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    Author Topic: Carb Problem  (Read 1757 times)

    Offline cudabob496

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    Carb Problem
    « on: April 04, 2014 - 11:04:01 PM »
    My Holley 850 DP has been great for 14 years.  In the last month it is backfiring
    more and more duing startup.  Float levels are good, and I recently replace the float
    needle/seat assemblies.  Thought timing may be too much, so backed off about 4 degrees,
    but still backfiring.  Any ideas?  Maybe I'm due for a total rebuild?

    PS Once it starts up, it seems to idle and run fine.
    « Last Edit: April 04, 2014 - 11:09:28 PM by cudabob496 »
    Logged
    72 Cuda, owned 25 years. 496, with ported Stage VI heads, .625 in solid roller, 254/258 at .050, 3500 stall, 3.91 rear. 850 Holley DP, Reverse manual valve body.

    1999 Trans Am, LS1, heads, cam, headers, stall, etc! Love to surprise the rice rockets with this one. They seem so confident, then it's "what the heck just happened?"

    2011 Kawasaki Z1000


    Offline cudabob496

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    Re: Carb Problem
    « Reply #1 on: April 05, 2014 - 01:24:11 AM »
    May have found problem. Have a new distributor cap and rotor, installed last year, but now their's a lotta moisture and corrosion in there.
    Cap has a vent hole that is capped.  Is that supposed to be unplugged, to vent the cap?

    about every piece of metal under the distributor cap has corrosion on it, and there is a lot of greenish copper corrorsion product
    on the floor of the distributor. And I live in a dry climate! Why didn't MSD tell me to yank the vent cap off my distributor cap?

    I came across this while troubleshooting a backfire, which can be caused by crossed wires, and the like.
    Everyone should look under their dist caps for moisture and corrosion.  Can't believe it was this bad
    after only 1 year of use!!
    « Last Edit: April 05, 2014 - 02:38:57 AM by cudabob496 »
    Logged
    72 Cuda, owned 25 years. 496, with ported Stage VI heads, .625 in solid roller, 254/258 at .050, 3500 stall, 3.91 rear. 850 Holley DP, Reverse manual valve body.

    1999 Trans Am, LS1, heads, cam, headers, stall, etc! Love to surprise the rice rockets with this one. They seem so confident, then it's "what the heck just happened?"

    2011 Kawasaki Z1000

    Offline dodj

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    Re: Carb Problem
    « Reply #2 on: April 05, 2014 - 01:42:24 PM »
    You don't remove the cap. There are air passages underneath. The cap is just so moisture cant drip into the vent.
    Logged
    Scott
    1973 Challenger  440 4 spd 
    2007.5 3500 6.7 Cummins Diesel, Anarchy tuned.
    Good friends don't let friends do stupid things. ........alone.

    Offline cudabob496

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    Re: Carb Problem
    « Reply #3 on: April 05, 2014 - 02:19:46 PM »
    Quote from: dodj on April 05, 2014 - 01:42:24 PM
    You don't remove the cap. There are air passages underneath. The cap is just so moisture cant drip into the vent.

    Well, old cap was on there for 10 years, and distributor was dry inside. This new one was on for 1 year,
    and lots of rust inside, so I think cap was not allowing venting.
    Logged
    72 Cuda, owned 25 years. 496, with ported Stage VI heads, .625 in solid roller, 254/258 at .050, 3500 stall, 3.91 rear. 850 Holley DP, Reverse manual valve body.

    1999 Trans Am, LS1, heads, cam, headers, stall, etc! Love to surprise the rice rockets with this one. They seem so confident, then it's "what the heck just happened?"

    2011 Kawasaki Z1000

    Offline cudabob496

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    Re: Carb Problem
    « Reply #4 on: April 05, 2014 - 05:18:46 PM »
    Dang, cleaned up distributor, but still backfiring. AFR shows running rich.
    Will investigate the power valve next.
    Logged
    72 Cuda, owned 25 years. 496, with ported Stage VI heads, .625 in solid roller, 254/258 at .050, 3500 stall, 3.91 rear. 850 Holley DP, Reverse manual valve body.

