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Thread: Lack of power in 4th

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  1. #1

    Default Lack of power in 4th

    Been battling issues with my truck, and have been down many rabbit holes. The upside is I have learned a ton and fixed a ton of stuff, but I'm still struggling to maintain 30 MPH in 4th gear with the original 230.

    Led me to find a takeout engine locally, priced right. I looked it over and the guy says he's done messing with it, hes just putting a diesel in. I ask what his issue is with the motor and low and behold, it struggles to maintain speed in 4th.

    For reference, both engines still have the original Holley carb, and have the civilian distributor swapped in, still running points, 12 volt converted. I haven't investigated as to whether or not he had the vacuum advance hooked up, but I've noticed little difference with or without in my testing. My carb was freshly rebuilt

    Has anyone else fought this battle and actually won? I have a pertronix kit coming to eliminate the points, but I guess part of me is questioning the carb, so I bought a NOS carb kit as well. I'm wondering if the power valve in the carb is malfunctioning when the motor is lugging.

    I have the stuff to swap the engine for a chevy v8 but I feel that its a disservice to this old truck.

  2. #2
    Join Date
    Feb 2004
    Location
    Silver Lake Sand Dunes MI
    Posts
    1,490

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    What is the compression of each cylinder? If they are good then look for issues like timing due to lack of vacuum advance, or fuel starvation (blocked fuel line, miss-adjusted carb) or an air restriction (blocked or dirty air filter). Also check to make sure the throttle cable is not kinked and is opening the carb fully. I had this issue once and it acted like the truck was down on top end power. Once I found the bad cable and replaced it, the truck ran like normal.
    Dave
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  3. #3

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    Quote Originally Posted by Beast View Post
    What is the compression of each cylinder? If they are good then look for issues like timing due to lack of vacuum advance, or fuel starvation (blocked fuel line, miss-adjusted carb) or an air restriction (blocked or dirty air filter). Also check to make sure the throttle cable is not kinked and is opening the carb fully. I had this issue once and it acted like the truck was down on top end power. Once I found the bad cable and replaced it, the truck ran like normal.
    Thanks for the quick reply.

    Front 5 cylinders has decent compression~120-130, cyl 6 is ~90.

    I've messed with the timing a ton, although I haven't verified the mechanical advance is functioning properly, I'll add that to my list. The original igniter didn't have a vacuum advance, I figured the distributor would be fine with it no hooked up.

    I have some fuel line concerns from a crummy tank, I ought to toss my boat tank in and try to see if that eliminates the issue.

    Throttle cable kinking is new to me, I's definitely have to check that out.

  4. #4
    Join Date
    Sep 1998
    Location
    North Central Wisconsin
    Posts
    11,520

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    There was a member with the same problem many years ago...what he found eventually was that the leaf spring like tensioner arm on the points set was in the wrong place and wasnt providing the correct tension on the points...basically it was on tghe wrong side of the mounting bracket and that was enough to keep the pressure down enough that as the load got higher, the points werent keeping up.

    In the following image, it is the flat metal plate that runs from the points past the arrow of #3 to the mount bolt that is in the line to the #4 arrow....hopefully that is clear...
    Lord send your Holy Ghost into our hearts and make the desire of our hearts Your Will.

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  5. #5

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    Quote Originally Posted by brute4c View Post
    There was a member with the same problem many years ago...what he found eventually was that the leaf spring like tensioner arm on the points set was in the wrong place and wasnt providing the correct tension on the points...basically it was on tghe wrong side of the mounting bracket and that was enough to keep the pressure down enough that as the load got higher, the points werent keeping up.

    In the following image, it is the flat metal plate that runs from the points past the arrow of #3 to the mount bolt that is in the line to the #4 arrow....hopefully that is clear...
    Not to contest an apparent success, Jon, but wouldn't that problem manifest itself at the same RPM in any gear? After all, it's the engine that's being made to work under a load, not the point spring (as I understand it).

  6. #6

    Default

    Quote Originally Posted by brute4c View Post
    There was a member with the same problem many years ago...what he found eventually was that the leaf spring like tensioner arm on the points set was in the wrong place and wasnt providing the correct tension on the points...basically it was on tghe wrong side of the mounting bracket and that was enough to keep the pressure down enough that as the load got higher, the points werent keeping up.

    In the following image, it is the flat metal plate that runs from the points past the arrow of #3 to the mount bolt that is in the line to the #4 arrow....hopefully that is clear...
    Interesting. Thats the kind of experience I love about forums, thank you.

    I've been into the ignition and rebuilt it all, so the probability of the spring being setup incorrectly is pretty high. I'll know for sure when I get the Pertronix points eliminator in.

    Nailhead: my thinking is that the spring not applying enough tension would contribute to a weak spark, which the engine would run ok with no load, add significant load and the engine wont run as well on the weak spark. Add higher RPM and I'd bet the spring would "float" like valve springs if there was inadequate tension.

    Parts should be in today, I'll let you know how it goes.

  7. #7

    Default

    The ignitor also has a mechanical advance using springs and centrifugal weights. There is no vacuum advance. if the mechanical advance is frozen or sticky that would also cause a problem as the throttle is opened.

  8. #8

    Default

    On top of the page is the Basic training section. There under faq is some good info that may help with your trucks problem. Good reading at the very least. Good luck . Scott

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