1406 runs awesome except misfire at cruise

Discussion in 'Engine Topic' started by bigblock665, Apr 3, 2012.

  1. bigblock665

    bigblock665 Veteran Member

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    My car misfires only seem to be under cruise conditions. If I unhook the vacuum advance the misfire is gone but when I hook it back up the miss comes back. The only thing I have not replaced in my dizzy is my pickup coil. And Iv cleaned the carb multiple times. The exact carb tuning and setup worked on my engine when it was new. Since dirt got into my carb it has never run the same again. Its either I still have dirt someplace in the carb or the pickup coil is going due to broken wires. They dont look to healthy. I really hate to take this carb off and take it apart again. Does this seem like a carb issue? It just does not make sense the advance will change a carb issue.... But this didn't all start till the carb got dirty...

    Any ideas or things to try would be appreciated... Im at my wits end with this car and just want it to run right. Im so mad at the thing I just want to see the car crushed.
     
  2. CorkyE

    CorkyE Veteran Member Lifetime Gold Member

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    It's either running too lean or it's pulling in too much timing at cruise. When I used to run vacuum advance, I would jet down until it started surging at cruise, then jet up a couple. I would guess an air bleed was slightly gummed up and when you cleaned the carb, it's running leaner now. If you have an adjustable stop on your vacuum advance, you can try pulling it back a bit.
     
  3. Andelin74

    Andelin74 Veteran Member

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    ^ +1

    Try backing up your timing a little bit and see what it does. If it starts performing poorly before losing the misfire it's likely too lean. I had a 1406 in the past though, and I know from the factory they were WAY rich, so i highly doubt its a fuel issue.
     
  4. bigblock665

    bigblock665 Veteran Member

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    well... The ignition is spot on timed and the carb was cleaned with agitated kerosine and blew out with 120 psi... Carb is spotless!!!!. Did not help. I put a set of metering rods and jets for a 1407 carb and richened it right up. Still had misfire. Put on a totally different carb "a 1407" and the misfire got worse. Still have this random mis at cruise, it also seems to be a serge when at full throttle going down the road. I ran my carb as rich as possible and had no luck. My ignition is running fine I even threw in a known good dizzy and the problem is still there. Im starting to think I ****ed up my cam maybe one of the exhaust lobes is flat. I did have one plug that was black on cyl 6 and the replacement plug did the same mis fire. So I know its not a bad plug issue. My vacuum on my vac. gauge looks good and has a strong signal. Is it possible I toasted my cam. would it cause my symptoms?
     
  5. CorkyE

    CorkyE Veteran Member Lifetime Gold Member

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    It's possible to have wiped a lobe, you should be able to see that with the valve covers removed. Another thing to check for is a vacuum leak. You can spray anything from WD-40 to carb cleaner around the intake gaskets and listen for a change in RPM's at idle. Some use a propane (unlit) torch to check.
     
  6. bigblock665

    bigblock665 Veteran Member

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    I have no vac leaks, I even blocked all vac accessories. My other question is could a cam make a misfire/like a poof sound out the exhaust? My vac is where it should be, I think an exhaust valve may not be opening as much as it should. I have good compression in all cylinders.

    It is the stock engine for my car and I believe 79 had issues with "soft cams"
     
  7. hhott71

    hhott71 R.I.P 11/19/18 Lifetime Gold Member

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    too much vacuum advance. The cruise mixture is light enough not to knock with too much timing, just misfire.
     
  8. Happy_Meal

    Happy_Meal Veteran Member

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    Double check to make sure the carb is sealing properly to the intake. I have that same carb on my 350 with the performer eps intake and had similar issues until somebody on here suggested I used a 1/8" spacer between the carb and intake with a gasket on both sides. I guess some of their carbs don't seal well and cause a tiny vacuum leak that won't necessarily show up on a vacuum gauge.
     
  9. bigblock665

    bigblock665 Veteran Member

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    cylinder 6 seems to turn black when all the other plugs look tan or clean. what could cause one plug to run black? Prolly where my mis is coming from. Compression looked fine on this cylinder... My ignition system down to the spark plug wire is firing properly from what the timing light shows. Iv changed multiple plugs in this cylinder and still no dice. Im getting good comp. in this cyl and Im not burning oil. so the piston and rings are fine. the only thing left is the cam.... And to be sure Im taking my carb off my engine and throwing on my buddys 1406 off his pickup that he drives daily. Same carb but his runs flawless... But I have a sneaking suspicion his carb will run the same way.

    So to recap my #6 plug loads up and seem to have a mis at all rpms retarding the timing does NOT cure it but fades the mis. Sound like something a whiped exhaust cam lobe will do? I dont think this engine is cammed but even at idle it shakes more than any cammed engine iv ever seen.

    I need help! Dont be afraid to tell me I have to replace a cam or a head ill buy a motor, block to heads. I dont want to but I will. I am just sick of dropping money and time into this engine. If its deff. a componant in my engine that will cause my problems then ill replace it. I just want to enjoy the car and cannot! Its my favorite car out of all the other vehicles i drive.
     
  10. bigblock665

    bigblock665 Veteran Member

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    Today I realized with the vac advance totally dis connected I still get the mis.
     

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