guy that left it in a field. The cylinder head looks pretty new so Im guessing that the head was replaced and they couldnt get it to start so they parked it. Iv checked the timing/firing order 3-4 times, Iv ran the valves and re-checked them. the only things I can find wrong are that there's no ballast resistor and low compression. If I crank it long enough the engine starts to fire but it back fires and has fire pop through the carb. I squirted oil in all 4 cylinder and compression went up to 120 so Im guessing the low compression has to be the culprit? I took the valve cover off and the cam and the crank line up. Iv had engines with low compression that have started and ran ok but never one that had 90psi and wouldnt start.
    Thanks.
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  • #2
    it could be that the cylinder walls are dry or the rings are stuck in the grooves. You might hit the cylinders with some atf or more oil to help keep the compression up, have you tried giving it a shot of carb cleaner to help it light off. The pop through the carb would leave me to believe the timing had too much advance or it's still really lean trying to start. I'd think that 90 on all 4 would be enough for it to fire off and run.

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    • #3
      90PSI should be enough to start it. I agree with the timing is the likely issue.

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      • #4
        cam a notch off?
        Don't worry about what you can't change.
        Do the best you can with what you have.
        Be honest, even if it hurts.

        "Socialism is a philosophy of failure, the creed of ignorance, and the gospel of envy; Its inherent virtue is the equal sharing of misery" ... Winston Churchill

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        • #5
          dist stabbed wrong, maybe 180 out?
          sigpic

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          • #6
            when my 4v cobra motor lost compression it would not start. it was like the motor was just free wheeling. it would hit maybe once or twice but would never run

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            • #7
              an engine should be putting out about what like 180 or so psi? that thing sounds like a boat anchor candidate

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              • #8
                No, 180psi is too high. Did you do the compression test with the throttle to the floor? I would also put a little oil in the cylinders and try again
                Scott Ganow
                Lone Star Performance
                16300 Midway Rd
                Addison, TX 75001
                214-630-5006

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                • #9
                  90 lbs is plenty for it to fire. Might not run great but it should at least fire up. Try some marvel mystery oil in the cylinders as let it sit for a bit. Also I agree sounds like the timing is a bit off.

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                  • #10
                    Thanks for the ideas guys. Im not sure why but this freaking truck is kicking my ass. Im sure there's a simple reason that Im over looking.

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                    • #11
                      Originally posted by 0393gt View Post
                      dist stabbed wrong, maybe 180 out?

                      Been there done that, it jumped the timing belt then wouldnt run. Spent a few nights troubleshooting firing problems.

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                      • #12
                        backfiring through the intake? That's usually a dead giveaway for timing.
                        "When the people find that they can vote themselves money, that will herald the end of the republic." -Benjamin Franklin
                        "A democracy will continue to exist up until the time that voters discover that they can vote themselves generous gifts from the public treasury." -Alexander Fraser Tytler

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                        • #13
                          if it's a overhead cam engine and they had the head off, I bet the cam isn't lined up correctly
                          Don't worry about what you can't change.
                          Do the best you can with what you have.
                          Be honest, even if it hurts.

                          "Socialism is a philosophy of failure, the creed of ignorance, and the gospel of envy; Its inherent virtue is the equal sharing of misery" ... Winston Churchill

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                          • #14
                            Originally posted by jyro View Post
                            if it's a overhead cam engine and they had the head off, I bet the cam isn't lined up correctly
                            and its probably got some nice valves that dont seal worth a crap...

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                            • #15
                              Originally posted by QIK46 View Post
                              and its probably got some nice valves that dont seal worth a crap...
                              This is what I was kinda thinking too that maybe some valve(s) are stuck open if it was left sitting.

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