    1999 Trans Am, LS1, heads, cam, headers, stall, etc! Love to surprise the rice rockets with this one. They seem so confident, then it's "what the heck just happened?"

    2011 Kawasaki Z1000

    Offline dodj

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    Re: Carb Problem
    « Reply #5 on: April 05, 2014 - 06:00:25 PM »
    Exhaust leak can cause backfiring too.
    Logged
    Scott
    1973 Challenger  440 4 spd 
    2007.5 3500 6.7 Cummins Diesel, Anarchy tuned.
    Good friends don't let friends do stupid things. ........alone.

    Offline cudabob496

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    Re: Carb Problem
    « Reply #6 on: April 05, 2014 - 06:12:42 PM »
    Quote from: dodj on April 05, 2014 - 06:00:25 PM
    Exhaust leak can cause backfiring too.

    Yep.  With the air cleaner off, its defintely shooting out the carb.  I've read if out the carb, could be lean condition, if
    out the exhaust, could be too rich.  But could be other things too. AFR showing rich, so will check power valve.
    AFR meters are very valuable if you want to know whats going on in carb.  May have to get new cap and rotor too.
    Logged
    72 Cuda, owned 25 years. 496, with ported Stage VI heads, .625 in solid roller, 254/258 at .050, 3500 stall, 3.91 rear. 850 Holley DP, Reverse manual valve body.

    1999 Trans Am, LS1, heads, cam, headers, stall, etc! Love to surprise the rice rockets with this one. They seem so confident, then it's "what the heck just happened?"

    2011 Kawasaki Z1000

    Offline jhaag

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    Re: Carb Problem
    « Reply #7 on: April 05, 2014 - 07:23:41 PM »
    check all the air bleeds. If you haven't touched it in years, could be blocked. Spray carb cleaner in them and a little compressed air.
    Logged
    love 70 Challengers

    Offline cudabob496

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    Re: Carb Problem
    « Reply #8 on: April 05, 2014 - 08:05:23 PM »
    Quote from: jhaag on April 05, 2014 - 07:23:41 PM
    check all the air bleeds. If you haven't touched it in years, could be blocked. Spray carb cleaner in them and a little compressed air.

    ok, thanks. I cleaned them a year ago, but haven't checked lately.
    Logged
    72 Cuda, owned 25 years. 496, with ported Stage VI heads, .625 in solid roller, 254/258 at .050, 3500 stall, 3.91 rear. 850 Holley DP, Reverse manual valve body.

    1999 Trans Am, LS1, heads, cam, headers, stall, etc! Love to surprise the rice rockets with this one. They seem so confident, then it's "what the heck just happened?"

    2011 Kawasaki Z1000

    Offline cudabob496

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    Re: Carb Problem
    « Reply #9 on: April 08, 2014 - 03:59:01 PM »
    Crazy, MSD says my cap might not be venting enough (duh, its a rusty mess), and I should drill a 5/32
    inch hole in the side of my cap (which is what I had done on the original cap, and why things were clean
    under cap for over 10 years.) Moral of the story, don't trust the vent on your MSD cap for keeping moisture
    out of your distributor! The moisture is created by spark ionization.
    « Last Edit: April 08, 2014 - 09:44:33 PM by cudabob496 »
    Logged
    72 Cuda, owned 25 years. 496, with ported Stage VI heads, .625 in solid roller, 254/258 at .050, 3500 stall, 3.91 rear. 850 Holley DP, Reverse manual valve body.

    1999 Trans Am, LS1, heads, cam, headers, stall, etc! Love to surprise the rice rockets with this one. They seem so confident, then it's "what the heck just happened?"

