Author Topic: 403 running weird  (Read 2874 times)

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Offline mattblack84

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403 running weird
« on: September 13, 2016, 09:34:57 AM »
Hi all, I have been beating my head on this for a while and so I decided to ask some experts.  I have a 79 Trans Am with the 403 Olds I have been having the hardest time getting it to idle right at low rpm it wants to almost surge slightly and run rough  wanting to almost stall.  With my vacuum Guage I see a 1 hg fluctuation.  What I have done so far  is sprayed carb cleaner to check for vacuum leaks found nothing checked the timing and made sure it was correct when I did try and adjust the idle air/fuel mixture screws I had a hell of a time getting a stable setting on the vacuum Guage I got the car kinda running right but there at 8 turns out I'm in the process now of checking vacuum diaphragms to double check but I'm beating my head over this so any help would be awesome thanks!

Offline Stringer

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Re: 403 running weird
« Reply #1 on: September 13, 2016, 08:09:44 PM »
Hard to tell.  It could be a multitude of things. What is the history on the car? When did it start? Other than looking for vacuum leaks what else have you done?

Offline mattblack84

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Re: 403 running weird
« Reply #2 on: September 14, 2016, 08:42:02 AM »
It's always had a weird run to it.  I've ruled out carb and engine because it was rebuilt and assembled by a trusted engine guy.  Right now I'm kinda leaning towards a low rpm misfire especially cause the vacuum readings are 1 hg of fluctuation. Tonight I'm gonna test to make sure I have the firing order right the vacuum advance is holding and not leaking then start disconnecting plugs to see if I hear a difference.

Offline Aus78Formula

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Re: 403 running weird
« Reply #3 on: September 14, 2016, 09:53:26 AM »
 "I've ruled out carb and engine because it was rebuilt and assembled by a trusted engine guy."

Don't get fooled into that, it can cost you thousands chasing other ideas before you realise you got screwed.

Offline Stringer

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Re: 403 running weird
« Reply #4 on: September 14, 2016, 10:46:04 AM »
Did you have the any ignition parts replaced...ignition modulator, distributor cap, etc.?

Offline mattblack84

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Re: 403 running weird
« Reply #5 on: September 14, 2016, 07:12:39 PM »
The engine was assembled by my dad and machined by my uncle the carb was done by my cousin.  For the ignition I did replace the module.  I'm going through the ignition tomarrow any pointers to check for misses.  I did check to see if I had leaking valves by putting paper to the tailpipes to see if it sucked in but I'm not getting any backfires or weirdness just idles and runs rough at low rpm smooth revving up.  If I find anything I will post it thanks all!

Offline Stringer

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Re: 403 running weird
« Reply #6 on: September 14, 2016, 07:33:37 PM »
How's your cousin with carbs?  Could be maladjusted float or wrong jets (maybe too lean), a gasket broken around the accelerator pump perhaps?  How are the streams when flicking the throttle?

Offline mattblack84

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Re: 403 running weird
« Reply #7 on: September 14, 2016, 08:53:49 PM »
With the carb I bought the kit from cliff Ruggles ordered it just how he said and I watched my cousin do it hes a retired Pontiac/Olds dealer mechanic.  I'm still leaning toward the missfire I have a few buddies coming over from work to help tomarrow but if I find the ignition in solid shape I'm gonna spend the 1000 dollars and go fuel injection or replace the qjet with a holley cause I'm ready to rip my hair out.

Offline mattblack84

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Re: 403 running weird
« Reply #8 on: September 15, 2016, 10:39:41 PM »
OK I feel like an idiot tonight I went through the ignition and found the plug wire for number 8 cylinder not quite pushed on.  My cruise control has a slight vacuum leak i found that not sure if that's normal?  also changed the plugs  reset the choke and fast idle  reset air fuel mixture and it's running quite smooth no more vacuum fluctuations so I think I might have found my gremlin.

Offline Aus78Formula

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Re: 403 running weird
« Reply #9 on: September 15, 2016, 11:15:58 PM »
Feeling stupid can be the preferred option to the other possibilities!

Offline Stringer

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Re: 403 running weird
« Reply #10 on: September 15, 2016, 11:53:33 PM »
Glad to hear it! Don't feel bad, it has happened to all of us.

Offline conan31214

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Re: 403 running weird
« Reply #11 on: October 31, 2016, 04:55:15 PM »
Process of elimination if the problem is not evident.

Offline mattblack84

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Re: 403 running weird
« Reply #12 on: June 02, 2019, 11:45:41 AM »
Update:  I rebuilt my 403, bigger camshaft, new keith black pistons, edelbrock intake beefed the old 403 up.  Still had trouble with the Rochester tried everything spent a lot of money.  I then looked into Fitech and bought a Fitech efi for up to 600 HP the gas tanks inc fuel injection tank, my brother works at inline tube so they designed me bigger then stock sizes stainless steel fuel line that would follow the stock path with AN fittings.  Went high pressure rubber from the end to the fuel tank and high pressure rubber from where the stock fuel pump was to the EFI.  Also added a pre and post filter.  Wow is all i can say car runs amazing!!  Looks totally stock until i take off the shaker.  took the orignal intake and carb lubed and boxed them up.  I was able to utilize the vacuum lines and ports in such a way that my air cleaner is still functioning and so is cruise control kit worked with AC i just had to buy a funky throttle and kick down bracket for EFI.  Everything fit too. Awesome is all i have to say!

Offline Aus78Formula

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Re: 403 running weird
« Reply #13 on: June 02, 2019, 04:53:01 PM »
Tell your brother to correct all the 403 listings being for a 79 TA only.
Good to hear you got it sorted and woke her up.

Offline mattblack84

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Re: 403 running weird
« Reply #14 on: June 03, 2019, 09:49:07 PM »
Yea I brought that up to him and they are working on it.