rough idel 1978 425 cad cdv?
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500, 472, 425, 368 Discussion, rough idel 1978 425 cad cdv? in Cadillac Engine Technical Discussion; hello all, well this is my 100th post here and i just have to say that this is the best ...
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    tripleyellow78cad's Avatar
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    rough idel 1978 425 cad cdv?

    hello all, well this is my 100th post here and i just have to say that this is the best informailon website i have ever seen for these ol" tire melters!
    anyway my 1978 425 cad cdv w/ 95k on the clock , runs very rough at idel
    and it started this after i first got this car 6 years ago and i cannot get it to run smooth, is this a common problem for these 425's?
    i have changed all if the following , dist cap,rotor , plugs, wires , fuel pump
    all of the vaccume lines , had the carb rebuilt,(new accelerator pump, gaskets ect) , this car i am also told by the smog shop that is runs extremily lean, this car has tons of power and runs great except at idel, acts like a huge vaccume leak, please help!!!

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    Re: rough idel 1978 425 cad cdv?

    Have you checked the vacuum? Also, I'd check the ignition timing. That should be a good place to start.
    And my old 425's were very smooth at idle if everything is set right.

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    Re: rough idel 1978 425 cad cdv?

    Quote Originally Posted by caddydaddy
    Have you checked the vacuum? Also, I'd check the ignition timing. That should be a good place to start.
    And my old 425's were very smooth at idle if everything is set right.
    vaccume is 20 in at idel , timing is set to factory specs,
    love the engine hate the rough idel,
    one other thing could it be that vacume switch in the head be bad?
    thanks

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    JTraik is offline Cadillac Owners Enthusiast
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    Re: rough idel 1978 425 cad cdv?

    Quote Originally Posted by tripleyellow78cad
    vaccume is 20 in at idel , timing is set to factory specs,
    love the engine hate the rough idel,
    one other thing could it be that vacume switch in the head be bad?
    thanks
    What vacuum switch are you talking about? You could also have a unevenly worn cam thats causing the problem.

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    jk1000 is offline Cadillac Owners Member
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    Re: rough idel 1978 425 cad cdv?

    I think a compression check is in order.
    Has the timing chain ever been replaced? While you can adjust ignition timing to compensate for a worn chain, your valve timing will always be late.
    Lean running? If it's too lean it will idle poorly.

    Good luck with this.

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    Re: rough idel 1978 425 cad cdv?

    Make sure that your distributor's mechanical advance isn't sticking. Search the archive for information. Check all vacuum lines for leakage. Spray carbonator spray and see if the idle rises to find leaks. Worn throttle shafts will leak vacuum too. A sticking EGR valve will crap up the idle. Intake manifold gasket is another place to spray and look for leaks. Another lumpy idle cause with this engine is carbon build-up on the valves near the stems. Very common problem. I like to use transmission fluid to clean it up. I've posted this proceedure here before so it should show up.
    A 425 will run just on the edge of lumpy if the timing is set advanced enough for optimum economy. The only time they run very smooth is when they don't have enough timing at idle of if they are fuel injected. These engines asked a little too much from a carbonator.

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    Re: rough idel 1978 425 cad cdv?

    Quote Originally Posted by The Ape Man
    Make sure that your distributor's mechanical advance isn't sticking. Search the archive for information. Check all vacuum lines for leakage. Spray carbonator spray and see if the idle rises to find leaks. Worn throttle shafts will leak vacuum too. A sticking EGR valve will crap up the idle. Intake manifold gasket is another place to spray and look for leaks. Another lumpy idle cause with this engine is carbon build-up on the valves near the stems. Very common problem. I like to use transmission fluid to clean it up. I've posted this proceedure here before so it should show up.
    A 425 will run just on the edge of lumpy if the timing is set advanced enough for optimum economy. The only time they run very smooth is when they don't have enough timing at idle of if they are fuel injected. These engines asked a little too much from a carbonator.
    thanks apeman , those are things i had not thought about yet, will try them asap,
    the timeing is set at 18 deg btdc as the book says, , mech advance works properly checked it today nice and smooth.

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    Re: rough idel 1978 425 cad cdv?

    what RPM is it running at idle?

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    Re: rough idel 1978 425 cad cdv?

    Quote Originally Posted by cadillacdeville
    what RPM is it running at idle?
    at 650 rpms , sprayed carb cleaner around the carb and the idel stayed the same, this engine does not always idel rough, with the a/c on and the tranny in gear it will do it but not always , most of the time it does it when the car is all warmed up.

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    Re: rough idel 1978 425 cad cdv?

    A sometimes rough idle on a clodrajet can be caused by the secondary throttle sticking slightly open. Just another thing to look at.

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    Re: rough idel 1978 425 cad cdv?

    That does sound like what it probably is when I replaced the carb on my 77 after I got it the secondarys were frozen shut by carbon bildup looked like they had never been used of course the car was owned by a 70 or 80 year old lady who more than likely never used all 4 barrels I could see how they could get stuck slightly open but here are a few things the 77 FSM recommends checking under (stalls or rough idle when hot) Ignition wires for cracking, hardness and proper connections engin timing and idle speed
    the P.C.V. valve for proper operation
    then if nothing is found there it says to remove vacuum hose from E.G.R. valve and connect an extra hose from any manifold source to valve If RPM doesn't drop the valve is leaking if thats found to be the problem let me know theres a page and a half of EGR system diagnosis most of it causing rough idle, stalling, detonation and poor fuel economy if its not the valve it says to run a cylinder compression check timing should be 18*BTDC @1400rpm idle should be 600 RPM and gap on the plugs should be .060

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