    2011 Kawasaki Z1000

    Offline erat340

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    Re: Carb Problem
    « Reply #10 on: April 08, 2014 - 09:09:20 PM »
    Try this... spray a light mist of WD40 on the inside of the dist cap and see if it runs better, old trick of mine.
    Logged

    Offline cudabob496

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    Re: Carb Problem
    « Reply #11 on: April 08, 2014 - 11:03:24 PM »
    Quote from: erat340 on April 08, 2014 - 09:09:20 PM
    Try this... spray a light mist of WD40 on the inside of the dist cap and see if it runs better, old trick of mine.

    Something to keep in mind!

    My guess is my distributor got all messed up from moisture and corrosion, which then caused the backfire, which blew my power valve.
    I told MSD they need to redesign thier product so this does not happen. Their vent is inadequate.
    I live in a dry area, with low humidity, and my distributor internals still rusted up!
    Logged
    72 Cuda, owned 25 years. 496, with ported Stage VI heads, .625 in solid roller, 254/258 at .050, 3500 stall, 3.91 rear. 850 Holley DP, Reverse manual valve body.

    1999 Trans Am, LS1, heads, cam, headers, stall, etc! Love to surprise the rice rockets with this one. They seem so confident, then it's "what the heck just happened?"

    2011 Kawasaki Z1000

    Offline cudabob496

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    Re: Carb Problem
    « Reply #12 on: April 12, 2014 - 10:57:21 PM »
    Well, problem solved, thankfully. New cap and rotor, and new power valve, and all seems back to normal.
    That MSD cap and rotor was only in there for about 1500 miles, and corroded itself to death, because the vent
    either wasn't working, or is too small.  Gonna see if I can get my money back for those. 

    I read somewhere that 90% of problems with an engine are usually electrical, with fuel related issues
    being only about 10%. Beginning to believe it.

    Anyway, make sure to vent your distributor cap. I put two 1/4 inch holes on the side of this new one. The side
    facing the headers, so dirt can't freely fall in there.
    Logged
    72 Cuda, owned 25 years. 496, with ported Stage VI heads, .625 in solid roller, 254/258 at .050, 3500 stall, 3.91 rear. 850 Holley DP, Reverse manual valve body.

    1999 Trans Am, LS1, heads, cam, headers, stall, etc! Love to surprise the rice rockets with this one. They seem so confident, then it's "what the heck just happened?"

    2011 Kawasaki Z1000

    Offline dodj

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    Re: Carb Problem
    « Reply #13 on: April 13, 2014 - 12:11:33 AM »
    Two 1/4" holes?!? Think that's enough?


    Think you went kinda extreme there.....
    Logged
    Scott
    1973 Challenger  440 4 spd 
    2007.5 3500 6.7 Cummins Diesel, Anarchy tuned.
    Good friends don't let friends do stupid things. ........alone.

    Offline cudabob496

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    • Posts: 8024
    Re: Carb Problem
    « Reply #14 on: April 13, 2014 - 03:35:31 AM »
    Quote from: dodj on April 13, 2014 - 12:11:33 AM
    Two 1/4" holes?!? Think that's enough?


    Think you went kinda extreme there.....

    I had one 3/8 hole on my old MSD cap, and it lasted 12 years.  These two 1/4 inch holes are about 2 inches apart.  Don't see
    that it can hurt anything, and should help keep things dry.

    I assume, with 8 pickups, and several thousand RPM, there is a significant spark show going on inside the distributor cap, with the build
    up of humidity.  I suppose the size of the vent hole(s) will determine how quickly things dry out in there, after the car is shut off. MSD is
    known for making some big sparks, with a lot more energy than a standard stock distributor.

    PS, good Mopar story:
    https://www.youtube.com/watch?v=zlnocas2IjM
    « Last Edit: April 13, 2014 - 04:56:46 AM by cudabob496 »
    Logged
    72 Cuda, owned 25 years. 496, with ported Stage VI heads, .625 in solid roller, 254/258 at .050, 3500 stall, 3.91 rear. 850 Holley DP, Reverse manual valve body.

    1999 Trans Am, LS1, heads, cam, headers, stall, etc! Love to surprise the rice rockets with this one. They seem so confident, then it's "what the heck just happened?"

    2011 Kawasaki Z1000
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